RRB Technician Grade III 2024-25 29 December 2024 Shift 1 GK Questions – with answers & solutions

Correct Answer: Option 1 — ATP
ATP (Adenosine Triphosphate) is called the energy currency of the cell. It is manufactured mainly in the mitochondria during cellular respiration, when glucose is oxidised step by step and the energy released is locked up in the high-energy phosphate bonds of ATP.
Whenever a cell needs energy, the terminal phosphate bond of ATP is broken by water (hydrolysis) to give ADP + inorganic phosphate, releasing about 30.5 kJ/mol of usable energy. This energy drives muscle contraction, protein synthesis, nerve impulse conduction and active transport across membranes.
❌ Why the other options are incorrect
✗ DNA — Deoxyribonucleic acid is the hereditary material of the cell; it stores and transmits genetic information and controls protein synthesis, but it is not an energy carrier.
✗ RNA — Ribonucleic acid carries the genetic message (mRNA) and takes part in protein synthesis (tRNA, rRNA); it does not supply energy for cell work.
✗ NAD — Nicotinamide adenine dinucleotide is only a hydrogen / electron carrier (coenzyme); it hands electrons to the electron transport chain, where ATP is finally produced.
📌 SSC & Railway Exam Facts
• ATP was discovered by Karl Lohmann (1929); the idea of ATP as energy currency is linked with Fritz Lipmann.
• Mitochondrion is the powerhouse of the cell; ATP is synthesised on its inner folds called cristae.
• Aerobic respiration of one glucose molecule yields 38 ATP (net 36 in eukaryotes), while anaerobic respiration yields only 2 ATP.
• In plants, ATP is also produced in the chloroplast during the light reaction of photosynthesis.
• GTP is another high-energy compound, used in protein synthesis and in the Krebs cycle.

Correct Answer: Option 3 — It has a single chromosome.
A prokaryotic cell is a primitive cell that has no true nucleus. Its genetic material — a single, circular chromosome of naked DNA — lies free in the cytoplasm in a region called the nucleoid, which has no nuclear membrane and no nucleolus.
Prokaryotes also lack all membrane-bound organelles such as mitochondria, plastids, Golgi apparatus, endoplasmic reticulum and lysosomes. Only 70S ribosomes are present. Bacteria, cyanobacteria (blue-green algae), mycoplasma and archaebacteria are prokaryotes.
❌ Why the other options are incorrect
✗ It has more than one chromosome — wrong; multiple chromosomes are a feature of eukaryotic cells. A bacterium has only one chromosome (plus small plasmids).
✗ Nuclear region is well defined and surrounded by nuclear membrane — wrong; this describes the true nucleus of a eukaryotic cell. A prokaryote has only an undefined nucleoid.
✗ Membrane bound cell organelles are present — wrong; membrane-bound organelles are absent in prokaryotes and are a defining feature of eukaryotes.
📌 SSC & Railway Exam Facts
• The words prokaryote and eukaryote were popularised by Édouard Chatton; karyon means nucleus.
• Prokaryotic ribosome = 70S; eukaryotic ribosome = 80S.
• Bacterial cell wall is made of peptidoglycan (murein); plant cell wall is made of cellulose.
• Infoldings of the prokaryotic plasma membrane are called mesosomes; they help in respiration and cell division.
• Mycoplasma is the smallest cell, and it is the only prokaryote without a cell wall.

Correct Answer: Option 1 — saliva
Saliva is not secreted by the stomach at all. It is produced by the three pairs of salivary glands in the buccal cavity (mouth) — parotid, submandibular and sublingual. Saliva contains the enzyme salivary amylase (ptyalin), which begins the digestion of starch into maltose in the mouth itself.
The gastric glands in the stomach wall release gastric juice, which has exactly three components:
• Hydrochloric acid (HCl) — makes the medium acidic (pH about 1.5–2), kills germs and activates pepsinogen.
• Pepsin — the protein-digesting enzyme, active only in acidic medium.
• Mucus — protects the inner lining of the stomach from the acid and from self-digestion.
❌ Why the other options are incorrect
✗ Pepsin — it IS released by gastric glands; secreted as inactive pepsinogen, converted to active pepsin by HCl, and it breaks proteins into peptones.
✗ Mucus — it IS released by the gastric glands; without it the acid would cause ulcers in the stomach wall.
✗ Hydrochloric acid — it IS released by the oxyntic (parietal) cells of the gastric glands and gives the stomach its acidic medium.
📌 SSC & Railway Exam Facts
• The stomach is the widest part of the alimentary canal; it is J-shaped.
• Salivary amylase works best in a slightly alkaline medium; pepsin works best in an acidic medium.
• Trypsin (pancreatic juice) and erepsin (intestinal juice) also digest proteins, but in the alkaline small intestine.
• Excess HCl causes acidity and peptic ulcers; Helicobacter pylori bacteria are a common cause of ulcers.
• Rennin in the gastric juice of infants curdles milk protein (casein).

Correct Answer: Option 1 — Both amino acids and sucrose
The phloem is the food-conducting vascular tissue of plants. The movement of soluble food through it is called translocation, and it is carried out by sieve tubes with the help of companion cells.
Phloem transports sucrose, the main soluble product of photosynthesis, from the source (leaf) to the sink (root, fruit, seed, growing bud). Along with it, the phloem also carries amino acids and other substances such as hormones. Unlike the xylem, translocation in phloem is bidirectional (both upward and downward) and needs energy from ATP — sugar is loaded into the sieve tube using ATP, water then enters by osmosis, and the resulting pressure pushes the sap along.
❌ Why the other options are incorrect
✗ Only Water — pure water and dissolved minerals are conducted by the xylem, and that movement is unidirectional (root to leaf).
✗ Only Amino acids — amino acids are carried, but so is sucrose; restricting it to amino acids alone is incomplete.
✗ Only Sucrose — sucrose is carried, but the phloem also moves amino acids and other organic substances.
📌 SSC & Railway Exam Facts
• Xylem — water and minerals, upward, uses transpiration pull, made of dead cells (tracheids, vessels).
• Phloem — food, both ways, uses ATP energy, made mostly of living cells (sieve tubes, companion cells, phloem parenchyma, phloem fibres).
• The pressure-flow (mass-flow) hypothesis of translocation was given by Ernst Münch (1930).
• Sucrose is the chief transported sugar; starch is the chief stored form.
• Removing a ring of bark (girdling) stops phloem transport and proves that food moves through the bark.

Correct Answer: Option 4 — Plasma membrane, nucleus and cytoplasm
NCERT states that every cell, whether of a plant or an animal, shows three basic features that make all its internal activities and its dealings with the outside world possible:
• Plasma membrane — the outermost living, selectively permeable boundary; it separates the cell from its surroundings and controls the entry and exit of materials by diffusion, osmosis and active transport.
• Nucleus — the control centre of the cell; it holds the chromosomes (DNA), directs all cell activities and passes on hereditary characters.
• Cytoplasm — the jelly-like fluid between the membrane and the nucleus in which all organelles float and in which most metabolic reactions (like glycolysis) take place.
❌ Why the other options are incorrect
✗ Plasma membrane, nucleus and Golgi apparatus — the Golgi apparatus is only one organelle (packaging and dispatch); it is absent in prokaryotes, so it cannot be a universal feature.
✗ Mitochondria, nucleus and cytoplasm — mitochondria are absent in prokaryotes and in mammalian RBCs; the plasma membrane, not mitochondria, is the third basic feature.
✗ Mitochondria, nucleus and Golgi apparatus — both mitochondria and Golgi apparatus are membrane-bound organelles missing from prokaryotic cells.
📌 SSC & Railway Exam Facts
• The cell was discovered by Robert Hooke (1665) in a slice of cork; he saw only the dead cell walls.
• Anton van Leeuwenhoek first saw living cells; Robert Brown (1831) discovered the nucleus.
• Cell Theory was given by Schleiden and Schwann (1838–39) and completed by Rudolf Virchow — *Omnis cellula-e cellula*.
• The plasma membrane is made of lipids and proteins; the fluid mosaic model was proposed by Singer and Nicolson (1972).
• Cytoplasm + nucleoplasm = protoplasm, called the physical basis of life by J. E. Purkinje.

Correct Answer: Option 4 — Photosynthesis
Photosynthesis is the process by which autotrophs (green plants, algae, cyanobacteria) take in simple inorganic substances — carbon dioxide from the air and water from the soil — and, using sunlight trapped by chlorophyll, convert them into carbohydrates (glucose), a stored form of chemical energy. Oxygen is released as a by-product.
6CO₂ + 12H₂O → (sunlight / chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O
The main events are: absorption of light energy by chlorophyll, conversion of light energy to chemical energy and splitting of water (photolysis) into hydrogen and oxygen, and reduction of CO₂ to carbohydrates. Extra glucose is stored as starch in the plant and as glycogen in animals that eat the plant.
❌ Why the other options are incorrect
✗ Movement — a general sign of life (locomotion in animals, tropic and nastic movements in plants); it does not make food.
✗ Digestion — the breakdown of complex food into simpler absorbable forms by enzymes; it is a heterotrophic process that releases energy rather than storing it.
✗ Circulation — the transport of blood, gases, food and wastes within the body; it distributes materials but does not synthesise them.
📌 SSC & Railway Exam Facts
• Chlorophyll is the green pigment in the chloroplast; the chloroplast is called the kitchen of the cell.
• Chlorophyll contains magnesium (Mg); haemoglobin contains iron (Fe).
• Photosynthesis occurs in two phases — the light reaction in the grana and the dark reaction (Calvin cycle) in the stroma.
• Gaseous exchange in leaves takes place through tiny pores called stomata, guarded by guard cells.
• Van Helmont, Joseph Priestley and Julius von Sachs performed the classic experiments on photosynthesis; the word was coined by Charles Barnes.

Correct Answer: Option 3 — cell
The cell is the structural and functional unit of life — the smallest unit that can carry out all the activities of a living being. Inside it, different cell organelles such as mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes and plastids work together like the departments of a factory, each with its own duty, all enclosed by the plasma membrane and controlled by the nucleus.
The levels of organisation in the body follow a clear ladder:
Cell → Tissue → Organ → Organ system → Organism
So organelles build the cell, and only after that do cells build tissues.
❌ Why the other options are incorrect
✗ Organ system — a group of organs working together for one major function, e.g. the digestive system (mouth, stomach, intestine, liver); it is the fourth level, far above organelles.
✗ Organ — a structure made of different tissues working together, e.g. heart, stomach, leaf, root; it is formed by tissues, not directly by organelles.
✗ Tissue — a group of similar cells performing a common function, e.g. muscle tissue, xylem; it is formed by cells, which themselves are made of organelles.
📌 SSC & Railway Exam Facts
• The term cell was coined by Robert Hooke (1665) from the Latin *cellula* meaning 'a little room'.
• Ostrich egg is the largest cell; Mycoplasma the smallest; the nerve cell is the longest cell in the human body.
• Amoeba, Paramecium and Chlamydomonas are unicellular; humans and trees are multicellular.
• An organelle without a membrane is the ribosome; mitochondria and plastids are double-membraned.
• Cell organelles were clearly seen only after the invention of the electron microscope.

Correct Answer: Option 2 — stomach
Pepsin is secreted by the gastric glands present in the wall of the stomach. It is released in an inactive form called pepsinogen, which is converted into active pepsin by the hydrochloric acid (HCl) of the gastric juice.
Pepsin is a proteolytic (protein-digesting) enzyme. It breaks large proteins into smaller units called peptones and proteoses. It works only in a strongly acidic medium (optimum pH about 1.5–2), which is exactly why the stomach secretes HCl. The mucus secreted alongside protects the stomach wall from being digested by its own pepsin.
❌ Why the other options are incorrect
✗ Duodenum — the first part of the small intestine; it receives bile (from liver) and pancreatic juice containing trypsin, amylase and lipase, and works in an alkaline medium.
✗ Ileum — the last part of the small intestine, mainly the site of absorption through finger-like villi; it secretes intestinal juice (succus entericus), not pepsin.
✗ Mouth — secretes saliva with salivary amylase (ptyalin), which digests starch, not protein.
📌 SSC & Railway Exam Facts
• Protein digestion: Pepsin (stomach) → Trypsin (pancreas) → Erepsin/peptidases (intestine) → amino acids.
• Pepsin was discovered by Theodor Schwann (1836) — the first animal enzyme to be discovered.
• Bile has no enzyme; it only emulsifies fats and makes the medium alkaline.
• The pancreas is a mixed gland — exocrine (pancreatic juice) and endocrine (insulin, glucagon).
• Rennin curdles milk in infants; lipase digests fats into fatty acids and glycerol.

Correct Answer: Option 2 — cell wall
The cell wall is a rigid, non-living covering found outside the plasma membrane in plant cells, fungi and bacteria. In plants it is made mainly of cellulose, a tough carbohydrate, along with hemicellulose and pectin.
Its functions are to give the cell shape, rigidity and mechanical strength, to protect it against mechanical injury and infection, and above all to allow plant cells to survive hypotonic surroundings — when a plant cell absorbs a lot of water by osmosis and swells, the strong wall exerts an opposing turgor pressure and stops the cell from bursting. The cell wall is fully permeable, so it does not control what enters the cell.
❌ Why the other options are incorrect
✗ Nuclear membrane — the double membrane with pores around the nucleus, present inside the cell, not outside the plasma membrane.
✗ Selectively permeable membrane — this is another name for the plasma membrane itself, which lies inside the cell wall and decides which substances enter or leave.
✗ Cell envelope — a term used for the multi-layered covering of bacteria (cell wall + membrane + capsule/glycocalyx); it is not the standard NCERT name for the rigid plant covering.
📌 SSC & Railway Exam Facts
• Plant cell wall — cellulose; fungal cell wall — chitin; bacterial cell wall — peptidoglycan.
• Animal cells have no cell wall, which is why they burst (haemolysis) in very dilute solutions.
• Adjacent plant cells stay connected through fine cytoplasmic threads called plasmodesmata.
• Loss of water makes the cell contents shrink away from the wall — plasmolysis.
• The cell wall was first seen by Robert Hooke in dead cork cells.

Correct Answer: Option 3 — Cracking of voice
Cracking or breaking of the voice is a secondary sexual character of boys, not girls. At puberty the male hormone testosterone, secreted by the testes, enlarges the larynx (voice box) and thickens the vocal cords, so the boy's voice becomes deep and 'cracks' for a while.
In girls, puberty is controlled by oestrogen secreted by the ovary. Its signs are enlargement of the breasts, widening of the hips, growth of hair in the armpits and pubic region, oily skin with pimples, and the beginning of the menstrual cycle (menarche) — the sure sign that the ovary has started releasing eggs.
❌ Why the other options are incorrect
✗ Oily skin — at puberty the sebaceous (oil) glands become very active in both girls and boys, causing oily skin and acne; it IS a sign of maturity.
✗ Increase in breast size — a definite female secondary sexual character caused by oestrogen; the mammary glands develop for future lactation.
✗ Menstruation — the monthly shedding of the uterine lining, beginning at menarche; it is the clearest sign of sexual maturity in girls.
📌 SSC & Railway Exam Facts
• Puberty in girls usually begins at 10–12 years and in boys at 12–14 years.
• Oestrogen and progesterone — female hormones (ovary); testosterone — male hormone (testis).
• The menstrual cycle lasts about 28 days; ovulation occurs around the 14th day.
• The first menstruation is menarche; its permanent stopping at about 45–50 years is menopause.
• Both ovary and testis are mixed glands — they make gametes as well as hormones.

Correct Answer: Option 4 — Geotropism
Geotropism (gravitropism) is the directional growth movement of a plant part in response to the pull of gravity. It explains both the upward and the downward growth of a plant:
• The stem grows upward, away from gravity — this is negative geotropism.
• The root grows downward, towards gravity — this is positive geotropism.
The response is controlled by the plant hormone auxin. Under gravity, auxin collects on the lower side of the organ. In the stem, extra auxin makes the lower side elongate faster, bending the shoot up; in the root, the same high concentration inhibits growth on the lower side, so the root curves down.
❌ Why the other options are incorrect
✗ Phototropism — growth towards or away from light; the shoot is positively phototropic and the root negatively phototropic. It explains bending towards a window, not up-and-down growth.
✗ Chemotropism — growth towards a chemical; the classic example is the pollen tube growing towards the ovule through the style.
✗ Hydrotropism — growth towards water; roots bend towards a moist region of soil. It relates to moisture, not to gravity.
📌 SSC & Railway Exam Facts
• Tropic movements are directional and growth-dependent; nastic movements (e.g. the drooping of *Mimosa pudica* leaves) are non-directional and not due to growth.
• Auxin was the first plant hormone discovered; Charles Darwin and Francis Darwin did the pioneering phototropism experiments.
• Gibberellin — stem elongation; Cytokinin — cell division; Abscisic acid (ABA) — stress/growth inhibitor, closes stomata.
• Ethylene is the only gaseous plant hormone; it ripens fruits.
• Thigmotropism is growth in response to touch — seen in the tendrils of climbers like pea and grapevine.

Correct Answer: Option 2 — At maturity, the rate of general body growth in humans begins to slow down.
In humans, sexual maturity (puberty) marks a clear change of priority in the body. NCERT states that as an individual approaches maturity, the rate of general body growth begins to slow down, while the reproductive tissues begin to mature. The body has largely finished building itself, so its resources are now directed towards gamete formation and the development of secondary sexual characters.
This change is driven by hormones — testosterone from the testes in boys and oestrogen from the ovaries in girls, both under the control of the pituitary gland.
❌ Why the other options are incorrect
✗ Growth gets faster at maturity — wrong; the rapid growth spurt happens before and during early puberty. At maturity the growth rate falls and finally stops when the bone epiphyseal plates close.
✗ Reproductive tissue stops to mature — wrong; exactly the opposite happens. At maturity the reproductive tissues begin to mature, so that sperms and ova can be produced.
✗ Germ-cells from one individual join together — wrong; that would be self-fusion. In sexual reproduction, germ cells come from two different individuals (a male and a female), which is the very source of variation.
📌 SSC & Railway Exam Facts
• Gametes are formed by meiosis and are haploid (n); their fusion restores the diploid (2n) number in the zygote.
• In humans 2n = 46 chromosomes (23 pairs); the gamete has 23.
• Sexual reproduction produces variation — the raw material of evolution; asexual reproduction gives near-identical clones.
• Human sperms are produced in the seminiferous tubules of the testis; eggs mature in the ovary.
• The testes lie outside the body in the scrotum because sperm formation needs a temperature about 2–3 °C lower than body temperature.

Correct Answer: Option 4 — abiotic components
An ecosystem is a self-sustaining unit in which living organisms interact with one another and with their physical surroundings. It has two components:
• Abiotic (non-living) components — the physical and chemical factors of the environment: temperature, rainfall, humidity, wind, light, soil, water, air and minerals. They decide which organisms can live in a place.
• Biotic (living) components — producers (green plants), consumers (herbivores, carnivores, omnivores, parasites) and decomposers (bacteria and fungi).
Since temperature, rainfall, wind, soil and minerals are all non-living, they are abiotic components.
❌ Why the other options are incorrect
✗ Living components — these are the organisms themselves; temperature, wind and minerals are clearly non-living.
✗ Biotic components — the biotic group covers only living things — plants, animals and microbes — not physical factors.
✗ Decomposers — a sub-group of the biotic component; bacteria and fungi that break down dead remains and return nutrients to the soil.
📌 SSC & Railway Exam Facts
• The term ecosystem was coined by A. G. Tansley (1935).
• Natural ecosystems — forest, grassland, lake, ocean; artificial (man-made) — crop field, garden, aquarium.
• The ocean is the largest ecosystem; the forest is the largest terrestrial ecosystem.
• Energy flow in an ecosystem is unidirectional, while nutrients move in cycles (biogeochemical cycles).
• Decomposers are also called saprophytes; without them nutrients would stay locked in dead bodies.

Correct Answer: Option 1 — primary consumer
The honeybee feeds directly on the nectar and pollen of flowers, that is, directly on a producer (green plant). Any organism that eats a producer is a herbivore, and a herbivore occupies the second trophic level of a food chain and is called a primary consumer.
Honey is only the processed and stored food made by the bee from nectar — it is not food manufactured from carbon dioxide, water and sunlight. Making one's own food from inorganic raw materials is the job of an autotroph, and the honeybee cannot do that; it is a heterotroph.
A typical chain: Flower (producer) → Honeybee (primary consumer) → Bee-eater bird (secondary consumer) → Hawk (tertiary consumer).
❌ Why the other options are incorrect
✗ Producer — producers are autotrophs like green plants and algae that make food by photosynthesis; the bee only collects and concentrates food already made by the plant.
✗ Decomposer — bacteria and fungi that break down dead bodies and wastes into simple substances; the honeybee feeds on living flowers, not on dead matter.
✗ Secondary consumer — an animal that eats a primary consumer, i.e. a small carnivore like a frog or snake; the bee eats plant products, not animals.
📌 SSC & Railway Exam Facts
• Rearing of honeybees for honey and wax is apiculture; the common Indian species is *Apis cerana indica*.
• Honeybees are also the most important pollinating agents, which is vital for crop yield.
• Grazing food chain begins with a green plant; the detritus food chain begins with dead organic matter.
• Only about 10% of energy passes from one trophic level to the next — Lindeman's 10 per cent law.
• Sericulture — silkworm rearing; Pisciculture — fish farming; Apiculture — beekeeping.

Correct Answer: Option 2 — Displacement reaction
In a displacement reaction a more reactive element takes the place of a less reactive element in its compound, setting the less reactive one free. Its general pattern is A + BC → AC + B, and a free element must take part on the reactant side.
Whether one metal can push out another is decided entirely by the reactivity series. The higher a metal stands in the series, the stronger its tendency to lose electrons and form ions, so it can throw a lower metal out of its salt solution. A metal cannot displace a metal placed above it.
Dipping a strip of zinc into blue copper sulphate solution shows this beautifully: the blue colour fades and a reddish-brown deposit of copper collects on the zinc.
⚗️ The Reaction
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Zinc is above copper in the reactivity series, so it displaces copper from the sulphate.
• The blue copper sulphate solution turns colourless as ZnSO₄ is formed.
• The reverse, Cu + ZnSO₄, never happens — copper is the less reactive metal.
❌ Why the other options are incorrect
✗ Combination reaction — two or more substances join to give a single product, e.g. CaO + H₂O → Ca(OH)₂; here two separate products appear.
✗ Decomposition reaction — a single compound breaks up into simpler substances, e.g. 2H₂O → (electricity) 2H₂ + O₂; here two reactants are present.
✗ Neutralisation reaction — an acid reacts with a base to give salt and water; no free metal and no reactivity series is involved.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• A + BC → AC + B = displacement; AB + CD → AD + CB = double displacement.
• Fe + CuSO₄ → FeSO₄ + Cu (blue → pale green) is the classic PYQ example.
• Metals above hydrogen displace H₂ from dilute acids; Cu, Hg, Ag and Au do not.
• The thermite reaction, 2Al + Fe₂O₃ → 2Fe + Al₂O₃ + heat, is a highly exothermic displacement used to weld railway tracks.

Correct Answer: Option 4 — pure gold
The carat (also written *karat*) is the unit used to express the purity of gold. The whole scale runs to 24, so 24-carat gold means 24 parts of gold out of 24 parts of metal — that is 100% (about 99.9%) pure gold.
The purity is worked out as percentage of gold = (carat ÷ 24) × 100. Hence 22-carat gold = (22/24) × 100 = 91.6% gold, which is why hallmarked jewellery carries the stamp 916.
Pure 24-carat gold is extremely soft, malleable and ductile, so ornaments cannot be made from it — it would bend out of shape. To harden it, gold is alloyed with copper or silver, and this alloying is exactly what lowers the carat value. Coins, bars and biscuits are made of 24-carat gold; jewellery is usually 22-, 18- or 14-carat.
❌ Why the other options are incorrect
✗ wrought gold — "wrought" describes a metal that has been hammered or worked into shape; the purest commercial form of iron is called wrought iron, not gold.
✗ impure gold — impure gold is gold mixed with other metals, which is precisely less than 24 carat (22, 18, 14 carat, etc.).
✗ spongy gold — "spongy" is the term used for the porous mass of iron obtained in a blast furnace (spongy iron), not a description of pure gold.
📌 SSC & Railway Exam Facts
• 24 carat = 99.9% pure; 22 carat = 91.6% (916 gold); 18 carat = 75%; 14 carat ≈ 58.5%.
• Gold's symbol Au comes from the Latin aurum; its atomic number is 79.
• Gold is the most malleable and most ductile metal — 1 g can be drawn into a wire about 2 km long.
• Gold dissolves only in aqua regia (3 parts conc. HCl : 1 part conc. HNO₃).
• Purity in India is certified by the BIS hallmark; the gold–copper alloy is often called coloured or standard gold.

Correct Answer: Option 4 — Sodium
How a metal behaves with water is a direct reading of its place in the reactivity series. Only the most reactive metals — potassium, sodium and calcium — react with cold water; the middle metals need hot water or steam, and the least reactive ones do not react at all.
Sodium reacts with cold water violently and exothermically, giving sodium hydroxide and hydrogen gas. So much heat is released that the hydrogen catches fire immediately, and the small piece of sodium darts about on the surface. Because of this, sodium and potassium are always stored under kerosene.
⚗️ The Reaction
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + heat
• The solution formed is alkaline, so it turns red litmus blue and phenolphthalein pink.
• The heat evolved ignites the hydrogen, which is why the reaction looks explosive.
• Calcium also reacts with cold water but far more gently: Ca + 2H₂O → Ca(OH)₂ + H₂, and the calcium floats on the bubbles.
❌ Why the other options are incorrect
✗ Iron — does not react with cold or hot water; it reacts only with steam: 3Fe + 4H₂O → (steam) Fe₃O₄ + 4H₂.
✗ Aluminium — a tough, thin layer of aluminium oxide (Al₂O₃) protects it, so it does not attack cold water; it reacts only with steam: 2Al + 3H₂O → Al₂O₃ + 3H₂.
✗ Zinc — lies below aluminium in reactivity and reacts only with steam: Zn + H₂O → ZnO + H₂.
📌 SSC & Railway Exam Facts
• Cold water — K, Na, Ca; hot water — Mg; steam — Al, Zn, Fe; no reaction — Pb, Cu, Ag, Au.
• Potassium reacts most violently of all; sodium and potassium are kept in kerosene to stop them reacting with air and moisture.
• Metal + water → metal hydroxide (or oxide) + hydrogen.
• Magnesium in hot water floats because bubbles of H₂ stick to its surface.
• Sodium's symbol Na comes from natrium; NaOH is caustic soda.

Correct Answer: Option 2 — Both statements I and II are correct.
Statement II is correct. Non-metals form acidic oxides. When such an oxide dissolves in water it gives an acid — for example CO₂ + H₂O → H₂CO₃ (carbonic acid) and SO₂ + H₂O → H₂SO₃ (sulphurous acid). That is why non-metallic oxides turn blue litmus red.
Statement I is also correct. Being acidic, a non-metallic oxide neutralises a base, and every neutralisation gives salt and water. So the general rule is acidic oxide + base → salt + water, exactly parallel to acid + base → salt + water.
Since both statements are true, the answer is Both statements I and II are correct.
⚗️ The Reaction
CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)
• CO₂, a non-metallic oxide, behaves as an acid towards the base NaOH.
• The products are a salt (sodium carbonate) and water — proving Statement I.
• The classic laboratory test uses lime water: CO₂ + Ca(OH)₂ → CaCO₃(s) + H₂O, which turns it milky.
❌ Why the other options are incorrect
✗ Only statement II is correct — Statement I is equally true; an acidic oxide reacting with a base does yield salt + water.
✗ Neither statement I nor II is correct — both are standard NCERT statements, so this is plainly wrong.
✗ Only statement I is correct — Statement II is also true; the acidic nature of non-metallic oxides is the very reason Statement I works.
📌 SSC & Railway Exam Facts
• Metal oxides are basic (Na₂O, CaO, MgO); non-metal oxides are acidic (CO₂, SO₂, SO₃, NO₂, P₂O₅).
• Amphoteric oxides react with both acids and bases: Al₂O₃, ZnO, PbO, SnO.
• Neutral oxides are CO, N₂O, NO and H₂O.
• SO₂ and NO₂ dissolve in rain water to give H₂SO₄ and HNO₃ — the cause of acid rain.
• Acid + base → salt + water is a neutralisation and also a double displacement reaction.

Correct Answer: Option 2 — Soap
A soap is defined as the sodium or potassium salt of a long chain carboxylic acid (fatty acid). A typical example is sodium stearate, C₁₇H₃₅COONa, obtained from stearic acid.
A soap molecule has two ends of opposite nature. The long hydrocarbon chain is the hydrophobic (water-hating), oil-loving tail, while the –COO⁻Na⁺ group is the hydrophilic (water-loving) head. The tails bury themselves in the grease while the heads stay in the water, so the dirt is lifted off the cloth and carried away.
Soap is manufactured by saponification — boiling an oil or fat with sodium hydroxide. Sodium soaps are hard soaps (bathing and washing bars) while potassium soaps are soft soaps (shaving creams, liquid soaps).
⚗️ The Reaction — Saponification
(C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃
• An ester (fat/oil) is hydrolysed by alkali to give soap plus glycerol.
• The by-product glycerol (C₃H₅(OH)₃) is used in medicines, cosmetics and explosives.
• Common salt is added at the end — salting out — to make the soap separate and float.
❌ Why the other options are incorrect
✗ Both detergent and soap — a detergent is not a salt of a carboxylic acid, so the two cannot be grouped together here.
✗ Micelle — a micelle is the spherical cluster that soap molecules form in water with their tails pointing inwards; it is a structure, not a chemical class.
✗ Detergent — a detergent is the ammonium or sulphonate salt of a long chain benzene sulphonic acid or sulphate, e.g. sodium alkyl benzene sulphonate.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of fatty acid; detergent = salt of sulphonic acid/sulphate.
• Soap does not lather in hard water — Ca²⁺ and Mg²⁺ ions form an insoluble scum; detergents work in hard water.
• Soap making is saponification; the by-product is glycerol.
• Micelle formation is the reason soap cleans; micelles scatter light and show the Tyndall effect.
• Detergents with branched chains are non-biodegradable and cause water pollution; straight-chain ones are biodegradable.

Correct Answer: Option 1 — Catenation
Catenation is the ability of an element to form bonds with other atoms of the same element, producing long chains, branched chains and closed rings. Carbon shows catenation to a far greater extent than any other element, and this single property is the reason an entire branch — organic chemistry — exists.
Two factors make carbon so good at it. First, carbon is a very small atom, so the C–C bond is short and exceptionally strong (bond energy about 348 kJ/mol), making the chains stable. Second, carbon is tetravalent — each atom can form four covalent bonds, so after joining two carbons there are still bonds left for hydrogen or other atoms.
Together, catenation + tetravalency explain why over ten million carbon compounds are known — more than the compounds of all other elements put together. The chains may be straight (n-butane), branched (isobutane) or cyclic (cyclohexane, benzene).
❌ Why the other options are incorrect
✗ Cyclisation — this is only the closing of a chain into a ring; it is one *outcome* of catenation, not the name of the general self-linking property.
✗ Metallic bonding — the attraction between positive metal kernels and a sea of delocalised electrons; carbon is a non-metal and bonds covalently.
✗ Polymerisation — the joining of many small monomer units into a giant molecule (polythene, PVC); it needs repeating monomers and is not carbon's basic atom-to-atom bonding ability.
📌 SSC & Railway Exam Facts
• Catenation + tetravalency are the two reasons for the huge number of carbon compounds.
• Order of catenation: C >> Si > Ge ≈ Sn; silicon catenates only up to about 7–8 atoms.
• C–C bond energy ≈ 348 kJ/mol; carbon's atomic number is 6, electronic configuration 2, 4.
• Carbon almost always forms covalent bonds because losing or gaining 4 electrons needs too much energy.
• Allotropes of carbon: diamond (hardest natural substance), graphite (conducts electricity), fullerene (C₆₀, buckminsterfullerene).

Correct Answer: Option 3 — Colloid is a homogeneous mixture.
This is a NOT-type question, so we must pick the false statement. A colloid is a heterogeneous mixture in which the dispersed particles measure 1–1000 nm. To the naked eye a colloid *looks* uniform, which is why students wrongly call it homogeneous — but under a powerful microscope two distinct parts are visible, the dispersed phase and the dispersion medium. Hence Option 3 is not a property of a colloid and is the required answer.
The other three statements are all genuine properties. Colloidal particles are too small to be trapped by filter paper, so ordinary filtration fails; they are also light enough to stay suspended, so they do not settle on standing — a colloid is therefore a stable mixture. They do, however, scatter a beam of light, giving the Tyndall effect.
❌ Why the other options are incorrect
✗ Colloid is a heterogeneous mixture — this is true; a colloid has two clearly separate phases, so it cannot be the answer to a NOT question.
✗ Particles of colloids do not settle down when left undisturbed — true; unlike a suspension, a colloid is stable and shows no sediment.
✗ Colloids cannot be separated by filtration — true; the particles pass through filter paper, and centrifugation must be used instead.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect is shown by colloids and suspensions, never by a true solution.
• A suspension settles and can be filtered; a colloid does neither — separate it by centrifugation.
• Colloid types: sol (paint), gel (jelly, butter), emulsion (milk), foam (shaving cream), aerosol (fog, smoke).
• Milk is an emulsion; smoke is a solid dispersed in a gas.

Correct Answer: Option 4 — Only A
Check each statement against the reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
A is correct. Both zinc and lead stand above copper in the series, so both are more reactive than copper. This is why zinc and lead can displace copper from copper sulphate solution, while copper cannot displace either of them.
B is incorrect — it is simply the opposite of A, and A and B can never both be true.
C is incorrect. When an iron nail is dipped in blue copper sulphate solution, iron (being above copper) displaces the copper, which deposits on the nail as a reddish-brown / brownish layer — not yellow. At the same time the solution changes from blue to pale green because iron(II) sulphate is formed.
Only statement A survives, so the answer is Only A.
⚗️ The Reaction
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• The nail gets a reddish-brown coating of copper, never a yellow one.
• The solution turns from blue (CuSO₄) to pale green (FeSO₄).
• This is a displacement reaction and also a redox reaction: Fe is oxidised, Cu²⁺ is reduced.
❌ Why the other options are incorrect
✗ Only C — C is false; the nail turns brownish, not yellow, and A is true, so C alone cannot be the answer.
✗ Only A and B — A and B are directly contradictory statements; both can never be correct together.
✗ Only A and C — A is correct but C is wrong because of the yellow colour claim.
📌 SSC & Railway Exam Facts
• CuSO₄·5H₂O = blue vitriol (blue); FeSO₄·7H₂O = green vitriol (pale green); ZnSO₄·7H₂O = white vitriol.
• A metal displaces from solution only a metal placed below it in the reactivity series.
• Zn + CuSO₄ → ZnSO₄ + Cu and Pb + CuSO₄ → PbSO₄ + Cu both take place.
• Cu + FeSO₄ → no reaction, because copper is less reactive than iron.
• Most reactive metal in the series is potassium (K); least reactive is gold (Au).

Correct Answer: Option 4 — (i)-(b); (ii)-(c); (iii)-(a); (iv)-(d)
In the IUPAC system the suffix of a name announces the functional group present, while the prefix counts the carbon atoms (meth-, eth-, prop-, but-).
• (i) Ketone → (b) -one (–C=O). The carbonyl group sits inside the chain, between two carbon atoms. Example: propanone, CH₃COCH₃ (acetone) — the smallest possible ketone, since it needs at least 3 carbons.
• (ii) Alkene → (c) -ene (–C=C–). An unsaturated hydrocarbon with a carbon–carbon double bond. Example: ethene, C₂H₄; general formula CₙH₂ₙ.
• (iii) Aldehyde → (a) -al (–CHO). Here the carbonyl group is at the end of the chain and carries a hydrogen. Example: ethanal, CH₃CHO.
• (iv) Carboxylic acid → (d) -oic acid (–COOH). Example: ethanoic acid, CH₃COOH (acetic acid), the acid in vinegar.
The usual trap is swapping -al and -one, so remember: aldehyde → -al (both begin with *al*), and ketone → -one (both contain *one*).
❌ Why the other options are incorrect
✗ (i)-(c); (ii)-(d); (iii)-(b); (iv)-(a) — it calls a ketone an alkene and a carboxylic acid an aldehyde; every pair is misplaced.
✗ (i)-(c); (ii)-(b); (iii)-(a); (iv)-(d) — only (iii) and (iv) are right; ketone and alkene have been interchanged.
✗ (i)-(b); (ii)-(c); (iii)-(d); (iv)-(a) — ketone and alkene are right, but aldehyde and carboxylic acid have been swapped.
📌 SSC & Railway Exam Facts
• Suffixes: alkane → -ane, alkene → -ene, alkyne → -yne, alcohol → -ol, aldehyde → -al, ketone → -one, acid → -oic acid.
• General formulas: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Ethanol (C₂H₅OH) is the alcohol in alcoholic drinks; ethanoic acid (CH₃COOH) is vinegar (5–8% solution).
• Glacial acetic acid is pure ethanoic acid, which freezes at 16.6 °C.
• Consecutive members of a homologous series differ by –CH₂–, i.e. by 14 u.

Correct Answer: Option 1 — condensation and solidification respectively
Every change of state has its own name, and the direction of the change decides whether heat is absorbed or released.
Gas → liquid is called condensation (also liquefaction). Cooling a gas slows its particles, the intermolecular force of attraction grows strong enough to hold them together, and the gas turns into a liquid. Water droplets forming on a cold glass and the formation of clouds, dew and fog are everyday examples.
Liquid → solid is called solidification (also freezing or fusion in reverse). Cooling removes more energy until the particles can only vibrate about fixed positions. Water freezing into ice at 0 °C (273 K) is the standard example.
Both of these changes are exothermic — the substance gives out its latent heat to the surroundings. The reverse changes, melting and vaporisation, absorb latent heat.
❌ Why the other options are incorrect
✗ solidification and condensation respectively — the two names are in the wrong order; solidification is liquid → solid, not gas → liquid.
✗ fusion and vaporisation respectively — fusion (melting) is solid → liquid and vaporisation is liquid → gas; both are the opposite directions of what is asked.
✗ sublimation and fusion respectively — sublimation is solid → gas directly, without a liquid stage (camphor, naphthalene, dry ice, ammonium chloride, iodine).
📌 SSC & Railway Exam Facts
• Solid → liquid = melting/fusion; liquid → gas = vaporisation; gas → liquid = condensation; liquid → solid = freezing/solidification.
• Solid → gas = sublimation; gas → solid = deposition.
• Melting point of ice = 0 °C = 273 K; boiling point of water = 100 °C = 373 K.
• Latent heat of fusion of ice = 3.34 × 10⁵ J/kg; latent heat of vaporisation of water = 22.5 × 10⁵ J/kg.
• Temperature stays constant during a change of state because the heat supplied is used as latent heat.

Correct Answer: Option 1 — Salt and hydrogen gas
The standard rule of the chapter is acid + metal → salt + hydrogen gas. A metal placed above hydrogen in the reactivity series displaces the H⁺ ions of a dilute acid, so hydrogen escapes as a gas and the metal takes its place to form a salt.
The evolved gas is identified by the classic pop test: bring a burning candle or splinter near the mouth of the tube and the hydrogen burns with a squeaky pop sound.
⚗️ The Reaction
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from hydrochloric acid.
• The salt formed is zinc chloride; brisk effervescence of H₂ is seen.
• Other examples: Mg + H₂SO₄ → MgSO₄ + H₂ and 2Na + 2HCl → 2NaCl + H₂.
❌ Why the other options are incorrect
✗ Salt and water — these are the products of neutralisation, i.e. acid + base → salt + water, not of an acid reacting with a metal.
✗ Only Hydrogen gas — the answer is incomplete; along with hydrogen a salt of the metal is always formed and stays in solution.
✗ Only Salt — also incomplete; the brisk effervescence of hydrogen is the very sign that the reaction is taking place.
📌 SSC & Railway Exam Facts
• Acid + metal → salt + H₂; acid + metal carbonate → salt + water + CO₂; acid + metal hydrogen carbonate → salt + water + CO₂; acid + base → salt + water.
• Cu, Hg, Ag and Au lie below hydrogen and do not liberate H₂ from dilute acids.
• Dilute nitric acid normally gives no H₂ — being an oxidising acid it gives oxides of nitrogen; magnesium and manganese are the exceptions.
• Hydrogen is confirmed by the pop sound test; CO₂ is confirmed by turning lime water milky.
• Metal oxides are basic, so metal oxide + acid → salt + water as well.

Correct Answer: Option 4 — Electrolytic refining
The metal obtained after reduction of its ore is still impure, so the last step of metallurgy is refining. The most widely used and most reliable method is electrolytic refining, used for copper, zinc, tin, nickel, silver, gold and aluminium.
The arrangement is easy to remember:
• Anode (+) — a thick block of the impure metal.
• Cathode (−) — a thin strip of the pure metal.
• Electrolyte — a solution of a salt of the same metal, e.g. acidified CuSO₄ for copper.
When current is passed, the impure anode dissolves into the solution as metal ions, and an equivalent amount of pure metal is deposited on the cathode. The soluble impurities go into the solution, while the insoluble impurities — often valuable metals such as silver, gold and platinum — fall to the bottom below the anode. This deposit is called anode mud (anode sludge).
❌ Why the other options are incorrect
✗ Painting — this only puts a protective coat on the metal surface to prevent rusting; it does not remove any impurity.
✗ Oiling — greasing or oiling also merely keeps air and moisture away to stop corrosion of tools and machine parts.
✗ Galvanisation — coating iron or steel with a layer of zinc to prevent rusting; again a protection method, not a purification method.
📌 SSC & Railway Exam Facts
• Impure metal = anode; pure metal = cathode; salt solution of the metal = electrolyte.
• The impurities collecting below the anode are the anode mud, from which Ag, Au and Pt are recovered.
• Order of metallurgy: concentration → roasting/calcination → reduction → refining.
• Roasting = heating a sulphide ore in excess air; calcination = heating a carbonate ore in limited air.
• Other refining methods: distillation (Zn, Hg), liquation (Sn, Pb), poling (Cu), zone refining (Si, Ge).

Correct Answer: Option 3 — Oxygen gas
Passing electricity through a concentrated aqueous solution of sodium chloride (brine) is the industrially important chlor-alkali process. It gives exactly three products, and oxygen is not one of them — so Option 3 is the required answer.
The name itself is a memory aid: "chlor" stands for the chlorine liberated and "alkali" for the sodium hydroxide produced.
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Chlorine gas (Cl₂) is released at the anode.
• Hydrogen gas (H₂) is released at the cathode.
• Sodium hydroxide (NaOH) is left near the cathode in the solution.
• Only H₂ and Cl₂ are evolved as gases; no O₂ is liberated, because the Cl⁻ ions of the concentrated brine are discharged at the anode in preference to water.
❌ Why the other options are incorrect
✗ Hydrogen gas — it is formed, at the cathode; it is used as a fuel, for making ammonia and for hydrogenating oils into vanaspati ghee.
✗ Chlorine gas — it is formed, at the anode; it is used for disinfecting drinking water and for making PVC, CFCs and pesticides.
✗ Sodium hydroxide — it is formed in the solution; caustic soda is used in soaps and detergents, paper making and degreasing metals.
📌 SSC & Railway Exam Facts
• Chlor-alkali process: brine → NaOH + Cl₂ + H₂; Cl₂ at anode, H₂ at cathode.
• Oxygen is obtained by the electrolysis of water (2H₂O → 2H₂ + O₂), where H₂ and O₂ are collected in the ratio 2 : 1.
• NaOH = caustic soda; NaCl = common salt (rock salt); Na₂CO₃·10H₂O = washing soda; NaHCO₃ = baking soda.
• Cl₂ + dry Ca(OH)₂ → CaOCl₂ + H₂O gives bleaching powder.
• Common salt is the raw material for NaOH, washing soda, baking soda and bleaching powder.

Correct Answer: Option 4 — Sir Isaac Newton
Sir Isaac Newton was the first scientist to pass a narrow beam of sunlight through a glass prism and obtain a band of seven colours on a screen. He performed this famous experiment in 1666 in a darkened room at Cambridge and named the coloured band the spectrum. The splitting of white light into its component colours is called dispersion of light.
Newton then did a second, decisive experiment: he placed a second inverted prism in the path of the spectrum and the seven colours recombined into white light. This proved that the colours were already present inside sunlight — the prism only separates them, it does not create them.
Dispersion happens because the refractive index of glass is different for different colours. Violet has the shortest wavelength, so it is refracted the most; red has the longest wavelength, so it is refracted the least. The order of colours is therefore VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red.
❌ Why the other options are incorrect
✗ Prof CV Raman — an Indian physicist who discovered the Raman Effect (scattering of light), for which he won the Nobel Prize in 1930; he did not discover the spectrum.
✗ Albert Einstein — known for the photoelectric effect and the theory of relativity, work done more than two centuries after Newton's prism experiment.
✗ Archimedes — an ancient Greek scientist famous for the principle of buoyancy and the lever, not for optics of this kind.
📌 SSC & Railway Exam Facts
• Dispersion of white light through a prism was first shown by Newton (1666); the coloured band is the spectrum.
• Sequence of colours: VIBGYOR; violet bends most, red bends least.
• A rainbow is a natural spectrum formed by refraction + total internal reflection + dispersion in water droplets.
• Refractive index of glass ≈ 1.5, of water ≈ 1.33; the speed of light in vacuum = 3 × 10⁸ m/s.
• CV Raman — Raman Effect (1928), Nobel Prize 1930; National Science Day is celebrated on 28 February.

Correct Answer: Option 1 — a concave mirror
A dentist uses a concave mirror because it can form a virtual, erect and magnified (enlarged) image. When an object is placed between the pole (P) and the focus (F) of a concave mirror, the reflected rays diverge and appear to come from behind the mirror, producing an enlarged upright image.
The dentist holds the small concave mirror very close to the tooth — that is, the tooth lies within the focal length — so the tooth appears much bigger, and tiny cavities become easy to see. The same mirror also converges light from the dentist's lamp onto the tooth, so the area being examined is brightly illuminated. Both effects come from the fact that a concave mirror is a converging mirror.
For this reason a concave mirror is also used as a shaving mirror and a make-up mirror, and by an ENT specialist as a head mirror.
❌ Why the other options are incorrect
✗ a convex mirror — it always gives a virtual, erect but diminished image; a smaller tooth is useless for examination. It is used as a rear-view mirror.
✗ a plane mirror — it gives an image of the same size as the object, with no magnification at all.
✗ a plano-convex mirror — this is not a standard mirror used in dentistry; plano-convex refers to a lens shape, and a convex reflecting surface would still diminish the image.
📌 SSC & Railway Exam Facts
• Concave mirror: object between P and F → image is virtual, erect and magnified.
• Uses of a concave mirror — dentist's mirror, shaving mirror, torch and headlight reflectors, solar cookers, telescopes.
• Uses of a convex mirror — vehicle rear-view mirrors, shop security mirrors, street-light reflectors (wide field of view).
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u = h′/h; f = R/2.
• Concave mirror has negative f; convex mirror has positive f (New Cartesian Sign Convention).

Correct Answer: Option 3 — directly proportional to its mass and square of its velocity
📖 Given
• Mass of the body = m
• Velocity of the body = v
✍️ Formula
KE = ½mv²
🔢 Solution
• From KE = ½mv², with v kept constant, KE ∝ m — doubling the mass doubles the kinetic energy.
• With m kept constant, KE ∝ v² — doubling the velocity makes the kinetic energy 4 times, and tripling it makes the KE 9 times.
• So the kinetic energy is directly proportional to the mass and to the square of the velocity.
• Derivation: a body starting from rest reaches velocity v after a displacement s, where v² = 2as. Work done, W = F × s = ma × (v²/2a) = ½mv², which is stored as kinetic energy.
In plain language: a heavier body moving at the same speed carries more energy, but speed matters much more than mass, because it enters as a square.
❌ Why the other options are incorrect
✗ directly proportional to its velocity but inversely proportional to its mass — both parts are wrong; KE rises with mass and rises with v², not with v.
✗ directly proportional to its mass but inversely proportional to its velocity — the mass part is right, but KE increases with velocity, it never decreases.
✗ inversely proportional to its mass and square of its velocity — this would mean a fast heavy truck has almost no energy, which is clearly false.
📌 SSC & Railway Exam Facts
• KE = ½mv²; in terms of momentum, KE = p²/2m and p = √(2mKE).
• Potential energy near the Earth's surface, PE = mgh.
• SI unit of energy is the joule (J); 1 J = 1 N m, 1 J = 10⁷ erg, 1 kWh = 3.6 × 10⁶ J.
• Work–energy theorem: net work done on a body = change in its kinetic energy.
• If the speed of a vehicle is doubled, its braking distance becomes four times — a direct consequence of KE ∝ v².

Correct Answer: Option 4 — 10g J
📖 Given
• Mass of the luggage, m = 10 kg
• Height through which it is raised, h = 1 m
• Acceleration due to gravity = g (in SI unit)
✍️ Formula
W = F × s = mgh
🔢 Solution
• The porter must apply an upward force equal to the weight of the luggage, F = mg = 10 × g = 10g N.
• The luggage is displaced upward through s = h = 1 m, in the same direction as the applied force, so θ = 0°.
• W = F × s = (10g) × 1
• W = 10g J
• If we take g = 9.8 m s⁻², this equals 98 J; with g = 10 m s⁻² it is 100 J.
In plain words, the work done is exactly the gain in gravitational potential energy of the luggage, because it has simply been raised 1 m higher.
❌ Why the other options are incorrect
✗ 15g J — this would need m × h = 15, for example a 15 kg load raised 1 m; the given data do not give 15.
✗ 18g J — no combination of the given values 10 kg and 1 m produces 18; it is an arbitrary number.
✗ 20g J — this is the error of doubling the answer, as if the height were 2 m instead of 1 m.
📌 SSC & Railway Exam Facts
• W = F s cos θ; work is positive for θ = 0°, negative for θ = 180° and zero for θ = 90°.
• Work done against gravity = mgh = gain in potential energy.
• A porter walking on level ground with a load on his head does zero work against gravity, since force (vertical) and displacement (horizontal) are perpendicular.
• SI unit of work and energy: joule (J); 1 J = 1 N m.
• Power = Work/Time, SI unit watt (W); 1 W = 1 J/s and 1 hp = 746 W.

Correct Answer: Option 4 — an erect and diminished image
A convex mirror is a diverging mirror — its reflecting surface bulges outward and its focus F and centre of curvature C both lie behind the mirror. Because of this, wherever the object is placed, the reflected rays always diverge and appear to meet behind the mirror. The image formed is therefore always virtual, erect and smaller than the object, and it lies between the pole P and the focus F.
Since the image is diminished, a very large area of the road can be squeezed into a small mirror. This gives a wide field of view, so the driver can see traffic over a much bigger stretch behind the vehicle than a plane mirror of the same size would show. The erect image also keeps the view natural — vehicles are not seen upside down.
The trade-off is that objects look farther away than they really are, which is why such mirrors carry the warning "objects in mirror are closer than they appear".
❌ Why the other options are incorrect
✗ an erect and an enlarged image — an enlarged image is given by a concave mirror when the object is between P and F; a convex mirror can never magnify.
✗ a narrower field of the view — exactly the opposite; the whole point of using a convex mirror is its wider field of view.
✗ a real image — a convex mirror never forms a real image for a real object; its image is always virtual and behind the mirror.
📌 SSC & Railway Exam Facts
• Convex mirror image: always virtual, erect, diminished, formed between P and F.
• Uses of a convex mirror — rear-view/side mirrors of vehicles, shop security mirrors, blind-turn road mirrors, street-light reflectors.
• For a convex mirror f and R are positive; for a concave mirror they are negative.
• f = R/2; mirror formula 1/v + 1/u = 1/f; magnification m = −v/u.
• For a convex mirror |m| < 1 always, and m is positive because the image is virtual and erect.

Correct Answer: Option 3 — −4 D
📖 Given
• Focal length of the lens, f = −25 cm
• To find: power P of the lens
✍️ Formula
P = 1/f, where f must be in metres and P is in dioptre (D)
🔢 Solution
• First convert the focal length into metres: f = −25 cm = −25/100 m = −0.25 m.
• P = 1/f = 1/(−0.25)
• P = −4 D
• The negative sign shows that the lens is a concave (diverging) lens.
In plain language, a lens of focal length 25 cm bends light fairly strongly, and the minus sign tells us it spreads the rays out instead of bringing them to a point.
❌ Why the other options are incorrect
✗ +0.04 D — this comes from dividing 1 by 25 without converting cm to metres, and the sign has also been dropped.
✗ −0.04 D — the sign is right, but the centimetre value was used directly instead of −0.25 m; 1/(−25) = −0.04.
✗ +4 D — the magnitude is correct but the sign is wrong; +4 D would mean a convex (converging) lens with f = +0.25 m.
📌 SSC & Railway Exam Facts
• P = 1/f (in metres); SI unit of power is the dioptre (D), and 1 D = 1 m⁻¹.
• Convex (converging) lens → f positive, P positive; concave (diverging) lens → f negative, P negative.
• For lenses in contact, powers simply add: P = P₁ + P₂ + P₃ …
• Lens formula 1/v − 1/u = 1/f; magnification m = v/u.
• Myopia (short-sightedness) is corrected by a concave lens; hypermetropia (long-sightedness) by a convex lens.

Correct Answer: Option 3 — F₂ > F₁
📖 Given
• First object: mass m₁ = 5 kg, acceleration a₁ = 10 m s⁻², force F₁
• Second object: mass m₂ = 10 kg, acceleration a₂ = 12 m s⁻², force F₂
✍️ Formula
F = ma (Newton's Second Law of Motion)
🔢 Solution
• F₁ = m₁ × a₁ = 5 × 10
• F₁ = 50 N
• F₂ = m₂ × a₂ = 10 × 12
• F₂ = 120 N
• Comparing the two: 120 N > 50 N, so F₂ > F₁.
• In fact F₂/F₁ = 120/50 = 2.4, that is F₂ = 2.4 × F₁.
In plain words, the second object is both heavier and is being given a larger acceleration, so naturally it needs the bigger force.
❌ Why the other options are incorrect
✗ F₂ = −2 × F₁ — a negative force here has no meaning; also the actual ratio is +2.4, not −2. This option comes from only comparing the masses (10/5 = 2) and adding a stray minus sign.
✗ F₁ = F₂ — the two forces are 50 N and 120 N, clearly unequal.
✗ F₁ > F₂ — reversed; 50 N is smaller than 120 N.
📌 SSC & Railway Exam Facts
• Newton's Second Law: F = ma, also F = dp/dt (rate of change of momentum).
• SI unit of force is the newton (N); 1 N = 1 kg m s⁻² and 1 N = 10⁵ dyne.
• First Law (law of inertia) defines force; Third Law — every action has an equal and opposite reaction.
• Momentum p = mv; its SI unit is kg m s⁻¹, and it is a vector quantity.
• The newton is a derived SI unit; force is measured with a spring balance.

Correct Answer: Option 4 — 1 m
📖 Given
• Power of the lens, P = +1 D
• To find: focal length f in metres
✍️ Formula
P = 1/f, so f = 1/P (f in metres, P in dioptre)
🔢 Solution
• f = 1/P
• f = 1/(+1)
• f = +1 m = 100 cm
• The positive sign of both P and f shows that the lens is a convex (converging) lens.
In plain language, a 1-dioptre lens is defined as the lens whose focal length is exactly 1 metre — this is the very definition of the dioptre, so the calculation is simply reading the definition backwards.
❌ Why the other options are incorrect
✗ 0.75 m — this would correspond to P = 1/0.75 ≈ 1.33 D, not 1 D.
✗ 0.5 m — this is the focal length of a +2 D lens, since 1/0.5 = 2 D.
✗ 0.25 m — this is the focal length of a +4 D lens, since 1/0.25 = 4 D; it comes from confusing the value with a much stronger lens.
📌 SSC & Railway Exam Facts
• 1 dioptre is the power of a lens of focal length 1 metre; 1 D = 1 m⁻¹.
• P = 1/f (metres) — never put f in centimetres in this formula.
• Convex lens → f and P positive; concave lens → f and P negative.
• Powers of thin lenses in contact add up: P = P₁ + P₂.
• An optician's prescription such as "−2.5 D" means a concave lens of focal length 40 cm, used to correct myopia.

Correct Answer: Option 4 — 29.4 N
📖 Given
• Mass of the football, m = 3 kg
• Acceleration due to gravity on the Earth, g = 9.8 m s⁻²
• To find: weight W on the Earth
✍️ Formula
W = mg
🔢 Solution
• W = m × g
• W = 3 × 9.8
• W = 29.4 N
• Weight is a force, so its SI unit is the newton (N), not the kilogram.
In plain language, the Earth pulls a 3 kg football with a force of 29.4 newton; the same ball taken to the Moon would weigh only about 4.9 N, because the Moon's g is nearly 1/6 of the Earth's, even though its mass stays 3 kg everywhere.
❌ Why the other options are incorrect
✗ 30.5 N — this is what you get by rounding g to about 10.17; using the given 9.8 gives exactly 29.4 N.
✗ 25 N — no combination of 3 kg with 9.8 m s⁻² gives 25 N; it is an arbitrary value.
✗ 35 N — this comes from multiplying 3 by about 11.7; again, not the given value of g.
📌 SSC & Railway Exam Facts
• W = mg — weight is a vector (a force) directed towards the centre of the Earth; mass is a scalar and stays the same everywhere.
• g ≈ 9.8 m s⁻² on the Earth, ≈ 1.63 m s⁻² on the Moon (1/6 of the Earth's), and 0 at the centre of the Earth.
• g = GM/R²; g is maximum at the poles and minimum at the equator.
• G = 6.67 × 10⁻¹¹ N m² kg⁻² is the universal gravitational constant, the same everywhere.
• Mass is measured with a beam balance, weight with a spring balance; 1 kgf = 9.8 N.

Correct Answer: Option 3 — 1:1
📖 Given
• Mass of the first body = m, mass of the second body = 4m
• Both are dropped from the same height h, so initial velocity u = 0
• Air resistance is neglected — this is free fall
✍️ Formula
h = ut + ½gt², and with u = 0 this becomes t = √(2h/g)
🔢 Solution
• For the first body: h = 0 + ½gt₁², so t₁ = √(2h/g).
• For the second body: h = 0 + ½gt₂², so t₂ = √(2h/g).
• The expression √(2h/g) contains only h and g — the mass does not appear at all.
• Therefore t₁ = t₂, and t₁ : t₂ = 1 : 1.
In plain language, in the absence of air resistance all bodies fall at the same rate, whatever their mass. This is exactly what Galileo demonstrated, and what astronaut David Scott repeated on the Moon by dropping a hammer and a feather together — both landed at the same moment.
❌ Why the other options are incorrect
✗ 4:1 — this assumes the lighter body takes 4 times longer, i.e. time depends on mass, which free fall contradicts.
✗ 1:2 — this comes from wrongly taking t ∝ √m and using √4 = 2; mass has no place in the equation.
✗ 1:4 — this assumes t ∝ m, i.e. the heavier body takes 4 times as long, which is doubly wrong.
📌 SSC & Railway Exam Facts
• For free fall from rest: v = gt, h = ½gt², v² = 2gh — none of them contains mass.
• Galileo Galilei is said to have shown this from the Leaning Tower of Pisa.
• In a vacuum a coin and a feather fall together; in air the feather lags because of air resistance.
• g ≈ 9.8 m s⁻² on the Earth; g is independent of the mass of the falling body.
• For a body dropped from rest, the distances covered in successive seconds are in the ratio 1 : 3 : 5 : 7 …

Correct Answer: Option 2 — 20/7 Ω
📖 Given
• R₁ = 5 Ω, R₂ = 10 Ω, R₃ = 20 Ω
• All three are connected in parallel
✍️ Formula
1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃
🔢 Solution
• 1/R(eq) = 1/5 + 1/10 + 1/20
• Taking LCM = 20: 1/R(eq) = 4/20 + 2/20 + 1/20
• 1/R(eq) = 7/20
• R(eq) = 20/7 Ω
• As a decimal this is about 2.86 Ω.
Notice the useful check: in a parallel combination the equivalent resistance is always smaller than the smallest resistor, and here 20/7 ≈ 2.86 Ω is indeed less than 5 Ω.
❌ Why the other options are incorrect
✗ 35 Ω — this is 5 + 10 + 20, the formula for a series connection, not parallel.
✗ 40/7 Ω — the LCM step has been done wrongly; the correct sum of reciprocals is 7/20, giving 20/7, not 40/7.
✗ 20 Ω — this is simply the largest resistor copied down; a parallel combination can never equal or exceed it.
📌 SSC & Railway Exam Facts
• Series: R(eq) = R₁ + R₂ + R₃, the same current flows through each resistor.
• Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃, the same voltage is across each resistor.
• For two resistors in parallel there is a shortcut: R(eq) = R₁R₂/(R₁ + R₂).
• n equal resistors each of R in parallel give R/n; in series they give nR.
• Ohm's law: V = IR; domestic appliances are connected in parallel so that each gets the full 220 V and can be switched independently.

Correct Answer: Option 2 — The mass of an object is constant and does not change from place to place.
Mass is the quantity of matter contained in a body. It is a scalar quantity, measured in kilograms (kg) with a beam balance, and it is a constant property of the body. A 5 kg bag of rice is 5 kg on the Earth, 5 kg on the Moon and 5 kg in deep space.
Weight, on the other hand, is the force with which the Earth (or any planet) attracts the body, given by W = mg. It is a vector, measured in newton (N) with a spring balance. Since g changes from place to place — it is larger at the poles, smaller at the equator, smaller still at high altitude, and only about 1/6 of its Earth value on the Moon — weight is not constant.
That is why an astronaut's mass is the same on the Moon as on Earth, but his weight there is only one-sixth.
❌ Why the other options are incorrect
✗ The weight of an object is the power with which it is attracted towards the earth. — weight is a force, not power. Power is the rate of doing work, measured in watt.
✗ The weight of an object is constant and does not change from place to place. — false; W = mg changes wherever g changes, e.g. on the Moon it is about 1/6.
✗ The weight is equal to the product of velocity and acceleration due to gravity. — wrong formula; weight is mass × g, not velocity × g. That product would not even have the units of force.
📌 SSC & Railway Exam Facts
• Mass — scalar, SI unit kg, measured by a beam balance, constant everywhere.
• Weight — vector (a force), SI unit newton (N), measured by a spring balance, varies with g.
• W = mg; 1 kgf = 9.8 N; on the Moon weight is about 1/6 of that on the Earth.
• g is maximum at the poles and minimum at the equator; g = 0 at the centre of the Earth.
• In a freely falling lift a person feels weightless, because the lift and the person fall with the same acceleration g.

Correct Answer: Option 4 — −2.0
📖 Given
• Object distance, u = −20 cm (the object is always in front of the mirror)
• Image distance, v = −40 cm (the image is on the same side, i.e. in front of the mirror)
• Mirror is concave
✍️ Formula
m = −v/u
🔢 Solution
• Apply the New Cartesian Sign Convention: distances measured against the incident light, i.e. in front of the mirror, are negative. So u = −20 cm and v = −40 cm.
• m = −v/u = −(−40)/(−20)
• m = −(40/20)
• m = −2.0
• The negative magnification means the image is real and inverted; the magnitude 2 means it is twice the size of the object.
In plain language, the object lies between F and C of the concave mirror, so a real, inverted, enlarged image is formed beyond C — exactly what m = −2.0 tells us.
❌ Why the other options are incorrect
✗ +0.4 — this comes from taking the ratio the wrong way round (20/40 = 0.5 mishandled) and also giving the image a virtual, erect nature it does not have.
✗ −0.4 — again u and v have been interchanged; that would describe a diminished image, but here the image is larger than the object.
✗ +2.0 — the magnitude is correct, but the positive sign would mean a virtual, erect image. An image formed on the same side as the object is real and inverted, so m must be negative.
📌 SSC & Railway Exam Facts
• m = −v/u = h′/h; m negative → real, inverted image; m positive → virtual, erect image.
• |m| > 1 means enlarged, |m| < 1 means diminished, |m| = 1 means the same size.
• Mirror formula 1/v + 1/u = 1/f; here 1/(−40) + 1/(−20) = 1/f gives f = −13.3 cm, confirming a concave mirror.
• A concave mirror gives an enlarged real image when the object lies between F and C.
• Plane mirror: m = +1 always — image is virtual, erect and of the same size, and laterally inverted.