RRB Technician Grade III 2024-25 28 December 2024 Shift 3 GK Questions – with answers & solutions

Correct Answer: Option 1 — Variation
Variation means the differences that appear among the individuals of a species. It arises from small inaccuracies in DNA copying during reproduction and, in sexual reproduction, from the fresh combinations of genes formed when two gametes fuse.
As NCERT states, variations are useful for the survival of species over time. If a niche changes drastically — the temperature of a water body rises, or a new disease appears — a population of identical individuals could be wiped out entirely. But if some individuals carry a variation, say heat tolerance, they survive and reproduce. Variation is therefore the raw material of natural selection and the basis of evolution.
❌ Why the other options are incorrect
✗ Population — simply the number of individuals of a species living in an area; a large population without variation still perishes if conditions change.
✗ Budding — a method of asexual reproduction (yeast, Hydra) that produces genetically identical clones, hence it actually reduces variation.
✗ Reproduction — it creates copies and maintains the species, but it is the variation generated during reproduction, not the copying itself, that ensures long-term survival.
📌 SSC & Railway Exam Facts
• Sexual reproduction produces more variation than asexual reproduction.
• Charles Darwin gave the theory of natural selection in *The Origin of Species* (1859).
• Gregor Mendel, the Father of Genetics, worked on the garden pea (Pisum sativum).
• Speciation is the formation of new species; genetic drift and geographical isolation aid it.
• Acquired characters are not inherited — proved by August Weismann's mice-tail experiment.

Correct Answer: Option 3 — peripheral nervous system
The Peripheral Nervous System (PNS) is made of all the nerves that branch out of the central nervous system to every part of the body. It comprises 12 pairs of cranial nerves arising from the brain and 31 pairs of spinal nerves arising from the spinal cord.
The PNS is the communication link of the body: its sensory (afferent) nerves carry impulses from receptors in the skin, eyes, ears, nose and tongue to the brain and spinal cord, while its motor (efferent) nerves carry commands from them to the muscles and glands, which act as effectors.
❌ Why the other options are incorrect
✗ central nervous system — made of only the brain and spinal cord; it processes information, and the nerves leaving it are not part of it.
✗ autonomic nervous system — a subdivision of the PNS that controls involuntary actions such as heartbeat, breathing and digestion through its sympathetic and parasympathetic parts.
✗ endocrine system — the system of ductless glands that coordinates the body chemically through hormones, not through nerve impulses.
📌 SSC & Railway Exam Facts
• Human nervous system = CNS (brain + spinal cord) + PNS (cranial + spinal nerves).
• 12 pairs of cranial nerves; 31 pairs of spinal nerves.
• The neuron is the structural and functional unit of the nervous system — the longest cell of the body.
• Reflex arc is completed at the spinal cord, which is why the reaction is so quick.
• Cerebrum — thinking and memory; cerebellum — balance and posture; medulla — heartbeat, breathing, blood pressure.

Correct Answer: Option 1 — Seed
After pollination, the pollen grain lands on the stigma and grows a pollen tube down the style into the ovary, where it reaches the ovule. The male gamete fuses with the female gamete (egg) in fertilisation to form the zygote.
The zygote then divides to form the embryo, and the whole ovule — now containing the embryo along with stored food and a protective coat — develops into the seed. At the same time the ovary ripens and becomes the fruit, while the sepals, petals, stamens and style usually wither and fall off.
❌ Why the other options are incorrect
✗ Fruit — the fruit develops from the ovary, not from the ovule; the ovary wall becomes the pericarp.
✗ Sepal — the outermost green whorl of the flower that protects the bud; it is present before fertilisation and is not formed from the ovule.
✗ Anther — the male part that produces pollen grains; it withers away after pollination and never forms a seed.
📌 SSC & Railway Exam Facts
• Ovule → seed; ovary → fruit; zygote → embryo; integument → seed coat.
• A seed has a seed coat, one or two cotyledons and an embryo (plumule + radicle).
• Plumule grows into the shoot and radicle into the root.
• Fruits formed without fertilisation are parthenocarpic (e.g. banana) and are seedless.
• In flowering plants (angiosperms) double fertilisation occurs, forming the zygote and the endosperm.

Correct Answer: Option 4 — prokaryotes
Organisms whose cells have no nuclear membrane are called prokaryotes (*pro* = primitive, *karyon* = nucleus). Their genetic material — a single circular DNA molecule — lies naked in the cytoplasm in a region called the nucleoid, without any covering membrane.
Prokaryotic cells also lack membrane-bound organelles such as mitochondria, plastids, Golgi apparatus, endoplasmic reticulum and lysosomes. They contain only 70S ribosomes. Bacteria and blue-green algae (cyanobacteria) are the standard examples. In contrast, eukaryotes have a true nucleus bounded by a double nuclear membrane with nuclear pores.
❌ Why the other options are incorrect
✗ eukaryotes — exactly the opposite; these organisms do have a well-defined nucleus with a nuclear membrane, e.g. plants, animals, fungi and Amoeba.
✗ cytoplasm — not a class of organisms at all; it is the jelly-like fluid between the plasma membrane and the nucleus in which organelles float.
✗ chromatin — the thread-like DNA–protein network inside the nucleus that condenses into chromosomes during cell division; it is a structure, not an organism.
📌 SSC & Railway Exam Facts
• Prokaryotes: bacteria, cyanobacteria (blue-green algae), mycoplasma — mycoplasma is the smallest cell.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Prokaryotes divide by binary fission; eukaryotes by mitosis and meiosis.
• Robert Hooke discovered the cell (1665); Robert Brown discovered the nucleus (1831).
• The cell theory was given by Schleiden and Schwann; Virchow added *Omnis cellula e cellula*.

Correct Answer: Option 2 — Condom – Mechanical barrier
Contraceptive methods are grouped by how they work. Barrier methods physically stop the sperm from meeting the egg. The condom, a thin rubber sheath worn on the penis (and the female condom used in the vagina), is therefore a mechanical barrier — and it is the only method that also protects against sexually transmitted diseases (STDs) such as AIDS and syphilis.
The other three categories are chemical methods (oral pills that change the hormone balance so that no egg is released), intra-uterine devices or IUDs (Copper T, loop — placed in the uterus to prevent implantation), and surgical methods (vasectomy in males, tubectomy in females).
❌ Why the other options are incorrect
✗ Contraceptive pills – Uterine device — wrong; pills are taken orally and act by altering the hormonal balance, they are not a device placed in the uterus.
✗ Tubectomy – Chemical barrier — wrong; tubectomy is a surgical method in which the woman's fallopian tubes are blocked or cut.
✗ Copper T – Hormonal imbalance — wrong; Copper T is an intra-uterine device (IUD) placed in the uterus; it works mechanically, not by hormones.
📌 SSC & Railway Exam Facts
• Vasectomy — the vas deferens is cut in the male; Tubectomy — the fallopian tube is cut in the female.
• IUDs may cause side effects such as irritation and bleeding in the uterus.
• Contraception also helps in family planning and controlling population growth.
• Female foeticide / prenatal sex determination is a punishable offence in India.
• AIDS is caused by HIV and spreads mainly through sexual contact, infected blood and needles.

Correct Answer: Option 1 — Germination
Germination is the process in which a seed, containing the dormant embryo, sprouts and grows into a young plant called a seedling when suitable conditions are available. The three conditions needed are water (moisture), air (oxygen) and a suitable temperature.
When water enters through the micropyle, the seed swells, enzymes become active and the stored food in the cotyledons or endosperm is digested and supplied to the embryo. The radicle comes out first and forms the root, then the plumule grows upward to form the shoot. Until the first leaves are formed, the seedling lives entirely on the food stored in the seed.
❌ Why the other options are incorrect
✗ Regeneration — the ability of some simple organisms such as Planaria and Hydra to grow a complete body from a cut piece, using specialised cells; it has nothing to do with seeds.
✗ Plantation — merely the planting of trees or crops by human beings over an area; it is an agricultural activity, not a biological process inside the seed.
✗ Fermentation — anaerobic respiration in yeast and some bacteria in which glucose breaks down into ethanol and CO₂, releasing very little energy.
📌 SSC & Railway Exam Facts
• Conditions for germination: water, oxygen and suitable temperature; sunlight is not essential initially.
• Epigeal germination — cotyledons come above the soil (bean, castor); hypogeal — they stay below (pea, maize).
• Micropyle is the small pore through which water enters the seed.
• A seed remains alive but inactive in dormancy until conditions become favourable.
• Monocot seeds have one cotyledon (maize, wheat); dicot seeds have two (gram, pea).

Correct Answer: Option 3 — 1-d, 2-a, 3-b, 4-c
Hormones are chemical messengers secreted in tiny amounts by endocrine (ductless) glands directly into the blood, which carries them to their target organs.
1 – d. Thyroxin, from the thyroid gland, regulates the metabolism of carbohydrates, proteins and fats; iodine is essential for its formation.
2 – a. Growth hormone, from the anterior pituitary, controls the overall growth and development of the body, especially of bones and muscles.
3 – b. Testosterone, from the testes, brings about development of the male sex organs and the secondary sexual characters at puberty.
4 – c. Insulin, from the beta cells of the pancreas, lowers and thereby controls the level of blood sugar (glucose).
❌ Why the other options are incorrect
✗ 1-a, 2-c, 3-b, 4-d — wrong; thyroxin does not control growth (that is GH), and insulin does not control fat–protein metabolism (that is thyroxin).
✗ 1-b, 2-a, 3-d, 4-c — wrong; thyroxin has no role in male sex organ development, which is the work of testosterone.
✗ 1-d, 2-b, 3-c, 4-a — wrong; growth hormone is not linked to male sex organs, and testosterone does not control blood sugar.
📌 SSC & Railway Exam Facts
• Deficiency of insulin causes diabetes mellitus; insulin was discovered by Banting and Best.
• Excess GH in childhood → gigantism; deficiency → dwarfism; excess in adults → acromegaly.
• Iodine deficiency → goitre; the thyroid is the largest endocrine gland.
• Oestrogen is the female sex hormone from the ovary; testosterone is the male one from the testes.
• Adrenaline, from the adrenal gland, is called the emergency / fight-or-flight hormone.

Correct Answer: Option 3 — Ovary
The ovary is the swollen basal part of the pistil (carpel), the female reproductive organ of a flower. It encloses one or more ovules. After fertilisation, the ovary grows rapidly, its wall thickens into the pericarp, and it ripens into the fruit, while the ovules inside it become the seeds.
At the same time the parts that have finished their job — the sepals, petals, stamens, style and stigma — usually dry up and fall off. The fruit protects the developing seeds and often helps in their dispersal by wind, water or animals.
❌ Why the other options are incorrect
✗ Anther — the male part at the top of the stamen that produces pollen grains; it withers after pollination and never becomes a fruit.
✗ Petal — the coloured whorl (corolla) whose job is to attract insects for pollination; it falls off after fertilisation.
✗ Pollen tube — a temporary tube that grows from the pollen grain to carry the male gamete to the ovule; it disappears once fertilisation is over.
📌 SSC & Railway Exam Facts
• Ovary → fruit; ovule → seed; zygote → embryo; ovary wall → pericarp.
• A fruit formed only from the ovary is a true fruit (mango, tomato); one involving the thalamus is a false fruit (apple, cashew).
• Parthenocarpic fruits (banana) form without fertilisation and are seedless.
• Coconut is a fibrous drupe; the edible part of mango is the mesocarp.
• Pistil = stigma + style + ovary; stamen = anther + filament.

Correct Answer: Option 2 — Nephron
The nephron is the basic structural and functional filtration unit of the kidney. Each human kidney contains about one million nephrons. A nephron consists of a cup-shaped Bowman's capsule enclosing a ball of capillaries called the glomerulus, followed by a long tubule that ends in the collecting duct.
Urine formation occurs in three steps: glomerular filtration, in which blood entering under high pressure is filtered so that water, glucose, salts, amino acids and urea pass into the capsule; selective reabsorption, in which useful substances such as glucose, amino acids, salts and most water are taken back into the blood; and secretion, in which extra wastes are added to the tubule. What remains is urine.
❌ Why the other options are incorrect
✗ Seminiferous tubule — the coiled tubule inside the testis where sperms are produced; it has no excretory role.
✗ Neuron — the structural and functional unit of the nervous system, which conducts electrical impulses; it does not filter blood.
✗ Sperm — the male gamete, a single reproductive cell with a head, middle piece and tail; it is not a unit of any organ.
📌 SSC & Railway Exam Facts
• The excretory system: two kidneys, two ureters, urinary bladder and urethra.
• The main nitrogenous waste in humans is urea, made in the liver from ammonia.
• Haemodialysis (artificial kidney) is used when the kidneys fail.
• Glomerulus + Bowman's capsule = Malpighian body/corpuscle.
• An adult human produces about 1 to 1.8 litres of urine per day.

Correct Answer: Option 3 — Both a) and b)
Statement a) is correct. On the basis of whether micro-organisms can break them down, wastes are classified into two types — biodegradable wastes (vegetable peels, paper, cow dung, cotton, wood), which are decomposed by bacteria and fungi, and non-biodegradable wastes (plastic, polythene, glass, metal, DDT), which are not.
Statement b) is also correct. Waste should be segregated at source: the wet (biodegradable) waste can be turned into compost or biogas, while the dry (non-biodegradable) waste can be sent for recycling. Mixing the two makes both treatment and recycling difficult, which is why municipal bodies supply separate green and blue bins.
❌ Why the other options are incorrect
✗ Only a) — incomplete; separate collection of dry and wet waste is a correct and strongly recommended practice.
✗ Only b) — incomplete; the two-fold classification of waste in statement a) is a standard NCERT fact.
✗ Neither a) nor b) — clearly wrong, as both statements are scientifically true.
📌 SSC & Railway Exam Facts
• Green bin — wet/biodegradable waste; blue bin — dry/recyclable waste.
• Vermicomposting uses earthworms to turn organic waste into manure.
• Biogas is mainly methane (CH₄), obtained from cow dung in a gobar gas plant.
• Swachh Bharat Abhiyan was launched on 2 October 2014.
• The three Rs of waste management — Reduce, Reuse, Recycle.

Correct Answer: Option 3 — placenta
After the fertilised egg is implanted in the wall of the uterus, a special disc-like tissue called the placenta develops. It is the connecting link between the mother and the developing embryo.
On the embryo's side the placenta bears finger-like villi, which greatly increase the surface area and are bathed in the mother's blood spaces. Through this arrangement, glucose, amino acids and oxygen pass from the mother's blood into the embryo, and the embryo's waste products such as carbon dioxide and urea pass back into the mother's blood to be thrown out by her body. The embryo is attached to the placenta by the umbilical cord.
❌ Why the other options are incorrect
✗ scrotum — a male structure, the skin pouch outside the abdomen that holds the testes at a lower temperature; it has no role in nutrition of an embryo.
✗ ovary — the female primary sex organ; it produces the ovum and the hormones oestrogen and progesterone, but it does not nourish the embryo.
✗ oviduct (fallopian tube) — the tube where fertilisation takes place and through which the zygote travels to the uterus; it neither nourishes nor houses the embryo.
📌 SSC & Railway Exam Facts
• The human gestation period is about 9 months (280 days).
• Implantation of the embryo occurs in the endometrium of the uterus.
• The umbilical cord connects the foetus to the placenta.
• Menstruation occurs when the ovum is not fertilised and the uterine lining breaks down.
• Fertilisation in humans takes place in the fallopian tube (oviduct), not in the uterus.

Correct Answer: Option 1 — zygotes
In asexual reproduction a single parent produces offspring without the formation or fusion of gametes. A zygote is formed only when a male gamete fuses with a female gamete during fertilisation — that is sexual reproduction. Hence no zygote is ever formed in asexual reproduction.
Because no gametes mix, the offspring are genetically identical clones of the parent and show very little variation. Asexual reproduction is quick, needs only one parent, and is common in bacteria, Amoeba, yeast, Hydra, Planaria, Rhizopus and Spirogyra.
❌ Why the other options are incorrect
✗ buds — buds are formed in asexual reproduction; yeast and Hydra reproduce by budding, the bud growing on the parent and then detaching.
✗ leaves — leaves are involved; Bryophyllum produces buds on its leaf margins which fall off and grow into new plants — a case of vegetative propagation.
✗ spores — spores are formed asexually; Rhizopus (bread mould) forms spores inside sporangia on erect hyphae, which germinate on a suitable surface.
📌 SSC & Railway Exam Facts
• Modes of asexual reproduction: fission, budding, fragmentation, regeneration, spore formation, vegetative propagation.
• Zygote → embryo → new individual is the pathway of sexual reproduction only.
• Fragmentation — Spirogyra; regeneration — Planaria; binary fission — Amoeba.
• Asexual offspring are clones; Dolly the sheep was the first cloned mammal (1996).
• Sexual reproduction creates variation, which is the basis of evolution.

Correct Answer: Option 4 — Cytoplasm
The breakdown of the six-carbon sugar glucose into two molecules of the three-carbon compound pyruvate (pyruvic acid) is called glycolysis, and it takes place in the cytoplasm of the cell. This very first step is common to both aerobic and anaerobic respiration and does not require oxygen.
What happens to the pyruvate next depends on oxygen. In the presence of oxygen it enters the mitochondria and is completely broken down into CO₂ and water, releasing a large amount of energy. In the absence of oxygen, pyruvate is converted into ethanol and CO₂ in yeast (fermentation) or into lactic acid in our muscle cells, which causes cramps after heavy exercise.
❌ Why the other options are incorrect
✗ Golgi-complex — it packages, modifies and dispatches materials made by the endoplasmic reticulum and forms lysosomes; it plays no part in glycolysis.
✗ Ribosome — the site of protein synthesis, made of RNA and protein; it does not break down glucose.
✗ Mitochondria — the site of the later stages of aerobic respiration (Krebs cycle and electron transport), which act on pyruvate after glycolysis, not on glucose.
📌 SSC & Railway Exam Facts
• Glycolysis — in the cytoplasm; Krebs cycle — in the mitochondrial matrix.
• Aerobic respiration yields 38 ATP per glucose; anaerobic yields only 2 ATP.
• Mitochondria is the powerhouse of the cell; ATP is the energy currency.
• Mitochondria have their own DNA and ribosomes and can make some of their own proteins.
• Lactic acid accumulation in muscles causes cramps; yeast fermentation is used in bread and alcohol making.

Correct Answer: Option 2 — chromosomes
A gene is a segment of DNA that carries the information for one character or trait. These genes are arranged in a definite linear order on thread-like structures called chromosomes, which lie inside the nucleus of the cell. Chromosomes are made of DNA and proteins (histones).
Every human body cell has 23 pairs (46) of chromosomes — one member of each pair coming from the father and the other from the mother. This is exactly why a child shows characters of both parents. Of these, 22 pairs are autosomes and 1 pair is the sex chromosomes (XX in a female, XY in a male), which decide the sex of the child.
❌ Why the other options are incorrect
✗ vacuoles — fluid-filled sacs that provide storage and turgidity; the large central vacuole of a plant cell stores water, sap and wastes. They carry no genes.
✗ endoplasmic reticulum — a membrane network for transport of materials and for making proteins (RER) and lipids (SER); genes are not located on it.
✗ lysosome — the suicide bag of the cell, full of powerful digestive enzymes that clear worn-out organelles; it has no genetic material.
📌 SSC & Railway Exam Facts
• DNA — Deoxyribonucleic acid; its double-helix model was given by Watson and Crick (1953).
• Gregor Mendel is the Father of Genetics; a gene's alternative forms are called alleles.
• The sex of a child is decided by the father, since the mother always gives an X.
• Chromosome number: human 46, pea 14, onion 16, housefly 12.
• The term gene was coined by Johannsen; chromosome was named by Waldeyer.

Correct Answer: Option 4 — Ca(OH)₂
A chemical formula is written by criss-crossing the valencies of the ions. Calcium forms the ion Ca²⁺ (valency 2) and the hydroxide radical is OH⁻ (valency 1). To make the compound electrically neutral, two OH⁻ ions are needed for every one Ca²⁺ ion, giving Ca(OH)₂. The bracket is essential — the subscript 2 outside it multiplies the whole OH group, so the formula stands for 1 Ca, 2 O and 2 H atoms.
Ca(OH)₂ is commonly called slaked lime or lime water. It is a strong base made by adding water to quick lime (CaO), an intensely exothermic reaction. Lime water is the standard laboratory test for carbon dioxide — it turns milky because insoluble CaCO₃ is formed. It is also used in whitewashing, in neutralising acidic soil and in making bleaching powder.
⚗️ The Reaction
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
• Quick lime (CaO) plus water gives slaked lime, Ca(OH)₂ — a combination and strongly exothermic reaction.
• Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O — this is why lime water turns milky with CO₂.
❌ Why the other options are incorrect
✗ KOH — this is potassium hydroxide (caustic potash); K⁺ has a valency of 1, so only one OH⁻ is needed.
✗ Mg(OH)₂ — this is magnesium hydroxide, the milk of magnesia used as an antacid; the metal is magnesium, not calcium.
✗ NaOH — this is sodium hydroxide (caustic soda), obtained from the chlor-alkali process; Na⁺ also has a valency of 1.
📌 SSC & Railway Exam Facts
• Quick lime = CaO, slaked lime = Ca(OH)₂, limestone/marble/chalk = CaCO₃, gypsum = CaSO₄·2H₂O.
• Plaster of Paris = CaSO₄·½H₂O, made by heating gypsum to about 100 °C (373 K).
• Caustic soda = NaOH, caustic potash = KOH, milk of magnesia = Mg(OH)₂ (an antacid).
• Bleaching powder = CaOCl₂, made by passing Cl₂ over dry slaked lime.
• Bases turn red litmus blue, taste bitter, feel soapy, and have pH > 7.

Correct Answer: Option 4 — Carbon dioxide and water
Methane (CH₄) is the first member of the alkane (saturated hydrocarbon) series. Like every hydrocarbon, it burns in a sufficient supply of oxygen to give carbon dioxide and water, releasing a large amount of heat and light. The carbon of the fuel ends up as CO₂ and its hydrogen as H₂O — that is the general rule for the complete combustion of any hydrocarbon.
Methane is the main constituent of natural gas, CNG and biogas (gobar gas). Being a saturated hydrocarbon, it burns with a clean blue flame and leaves no soot, which makes it an excellent domestic fuel.
⚗️ The Reaction
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + heat + light
• Balance check — C: 1 = 1, H: 4 = 4, O: 4 = 4. The coefficient 2 before O₂ and H₂O is essential.
• This is an oxidation (combustion) reaction and is strongly exothermic.
• In an insufficient supply of oxygen the combustion is incomplete and poisonous carbon monoxide (CO) plus soot are formed instead.
❌ Why the other options are incorrect
✗ Carbon monoxide only — CO appears only in incomplete combustion, and even then water is also formed, since the hydrogen of methane must go somewhere.
✗ Carbonic acid — H₂CO₃ is formed when CO₂ dissolves in water (as in soda water); it is not a product of burning.
✗ Carbon monoxide and water — these are the products of incomplete combustion in a limited supply of oxygen; the question specifies burning with oxygen, so the complete products are CO₂ and H₂O.
📌 SSC & Railway Exam Facts
• Complete combustion of a hydrocarbon → CO₂ + H₂O; incomplete combustion → CO + soot (C) + H₂O.
• Methane CH₄ is the first alkane; the series runs CH₄, C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane).
• CNG and biogas are mainly methane; LPG is mainly butane.
• CO is a poisonous gas — it binds to haemoglobin and prevents oxygen transport.
• Methane is a greenhouse gas and is also called marsh gas; the general alkane formula is CₙH₂ₙ₊₂.

Correct Answer: Option 1 — CO
The prefix "mono" means one. Carbon monoxide therefore contains one carbon atom joined to one oxygen atom, and its molecular formula is simply CO. Reading the name is enough to write the formula: *mono*-oxide → one oxygen.
CO is formed by the incomplete combustion of carbon and carbon-containing fuels — burning coal, charcoal or petrol in a limited supply of oxygen. It is a colourless, odourless and highly poisonous gas. Breathing it is dangerous because CO binds to the haemoglobin of blood about 200–250 times more strongly than oxygen does, forming carboxyhaemoglobin and stopping oxygen from reaching the tissues. This is why a coal sigri must never be burnt in a closed room.
❌ Why the other options are incorrect
✗ C₂O — no such common oxide of carbon exists; the subscript 2 on carbon would mean two carbon atoms, which the name "monoxide" does not indicate.
✗ CO₂ — this is carbon dioxide, where "di" means two oxygen atoms; it is the product of complete combustion and of respiration.
✗ CO₃ — this is not a molecule but the carbonate radical, CO₃²⁻, which exists only inside compounds such as CaCO₃ and Na₂CO₃.
📌 SSC & Railway Exam Facts
• Prefixes: mono = 1, di = 2, tri = 3, tetra = 4 — CO, CO₂, SO₂, SO₃, CCl₄.
• CO is poisonous because it forms carboxyhaemoglobin with blood haemoglobin.
• Incomplete combustion → CO; complete combustion → CO₂.
• CO₂ turns lime water milky (forming CaCO₃) — the standard laboratory test for the gas.
• CO₂ is a greenhouse gas and is used in fire extinguishers; solid CO₂ is dry ice.

Correct Answer: Option 1 — Carbon dioxide
Coke is an almost pure form of carbon — a grey, hard, porous solid obtained by the destructive distillation of coal (heating coal strongly in the absence of air). Since coke is essentially carbon, burning it in air simply oxidises that carbon, and the gas produced in the main is carbon dioxide (CO₂).
Coke burns without a flame, giving only a red glow, because it is a solid fuel that produces no gaseous vapours. It has a high calorific value, so it is widely used as a fuel and, more importantly, as a reducing agent in the blast furnace for extracting iron from its ore.
⚗️ The Reaction
C(s) + O₂(g) → CO₂(g) + heat
• One carbon atom joins one oxygen molecule, so the equation is already balanced: C 1 = 1, O 2 = 2.
• It is at once a combination, an oxidation and a combustion reaction, and is strongly exothermic.
• In a limited supply of air the product is poisonous carbon monoxide (CO) instead — 2C + O₂ → 2CO.
❌ Why the other options are incorrect
✗ Nitrogen dioxide — NO₂ comes from the oxides of nitrogen formed at very high temperatures in engines, or from burning nitrogen-rich matter; coke contains carbon, not nitrogen, as its main element.
✗ Oxygen — oxygen is consumed during burning, not produced; burning is a reaction with oxygen.
✗ Sulphur dioxide — SO₂ is produced when sulphur-bearing fuels (such as raw coal or crude petroleum) burn; coke is purified carbon, and SO₂ is only a trace impurity, not the main gas.
📌 SSC & Railway Exam Facts
• Coke = pure carbon, obtained by the destructive distillation of coal; other products are coal tar and coal gas.
• Coke burns without a flame (only a glow), like charcoal; substances that vaporise (LPG, kerosene, wax) burn with a flame.
• Coke is the reducing agent in the blast furnace: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
• SO₂ and NO₂ cause acid rain, producing H₂SO₄ and HNO₃.
• Calorific value is measured in kJ/kg; hydrogen has the highest of all fuels (≈ 150,000 kJ/kg).

Correct Answer: Option 1 — Gold
Ductility is the property of a metal by which it can be drawn into thin wires. Gold (Au) is the most ductile metal known — a single gram of gold can be drawn into a wire about 2 km long. Gold is also the most malleable metal, meaning it can be beaten into the thinnest sheets (gold leaf, only a few hundred atoms thick).
The reason is the metallic bond. In a metal the layers of positive ions are held together by a sea of free electrons; when a stress is applied the layers can slide over one another without the bond breaking, because the mobile electrons simply re-adjust. In gold these layers slide very easily, so it neither cracks nor snaps.
Because of this, gold is used in jewellery, in fine electronic contacts and in spacecraft coatings. Silver is the second most ductile metal.
❌ Why the other options are incorrect
✗ Lead — lead is malleable but very poorly ductile; it has low tensile strength and the wire breaks on drawing, which is why lead wires are not made.
✗ Sodium — an alkali metal so soft that it can be cut with a knife; it is not drawn into wires, and it is so reactive that it is stored under kerosene.
✗ Lithium — the lightest metal (density 0.53 g/cm³), also soft and highly reactive; it is used in batteries, not for wire-drawing.
📌 SSC & Railway Exam Facts
• Gold is the most ductile and most malleable metal; silver comes second.
• Ductility = drawn into wires; Malleability = beaten into sheets.
• Best conductor of electricity: silver > copper > gold > aluminium.
• Hardest metal = tungsten (W); softest metals = caesium and sodium (cut with a knife).
• Lithium is the lightest metal; mercury is the only metal that is liquid at room temperature; osmium is the densest.

Correct Answer: Option 3 — Yellow flame
Unsaturated hydrocarbons are those containing a double bond (alkenes, CₙH₂ₙ) or a triple bond (alkynes, CₙH₂ₙ₋₂), such as ethene (C₂H₄) and ethyne (C₂H₂). Compared with saturated compounds they have a much higher percentage of carbon in the molecule.
When they burn in air, the oxygen available is not enough to oxidise all that carbon. The combustion is therefore incomplete, and unburnt carbon particles (soot) are left in the flame. These hot glowing carbon particles make the flame yellow and luminous, and they deposit black soot on any vessel held over it.
Saturated hydrocarbons (alkanes, CₙH₂ₙ₊₂) such as methane have relatively less carbon, burn completely and therefore give a clean blue flame with no soot — which is why LPG and CNG are good domestic fuels.
❌ Why the other options are incorrect
✗ Green flame — a green flame is a flame test colour given by the salts of copper and barium, not by burning hydrocarbons.
✗ Red flame — red or crimson is the flame test colour of strontium and lithium salts; it is unrelated to the saturation of a compound.
✗ Clean flame — a clean blue flame is given by saturated hydrocarbons, which burn completely; unsaturated ones do the opposite and smoke.
📌 SSC & Railway Exam Facts
• Saturated → clean blue flame; Unsaturated → yellow, sooty (luminous) flame.
• Alkane CₙH₂ₙ₊₂ (single bond), Alkene CₙH₂ₙ (double bond), Alkyne CₙH₂ₙ₋₂ (triple bond).
• A sooty flame is the simple test to tell an unsaturated compound from a saturated one; the chemical test is decolourisation of bromine water.
• A blackened cooking vessel means the fuel is burning incompletely owing to an insufficient air supply.
• Ethyne (acetylene, C₂H₂) burnt in pure oxygen gives the very hot oxy-acetylene flame (≈ 3000 °C) used for welding.

Correct Answer: Option 2 — Milk is not a colloidal solution.
Milk is one of the best-known colloids. It is an emulsion — tiny droplets of liquid fat (the dispersed phase) spread through water (the dispersion medium). The fat droplets are 1–1000 nm across, so they make milk look opaque and white, they scatter light (Tyndall effect), and they do not settle on standing. Saying that milk is not a colloidal solution is therefore false, which is exactly what the question asks for.
The cream of milk can be separated only by centrifugation, never by filtration — a further proof that milk is a colloid and not a suspension.
❌ Why the other options are incorrect
✗ Colloidal solutions are a heterogeneous mixture — true; although a colloid looks uniform, its particles form a separate phase, so it is classified as heterogeneous.
✗ Fog is an example of a colloidal solution — true; fog is an aerosol, with tiny water droplets dispersed in air (liquid in gas).
✗ Components cannot be separated by the process of filtration — true; colloidal particles are far too small to be held back by filter paper and pass straight through, so centrifugation must be used.
📌 SSC & Railway Exam Facts
• Milk = emulsion (liquid in liquid); fog/cloud = aerosol (liquid in gas); smoke = solid in gas.
• Butter and cheese = gel (liquid in solid); shaving/whipped cream = foam (gas in liquid); ink and blood = sol.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Colloids are separated by centrifugation, suspensions by filtration.
• Tyndall effect is shown by colloids and suspensions; a true solution never shows it.

Correct Answer: Option 4 — Soap
A soap is the sodium or potassium salt of a long-chain fatty acid, for example sodium stearate, C₁₇H₃₅COONa. Its molecule has two very different ends:
(i) a long hydrocarbon tail which is non-polar, water-repelling (hydrophobic) but oil-loving (oleophilic); and (ii) a short ionic head, −COO⁻Na⁺, which is polar and water-loving (hydrophilic). A molecule with both a water-loving and a water-hating end is described as amphipathic.
When soap is put in water, the molecules arrange themselves into clusters called micelles: the hydrophobic tails point inwards and trap the oil or grease of the dirt, while the hydrophilic heads face outwards into the water. The grease is thus held in the middle of the micelle and is carried away with the rinsing water — this is how soap cleans.
❌ Why the other options are incorrect
✗ Water — H₂O is a single polar molecule; it is hydrophilic all over and has no water-hating end, which is exactly why it alone cannot wash away grease.
✗ Cream — cream is a colloid (an emulsion) of fat in water, a mixture; it is not a single molecule with two distinct ends.
✗ Perfume — a solution of fragrant oils in alcohol; it evaporates quickly but its molecules have no hydrophilic–hydrophobic double nature.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of a fatty acid; the cleaning cluster it forms is called a micelle.
• Saponification = heating a fat or oil with NaOH to give soap + glycerol.
• Soap does not lather well in hard water (which has Ca²⁺ and Mg²⁺ ions) and forms an insoluble scum; detergents work even in hard water.
• Hydrophilic = water-loving head; Hydrophobic = water-hating hydrocarbon tail.
• Detergents are ammonium or sulphonate salts of long-chain hydrocarbons and are used in shampoos and washing powders.

Correct Answer: Option 2 — 100°C (212°F)
The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the atmospheric pressure, so that the liquid changes rapidly into vapour throughout its bulk, not merely at the surface. For water at normal atmospheric pressure (1 atm = 760 mm of Hg) this temperature is 100 °C, which equals 373 K and 212 °F.
The Fahrenheit value checks out with the standard conversion °F = (°C × 9/5) + 32, giving (100 × 9/5) + 32 = 180 + 32 = 212 °F.
While the water boils, its temperature stays fixed at 100 °C even though heat is still being supplied, because that heat is absorbed as the latent heat of vaporisation (22.5 × 10⁵ J/kg). The boiling point falls as the pressure falls — on a high hill water boils below 100 °C, and cooking there takes longer — while a pressure cooker raises the pressure and so raises the boiling point, cooking food faster.
❌ Why the other options are incorrect
✗ 80°C (176°F) — water only evaporates at this temperature from its surface; it does not boil, as the vapour pressure is still below atmospheric pressure.
✗ 50°C (122°F) — merely warm water; boiling at 50 °C would need the pressure to be lowered to about one-eighth of an atmosphere.
✗ 0°C (32°F) — this is the melting/freezing point of water (273 K), where ice turns into water, not the boiling point.
📌 SSC & Railway Exam Facts
• Boiling point of water = 100 °C = 373 K = 212 °F; melting point of ice = 0 °C = 273 K = 32 °F.
• Conversions: K = °C + 273, °F = (°C × 9/5) + 32; −40 °C = −40 °F.
• Boiling point decreases with decreasing pressure — food is cooked in a pressure cooker at a higher pressure, and so faster, on hills.
• Temperature stays constant during boiling; the heat supplied becomes latent heat of vaporisation = 22.5 × 10⁵ J/kg.
• Boiling is a bulk phenomenon at a fixed temperature; evaporation is a surface phenomenon at all temperatures.

Correct Answer: Option 3 — CaCO₃ + H₂O
When carbon dioxide is passed through calcium hydroxide solution (lime water), the two react to give calcium carbonate and water. Calcium carbonate is insoluble in water, so it appears as a fine white precipitate that makes the clear lime water turn milky. This is the standard laboratory test for carbon dioxide gas.
CO₂ is an acidic oxide and Ca(OH)₂ is a base, so this is in effect a neutralisation: an acidic oxide + base → salt + water.
If excess CO₂ is passed on, the milkiness disappears again, because the insoluble CaCO₃ dissolves as soluble calcium hydrogencarbonate, Ca(HCO₃)₂.
⚗️ The Reaction
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l)
• Balance check — Ca: 1 = 1, C: 1 = 1, O: 4 = 4, H: 2 = 2. The equation is balanced as written.
• The (s) and the white milkiness show that CaCO₃ comes out as a precipitate.
• With excess CO₂: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂, and the milkiness vanishes.
• This is also the reaction that hardens whitewash — the CaCO₃ film gives the shiny white finish on a wall.
❌ Why the other options are incorrect
✗ H₂CO₃ + H₂O — carbonic acid is formed when CO₂ dissolves in water alone (CO₂ + H₂O → H₂CO₃, as in soda water), not when it reacts with a base.
✗ CO + H₂O — carbon monoxide is a product of the incomplete combustion of carbon; CO₂ is not reduced to CO by a base.
✗ CaO + H₂O — CaO (quick lime) with water is the reverse change: it produces Ca(OH)₂ rather than being produced from it.
📌 SSC & Railway Exam Facts
• Lime water turning milky is the test for CO₂; the milkiness is insoluble CaCO₃.
• Excess CO₂ removes the milkiness by forming soluble Ca(HCO₃)₂.
• CaCO₃ is the formula of limestone, marble and chalk; heating it gives CaO + CO₂ (decomposition).
• Ca(HCO₃)₂ and Mg(HCO₃)₂ cause the temporary hardness of water, removed by boiling.
• Whitewashing uses Ca(OH)₂, which sets into a shiny CaCO₃ coat with the CO₂ of the air.

Correct Answer: Option 4 — BaSO₄ (s) + 2NaCl (aq)
This is a double displacement reaction of the pattern AB + CD → AD + CB: the two compounds simply exchange their ions. Ba²⁺ pairs with SO₄²⁻ to give barium sulphate, and Na⁺ pairs with Cl⁻ to give sodium chloride.
The radicals keep their own valencies, so the formulae must be BaSO₄ (both ions are divalent) and NaCl (both are monovalent). Since two Na⁺ ions are released by one Na₂SO₄, two NaCl are formed, which is why the coefficient 2 is needed before NaCl.
BaSO₄ is insoluble in water and separates out as a thick white precipitate — the (s) in the answer. A double displacement that gives an insoluble solid is also called a precipitation reaction.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
• Balance check — Na: 2 = 2, S: 1 = 1, O: 4 = 4, Ba: 1 = 1, Cl: 2 = 2. Fully balanced.
• Ba²⁺ + SO₄²⁻ → BaSO₄ (white precipitate); Na⁺ + Cl⁻ → NaCl (stays dissolved).
• No element changes its oxidation number, so this is not a redox reaction — only the partners change.
❌ Why the other options are incorrect
✗ 2BaSO₂ (s) + 2NaCl (aq) + 2O₂ (g) — wrong on two counts: the sulphate radical is SO₄²⁻, not SO₂, and no oxygen gas is released in a double displacement.
✗ BaSO₂ (s) + NaCl (aq) + O₂ (g) — again invents BaSO₂ and free O₂; it also leaves Na and Cl unbalanced (2 Na on the left, 1 on the right).
✗ BaSO₄ (s) + 2NaCl₂ (aq) — the product BaSO₄ is right, but NaCl₂ does not exist: Na⁺ has a valency of 1, so it can hold only one Cl⁻. This is the classic trap of blindly copying the subscript 2 from BaCl₂.
📌 SSC & Railway Exam Facts
• Double displacement: AB + CD → AD + CB; if an insoluble solid forms it is a precipitation reaction.
• White precipitates: BaSO₄, AgCl, PbSO₄, CaCO₃; yellow: PbI₂, AgBr.
• AgNO₃ + NaCl → AgCl↓ (white) + NaNO₃ is the other classic PYQ example.
• BaSO₄ is used as the barium meal for X-rays of the stomach, being insoluble and so non-toxic.
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.

Correct Answer: Option 1 — K, L, M, N, O….
In Niels Bohr's model of the atom (1913), electrons revolve around the nucleus only in certain fixed, discrete paths called orbits, shells or energy levels. These shells are numbered n = 1, 2, 3, 4, 5 … and are named by the letters K, L, M, N, O … in that order.
An electron moving in one of these shells does not radiate energy, so it never spirals into the nucleus — this was Bohr's correction of the main defect of Rutherford's model. Energy is absorbed or emitted only when an electron jumps from one shell to another, and the amount is a fixed quantum, E₂ − E₁.
The maximum number of electrons a shell can hold is given by the Bohr–Bury rule, 2n² — so K = 2, L = 8, M = 18, N = 32 — while the outermost shell can never hold more than 8 electrons.
❌ Why the other options are incorrect
✗ W, X, Y, Z, A…. — these letters are not used for shells at all; X, Y, Z are ordinary algebraic symbols in chemistry, not shell names.
✗ R, S T, U, V…. — no shell carries these letters; s, p, d, f (small letters) are the names of sub-shells/orbitals, which is a different idea altogether.
✗ A, B, C, D, E…. — shell naming begins at K, not at A; the sequence was chosen historically from X-ray spectroscopy, where the innermost series was labelled the K-series.
📌 SSC & Railway Exam Facts
• Shells: K (n=1), L (n=2), M (n=3), N (n=4), O (n=5); capacity by 2n² = 2, 8, 18, 32, 50.
• The outermost (valence) shell can hold a maximum of 8 electrons — the octet rule.
• Valency is decided by the number of electrons in the outermost shell.
• Bohr's model corrected Rutherford's defect: electrons in a fixed orbit do not radiate energy.
• Niels Bohr received the Nobel Prize in Physics, 1922; sub-shells are named s, p, d, f.

Correct Answer: Option 1 — Colloids
The Tyndall effect is the scattering of a beam of light by the particles of a mixture, which makes the path of the beam visible. A mixture can show it only if its particles are large enough to intercept light, that is 1 nm and above.
Colloidal particles measure 1–1000 nm, comparable to the wavelength of visible light (400–700 nm), so they scatter the beam strongly and the path of light can be seen from the side. This is the standard laboratory test used to distinguish a colloid from a true solution. Suspensions, whose particles are even bigger, show it as well.
Familiar sights: a beam of sunlight in a dusty room, car head-lights in fog, the light of a cinema projector in a smoky hall, and sunlight filtering through the mist in a dense forest.
❌ Why the other options are incorrect
✗ Gases — a pure gas is homogeneous and its molecules are far smaller than 1 nm, so it does not scatter a beam; only a gas carrying dust or water droplets (which is then a colloid — smoke or fog) appears to do so.
✗ Solutions — in a true solution the particles are smaller than 1 nm; they cannot scatter light, so a true solution shows no Tyndall effect and stays perfectly transparent.
✗ Metals — metals are pure substances (elements), not mixtures with dispersed particles; light is reflected from a polished metal surface, which is a different phenomenon.
📌 SSC & Railway Exam Facts
• Tyndall effect = scattering of light by colloidal (and suspension) particles; named after John Tyndall.
• Colloid → shows it; true solution → never shows it. This is the standard test to tell the two apart.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Scattering also makes the sky blue and the setting Sun red (Rayleigh scattering ∝ 1/λ⁴).
• Milk, fog, smoke, ink, blood, cheese, butter are everyday colloids; all show the Tyndall effect.

Correct Answer: Option 3 — 10
📖 Given
• Current, I = 1 A
• Time, t = 10 s
• To find: charge Q in coulombs
✍️ Formula
I = Q / t, therefore Q = I × t
🔢 Solution
• Q = I × t
• Q = 1 × 10
• Q = 10 C
So 10 coulomb of charge passes through the conductor. This follows straight from the definition of current: 1 ampere is 1 coulomb of charge flowing per second, so in 10 seconds a 1 A current carries 10 coulomb.
❌ Why the other options are incorrect
✗ 0.1 — this is what you get if you divide the current by the time (1 ÷ 10) instead of multiplying; that operation has no physical meaning here.
✗ 100 — this comes from using t = 100 s or from multiplying 10 by 10; the time given is only 10 s.
✗ 1 — this is the value of the current, not the charge; the charge grows with time and after 10 s it is ten times larger.
📌 SSC & Railway Exam Facts
• I = Q/t; SI unit of charge is the coulomb (C), of current the ampere (A).
• 1 A = 1 C/s, and 1 C is the charge of about 6.25 × 10¹⁸ electrons.
• Charge on one electron = −1.6 × 10⁻¹⁹ C; on one proton = +1.6 × 10⁻¹⁹ C.
• Current is measured with an ammeter, connected in series; voltage with a voltmeter, connected in parallel.
• Conventional current flows from positive to negative terminal, opposite to the actual flow of electrons.

Correct Answer: Option 4 — If an object is in motion, applied force is directly proportional to the acceleration.
Newton's Second Law of Motion states that the rate of change of momentum of a body is directly proportional to the applied unbalanced force, and the change takes place in the direction of the force.
Starting from momentum p = mv:
F ∝ Δp/Δt = m(v − u)/t = ma
So for a body of constant mass, F = ma. With the mass fixed, this means the applied force is directly proportional to the acceleration produced — exactly what option 4 says.
The same law fixes the size of the SI unit of force: 1 newton is the force that produces an acceleration of 1 m/s² in a mass of 1 kg, so 1 N = 1 kg m/s².
❌ Why the other options are incorrect
✗ The rate of change of momentum of an object is independent of the applied unbalanced force. — this is the exact opposite of the second law; the rate of change of momentum is directly proportional to that force.
✗ Applied force on a body is inversely proportional to the acceleration of the body. — the relation F = ma is a direct proportionality, not an inverse one; more force gives more acceleration.
✗ An object at rest will remain at rest when external force is not applied. — this is the statement of Newton's First Law (law of inertia), not the second law.
📌 SSC & Railway Exam Facts
• Second law: F = ma = Δp/Δt; force and acceleration are in the same direction.
• 1 newton = 1 kg m/s²; the CGS unit is the dyne, and 1 N = 10⁵ dyne.
• Momentum p = mv, a vector, SI unit kg m/s.
• Impulse = F × t = change in momentum; this is why cricketers draw their hands back while catching a ball.
• The second law gives a quantitative measure of force; the first law only defines force qualitatively.

Correct Answer: Option 2 — 20 N
📖 Given
• Pressure, P = 10 Nm⁻²
• Area, A = 2 m²
• To find: thrust F
✍️ Formula
Pressure = Thrust / Area, i.e. P = F/A, therefore F = P × A
🔢 Solution
• F = P × A
• F = 10 × 2
• F = 20 N
So a thrust of 20 N acts on the body. Thrust is simply the force acting perpendicular to a surface, and pressure is that thrust spread over each square metre of the surface.
❌ Why the other options are incorrect
✗ 5 N — this comes from dividing the pressure by the area (10 ÷ 2) instead of multiplying; that gives pressure per unit area, which is meaningless here.
✗ 2 N — this is just the area value written with the unit of force; area is not thrust.
✗ 10 N — this is the pressure value carried over as if it were the force; it ignores the area altogether, i.e. it assumes A = 1 m².
📌 SSC & Railway Exam Facts
• P = F/A; SI unit of pressure is the pascal (Pa) and 1 Pa = 1 N/m².
• Thrust is a force (unit newton), while pressure is force per unit area — a vector and a scalar respectively.
• For a fixed force, smaller area means larger pressure — this is why a knife is sharpened and a nail is pointed.
• Pressure in a liquid at depth h: P = hρg; it acts equally in all directions.
• Atmospheric pressure = 1.013 × 10⁵ Pa = 1 atm = 760 mm of Hg, measured with a barometer.

Correct Answer: Option 1 — 5 N
📖 Given
• Mass of the box, m = 10 kg
• Initial velocity, u = 5 m s⁻¹
• Final velocity, v = 15 m s⁻¹
• Time for which the force acts, t = 20 s
✍️ Formula
a = (v − u) / t and F = ma
🔢 Solution
• a = (15 − 5) / 20
• a = 10 / 20 = 0.5 m s⁻²
• F = m × a = 10 × 0.5
• F = 5 N
So a steady force of 5 newton is enough to speed the box up by 10 m s⁻¹ over 20 seconds. The same answer follows from the momentum form: F = (mv − mu)/t = (150 − 50)/20 = 5 N.
❌ Why the other options are incorrect
✗ 10 N — this is obtained by using the change in velocity (10) directly as the acceleration, forgetting to divide by the time of 20 s.
✗ 1 N — this comes from wrongly dividing again by the mass or from taking a = 0.1 m s⁻²; the correct acceleration is 0.5 m s⁻².
✗ 4 N — this comes from a slip such as using u = 7 m s⁻¹ or a wrong subtraction; the correct velocity change is 15 − 5 = 10 m s⁻¹.
📌 SSC & Railway Exam Facts
• Newton's Second Law: F = ma = (mv − mu)/t; 1 N = 1 kg m s⁻².
• Impulse = F × t = change in momentum; SI unit N s, same as kg m/s.
• Equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• If the force acts opposite to the motion, the acceleration is negative (retardation).
• Momentum is conserved in the absence of an external unbalanced force.

Correct Answer: Option 3 — image is real
For a spherical mirror the magnification is
m = h'/h = −v/u
where h' is the image height and h the object height.
The sign of m tells us about the nature and orientation of the image, while the numerical value of m tells us about the size.
• m is negative → the image is real and inverted (it is formed in front of the mirror and can be caught on a screen).
• m is positive → the image is virtual and erect (it is formed behind the mirror and cannot be caught on a screen).
Since a real image formed by a mirror is always inverted, h' is measured below the axis and comes out negative, which is exactly why m is negative. A concave mirror gives a negative m whenever the object lies beyond the focus.
❌ Why the other options are incorrect
✗ image is enlarged — enlargement is decided by the magnitude of m, not its sign: |m| > 1 means enlarged, whatever the sign.
✗ image is diminished — again a matter of magnitude: |m| < 1 means diminished; a diminished image may be real (negative m) or virtual (positive m).
✗ image is virtual — a virtual image always has a positive magnification, since it is erect; this is the opposite of what a negative sign means.
📌 SSC & Railway Exam Facts
• Magnification m = h'/h = −v/u for mirrors; for lenses m = h'/h = v/u.
• Negative m → real, inverted; positive m → virtual, erect.
• |m| > 1 → enlarged, |m| = 1 → same size, |m| < 1 → diminished.
• A convex mirror always gives 0 < m < 1 — a virtual, erect, diminished image.
• Mirror formula: 1/v + 1/u = 1/f; R = 2f; magnification has no unit.

Correct Answer: Option 3 — 2 A
📖 Given
• Resistance of the lamp, R₁ = 10 Ω
• Resistance of the conductor, R₂ = 2 Ω
• Supply voltage, V = 24 V
• The two are connected in series
✍️ Formula
Series: R(eq) = R₁ + R₂, and Ohm's law: I = V / R(eq)
🔢 Solution
• R(eq) = 10 + 2
• R(eq) = 12 Ω
• I = V / R(eq) = 24 / 12
• I = 2 A
So a current of 2 ampere flows through the whole circuit. In a series circuit the same current passes through every component, so both the lamp and the conductor carry 2 A. The voltage divides as 20 V across the lamp and 4 V across the conductor.
❌ Why the other options are incorrect
✗ 1 A — this is obtained if the total resistance is wrongly taken as 24 Ω (for example by adding the voltage to the resistances).
✗ 4 A — this comes from taking R(eq) = 6 Ω, for example by wrongly combining 10 Ω and 2 Ω in parallel-like fashion instead of adding them; in series the total is 10 + 2 = 12 Ω.
✗ 3 A — this comes from taking R(eq) = 8 Ω, i.e. wrongly subtracting 2 from 10 instead of adding, which is the parallel-style mistake.
📌 SSC & Railway Exam Facts
• Ohm's law: V = IR; SI unit of resistance is the ohm (Ω).
• Series: R(eq) = R₁ + R₂ + …, current same; Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + …, voltage same.
• R = ρl/A — resistance is directly proportional to length and inversely proportional to area of cross-section.
• Electric power P = VI = I²R = V²/R; here P = 24 × 2 = 48 W.
• Ohm's law is valid only at constant temperature; Georg Simon Ohm gave it in 1827.

Correct Answer: Option 2 — 1/v + 1/u = 1/f
The relation connecting the object distance u, the image distance v and the focal length f of a spherical mirror is called the mirror formula:
1/v + 1/u = 1/f
It holds for both concave and convex mirrors, provided the New Cartesian Sign Convention is used: all distances are measured from the pole, distances measured in the direction of the incident light are positive and those opposite to it are negative. So for a concave mirror f is negative, and for a convex mirror f is positive.
Remember also that f = R/2, i.e. the focal length is half the radius of curvature, and the magnification is m = −v/u.
A common trap is to mix this up with the lens formula, 1/v − 1/u = 1/f, which has a minus sign — mirrors use plus, lenses use minus.
❌ Why the other options are incorrect
✗ 1/v − 1/u = 1/2f — wrong on two counts: it uses the lens-type minus sign and an extra factor of 2 that does not belong in the formula.
✗ 1/v − 1/u = 1/f — this is the lens formula, not the mirror formula; using it for a mirror gives a wrong image position.
✗ 1/v + 1/u = 1/2f — the right-hand side must be 1/f, not 1/2f; the factor 2 belongs to the separate relation R = 2f.
📌 SSC & Railway Exam Facts
• Mirror formula: 1/v + 1/u = 1/f; Lens formula: 1/v − 1/u = 1/f.
• f = R/2 for a spherical mirror; R is the radius of curvature.
• Magnification: mirror m = −v/u, lens m = v/u.
• Power of a lens P = 1/f (metre), SI unit dioptre (D).
• A concave mirror is converging (f negative); a convex mirror is diverging (f positive) in the New Cartesian convention.

Correct Answer: Option 3 — Its state of motion remains unchanged.
Forces acting on a body are said to be balanced when their resultant (net) force is zero. The individual forces do not vanish — they simply cancel one another out.
By Newton's Second Law, F = ma, if the net force F = 0 then the acceleration a = 0. Zero acceleration means the velocity cannot change either in magnitude or in direction. So a body at rest stays at rest, and a body moving with uniform velocity keeps moving with the same speed in the same straight line. This is precisely the content of Newton's First Law of Motion.
A book lying on a table is the classic example: its weight acting downwards is exactly balanced by the normal reaction of the table acting upwards, so the book stays at rest. Balanced forces can still change the shape of a body, as when a rubber ball is squeezed from both sides.
❌ Why the other options are incorrect
✗ It speeds up. — speeding up needs a positive acceleration, which requires an unbalanced force along the direction of motion.
✗ It slows down. — slowing down is retardation, produced by an unbalanced force acting opposite to the motion, such as friction.
✗ It changes its direction of motion. — changing direction is also a change of velocity, and so needs an unbalanced force acting sideways, as in circular motion.
📌 SSC & Railway Exam Facts
• Balanced forces → net force zero → no acceleration; unbalanced forces → acceleration.
• Newton's First Law (Law of Inertia) describes exactly this situation.
• Balanced forces cannot change the state of motion but can change the shape of a body.
• Friction is an example of an unbalanced force that brings a moving body to rest.
• F = ma; SI unit of force is the newton (N), 1 N = 1 kg m/s².

Correct Answer: Option 3 — greater at the poles than at the equator
The acceleration due to gravity on the surface of the Earth is
g = GM / R²
so g is inversely proportional to the square of the Earth's radius R at that place.
The Earth is not a perfect sphere — it is an oblate spheroid, slightly flattened at the poles and bulging at the equator. Hence the polar radius (≈ 6357 km) is smaller than the equatorial radius (≈ 6378 km). A smaller R means a larger g, so g is maximum at the poles and minimum at the equator.
The Earth's rotation adds to the same conclusion: the centrifugal effect of rotation reduces the effective g most at the equator and not at all at the poles. Numerically, g ≈ 9.83 m/s² at the poles and g ≈ 9.78 m/s² at the equator.
❌ Why the other options are incorrect
✗ the same at the poles and equator — this would be true only if the Earth were a perfect non-rotating sphere; in reality g varies with latitude.
✗ negative at the equator and positive at the poles — g is an acceleration directed towards the centre of the Earth everywhere; it never becomes negative in this sense.
✗ greater at the equator than at the poles — exactly the reverse of the truth, since the equatorial radius is the larger one.
📌 SSC & Railway Exam Facts
• g = GM/R²; average value g = 9.8 m/s²; g is maximum at the poles, minimum at the equator.
• g decreases with height (g' = g(1 − 2h/R)) and with depth (g' = g(1 − d/R)), and is zero at the centre of the Earth.
• G = 6.67 × 10⁻¹¹ N m² kg⁻² is a universal constant; g is not a constant.
• Value of g on the Moon is about 1/6 of that on the Earth, i.e. 1.625 m/s².
• A body's weight W = mg is therefore slightly more at the poles than at the equator, though its mass stays the same.

Correct Answer: Option 3 — negative
When a stone is thrown upwards, we naturally take the upward direction as positive, because that is the direction of the initial velocity u.
But the acceleration due to gravity always acts downwards, towards the centre of the Earth. Since 'down' is the direction opposite to the chosen positive direction, g must be taken as negative (g = −9.8 m/s²) in the equations of motion.
That is why the stone decelerates as it rises: v = u − gt, and its velocity becomes zero at the highest point. The maximum height reached is h = u²/2g and the time to reach the top is t = u/g. On the way down, if we keep the same convention, the stone speeds up again because the acceleration and the (now downward) velocity point the same way.
❌ Why the other options are incorrect
✗ positive — g would be taken positive only if the downward direction were chosen as positive, as in a body falling freely from rest.
✗ infinite — g is a finite quantity, 9.8 m/s² near the Earth's surface; it can never be infinite.
✗ zero — g is zero only at the centre of the Earth or in a state of free fall inside a satellite; even at the highest point of the throw, where the velocity is zero, g is still 9.8 m/s² downwards.
📌 SSC & Railway Exam Facts
• Thrown up → g is negative (retardation); falling down → g is positive (acceleration).
• At the topmost point velocity v = 0 but acceleration g ≠ 0.
• Maximum height h = u²/2g; time of ascent = time of descent = u/g; total time of flight = 2u/g.
• A body returns to the thrower's hand with the same speed with which it was thrown (ignoring air resistance).
• g = 9.8 m/s² on Earth; g = GM/R²; astronauts feel weightless because they are in continuous free fall.

Correct Answer: Option 1 — concave mirror
A spherical mirror is a piece cut from a hollow sphere with one surface silvered. If the reflecting (polished) surface is the inner, hollow side — the side facing the centre of the sphere — the mirror is called a concave mirror. If the outer, bulging side reflects, it is a convex mirror.
A concave mirror is a converging mirror: a beam of light parallel to the principal axis is brought together at the principal focus F after reflection. Its focal length f = R/2, and in the New Cartesian sign convention f is negative.
Because of this converging property a concave mirror is used as a shaving mirror, a dentist's mirror, in torches, headlights and searchlights (bulb at the focus gives a parallel beam) and in solar cookers and reflecting telescopes.
❌ Why the other options are incorrect
✗ convex mirror — here the reflecting surface is the outer bulged side, away from the centre of the sphere; it is a diverging mirror and always gives a virtual, erect, diminished image.
✗ plano-convex mirror — 'plano-convex' describes a lens with one flat and one bulging face, not a spherical mirror of the kind described.
✗ plane mirror — a plane mirror is flat, so it has no centre of curvature and no focus; its focal length is taken as infinity.
📌 SSC & Railway Exam Facts
• Concave = converging (reflecting surface inward); Convex = diverging (reflecting surface outward).
• f = R/2; mirror formula 1/v + 1/u = 1/f; magnification m = −v/u.
• A convex mirror is used as a rear-view mirror because it gives a wide field of view and an erect image.
• A concave mirror is used in torches, solar cookers, shaving mirrors and reflecting telescopes.
• Only a concave mirror can give a real image; a convex mirror and a plane mirror always give virtual images.

Correct Answer: Option 2 — Zero
📖 Given
• A force F acts on a body
• The displacement s is perpendicular to the force, so the angle between them is θ = 90°
✍️ Formula
W = F s cos θ
🔢 Solution
• W = F × s × cos 90°
• cos 90° = 0
• W = F × s × 0
• W = 0 (zero)
So when the force and the displacement are at right angles, no work is done at all, however large the force and the displacement may be. The force in that case only changes the direction of motion, not the speed, so the body's kinetic energy stays the same.
Classic examples: a coolie carrying a load on his head while walking horizontally (the force is vertical, the displacement horizontal) and the Moon revolving around the Earth in a circular orbit, where the gravitational pull is always perpendicular to the motion.
❌ Why the other options are incorrect
✗ Positive — work is positive only when the angle is less than 90° (cos θ > 0), for example a body being pushed along the direction of the force.
✗ Infinite — work can never be infinite here; W = Fs cos θ is a finite product, and cos θ never exceeds 1.
✗ Negative — work is negative when the angle is more than 90° (cos θ < 0), for example work done against friction or by gravity on a body being lifted up.
📌 SSC & Railway Exam Facts
• W = F s cos θ: θ = 0° → maximum positive work, θ = 90° → zero work, θ = 180° → negative work.
• SI unit of work and energy is the joule (J); 1 J = 1 N m; CGS unit is the erg, 1 J = 10⁷ erg.
• In uniform circular motion the centripetal force does zero work, since it is always perpendicular to the velocity.
• Work is a scalar quantity, even though force and displacement are both vectors.
• Work–energy theorem: work done by the net force = change in kinetic energy.

Correct Answer: Option 4 — less than the resistance of the individual resistors
For two resistors joined in parallel,
1/R(eq) = 1/R₁ + 1/R₂, which gives R(eq) = R₁R₂ / (R₁ + R₂)
Since we are adding the reciprocals, the reciprocal of R(eq) is larger than either 1/R₁ or 1/R₂, so R(eq) must be smaller than both R₁ and R₂ — smaller even than the smallest resistance in the combination.
A quick check with numbers: for R₁ = 6 Ω and R₂ = 3 Ω,
R(eq) = (6 × 3)/(6 + 3) = 18/9 = 2 Ω, which is indeed less than 3 Ω.
The physical reason is that connecting resistors in parallel gives the current more paths to flow through, which is like widening a road — the effective area of cross-section increases, so the opposition to current falls.
❌ Why the other options are incorrect
✗ equal to the sum of the resistance of the resistors — this is the rule for a series combination, R(eq) = R₁ + R₂, the opposite arrangement.
✗ equal to the product of resistance of the individual resistors — the product appears only in the numerator of R₁R₂/(R₁+R₂); by itself it has the wrong unit, Ω².
✗ greater than the resistance of the individual resistors — again the reverse of the truth; only a series joining can raise the total resistance above each individual value.
📌 SSC & Railway Exam Facts
• Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + …; for two resistors R(eq) = R₁R₂/(R₁+R₂).
• n equal resistors of R in parallel give R/n; in series they give nR.
• In parallel the voltage is the same across each branch and the current divides.
• Household appliances are wired in parallel so each receives the full 220 V and works independently.
• Ohm's law V = IR; R = ρl/A, where ρ is resistivity with unit Ω m.