RRB Technician Grade III 2024-25 27 December 2024 Shift 2 GK Questions – with answers & solutions

Correct Answer: Option 1 — Paramecium
Paramecium is a unicellular (single-celled) organism whose entire body is covered with thousands of short, hair-like structures called cilia. That is why it is described as a ciliated or ciliary organism. The rhythmic beating of the cilia moves the animal through water and also sweeps food particles into the oral groove, where a food vacuole is formed.
Being unicellular, one single cell carries out every life process — nutrition, respiration, excretion, movement and reproduction. Because of its slipper-like outline it is often called the slipper animalcule.
❌ Why the other options are incorrect
✗ Leeches — multicellular blood-sucking annelid worms; they have suckers, not cilia, and belong to the phylum Annelida.
✗ Amoeba — it is indeed unicellular, but it moves and feeds by pseudopodia (false feet) of flowing cytoplasm; it has no cilia.
✗ Ticks — multicellular arachnids (eight-legged arthropods) that are external parasites of animals; they have jointed legs, not cilia.
📌 SSC & Railway Exam Facts
• Cilia — short and numerous (Paramecium); flagella — long and few (Euglena, sperm).
• Pseudopodia — Amoeba; parapodia — Nereis.
• Paramecium has two nuclei — a large macronucleus and a small micronucleus.
• Unicellular examples: Amoeba, Paramecium, Euglena, Chlamydomonas, yeast, bacteria.
• In humans, ciliated epithelium lines the trachea and pushes out dust and mucus.

Correct Answer: Option 1 — Rudolf Virchow
Rudolf Virchow, a German scientist, stated in 1855 that "Omnis cellula e cellula" — all cells arise from pre-existing cells. This expanded and completed the cell theory, which had been proposed by Schleiden (1838) and Schwann (1839).
The modern cell theory therefore says: (i) all living organisms are composed of cells and products of cells, (ii) the cell is the basic structural and functional unit of life, and (iii) every cell comes from a pre-existing cell by division. Virchow's addition ruled out the old idea of spontaneous generation of life from non-living matter.
❌ Why the other options are incorrect
✗ Robert Brown — in 1831 he discovered the nucleus in the cells of an orchid leaf.
✗ Purkinje — in 1839 he coined the term protoplasm for the fluid substance of the cell.
✗ Robert Hooke — in 1665 he observed a thin slice of cork under his microscope and coined the word "cell".
📌 SSC & Railway Exam Facts
• Cell theory — Schleiden (plants, 1838) + Schwann (animals, 1839); extended by Virchow (1855).
• Virchow is also called the Father of Modern Pathology.
• Leeuwenhoek (1674) first saw free living cells in pond water.
• Robert Hooke described cells in his book Micrographia.
• The cell is the structural and functional unit of life; the largest human cell is the ovum, the longest is the nerve cell.

Correct Answer: Option 4 — Pancreas
The pancreas is a large gland lying just below the stomach. It secretes pancreatic juice into the duodenum (the first part of the small intestine) through the pancreatic duct. This juice contains trypsin, which digests proteins, amylase, which digests starch, and lipase, which breaks down emulsified fats.
The pancreas is a mixed (hetero-crine) gland — as an exocrine gland it pours digestive enzymes into the gut, and as an endocrine gland its islets of Langerhans release the hormones insulin and glucagon straight into the blood.
❌ Why the other options are incorrect
✗ Salivary gland — secretes saliva containing the enzyme salivary amylase (ptyalin), which begins the digestion of starch in the mouth.
✗ Gastric gland — in the stomach wall; it secretes hydrochloric acid, the protein-digesting enzyme pepsin and protective mucus — but not trypsin.
✗ Liver — the largest gland of the body; it secretes bile, which contains no enzyme and only emulsifies fats and makes the food alkaline.
📌 SSC & Railway Exam Facts
• Trypsin → proteins; Amylase → starch; Lipase → fats — all from pancreatic juice.
• The pancreas is a mixed gland (exocrine + endocrine).
• Pepsin works in an acidic medium, trypsin in an alkaline medium.
• Bile is made in the liver and stored in the gall bladder.
• Final digestion and absorption occur in the small intestine through finger-like villi.

Correct Answer: Option 1 — Bacteria
Biodegradable substances are those which can be broken down into simpler, harmless substances by the action of micro-organisms such as bacteria and fungi. These micro-organisms are the decomposers of the ecosystem. They secrete enzymes onto dead plant and animal matter, rot it, and return the nutrients to the soil so that plants can absorb them again.
Without decomposers the nutrient cycle would stop and dead bodies and wastes would pile up. Vegetable peels, paper, cotton, wood, cow dung and sewage are biodegradable; plastic, glass, aluminium cans, DDT and radioactive waste are non-biodegradable.
❌ Why the other options are incorrect
✗ Wind — wind only disperses and physically scatters waste; it cannot chemically break organic matter into simpler substances.
✗ Sunlight — sunlight can help dry or fade some materials, but decomposition of biodegradable waste is a biological action, not a purely physical one.
✗ Virus — a virus can multiply only inside a living host cell; it is not a decomposer and does not rot dead organic matter.
📌 SSC & Railway Exam Facts
• Decomposers — bacteria and fungi; they are also called micro-organisms of the soil.
• Biodegradable — food waste, paper, cotton, jute, cow dung. Non-biodegradable — plastic, glass, DDT, metals.
• Composting and vermicomposting (using earthworms) turn biodegradable waste into manure.
• Biogas is produced by the anaerobic decomposition of cow dung by bacteria; it is mainly methane (CH₄).
• Non-biodegradable pesticides show biomagnification along a food chain.

Correct Answer: Option 4 — the left atrium
Oxygen-rich (oxygenated) blood from the lungs enters the heart at the left atrium, the thin-walled upper chamber on the left side. It is brought there by the pulmonary veins. The left atrium relaxes while it is collecting this blood, then contracts so that the blood passes down into the left ventricle.
The left ventricle has the thickest muscular wall of all four chambers, because it must pump the oxygenated blood with great force into the aorta and onward to the whole body. The right atrium meanwhile receives deoxygenated blood from the body through the vena cava, and the right ventricle sends it to the lungs through the pulmonary artery.
❌ Why the other options are incorrect
✗ the pulmonary arteries — they carry deoxygenated blood away from the right ventricle to the lungs; they are not a chamber of the heart.
✗ the pulmonary veins — they are the vessels that bring oxygen-rich blood from the lungs, but the blood "comes into" the heart at a chamber, the left atrium.
✗ the left ventricle — blood reaches it only after the left atrium contracts; it is the pumping chamber, not the receiving chamber.
📌 SSC & Railway Exam Facts
• The human heart has four chambers — two atria (upper) and two ventricles (lower).
• Pulmonary artery is the only artery carrying deoxygenated blood; pulmonary vein the only vein carrying oxygenated blood.
• Double circulation — blood passes through the heart twice in one complete cycle.
• The left ventricle wall is the thickest; valves prevent backflow of blood.
• Normal human heartbeat is about 72 times per minute; blood pressure about 120/80 mm Hg.

Correct Answer: Option 2 — female gamete
In sexual reproduction two germ-cells (gametes) fuse. To make the fusion efficient, the two gametes are of different sizes and roles. One germ-cell is small and motile so that it can travel to the other; conventionally this is called the male gamete. The other germ-cell is larger and contains stored food, and it is conventionally called the female gamete.
The stored food is needed because the zygote formed after fertilisation must divide and grow before it can obtain nutrition from outside. In humans the female gamete is the ovum (egg) produced by the ovary, and the male gamete is the sperm produced by the testis; in flowering plants they are the egg cell in the ovule and the pollen grain.
❌ Why the other options are incorrect
✗ motile gamete — motility is the property of the male gamete (sperm), which is small and carries almost no food store.
✗ immature egg — an immature germ cell is an oocyte, still developing; the term used for the food-storing mature gamete is female gamete.
✗ male gamete — it is the small, motile cell whose job is only to deliver the DNA, not to supply food.
📌 SSC & Railway Exam Facts
• Fusion of male and female gametes = fertilisation; the product is the zygote.
• Gametes are haploid (n); the zygote is diploid (2n) — in humans n = 23, 2n = 46.
• Gametes are formed by meiosis (reduction division).
• Human ovum is the largest cell in the body; sperm is the smallest.
• Ovary also secretes oestrogen and progesterone; testis secretes testosterone.

Correct Answer: Option 3 — vas deferens
The vas deferens (sperm duct) is the tube that carries sperms from the epididymis of the testis towards the urethra. If this tube is surgically cut and tied, sperms can no longer leave the testis and reach the semen, so sperm transfer is prevented. This operation is the male sterilisation method called vasectomy.
Vasectomy is a permanent surgical method of contraception. It stops the sperms only; the man's testosterone, sexual function and semen volume are unaffected, because most of the fluid of semen comes from the seminal vesicles and the prostate. The corresponding female operation, in which the fallopian tubes are cut and tied, is called tubectomy.
❌ Why the other options are incorrect
✗ fallopian tube — the oviduct of the female; it carries the egg to the uterus and is the site of fertilisation. It does not exist in males.
✗ prostate gland — a male accessory gland that only adds fluid to the semen, making the sperms easier to transport and nourish; cutting it would not stop sperm transfer.
✗ seminal vesicle — another accessory gland that adds a fructose-rich secretion to the semen for the sperms' energy; it does not conduct sperms.
📌 SSC & Railway Exam Facts
• Vasectomy — male sterilisation (vas deferens cut); Tubectomy — female sterilisation (fallopian tube cut).
• Sperms are produced in the testes and mature and are stored in the epididymis.
• Testes lie in the scrotum, outside the body, because sperm formation needs a temperature about 2–3 °C lower than body temperature.
• Semen = sperms + secretions of seminal vesicles and prostate.
• Other contraceptive methods: barrier (condom), oral pills (hormonal), IUCD (Copper-T).

Correct Answer: Option 1 — DNA copying
Reproduction begins with the copying of DNA in the cell, because DNA carries the blueprint for building the body. The copying is remarkably accurate but never perfect — small errors (mutations) creep in. These errors change the information slightly, and that is the basic source of variation among individuals of a species.
Variation is valuable in the long run. If a population is suddenly hit by a change such as extreme heat, drought or a new disease, the individuals that happen to carry a suitable variation survive and reproduce. Thus variation gives the species a chance of survival and is the raw material for natural selection and evolution.
❌ Why the other options are incorrect
✗ differentiation of white blood cells — WBCs are part of the immune defence of the body; their differentiation is a body function, not an inheritable source of variation.
✗ formation of red blood cells — mature human RBCs have no nucleus and no DNA, so they cannot pass on any change to the next generation.
✗ formation of neurons — neurons conduct nerve impulses; errors here would affect the individual, not the inherited variation of the species.
📌 SSC & Railway Exam Facts
• DNA = Deoxyribo Nucleic Acid, the hereditary material; its double-helix model was given by Watson and Crick (1953).
• A permanent change in the DNA sequence is a mutation.
• Asexual reproduction gives very little variation; sexual reproduction gives much more.
• Charles Darwin proposed natural selection in *On the Origin of Species*.
• Gregor Mendel is the Father of Genetics; he worked on the garden pea (Pisum sativum).

Correct Answer: Option 1 — Both proteins and lipids
The plasma membrane (cell membrane) is the outermost covering of an animal cell and lies just inside the cell wall in a plant cell. It is made of lipids and proteins — a bilayer of phospholipid molecules in which protein molecules are embedded. This arrangement is described by the fluid mosaic model of Singer and Nicolson.
The membrane is selectively permeable (semi-permeable) — it allows only some substances to pass. Gases such as O₂ and CO₂ move across it by diffusion, water moves by osmosis, and larger particles are taken in by endocytosis, the flexibility of the lipid layer allowing the membrane to fold, as in Amoeba.
❌ Why the other options are incorrect
✗ Only lipids — a purely lipid sheet could not transport substances selectively; the carrier and channel proteins are essential for that.
✗ Only proteins — proteins alone cannot form a continuous flexible barrier; the phospholipid bilayer provides the basic framework.
✗ Only cellulose — cellulose builds the cell wall of plant cells, which is rigid, freely permeable and is outside the plasma membrane.
📌 SSC & Railway Exam Facts
• Fluid mosaic model — given by Singer and Nicolson (1972).
• The plasma membrane is selectively permeable; the cell wall is fully permeable.
• Diffusion moves gases; osmosis moves water across a semi-permeable membrane.
• Plant cell wall is cellulose; fungal wall is chitin; bacterial wall is peptidoglycan.
• Endocytosis (Amoeba engulfing food) is possible only because the membrane is flexible.

Correct Answer: Option 4 — Nervous tissue
The spinal cord, like the brain and the nerves, is built of nervous tissue. Its unit is the neuron (nerve cell), the longest cell of the human body, made of a cell body (cyton), branched dendrites that receive the stimulus, and a long axon that carries the impulse onward.
Nervous tissue is highly specialised for conduction — it receives a stimulus, converts it into an electrical impulse and transmits it very rapidly. The spinal cord, enclosed in the vertebral column, joins the brain to the rest of the body and is also the centre of reflex actions, in which the response is produced without waiting for the brain.
❌ Why the other options are incorrect
✗ Muscular tissue — made of contractile muscle fibres; it produces movement, but does not conduct impulses.
✗ Epithelial tissue — the covering and lining tissue of skin, gut and ducts; it protects, absorbs and secretes.
✗ Connective tissue — binds and supports organs and transports materials (bone, cartilage, blood, tendon, ligament); it only protects and cushions the cord, it does not form it.
📌 SSC & Railway Exam Facts
• Neuron is the structural and functional unit of the nervous system; the gap between two neurons is the synapse.
• CNS = brain + spinal cord; PNS = cranial + spinal nerves.
• A reflex arc is completed in the spinal cord, giving a quick, automatic response.
• Humans have 31 pairs of spinal nerves and 12 pairs of cranial nerves.
• The spinal cord runs inside the vertebral column (backbone) and is bathed in cerebrospinal fluid.

Correct Answer: Option 4 — sperms
In human beings fertilisation is internal, so the male gametes must be delivered into the female body. The penis is the male copulatory organ; during copulation it transfers semen, which carries the sperms, into the vagina of the female. From there the sperms swim up through the uterus into the fallopian tube, where one of them fuses with the ovum.
Sperms are made in the testes, mature and are stored in the epididymis, and travel through the vas deferens. On the way, the seminal vesicles and the prostate gland add fluids that nourish the sperms and make their transport easier; this mixture is the semen.
❌ Why the other options are incorrect
✗ polar bodies — small non-functional cells cast off during the meiotic division of the oocyte in the female; they degenerate and are never transferred.
✗ eggs — the ovum is the female gamete, released by the ovary into the fallopian tube; it is never handled by the male organ.
✗ zygotes — the zygote is formed inside the female body after fertilisation, in the fallopian tube; it is not transferred by the penis.
📌 SSC & Railway Exam Facts
• Male reproductive organs: testes, epididymis, vas deferens, seminal vesicle, prostate, urethra, penis.
• Fertilisation occurs in the fallopian tube (oviduct); the embryo implants in the uterus.
• Placenta supplies the embryo with nutrition and oxygen and removes its wastes.
• Human gestation period is about 9 months (280 days).
• Testosterone is the male sex hormone; oestrogen and progesterone are female sex hormones.

Correct Answer: Option 3 — Mitochondria
Mitochondria are double-membrane organelles that possess their own DNA and ribosomes. Because of this they can synthesise some of their own proteins and are therefore called semi-autonomous organelles. Their inner membrane is deeply folded into cristae, giving a large surface area for the enzymes of respiration.
Mitochondria are the powerhouse of the cell: they carry out aerobic respiration and release energy in the form of ATP (adenosine triphosphate), the energy currency of the cell. In plants the plastids (chloroplasts) also contain their own DNA and ribosomes.
❌ Why the other options are incorrect
✗ Endoplasmic reticulum — a membrane network; RER (with ribosomes on its surface) helps in protein transport and SER in lipid synthesis and detoxification, but it has no DNA of its own.
✗ Golgi apparatus — stacks of membrane-bound cisternae that modify, pack and dispatch materials and form lysosomes; no DNA, no ribosomes.
✗ Lysosomes — membrane-bound "suicide bags" filled with powerful digestive enzymes that clear worn-out organelles; they too lack DNA.
📌 SSC & Railway Exam Facts
• Mitochondria and plastids are the only organelles with their own DNA and ribosomes.
• Mitochondria = powerhouse of the cell; ATP = energy currency.
• Lysosome = suicidal bag; Golgi body = secretory/packaging organelle.
• Ribosome = site of protein synthesis; found free or on the RER.
• Mitochondria and plastids are absent in prokaryotes such as bacteria.

Correct Answer: Option 3 — Only (i) and (iii)
Respiration begins in the cytoplasm, where glucose (6C) is broken into two molecules of pyruvate (3C) — this step is glycolysis. What happens to the pyruvate next depends on the availability of oxygen.
In the absence of oxygen (anaerobic respiration), pyruvate is converted in yeast into ethanol and carbon dioxide; this is fermentation, so statement (i) is correct. In our muscle cells during vigorous exercise, the sudden lack of oxygen makes pyruvate turn into lactic acid, which causes cramps; statement (iii) is also correct. In the presence of oxygen (aerobic respiration) pyruvate is broken into carbon dioxide and water inside the mitochondria, not the chloroplast — so statement (ii) is wrong.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — wrongly accepts (ii); the aerobic break-down of pyruvate occurs in the mitochondria.
✗ Only (i) and (ii) — again includes the false statement (ii) and leaves out the true statement (iii).
✗ Only (i) — too narrow; statement (iii) about lactic acid in muscle cells is perfectly correct.
📌 SSC & Railway Exam Facts
• Glycolysis occurs in the cytoplasm; further aerobic break-down in the mitochondria.
• Aerobic respiration releases much more energy than anaerobic respiration.
• Chloroplast is the site of photosynthesis, not of respiration.
• Muscle cramps are caused by the accumulation of lactic acid.
• Energy released in respiration is stored as ATP; CO₂ + H₂O + energy are the products of aerobic respiration.

Correct Answer: Option 4 — Three
Amphibians such as the frog, toad and salamander have a three-chambered heart — two atria and a single ventricle. Oxygenated blood from the lungs and skin enters the left atrium and deoxygenated blood from the body enters the right atrium, but because there is only one ventricle the two kinds of blood mix to some extent before being pumped out.
This mixing is tolerable for amphibians and many reptiles because they are cold-blooded (poikilothermic) — they do not use energy to keep their body temperature constant, so their oxygen demand is lower. Birds and mammals, being warm-blooded, need a four-chambered heart that keeps oxygenated and deoxygenated blood completely separate.
❌ Why the other options are incorrect
✗ Four — a four-chambered heart is found in birds, mammals and crocodiles, which need fully separated blood streams.
✗ One — a single-chambered heart does not occur in vertebrates; fishes have two chambers, not one.
✗ Two — two chambers (one atrium + one ventricle) are found in fishes, which pump only deoxygenated blood to the gills — single circulation.
📌 SSC & Railway Exam Facts
• Fish — 2 chambers; Amphibians and most reptiles — 3; Crocodile, birds and mammals — 4.
• Fishes show single circulation; birds and mammals show double circulation.
• Crocodile is the only reptile with a four-chambered heart.
• Cold-blooded — fish, amphibians, reptiles; warm-blooded — birds and mammals.
• In humans, the left ventricle wall is thickest as it pumps blood to the whole body through the aorta.

Correct Answer: Option 1 — Alkane
A carbon compound is called saturated when every carbon–carbon linkage in it is a single covalent bond (C–C), so each carbon holds the maximum possible number of hydrogen atoms and the molecule cannot take up any more. Such hydrocarbons are the alkanes, general formula CₙH₂ₙ₊₂ — CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane).
Compounds with a double bond (C=C) or a triple bond (C≡C) are unsaturated, because two carbons are sharing extra electron pairs instead of bonding to hydrogen; such molecules can add hydrogen and become saturated.
Saturated and unsaturated compounds are told apart in the laboratory by their flame: alkanes burn with a clean blue flame, while unsaturated hydrocarbons burn with a yellow, sooty flame because of their higher percentage of carbon. Unsaturated compounds also decolourise bromine water, whereas alkanes do not — the standard test.
❌ Why the other options are incorrect
✗ Alkyne — an unsaturated hydrocarbon containing a carbon–carbon triple bond (C≡C), general formula CₙH₂ₙ₋₂, e.g. ethyne, C₂H₂.
✗ Alkene — an unsaturated hydrocarbon containing a carbon–carbon double bond (C=C), general formula CₙH₂ₙ, e.g. ethene, C₂H₄.
✗ Ethylene — this is simply the common name of ethene (C₂H₄), an alkene; it carries a double bond and so is unsaturated, not saturated.
📌 SSC & Railway Exam Facts
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Alkanes are also called paraffins; their typical reaction is substitution (with Cl₂ in sunlight).
• Unsaturated hydrocarbons give addition reactions and decolourise bromine water — alkanes do not.
• Hydrogenation of oil (vegetable oil + H₂ over nickel catalyst) converts unsaturated oil into saturated vanaspati ghee.
• Ethylene (ethene) is the gas used for the artificial ripening of fruits.

Correct Answer: Option 4 — 1, 2 and 3
All three statements describe standard physical properties of metals, so every one of them is correct.
Statement 1 — Malleability. Metals can be hammered or beaten into thin sheets without breaking. Gold and silver are the most malleable metals; aluminium foil is used to wrap food and iron sheets are rolled for car bodies.
Statement 2 — Ductility. Metals can be drawn into thin wires. Gold is the most ductile metal — about 2 km of wire can be drawn from 1 g of gold; copper and aluminium wires carry our electricity.
Statement 3 — Thermal conductivity. Metals possess free (delocalised) electrons, which carry heat rapidly through the lattice. Silver is the best conductor of heat, followed by copper, which is why cooking utensils are made of copper and aluminium. Metals are also lustrous, sonorous, hard, and have high melting points and densities.
❌ Why the other options are incorrect
✗ Only 1 — it ignores statements 2 and 3; ductility and thermal conductivity are equally standard metallic properties.
✗ Only 1 and 3 — statement 2 is also correct; drawing a metal into wire is exactly what ductility means.
✗ Only 1 and 2 — statement 3 is correct as well; the free electrons of a metal make it a good conductor of both heat and electricity.
📌 SSC & Railway Exam Facts
• Most malleable and most ductile metal — gold (Au); silver is next.
• Best conductors of heat and electricity: silver > copper > gold > aluminium.
• Poorest metallic conductors — lead and mercury; mercury is the only liquid metal.
• Sonorous metals ring when struck — that is why school bells are made of metal.
• Exceptions to remember: sodium and potassium are soft (cut with a knife), gallium and caesium melt in the palm, and graphite (a non-metal) conducts electricity.

Correct Answer: Option 2 — Alkaline potassium permanganate
An alcohol can be converted into the corresponding carboxylic acid by oxidation, which means adding oxygen to the –OH group. The reagents that supply this oxygen are called oxidising agents, and the two used in the NCERT course are alkaline potassium permanganate (alkaline KMnO₄) and acidified potassium dichromate (acidified K₂Cr₂O₇).
When ethanol is warmed with alkaline KMnO₄, the purple colour of the permanganate disappears as it is consumed, and ethanoic acid (acetic acid) is formed. Because these reagents add oxygen to another substance, they are also called oxidising agents in a general sense.
⚗️ The Reaction
CH₃CH₂OH → (alkaline KMnO₄ + heat) CH₃COOH
• Ethanol (CH₃CH₂OH) is oxidised to ethanoic acid (CH₃COOH) — the –CH₂OH group becomes –COOH.
• The same change is brought about by acidified K₂Cr₂O₇, where the orange dichromate turns green.
• A 5–8% solution of ethanoic acid in water is vinegar, and pure ethanoic acid freezes in cold weather to give glacial acetic acid.
❌ Why the other options are incorrect
✗ Zinc — zinc is a reducing agent, not an oxidising agent; with acids it liberates H₂ gas (Zn + 2HCl → ZnCl₂ + H₂).
✗ Palladium — Pd is a hydrogenation catalyst, used to add hydrogen across double bonds — the opposite of oxidation.
✗ Nickel — Ni is the catalyst used in the hydrogenation of vegetable oils into vanaspati ghee; it reduces, it does not oxidise.
📌 SSC & Railway Exam Facts
• Oxidising agents for alcohols: alkaline KMnO₄ and acidified K₂Cr₂O₇.
• KMnO₄ is purple and gets decolourised; K₂Cr₂O₇ is orange and turns green.
• Ethanol → ethanoic acid on oxidation; hence alcohols are said to undergo oxidation to acids.
• Vinegar = 5–8% ethanoic acid; glacial acetic acid freezes at about 17 °C.
• Catalysts for addition (hydrogenation): Ni, Pd, Pt.

Correct Answer: Option 4 — Only (b) is correct.
The paper has swapped the names of (a) and (c). Statement (b) alone names its reaction correctly.
(b) Esterification is correct. A carboxylic acid heated with an alcohol in the presence of a few drops of conc. H₂SO₄ (acid catalyst) gives a sweet-smelling ester. Esters give fruits their fragrance and are used in perfumes and flavouring agents.
(a) is wrongly named. Heating ethanol with conc. H₂SO₄ at 443 K removes a molecule of water and gives ethene — that is a dehydration reaction, and conc. H₂SO₄ acts as a dehydrating agent, not saponification.
(c) is wrongly named. Heating an ester with sodium hydroxide splits it back into the sodium salt of the carboxylic acid and an alcohol — that is saponification, the very reaction by which soap is made, not dehydration.
⚗️ The Reactions
CH₃COOH + C₂H₅OH → (conc. H₂SO₄) CH₃COOC₂H₅ + H₂O (esterification)
CH₃CH₂OH → (conc. H₂SO₄, 443 K) CH₂=CH₂ + H₂O (dehydration)
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (saponification)
• Esterification is reversible; saponification is its alkaline hydrolysis, the reverse step.
• In soap making, the ester is a fat or oil and the product sodium salt of a fatty acid is the soap.
❌ Why the other options are incorrect
✗ Only (a) is correct — (a) actually describes dehydration, so it is mismatched; and it wrongly leaves out the correct pair (b).
✗ (a), (b) and (c) are correct — only (b) is correctly matched; the names in (a) and (c) have been interchanged.
✗ Only (c) is correct — (c) actually describes saponification, not dehydration, so it is mismatched.
📌 SSC & Railway Exam Facts
• Ester + NaOH → soap + alcohol is called saponification.
• Acid + alcohol → ester + water (conc. H₂SO₄ catalyst) is esterification; esters smell sweet.
• Conc. H₂SO₄ is a dehydrating agent; ethanol at 443 K gives ethene.
• Soap = sodium/potassium salt of a long-chain fatty acid; it cleans by forming micelles.
• Ethanol at 373 K with conc. H₂SO₄ gives ether, at 443 K it gives ethene — a common trap.

Correct Answer: Option 1 — Silver
The reactivity series (activity series) arranges metals in the decreasing order of their chemical reactivity, that is, in the order of how readily they lose electrons and form positive ions:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au
Copper (Cu) lies below hydrogen and is therefore a fairly unreactive metal, but it still stands above mercury, silver and gold. Of the four options, only silver (Ag) lies below copper, so copper is more reactive than silver. Calcium, sodium and potassium all lie far above copper and are much more reactive than it.
This order is verified experimentally: copper displaces silver from silver nitrate solution, the blue Cu²⁺ colour appearing while shining silver is deposited on the copper wire.
⚗️ The Reaction
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
• Copper displaces silver because it is higher in the reactivity series — proof that Cu is more reactive than Ag.
• The colourless solution turns blue due to Cu(NO₃)₂, and greyish-white silver settles on the copper.
• The reverse reaction — silver displacing copper — never happens, since Ag is below Cu.
❌ Why the other options are incorrect
✗ Calcium — Ca stands far above Cu in the series; it even reacts with cold water to give Ca(OH)₂ + H₂, which copper never does.
✗ Sodium — Na is among the most reactive metals; it reacts violently with cold water and is stored under kerosene.
✗ Potassium — K is the most reactive metal of the series; its reaction with water is so vigorous that the hydrogen catches fire.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• Metals below hydrogen (Cu, Hg, Ag, Au) do not liberate H₂ from dilute acids.
• Gold and platinum are the least reactive and occur in the native (free) state.
• Na and K are stored in kerosene; phosphorus is stored in water.
• Fe + CuSO₄ → FeSO₄ + Cu — iron displaces copper, so iron vessels cannot store copper sulphate solution.

Correct Answer: Option 3 — The gaseous state
Compressibility is the property by which the volume of a substance can be reduced by applying pressure. It depends entirely on the intermolecular space between the particles — the more empty space there is, the more the particles can be pushed together.
In a gas the particles are very far apart, the force of attraction between them is negligible, and they move randomly at high speed. This huge intermolecular space makes gases highly compressible — a large volume of gas can be squeezed into a small cylinder. That is exactly why LPG (liquefied petroleum gas) is supplied in cylinders, CNG (compressed natural gas) runs vehicles, and oxygen is carried in cylinders in hospitals.
In solids the particles are tightly packed with almost no free space, so solids are practically incompressible. In liquids the particles are close but can slide, so liquids are only slightly compressible — far less than gases.
❌ Why the other options are incorrect
✗ The liquid state — liquids are only slightly compressible; their particles are already close together, which is why hydraulic brakes work on the near-incompressibility of liquids.
✗ The solid state — a solid has the smallest intermolecular space and the strongest force of attraction, so it is almost incompressible and rigid.
✗ Both solid state and liquid state — neither is highly compressible; this option confuses fluidity with compressibility.
📌 SSC & Railway Exam Facts
• Compressibility and intermolecular space: gas > liquid > solid; intermolecular force: solid > liquid > gas.
• LPG = Liquefied Petroleum Gas (mainly butane); CNG = Compressed Natural Gas (mainly methane).
• Rate of diffusion: gas > liquid > solid.
• Gases exert pressure on the walls of the container because their particles collide with the walls.
• Liquids and gases together are called fluids because both can flow.

Correct Answer: Option 4 — same atomic mass
Isobars are atoms of different elements that have different atomic numbers (Z) but the same mass number (A), that is, the same atomic mass. Since A = number of protons + number of neutrons, isobars manage to reach the same total by having different numbers of protons balanced by different numbers of neutrons.
The classic example is calcium-40 (₂₀Ca⁴⁰) and argon-40 (₁₈Ar⁴⁰). Calcium has 20 protons + 20 neutrons, argon has 18 protons + 22 neutrons — both add up to 40. Because their atomic numbers differ, they are different elements with completely different chemical properties.
Do not confuse isobars with isotopes, which are atoms of the same element (same Z) with different mass numbers, such as ¹H, ²H, ³H or ³⁵Cl and ³⁷Cl.
❌ Why the other options are incorrect
✗ same number of electrons — in a neutral atom electrons = protons = atomic number; since isobars have different atomic numbers, their electron counts must also differ.
✗ different atomic mass — this is the opposite of the definition; a different mass number would make them ordinary unrelated atoms, not isobars.
✗ same number of protons — the number of protons is the atomic number, and the question itself says isobars have different atomic numbers; equal protons would make them isotopes of one element.
📌 SSC & Railway Exam Facts
• Isotopes — same atomic number, different mass number (e.g. ¹H, ²H, ³H).
• Isobars — different atomic number, same mass number (e.g. ₂₀Ca⁴⁰ and ₁₈Ar⁴⁰).
• Isotones — same number of neutrons (e.g. ₆C¹⁴ and ₈O¹⁶).
• Mass number (A) = protons + neutrons; atomic number (Z) = protons = electrons in a neutral atom.
• Uses of isotopes: U-235 in nuclear reactors, Co-60 in cancer therapy, I-131 for goitre/thyroid.

Correct Answer: Option 3 — Aldehydes
In the IUPAC system the suffix of the name tells you the functional group. The suffix “-al” always stands for an aldehyde, whose functional group is –CHO (the carbonyl group attached to a hydrogen atom).
Butanal therefore has a four-carbon chain (but-) that is saturated (-an-) and ends in an aldehyde group (-al). Its formula is CH₃CH₂CH₂CHO (C₄H₈O). Note that the –CHO group is always at the end of the chain, so it is automatically carbon number 1.
The other common suffixes are worth memorising together: -ol → alcohol (–OH), -one → ketone (>C=O), -oic acid → carboxylic acid (–COOH), -ene → alkene (C=C) and -yne → alkyne (C≡C).
❌ Why the other options are incorrect
✗ Ketone — a ketone ends in “-one” (e.g. propanone, CH₃COCH₃) and its >C=O group sits between two carbon atoms, never at the chain end.
✗ Alcohol — an alcohol ends in “-ol” (e.g. butanol, C₄H₉OH) and contains the hydroxyl group –OH, not –CHO.
✗ Alkene — an alkene ends in “-ene” (e.g. butene, C₄H₈) and is defined by a carbon–carbon double bond, which is a bond type, not an oxygen-containing group.
📌 SSC & Railway Exam Facts
• Suffix key: -ol = alcohol, -al = aldehyde, -one = ketone, -oic acid = carboxylic acid.
• Functional groups: alcohol –OH, aldehyde –CHO, ketone >C=O, acid –COOH.
• The smallest aldehyde is methanal (HCHO, formaldehyde); its 40% solution is formalin, used to preserve specimens.
• Butanal = CH₃CH₂CH₂CHO; the –CHO group is always at carbon 1 of the chain.
• Aldehydes and ketones both contain the carbonyl (>C=O) group — the difference is its position in the chain.

Correct Answer: Option 4 — Atoms consist of a positively charged nucleus and negatively charged electrons.
In 1911 Ernest Rutherford directed a beam of fast alpha (α) particles — doubly-charged helium ions, He²⁺ — at a very thin gold foil and recorded where they landed on a zinc sulphide screen.
He observed that most α-particles passed straight through the foil, a small fraction were deflected through appreciable angles, and about 1 in 12,000 rebounded almost straight back. He famously said this was as incredible as a 15-inch shell bouncing back off a sheet of tissue paper.
His conclusions were: (i) the atom is mostly empty space, since most particles went straight through; (ii) all the positive charge and nearly all the mass are concentrated in a very small central region — the nucleus, since only a very few particles were strongly repelled; and (iii) the negatively charged electrons revolve around this nucleus in circular paths called orbits. The nucleus (≈10⁻¹⁵ m) is about 100,000 times smaller than the atom (≈10⁻¹⁰ m).
❌ Why the other options are incorrect
✗ Atoms are indivisible and are solid spheres — this is Dalton's atomic theory (1808); the discovery of the electron, proton and neutron showed that atoms are divisible.
✗ Atoms are composed of protons, neutrons, and electrons in a random arrangement — Rutherford's experiment proved a definite, ordered structure, not a random one; also the neutron was discovered only in 1932 by Chadwick, after this experiment.
✗ Atoms have a uniform positive charge spread throughout their volume — this was J. J. Thomson's plum-pudding (watermelon) model, and it is exactly the model that Rutherford's rebounding α-particles disproved.
📌 SSC & Railway Exam Facts
• Rutherford's α-scattering / gold foil experiment discovered the nucleus.
• α-particle = He²⁺ = 2 protons + 2 neutrons, mass 4 u, charge +2.
• Rutherford — Father of Nuclear Physics; Nobel Prize in Chemistry, 1908.
• Defect of his model: a revolving electron should radiate energy and fall into the nucleus — corrected by Niels Bohr (1913).
• Discoverers: electron — J. J. Thomson, proton — Goldstein/Rutherford, neutron — James Chadwick.

Correct Answer: Option 1 — Sodium hydroxide
The chlor-alkali process is the industrial electrolysis of a concentrated aqueous solution of sodium chloride, called brine. The name itself is the clue: “chlor” for the chlorine produced and “alkali” for the alkali sodium hydroxide (NaOH) produced.
When electric current is passed through brine, chlorine gas (Cl₂) is liberated at the anode, hydrogen gas (H₂) at the cathode, and sodium hydroxide solution is left behind near the cathode. All three products are commercially valuable, which makes the process highly economical.
Their uses: NaOH for soaps, detergents, paper and degreasing metals; Cl₂ for water treatment, PVC, CFCs and bleaching powder; H₂ for fuels, margarine and ammonia (fertilisers).
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Cl₂ at the anode, H₂ at the cathode, NaOH remains in the solution near the cathode.
• Check the balance: 2 Na, 2 Cl, 4 H and 2 O appear on both sides.
• Cl₂ + dry Ca(OH)₂ → CaOCl₂ + H₂O turns the chlorine into bleaching powder.
❌ Why the other options are incorrect
✗ Sodium carbonate — washing soda, Na₂CO₃·10H₂O, is made by the Solvay (ammonia–soda) process, not by electrolysis of brine.
✗ Sodium chloride — common salt is the raw material (brine) that is fed into the process, not the product.
✗ Sodium bicarbonate — baking soda, NaHCO₃, is made by passing CO₂ and NH₃ through brine: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.
📌 SSC & Railway Exam Facts
• Chlor-alkali process: electrolysis of brine gives NaOH + Cl₂ + H₂.
• NaOH = caustic soda; Na₂CO₃·10H₂O = washing soda; NaHCO₃ = baking soda; NaCl = common salt.
• CaOCl₂ = bleaching powder, made from Cl₂ + dry slaked lime.
• Anode → Cl₂ (oxidation); cathode → H₂ (reduction) — a standard PYQ pairing.
• Baking soda is a mild, non-corrosive base used in antacids and baking powder.

Correct Answer: Option 2 — They have the same molecular formula but different structural arrangements.
Isomers are compounds that have the same molecular formula — the same number of atoms of each element — but a different arrangement of those atoms, and therefore different physical and chemical properties. The phenomenon itself is called isomerism.
The classic school example is butane, C₄H₁₀, which exists in two forms: n-butane, a straight chain of four carbons (CH₃–CH₂–CH₂–CH₃), and isobutane (2-methylpropane), a branched chain (CH₃–CH(CH₃)–CH₃). Both contain 4 carbon and 10 hydrogen atoms, yet n-butane boils at about −0.5 °C and isobutane at about −12 °C.
Isomerism is a direct consequence of carbon's tetravalency and its power of catenation — its ability to link with other carbon atoms in straight chains, branched chains and rings. This is a major reason why there are millions of carbon compounds. The first three alkanes — methane, ethane and propane — have no isomers; isomerism starts from butane (C₄H₁₀).
❌ Why the other options are incorrect
✗ They have different molecular formulas but the same structural arrangement — this reverses the definition; a different molecular formula makes them entirely different compounds, not isomers.
✗ They have the same molecular formula and the same structural arrangement — identical formula and identical structure means it is the same compound, not a pair of isomers.
✗ They have no carbon atoms in their structure — isomerism is discussed here for organic (carbon) compounds; the option contradicts the very subject of the question.
📌 SSC & Railway Exam Facts
• Isomers = same molecular formula, different structure (and hence different properties).
• C₄H₁₀ has 2 isomers, C₅H₁₂ has 3 isomers (n-, iso- and neo-pentane).
• Methane, ethane and propane have no isomers; isomerism begins at butane.
• Catenation (self-linking) and tetravalency of carbon explain the huge number of organic compounds.
• Isotopes, isobars, isotones belong to atomic structure; isomers belong to organic chemistry — do not mix them up.

Correct Answer: Option 1 — increased
Evaporation is the change of a liquid into vapour at any temperature below its boiling point, and it is a surface phenomenon — only those particles that are at the surface and possess enough kinetic energy can escape into the air.
Because escape happens only from the surface, increasing the surface area exposes more particles to the air, so more particles can escape per second and the rate of evaporation increases. This is why wet clothes spread out on a line dry much faster than the same clothes left in a bundle, and why hot tea is poured into a saucer to cool it quickly.
The rate of evaporation increases with surface area, temperature and wind speed, and decreases with rising humidity, since humid air is already close to saturation with water vapour.
❌ Why the other options are incorrect
✗ First increases then decreases — there is no such reversal; the relationship with surface area is a steady direct (increasing) one.
✗ constant — the rate cannot stay unchanged; more exposed surface always means more escaping particles, a fact demonstrated by drying clothes.
✗ decreased — this is the exact opposite of the truth; reducing the surface area (folding the clothes) is what slows evaporation down.
📌 SSC & Railway Exam Facts
• Evaporation increases with temperature, surface area and wind speed; it decreases with humidity.
• Evaporation is a surface phenomenon and causes cooling; boiling occurs throughout the liquid at a fixed temperature.
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg; latent heat of fusion of ice = 3.34 × 10⁵ J/kg.
• Water in an earthen pot (matka) cools because water evaporates through its pores.
• We feel cool under a fan because moving air speeds up the evaporation of sweat.

Correct Answer: Option 1 — C₆H₅OH
A homologous series is a family of organic compounds that have the same functional group, fit a single general formula, and in which each member differs from the next by one –CH₂– unit (14 u of molecular mass). Members show a gradual gradation in physical properties but similar chemical properties.
The alcohol series has the general formula CₙH₂ₙ₊₁OH: CH₃OH (methanol, n = 1), C₂H₅OH (ethanol, n = 2) and C₃H₇OH (propanol, n = 3) all fit it perfectly and differ from one another by exactly –CH₂–, so these three are true homologues.
C₆H₅OH is phenol, in which the –OH group is attached directly to a benzene ring (an aromatic ring), not to an open-chain alkyl group. It does not obey CₙH₂ₙ₊₁OH (for n = 6 the alcohol would be C₆H₁₃OH), so it belongs to a different series and is the odd one out.
❌ Why the other options are incorrect
✗ C₂H₅OH — ethanol, the second member of the alcohol series; it differs from CH₃OH by exactly one –CH₂–, so it is a homologue.
✗ CH₃OH — methanol, the first member of the same series; it fits CₙH₂ₙ₊₁OH with n = 1.
✗ C₃H₇OH — propanol, the third member; it differs from ethanol by one –CH₂– and fits the general formula with n = 3.
📌 SSC & Railway Exam Facts
• In a homologous series, successive members differ by –CH₂–, i.e. by 14 u of molecular mass.
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂, alcohol CₙH₂ₙ₊₁OH.
• CH₃OH = methanol (wood spirit) — highly poisonous, causes blindness; C₂H₅OH = ethanol, present in alcoholic drinks.
• C₆H₅OH = phenol (carbolic acid), an aromatic compound used as a disinfectant.
• Members of a homologous series have similar chemical properties but gradually changing physical properties.

Correct Answer: Option 1 — Joule
Work is said to be done when a force acting on a body produces a displacement in it. Numerically, W = F × s, where F is the force applied and s is the displacement in the direction of the force.
Putting the SI units into this formula, 1 unit of work = 1 newton × 1 metre = 1 N m. This unit is given the special name joule (J), in honour of the English physicist James Prescott Joule.
So 1 joule is the work done when a force of 1 newton moves a body through 1 metre in the direction of the force. Since work and energy are the same physical quantity, energy is also measured in joules.
❌ Why the other options are incorrect
✗ Kilogram — the SI unit of mass, a measure of the quantity of matter in a body, not of work.
✗ Newton — the SI unit of force; 1 N = 1 kg m/s². Force multiplied by displacement gives work.
✗ Meter — the SI unit of length or displacement, one of the two factors of work, not work itself.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit joule (J), CGS unit erg, and 1 J = 10⁷ erg.
• Work and energy are scalar quantities even though force and displacement are vectors.
• 1 calorie = 4.2 J; 1 kWh = 3.6 × 10⁶ J.
• Power = work/time, SI unit watt (W); 1 W = 1 J/s and 1 horsepower = 746 W.
• Zero work is done when displacement is zero or when the force is perpendicular to the displacement.

Correct Answer: Option 3 — 100 m
📖 Given
• Retardation (acceleration opposite to motion), a = −2 m s⁻²
• Time taken to stop, t = 10 s
• Final velocity, v = 0 (the cycle stops)
✍️ Formula
First find u from v = u + at, then use s = ut + ½at²
🔢 Solution
• 0 = u + (−2 × 10)
• 0 = u − 20
• u = 20 m/s
• s = (20 × 10) + ½ × (−2) × (10)²
• s = 200 − (1 × 100)
• s = 200 − 100
• s = 100 m
So Ram's cycle rolls another 100 metres after he squeezes the brakes before it finally comes to rest.
❌ Why the other options are incorrect
✗ 150 m — no combination of the given data gives this; it is a made-up value between 100 and 200.
✗ 50 m — this comes from using ½at² = ½ × 2 × 10² ÷ 2 wrongly, i.e. halving the answer once more.
✗ 200 m — this is only the first term ut = 20 × 10; the student forgot to subtract ½at² = 100 m.
📌 SSC & Railway Exam Facts
• Equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• A quicker route here is s = (u + v)/2 × t = (20 + 0)/2 × 10 = 100 m.
• Retardation is negative acceleration — the velocity decreases with time.
• SI unit of acceleration is m/s² (m s⁻²); it is a vector quantity.
• Stopping distance ∝ u², which is why doubling the speed makes braking distance four times longer.

Correct Answer: Option 1 — the atmospheric refraction of light from the star
The twinkling of stars is caused by atmospheric refraction of starlight. The earth's atmosphere is made of layers of different optical densities, and these layers keep changing because of wind and temperature variations.
Starlight entering the atmosphere therefore bends continuously and, since a star is practically a point source, its apparent position and the amount of light reaching the eye keep fluctuating. The star thus appears to twinkle — sometimes bright, sometimes dim.
Planets do not twinkle because a planet is much closer and acts as a collection of point sources (an extended source). The variations in light from the different points average out, so the total light reaching the eye stays steady.
❌ Why the other options are incorrect
✗ the atmospheric reflection of light from the stars — reflection sends light back from a surface; the atmosphere transmits and bends starlight, it does not reflect it back.
✗ the interference of light from different stars — stars are independent, incoherent sources, so light from two stars cannot produce a steady interference pattern.
✗ the interference of light from a star and moon — the moon only reflects sunlight; it is again incoherent with starlight and cannot cause twinkling.
📌 SSC & Railway Exam Facts
• Stars twinkle, planets do not — a classic one-mark question.
• Atmospheric refraction also causes the advanced sunrise and delayed sunset by about 2 minutes each.
• The sun appears flattened (oval) at sunrise and sunset due to refraction.
• Scattering of light by the atmosphere makes the sky blue and the setting sun red.
• The Tyndall effect is the scattering of light by colloidal particles.

Correct Answer: Option 3 — Work done/(Current × Time)
📖 Given
• The quantity required is voltage (potential difference), V
• Charge, Q, current, I, and time, t
✍️ Formula
V = W/Q and Q = I × t
🔢 Solution
• Potential difference is the work done per unit charge: V = W/Q
• Charge that flows in time t is Q = I t
• Substituting, V = W/(I × t)
• In words, Voltage = Work done/(Current × Time)
So a 12 V battery does 12 joules of work on every coulomb of charge it drives through the circuit.
❌ Why the other options are incorrect
✗ Work done × Charge — multiplying instead of dividing; this gives units of J C, which is not volt.
✗ Work done × Charge × Time — even further from the definition; voltage is work per unit charge, never work times charge.
✗ Work done/(Current × Charge) — the denominator must be charge (I × t), not current × charge, which would give J/(A C).
📌 SSC & Railway Exam Facts
• 1 volt = 1 joule/coulomb; 1 A = 1 C/s; 1 Ω = 1 V/A.
• Ohm's law: V = IR, given by Georg Simon Ohm.
• Voltage is measured by a voltmeter in parallel; current by an ammeter in series.
• Electrical energy W = VIt and electric power P = VI = I²R = V²/R.
• Charge on one electron = 1.6 × 10⁻¹⁹ C; 1 coulomb ≈ 6.25 × 10¹⁸ electrons.

Correct Answer: Option 2 — N m² kg⁻²
📖 Given
• Two masses m₁ and m₂ separated by a distance r
• The gravitational force between them is F, measured in newton (N)
✍️ Formula
Newton's law of universal gravitation: F = Gm₁m₂/r²
🔢 Solution
• Rearranging for G, G = Fr²/(m₁m₂)
• Unit of F = N, unit of r² = m², unit of m₁m₂ = kg × kg = kg²
• So unit of G = N × m²/kg² = N m² kg⁻²
Its numerical value is G = 6.67 × 10⁻¹¹ N m² kg⁻², and because it is the same everywhere in the universe it is called the universal gravitational constant.
❌ Why the other options are incorrect
✗ N m² kg⁻¹ — this would follow if only one mass appeared in the denominator; the formula has the product of two masses.
✗ N m² — this ignores the masses altogether; it is the unit you get from Fr² alone.
✗ N m² kg — here kg is multiplied instead of divided, which is dimensionally the opposite of the correct unit.
📌 SSC & Railway Exam Facts
• G = 6.67 × 10⁻¹¹ N m² kg⁻², first measured experimentally by Henry Cavendish.
• Newton's law of gravitation: F ∝ m₁m₂/r² — the inverse square law.
• g = GM/R² ≈ 9.8 m/s² on the earth's surface; g and G are entirely different quantities.
• G is a scalar and its value does not change with place, medium or temperature.
• Dimensional formula of G is [M⁻¹L³T⁻²].

Correct Answer: Option 3 — Both Statements I and II
Statement I is correct. Every current-carrying conductor produces a magnetic field around it — this is the magnetic effect of electric current, discovered by Hans Christian Oersted in 1820. In a circular coil the field lines are circles near the wire, and at the centre of the coil they become nearly straight and perpendicular to the plane of the coil. With n turns, the field at the centre becomes n times that of a single turn.
Statement II is also correct. A solenoid is a long cylindrical coil of many closely wound turns. When current flows, the field inside is strong and uniform, and the pattern outside is exactly like that of a bar magnet — one face acts as the north pole and the other as the south pole. Placing a soft iron core inside turns it into a powerful electromagnet.
Since both statements are true, the correct option is Both Statements I and II.
❌ Why the other options are incorrect
✗ Statement I only — it wrongly rejects Statement II, but a solenoid genuinely behaves like a bar magnet.
✗ Statement II only — it wrongly rejects Statement I, yet a current-carrying coil certainly produces a magnetic field.
✗ Neither Statement I nor II — both statements are standard NCERT facts, so rejecting both is wrong.
📌 SSC & Railway Exam Facts
• Field inside a solenoid: B = μ₀nI, where n is turns per unit length.
• Direction of the field is found by the right-hand thumb rule; for a coil use the clock rule.
• SI unit of magnetic field is the tesla (T); 1 T = 1 N/(A m).
• Electromagnets use soft iron; permanent magnets use steel or alnico.
• Fleming's left-hand rule → electric motor; Fleming's right-hand rule → generator.

Correct Answer: Option 4 — Concave mirror
📖 Given
• Magnification, m = +2.3
• The positive sign means the image is virtual and erect
• The magnitude 2.3 > 1 means the image is enlarged
✍️ Formula
m = h′/h = −v/u
🔢 Solution
• m is positive ⇒ image is virtual and erect
• |m| = 2.3, which is greater than 1 ⇒ image is bigger than the object
• Only a concave mirror can give a virtual, erect and magnified image — it does so when the object is placed between the pole P and the focus F
• Hence the mirror is a concave mirror
This is exactly the arrangement used in a shaving mirror and a dentist's mirror, where a magnified upright image is needed.
❌ Why the other options are incorrect
✗ Planoconvex mirror — a plano-convex shape is a lens term; it is not one of the standard spherical mirrors studied here.
✗ Plane mirror — a plane mirror always gives m = +1, an image exactly the same size as the object, never 2.3.
✗ Convex mirror — it always forms a virtual, erect but diminished image, so 0 < m < 1; it can never give m = 2.3.
📌 SSC & Railway Exam Facts
• m = −v/u = h′/h; positive m → virtual, erect, negative m → real, inverted.
• Mirror formula: 1/v + 1/u = 1/f, and R = 2f.
• Concave mirror uses: torch, headlight, solar cooker, shaving mirror, dentist's mirror.
• Convex mirror is used as a rear-view mirror for its wide field of view.
• A concave mirror gives a real, inverted image for every object position beyond the focus F.

Correct Answer: Option 2 — Only (i)
Statement (i) is NOT correct. Resistance definitely depends on temperature. From R = ρl/A, the resistivity ρ of a material changes with temperature. For metals, ρ and hence R increase as the temperature rises, because the vibrating ions scatter the electrons more. For semiconductors, resistance decreases with a rise in temperature. So calling resistance independent of temperature is wrong.
Statement (ii) is correct. By Ohm's law, V = IR, so at constant temperature I ∝ V. Increasing the voltage across a resistor increases the current through it in the same ratio; the V–I graph of an ohmic conductor is a straight line through the origin.
Statement (iii) is correct. The SI unit of resistance is the ohm (Ω), defined as 1 Ω = 1 volt/1 ampere.
Since only statement (i) is wrong, the answer is Only (i).
❌ Why the other options are incorrect
✗ Only (ii) — statement (ii) is a direct restatement of Ohm's law and is perfectly correct.
✗ Both (i) and (ii) — (i) is indeed wrong, but (ii) is correct, so this pair is not acceptable.
✗ Both (ii) and (iii) — both of these statements are correct, so neither belongs in a list of incorrect statements.
📌 SSC & Railway Exam Facts
• Ohm's law: V = IR — valid at constant temperature for ohmic conductors.
• R = ρl/A; resistance ∝ length and ∝ 1/area of cross-section.
• SI unit of resistance is ohm (Ω); of resistivity, ohm metre (Ω m).
• In metals resistance rises with temperature; in semiconductors it falls.
• Superconductors show zero resistance below a critical temperature.

Correct Answer: Option 1 — Different layers of the atmosphere have different density
The earth's atmosphere is not uniform. Its layers have different densities, and therefore different refractive indices, which go on changing with wind, pressure and temperature.
When starlight travels down through this uneven atmosphere it is refracted again and again, bending a little more each time towards the normal as it enters denser air. Because the refracting layers keep shifting, the direction in which the light finally reaches the eye also keeps shifting. The star therefore appears slightly displaced from its true position, and that apparent position keeps changing — which is also why the star seems to twinkle.
For the same reason a star near the horizon appears higher than it really is, and the sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset.
❌ Why the other options are incorrect
✗ Star doesn't have it's own light — false in fact; a star is a self-luminous body that produces its own light by nuclear fusion. It is the moon and planets that shine by reflected light.
✗ Different layers of the atmosphere are of same density — if the density were the same everywhere there would be no refraction at all, and the star's position would never shift.
✗ Stars are very close to the surface of the earth — stars are extremely far away; the nearest star after the sun, Proxima Centauri, is about 4.2 light years away.
📌 SSC & Railway Exam Facts
• Atmospheric refraction explains twinkling of stars, advanced sunrise and delayed sunset.
• Planets do not twinkle because they are extended sources, not point sources.
• Refractive index n = c/v; light bends towards the normal on entering a denser medium.
• The sun looks oval and red at sunrise and sunset due to refraction and scattering.
• The apparent flattening of the sun's disc is also an effect of atmospheric refraction.

Correct Answer: Option 4 — blue
When sunlight enters the earth's atmosphere it strikes molecules of air that are much smaller than the wavelength of visible light. Such tiny particles scatter light according to Rayleigh's law of scattering:
Amount of scattering ∝ 1/λ⁴
Since scattering is inversely proportional to the fourth power of the wavelength, the shorter wavelengths are scattered far more strongly. In the visible spectrum violet and blue have the shortest wavelengths, so they are scattered the most; red, with the longest wavelength, is scattered the least.
The eye is less sensitive to violet than to blue, and part of the violet is absorbed high in the atmosphere — so the scattered light we finally see makes the sky look blue.
❌ Why the other options are incorrect
✗ green — green lies in the middle of the visible spectrum (about 550 nm), so it is scattered less than blue.
✗ pink — pink is not a spectral colour of white light at all; it is a mixture, and does not appear in the VIBGYOR sequence.
✗ red — red has the longest wavelength (about 700 nm), so it is least scattered; that is why red is used for danger signals and why the setting sun looks red.
📌 SSC & Railway Exam Facts
• Rayleigh scattering: intensity ∝ 1/λ⁴ — shorter wavelength scatters more.
• Wavelength order VIBGYOR: violet shortest (≈ 400 nm), red longest (≈ 700 nm).
• Red light travels the farthest through fog, so danger signals are red.
• The sky looks black to an astronaut because there is no atmosphere to scatter light in space.
• The sun looks red at sunrise and sunset because blue light is scattered away over the long slant path.

Correct Answer: Option 3 — 10B
📖 Given
• Magnetic field at the centre of a single turn coil = B
• Number of turns in the new coil, n = 10
• Current I and radius r are kept the same
✍️ Formula
B = μ₀nI/2r — the field at the centre of a circular coil of n turns
🔢 Solution
• For one turn (n = 1): B = μ₀I/2r
• For ten turns (n = 10): B′ = μ₀ × 10 × I/2r
• B′ = 10 × (μ₀I/2r)
• B′ = 10B
Each turn carries the same current in the same direction, so the fields of all ten turns simply add up at the centre, making the field ten times stronger.
❌ Why the other options are incorrect
✗ 100B — this comes from wrongly squaring the number of turns; the field is proportional to n, not to n².
✗ B — the field would stay B only if the extra turns produced nothing, but every extra turn adds its own field.
✗ 5B — this is half the correct value, obtained by wrongly taking only 5 of the 10 turns into account.
📌 SSC & Railway Exam Facts
• Field at centre of a circular coil: B = μ₀nI/2r — proportional to n and I, inversely proportional to r.
• Field inside a long solenoid: B = μ₀nI, where n is turns per unit length.
• μ₀ = 4π × 10⁻⁷ T m/A is the permeability of free space.
• Direction of the field is given by the right-hand thumb rule.
• Oersted (1820) discovered that a current-carrying wire deflects a magnetic compass needle.

Correct Answer: Option 2 — behind the mirror at the focus F
A convex mirror always forms a virtual, erect and diminished image, and that image always lies behind the mirror, between the pole P and the focus F.
The image becomes highly diminished and point-sized only in one limiting case — when the object is placed at infinity. Rays coming from an object at infinity reach the convex mirror as a parallel beam; after reflection they diverge and appear to come from the principal focus F situated behind the mirror.
So the point-sized virtual image forms exactly at the focus F, behind the mirror. As the object is brought nearer from infinity, the image moves from F towards the pole P and grows a little in size, but it always stays virtual, erect and smaller than the object.
❌ Why the other options are incorrect
✗ in front of the mirror at the centre of curvature C — an image formed in front of a mirror is real; a convex mirror can never form a real image of a real object.
✗ behind the mirror at the centre of curvature C — the image of a convex mirror never goes beyond F; it stays in the region between P and F.
✗ in front of the mirror at the focus F — for a convex mirror the focus itself lies behind the mirror, not in front, and the image is virtual.
📌 SSC & Railway Exam Facts
• Convex mirror: image is always virtual, erect and diminished, formed between P and F.
• Object at infinity → image at F, point-sized; object anywhere else → image between P and F.
• For a convex mirror, f and R are taken positive by the sign convention, and f = R/2.
• Convex mirrors are used as rear-view mirrors and in shop security mirrors for a wide field of view.
• Concave mirror forms a real inverted image except when the object lies between P and F.

Correct Answer: Option 4 — Gravitational
The planets revolve around the sun because of the gravitational force of attraction exerted on them by the sun. According to Newton's universal law of gravitation, every body in the universe attracts every other body with a force
F = Gm₁m₂/r²
where G = 6.67 × 10⁻¹¹ N m² kg⁻².
This gravitational pull acts along the line joining the planet and the sun and provides exactly the centripetal force needed to keep the planet in its orbit. Without it, a planet would fly off along a straight line in accordance with Newton's first law of motion.
The same force keeps the moon revolving around the earth, holds artificial satellites in orbit and causes the tides in the sea.
❌ Why the other options are incorrect
✗ Frictional — friction acts only between surfaces in contact; there is no contact and practically no medium in the vacuum of space.
✗ Magnetic — magnetic force acts on magnets and moving charges. Planets are electrically neutral bodies and are not held by magnetism.
✗ Electrical — electrical force needs net charge; the sun and planets are overall neutral, so no net electrostatic force binds them.
📌 SSC & Railway Exam Facts
• Newton's law of gravitation: F = Gm₁m₂/r² — an inverse square law.
• Kepler's three laws: orbits are ellipses, equal areas in equal times, and T² ∝ r³.
• Centripetal force for a planet: F = mv²/r, always directed towards the centre.
• Gravitational force is the weakest of the four fundamental forces but acts over infinite range.
• Tides are caused mainly by the gravitational pull of the moon on the earth's oceans.