RRB Technician Grade III 2024-25 23 December 2024 Shift 2 GK Questions – with answers & solutions

Correct Answer: Option 3 — Only (i) and (iii)
Arteries are the thick vessels that carry blood away from the heart to the organs of the body. Blood leaves the heart in spurts and at high pressure, so arterial walls are thick, strong, muscular and elastic — never thin. Statement B is therefore wrong.
Veins collect blood from the organs and bring it back to the heart. Here the blood is no longer under high pressure, so veins have thin walls and are fitted with valves that permit blood to flow in one direction only and stop it from flowing backwards. Statement C is correct, and statement A is correct by the very definition of an artery.
Hence only A and C are correctly related to transportation.
❌ Why the other options are incorrect
✗ Only (i) — it rejects statement C, but the one-way valves of veins are a standard textbook fact.
✗ (i), (ii) and (iii) — it accepts B, yet arteries have thick elastic walls; thin walls belong to veins and capillaries.
✗ Only (i) and (ii) — it again accepts the false statement (ii) and drops the correct statement (iii).
📌 SSC & Railway Exam Facts
• The pulmonary artery is the only artery carrying deoxygenated blood; the pulmonary vein is the only vein carrying oxygenated blood.
• Blood circulation was discovered by William Harvey.
• The human heart has four chambers — two atria and two ventricles; the left ventricle has the thickest wall.
• Normal blood pressure is about 120/80 mm Hg, measured with a sphygmomanometer.
• Humans show double circulation — blood passes through the heart twice in one complete round.

Correct Answer: Option 2 — Vacuoles
Vacuoles are membrane-bound storage sacs filled with a watery fluid. In animal cells they are small and few, while in plant cells a single central vacuole may occupy 50–90 per cent of the cell volume. The membrane around the plant vacuole is called the tonoplast.
The large plant vacuole is filled with cell sap — water, sugars, amino acids, mineral salts, pigments and organic acids. Water stored in it creates turgor pressure, which presses the protoplasm against the cell wall and keeps soft herbaceous plants firm and erect. Vacuoles also store waste products that the plant cannot excrete.
❌ Why the other options are incorrect
✗ Golgi apparatus — present in both cell types as stacks of cisternae; it modifies, packages and dispatches materials made by the ER, and its size is not the distinguishing feature asked here.
✗ Plastids — these are found only in plant cells, so they cannot be described as "small in animal cells".
✗ Lysosomes — the suicide bags filled with digestive enzymes; they are prominent in animal cells and rare in plant cells, the opposite of what the question describes.
📌 SSC & Railway Exam Facts
• Amoeba uses a food vacuole to digest food and a contractile vacuole for osmoregulation.
• The plant vacuole membrane is the tonoplast; the fluid inside is cell sap.
• Loss of vacuolar water causes wilting; regaining it restores turgidity.
• Cell wall, plastids and a large central vacuole are the three structures found in plant cells but not in animal cells.
• Centrioles and lysosomes are typical of animal cells.

Correct Answer: Option 2 — Coal – digested in our body
Coal cannot be digested by the human body. Digestion happens only when a specific enzyme is available for a specific substance, because enzymes are highly specific in action — a key fits only its own lock. Our alimentary canal has amylase for starch, pepsin and trypsin for proteins and lipase for fats, but there is no enzyme for coal, which is chemically almost pure carbon.
Substances that living organisms and decomposer micro-organisms can break down are called biodegradable; those they cannot break down, such as coal, plastic, glass and DDT, are non-biodegradable and stay in the environment for centuries.
❌ Why the other options are incorrect
✗ Rice – digested in our body — a correct pair; rice is starch, digested by salivary amylase (ptyalin) and pancreatic amylase into glucose.
✗ Protein – digested in our body — a correct pair; proteins are digested by pepsin in the stomach and trypsin in the small intestine into amino acids.
✗ Plastic – does not breakdown by the action of bacteria — a correct pair; plastic is a non-biodegradable man-made polymer that bacteria cannot attack.
📌 SSC & Railway Exam Facts
• Biodegradable wastes — paper, food waste, cotton, wool, wood, cow dung; non-biodegradable — plastic, polythene, glass, metal cans, DDT.
• Decomposers (bacteria and fungi) are called nature's cleaners because they recycle biodegradable matter.
• Non-biodegradable pesticides build up along a food chain — this is biomagnification.
• The three R's of waste management are Reduce, Reuse and Recycle.
• Enzymes are biological catalysts and are mostly proteins; they work best at a specific pH and temperature.

Correct Answer: Option 1 — starch
During photosynthesis a green plant converts carbon dioxide and water into glucose using sunlight trapped by chlorophyll. Whatever glucose is not used immediately for respiration or growth is polymerised into starch, an insoluble carbohydrate stored in the leaves, stems, roots, tubers and seeds.
Starch is the ideal store because it is insoluble — it does not dissolve in cell sap, so it neither leaks away nor disturbs the osmotic balance of the cell. When energy is needed, enzymes hydrolyse starch back into glucose. Presence of starch in a leaf is the standard test for photosynthesis, shown by the iodine test in which starch turns blue-black.
❌ Why the other options are incorrect
✗ cellulose — also a glucose polymer, but it is a structural carbohydrate that builds the cell wall; it is not an energy reserve and humans cannot digest it.
✗ glycogen — the storage carbohydrate of animals and fungi, stored in the liver and muscles; it is called animal starch, not a plant reserve.
✗ glucose — the immediate product of photosynthesis, but being soluble and osmotically active it cannot be stockpiled in large amounts.
📌 SSC & Railway Exam Facts
• Photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of sunlight and chlorophyll.
• Photosynthesis occurs in the chloroplast, called the kitchen of the cell.
• Starch + iodine → blue-black colour; this is the classic starch test.
• Starch is stored as grains in the leucoplasts (amyloplasts) of plant cells.
• Gases enter the leaf through stomata, guarded by bean-shaped guard cells.

Correct Answer: Option 3 — Cytokinin
Cytokinins are plant hormones (phytohormones) that promote cell division (cytokinesis) — the very word cytokinin comes from *cytokinesis*. They are naturally present in greater concentration in regions of rapid cell division, such as fruits, seeds, root tips and shoot apices.
Besides triggering division, cytokinins delay the ageing of leaves, help in opening of stomata, and break the apical dominance imposed by auxins so that lateral buds can sprout. Zeatin, obtained from maize, is the best-known natural cytokinin, and the hormone is largely synthesised in the root tips and carried upward through the xylem.
❌ Why the other options are incorrect
✗ Abscisic acid (ABA) — the only growth inhibitor among these; it causes wilting, closure of stomata, dormancy of buds and seeds, and abscission (fall) of leaves. It is the stress hormone.
✗ Gibberellins — cause elongation of the stem, bolting, breaking of seed dormancy and parthenocarpy; discovered from the fungus *Gibberella fujikuroi*.
✗ Auxins — synthesised at the shoot tip; they cause cell elongation (not division) and are responsible for phototropism and apical dominance.
📌 SSC & Railway Exam Facts
• Five main plant hormones: Auxin, Gibberellin, Cytokinin, Abscisic acid and Ethylene.
• Ethylene is the only gaseous plant hormone; it ripens fruits.
• Auxin was the first plant hormone discovered; F. W. Went isolated it.
• ABA is the stress / anti-transpirant hormone; cytokinin is the anti-ageing hormone.
• Gibberellin makes dwarf plants grow tall; cytokinin is widely used in tissue culture media.

Correct Answer: Option 1 — Nephron
The nephron is the structural and functional unit of the kidney. Each human kidney contains about one million nephrons packed closely together, which is exactly what the phrase "densely packed kidney filtration unit" describes.
A nephron has two parts — the Malpighian body (a cluster of capillaries called the glomerulus enclosed in the cup-shaped Bowman's capsule) and a long renal tubule. Urine formation takes place in three steps: glomerular filtration of blood under pressure, selective reabsorption of glucose, amino acids, salts and most water in the tubule, and tubular secretion of extra wastes. The fluid that finally leaves is urine, which passes through the ureter to the urinary bladder.
❌ Why the other options are incorrect
✗ Neurone — the structural and functional unit of the nervous system, made of dendrites, cell body, axon and nerve endings; it conducts impulses, it does not filter blood.
✗ Capillary — the thinnest blood vessel, one cell thick, where exchange of gases and nutrients with tissues happens; it is only a part of the nephron's blood supply.
✗ Glomerulus — this is the tuft of capillaries inside the Bowman's capsule that does the filtering, but it is a part of the nephron, not the complete filtration unit.
📌 SSC & Railway Exam Facts
• Excretory system — 2 kidneys, 2 ureters, 1 urinary bladder, 1 urethra.
• Kidneys are bean-shaped, placed near the backbone; the right kidney is slightly lower than the left.
• An adult produces about 1–1.8 litres of urine per day; urine is about 95% water and contains urea.
• Urea is made in the liver from ammonia and removed by the kidneys — humans are ureotelic.
• Kidney failure is treated by haemodialysis, using an artificial kidney.

Correct Answer: Option 1 — Skin
Squamous epithelium is made of extremely thin, flat, plate-like cells fitted together like floor tiles. Where the surface has to bear constant friction and wear and tear, these flat cells are piled up in many layers — such an arrangement is called stratified squamous epithelium.
The skin is the classic example. Its outer covering is stratified squamous epithelium, and the outermost layers become keratinised (hardened) to make the skin waterproof and to protect the body against injury, infection, dehydration and heat loss.
❌ Why the other options are incorrect
✗ Oesophagus — its inner lining is also squamous in type, but the many-layered, wear-resistant stratified squamous covering is described in the textbook as the protective covering of the skin.
✗ Lining of the small intestine — made of columnar epithelium with microvilli, designed for absorption of digested food, not for protection.
✗ Lining of the kidney tubules — made of a single layer of cuboidal epithelium, suited to reabsorption and secretion; being single-layered it is not stratified.
📌 SSC & Railway Exam Facts
• Epithelium types — squamous, cuboidal, columnar, ciliated columnar, stratified squamous and glandular.
• Ciliated columnar epithelium lines the respiratory tract and pushes mucus and dust outward.
• Glandular epithelium forms glands that secrete enzymes, hormones and mucus.
• Skin is the largest organ of the human body; its pigment is melanin.
• Epithelium always rests on a basement membrane and has no blood vessels of its own.

Correct Answer: Option 4 — Sclerenchyma
Sclerenchyma is the hardest simple permanent tissue of plants. Its cells are dead, long and narrow, with no protoplasm, and their walls are thickened with lignin, a natural cement. The cells are so tightly packed that no intercellular spaces are left.
Its work is to give strength, hardness and mechanical support. It is therefore found in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts — exactly as stated in the question. The husk of a coconut is made of sclerenchyma fibres.
❌ Why the other options are incorrect
✗ Epidermis — the single outermost protective layer of the plant body; it bears stomata and a waxy cuticle to check water loss, but gives no mechanical strength.
✗ Parenchyma — living, thin-walled, loosely packed cells that store food and water; when it contains chlorophyll it is called chlorenchyma, and with large air cavities, aerenchyma.
✗ Apical meristem — dividing tissue at the tip of root and shoot responsible for increase in length (primary growth); it is not a supporting tissue.
📌 SSC & Railway Exam Facts
• Simple permanent tissues: Parenchyma, Collenchyma, Sclerenchyma; complex: Xylem and Phloem.
• Collenchyma gives flexibility — it lets tendrils and climber stems bend without breaking.
• Lignin is the chemical that hardens sclerenchyma cell walls.
• Coir from coconut husk and jute fibres are commercial sclerenchymatous fibres.
• Lateral meristem (cambium) increases girth; intercalary meristem lies at the base of leaves and internodes.

Correct Answer: Option 2 — Self-pollination
Pollination is the transfer of pollen grains from the anther of the stamen to the stigma of the pistil. When this transfer happens within the same flower, or between two flowers borne on the same plant, it is called self-pollination (autogamy).
Self-pollination is common in bisexual flowers such as pea, wheat, rice and tomato. It needs no external agent, so it is very reliable, but it produces little variation because both gametes come from the same parent. Pollination is only the first step; the actual fusion of gametes that follows inside the ovule is fertilisation.
❌ Why the other options are incorrect
✗ Spore formation — a method of asexual reproduction seen in *Rhizopus*, ferns and mosses, where spores in a sporangium grow into new individuals without any fusion.
✗ Fertilisation — the fusion of the male gamete with the egg to form a zygote; it happens after pollination, inside the ovule.
✗ Cross-pollination — transfer of pollen to the stigma of a different plant of the same species, carried by wind, water, insects or birds; it creates genetic variation.
📌 SSC & Railway Exam Facts
• Stamen = anther + filament; Pistil = stigma + style + ovary.
• After fertilisation the ovule becomes the seed and the ovary becomes the fruit.
• Pollination by wind is anemophily, by water hydrophily, by insects entomophily.
• Angiosperms show double fertilisation, discovered by S. G. Nawaschin.
• Unisexual flowers (papaya, watermelon) can show only cross-pollination.

Correct Answer: Option 4 — the body design of the organism
The method an organism uses to reproduce is decided by how its body is built. Organisms with a simple body design — a single cell or a loose mass of similar cells — can reproduce by simply splitting or budding, because any part of the body can grow into a new individual.
Thus Amoeba and Leishmania divide by fission, Hydra and yeast by budding, *Planaria* by regeneration, and *Rhizopus* by spore formation. Organisms with a complex body design, where cells are specialised into tissues and organs, cannot do this; they need specialised reproductive organs and adopt sexual reproduction, as in higher plants and animals.
❌ Why the other options are incorrect
✗ The respiratory pigment — molecules such as haemoglobin only carry oxygen in the blood; they have nothing to do with how an organism reproduces.
✗ The nervous system — it controls response and coordination; many organisms that reproduce successfully, such as bacteria and plants, have no nervous system at all.
✗ The nutrition strategy — being autotrophic or heterotrophic decides how food is obtained, not whether the organism divides, buds or forms gametes.
📌 SSC & Railway Exam Facts
• Binary fission — *Amoeba, Paramecium, bacteria*; multiple fission — *Plasmodium*.
• Budding — *Hydra, yeast*; Regeneration — *Planaria*; Fragmentation — *Spirogyra*.
• Vegetative propagation — potato by eyes, ginger by rhizome, *Bryophyllum* by leaf buds.
• Asexual reproduction gives clones (no variation); sexual reproduction creates variation, the raw material of evolution.
• DNA copying is the basic event common to every mode of reproduction.

Correct Answer: Option 2 — Plasma
Plasma is the straw-coloured liquid matrix of blood and makes up about 55 per cent of its volume. It is roughly 90–92 per cent water, the rest being proteins (albumin, globulin, fibrinogen), salts, glucose, hormones and vitamins.
Because it is a solvent, plasma transports food (digested nutrients), carbon dioxide and nitrogenous wastes such as urea in dissolved form. It also carries hormones, distributes heat evenly through the body and helps maintain blood pressure and pH. Blood is therefore called a fluid connective tissue — cells floating in a liquid matrix.
❌ Why the other options are incorrect
✗ Blood platelets (thrombocytes) — tiny cell fragments that plug a cut and start blood clotting, so that pressure in the system is not lost; they transport nothing.
✗ Red blood corpuscles (RBCs) — contain the pigment haemoglobin and specialise in carrying oxygen; in mammals they are biconcave and lack a nucleus.
✗ Lymph — a colourless fluid formed from leaked plasma; it carries lymphocytes and digested fats and drains tissue fluid back into blood, but it is not the main transport medium of blood.
📌 SSC & Railway Exam Facts
• Blood = Plasma (55%) + Formed elements (45%) — RBC, WBC and platelets.
• RBC lifespan ~120 days; they are made in the bone marrow and destroyed in the spleen (the graveyard of RBCs).
• WBCs are the body's soldiers; they fight infection.
• Blood groups A, B, AB, O were discovered by Karl Landsteiner; O⁻ is the universal donor, AB⁺ the universal recipient.
• Deficiency of haemoglobin causes anaemia; iron is its central metal atom.

Correct Answer: Option 2 — Connective tissue
A nerve is not a single cell. The long axons of many neurons, called nerve fibres, are bundled side by side and wrapped in sheaths of connective tissue to form a nerve, much like many wires wrapped together in one cable.
Connective tissue is an animal tissue whose cells lie far apart in an abundant intercellular matrix; its job is to bind, support, pack and protect other tissues. Around a nerve it holds the fibres together, gives them strength and protection, and carries the blood vessels that nourish them.
❌ Why the other options are incorrect
✗ Epithelial tissue — a covering and lining tissue with tightly packed cells and almost no matrix; it protects surfaces and helps in absorption and secretion, but it does not bind bundles of fibres.
✗ Meristematic tissue — a plant tissue of actively dividing cells found at root and shoot tips; it has no role in animal nerves at all.
✗ Conducting tissue — again a plant tissue, namely xylem and phloem, which transport water, minerals and food.
📌 SSC & Railway Exam Facts
• The neuron is the longest cell of the human body and the unit of the nervous system.
• A neuron has dendrites, cell body (cyton), axon and nerve endings.
• Connective tissues include areolar, adipose, bone, cartilage, blood, tendon and ligament.
• Tendon joins muscle to bone; ligament joins bone to bone.
• Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Correct Answer: Option 1 — Binary fission
Binary fission is the simplest kind of asexual reproduction, seen in unicellular organisms. The parent cell first duplicates its DNA, the nucleus divides, and then the cytoplasm splits, so that the body is divided into two equal daughter individuals. The parent body does not survive as a separate individual — it becomes the two offspring.
*Amoeba* divides in any plane, *Leishmania* divides in a definite plane along the whip-like flagellum, and *Bacteria* and *Paramecium* also multiply this way. In *Plasmodium*, the malarial parasite, the parent splits into many daughter cells at once — that is multiple fission.
❌ Why the other options are incorrect
✗ Regeneration — a fully new organism grows from a cut piece of the body, as in *Planaria* and *Hydra*; the pieces are not equal halves formed during cell division.
✗ Spore formation — thick-walled spores made inside a sporangium, as in *Rhizopus*, germinate into new individuals when conditions are right.
✗ Budding — a small bud appears on the parent, grows while attached and then detaches, as in yeast and Hydra; the two bodies are unequal.
📌 SSC & Railway Exam Facts
• Binary fission — *Amoeba, Paramecium, Leishmania, Bacteria*; multiple fission — *Plasmodium*.
• Fragmentation in *Spirogyra*; regeneration in *Planaria*; budding in yeast and Hydra.
• Asexual reproduction needs one parent and produces genetically identical clones.
• *Plasmodium* causes malaria, spread by the female Anopheles mosquito.
• Leishmania causes kala-azar, spread by the sandfly.

Correct Answer: Option 2 — The testes secrete the hormone testosterone.
The testes are the primary sex organs of the male. They perform two jobs at once: they produce the male gamete, the sperm, and they secrete the male sex hormone testosterone.
Testosterone controls the development of the male reproductive organs and brings about the secondary sexual characters at puberty — a deeper voice, growth of facial, armpit and pubic hair, broadening of the shoulders and enlargement of the muscles. It also regulates the production of sperms itself. The testes are held outside the body in a pouch called the scrotum, which keeps them about 2–2.5 °C cooler than the body.
❌ Why the other options are incorrect
✗ Higher temperature for sperm formation — the opposite is true; sperm formation needs a lower temperature, which is exactly why the testes lie outside the abdomen.
✗ Sperms are produced in the seminal vesicles — wrong; sperms are made in the seminiferous tubules of the testes. The seminal vesicle and prostate gland only add fluid that nourishes the sperms and makes their transport easier.
✗ The testes are located in the abdominal cavity — wrong; they descend before birth into the scrotum, outside the abdominal cavity.
📌 SSC & Railway Exam Facts
• Male reproductive system — testes, vas deferens, seminal vesicle, prostate gland, urethra, penis.
• Sperm + fluid of the accessory glands = semen.
• The female sex hormone from the ovary is oestrogen; the pregnancy hormone is progesterone.
• Sperm production begins at puberty and is controlled by FSH and LH from the pituitary gland.
• In humans the male gamete decides the sex of the child — an X-bearing sperm gives a girl, a Y-bearing sperm gives a boy.

Correct Answer: Option 1 — Compressed Natural Gas and Liquefied Petroleum Gas
CNG stands for Compressed Natural Gas. Natural gas is mainly methane (CH₄) — about 90% — and is compressed to roughly 200–250 bar before being stored in cylinders. It burns almost without smoke or soot, which is why CNG is used for buses, autos and cars in cities such as Delhi; it is also a raw material for making hydrogen and fertilisers.
LPG stands for Liquefied Petroleum Gas. It is obtained during the refining of petroleum and is essentially a mixture of butane (C₄H₁₀) and propane (C₃H₈), liquefied under pressure inside the cylinder. LPG itself is odourless, so a strong-smelling compound, ethyl mercaptan, is deliberately added so that a leak can be detected.
❌ Why the other options are incorrect
✗ Compressed Neutral Gas and Liquefied Petroleum Gas — the N in CNG stands for Natural, not "Neutral"; a common mis-expansion.
✗ Compressed Natural Gas and Liquefied Pressured Gas — the P in LPG stands for Petroleum, not "Pressured".
✗ Compressed Neutral Gas and Liquefied Pressured Gas — both expansions are wrong here.
📌 SSC & Railway Exam Facts
• The chief constituent of CNG / natural gas is methane (CH₄).
• LPG is mainly butane (C₄H₁₀) + propane (C₃H₈); ethyl mercaptan gives it the warning smell.
• Calorific value: LPG ≈ 55 kJ/g, CNG ≈ 50 kJ/g, kerosene ≈ 45 kJ/g.
• Hydrogen has the highest calorific value of all fuels — about 150 kJ/g.
• CNG is called a clean fuel because it releases almost no SO₂ or particulate matter.

Correct Answer: Option 2 — The rate of diffusion of liquids is higher than that of solids
Diffusion is the spontaneous intermixing of the particles of two substances on their own. Its rate depends on how freely the particles can move, which in turn depends on the intermolecular force and the space between particles.
In a solid the particles only vibrate about fixed positions — the intermolecular force is strongest and the particles are most closely packed, so diffusion is extremely slow. In a liquid the particles are still close together but can slide over one another, so diffusion is clearly faster than in solids (though slower than in gases). Copper sulphate crystals dissolving and colouring still water, or ink spreading in water, are the standard examples.
The order of the rate of diffusion is therefore gases > liquids > solids.
❌ Why the other options are incorrect
✗ Liquids are rigid — rigidity belongs to solids; liquids show fluidity and flow.
✗ Liquids have fixed shape — a liquid takes the shape of its container; only solids have a definite shape.
✗ Liquids have no fixed volume — liquids do have a fixed volume; it is gases that have neither fixed shape nor fixed volume.
📌 SSC & Railway Exam Facts
• Rate of diffusion: gases > liquids > solids.
• Liquids: definite volume, no definite shape; gases: neither.
• Diffusion becomes faster on heating because the kinetic energy of the particles rises.
• Compressibility order: gases > liquids > solids; density order: solids > liquids > gases.
• Gases diffuse fastest because of the largest intermolecular space and negligible force of attraction.

Correct Answer: Option 2 — 1, 2 and 3
A mixture is formed when two or more substances are simply mixed together in any proportion without any chemical reaction. A compound is formed when elements combine chemically in a fixed ratio by mass.
Statement 1 is correct — the constituents of a mixture can be taken in any ratio, so its composition is variable. Sugar solution may be weak or strong; air contains varying amounts of water vapour.
Statement 2 is correct — this is the Law of Constant Proportions (Proust's Law). Water is always H₂O, i.e. H : O = 1 : 8 by mass, whatever its source.
Statement 3 is correct — since no new substance is produced, every component of a mixture retains its own properties. In a mixture of iron filings and sulphur, the iron is still attracted by a magnet; in iron sulphide (FeS), formed by heating them, it is not.
❌ Why the other options are incorrect
✗ Only 2 and 3 — it wrongly rejects statement 1, yet variable composition is the very definition of a mixture.
✗ Only 1 — it rejects the Law of Constant Proportions and the retention of properties, both of which are true.
✗ Only 1 and 2 — it wrongly rejects statement 3; a mixture certainly shows its constituents' properties.
📌 SSC & Railway Exam Facts
• Mixture: variable composition, no energy change, separable by physical methods.
• Compound: fixed composition, energy change occurs, separable only by chemical methods.
• Law of Constant Proportions was given by Joseph Proust.
• Air is a homogeneous mixture; water (H₂O) is a compound — a classic exam pair.
• A homogeneous mixture of two or more metals is an alloy, e.g. brass = Cu + Zn.

Correct Answer: Option 4 — Solid particles are more closely packed than gas particles
The states of matter differ not in the kind of particles but in the arrangement, spacing and energy of those particles.
In a solid, the interparticle force of attraction is maximum and the interparticle space is minimum, so the particles are very closely packed in a regular pattern and can only vibrate about fixed positions. That is why solids have a definite shape, a definite volume, high density and are almost incompressible.
In a gas, the force of attraction is negligible and the interparticle space is enormous, so the particles move about randomly at high speed, filling the whole container. Hence gases have neither definite shape nor definite volume and are highly compressible — the very property that lets LPG and CNG be packed into cylinders.
❌ Why the other options are incorrect
✗ Solid particles are made of different elements than gas particles — the same substance can exist in all three states; ice, water and steam are all H₂O.
✗ Solid particles have more energy than gas particles — the kinetic energy order is gas > liquid > solid; solid particles have the least.
✗ Solid particles move more freely than gas particles — exactly the reverse; solid particles only vibrate, gas particles move freely.
📌 SSC & Railway Exam Facts
• Interparticle force: solid > liquid > gas; interparticle space: gas > liquid > solid.
• Kinetic energy of particles: gas > liquid > solid.
• Solid → definite shape and volume; liquid → definite volume only; gas → neither.
• Plasma and Bose–Einstein Condensate (BEC) are the fourth and fifth states of matter.
• BEC was predicted by S. N. Bose and Albert Einstein.

Correct Answer: Option 3 — Only 2
In a double displacement reaction two compounds exchange their ions to give two new compounds. The general form is AB + CD → AD + CB. When one product is an insoluble solid, the reaction is also called a precipitation reaction.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
• Na⁺ pairs with Cl⁻ and Ba²⁺ pairs with SO₄²⁻ — a clear exchange of ions.
• The white precipitate of barium sulphate (BaSO₄), shown by the (s), makes it a precipitation reaction as well.
Only reaction (2) shows such an exchange, so "Only 2" is the answer.
❌ Why the other options are incorrect
✗ Only 2 and 3 — reaction (3), Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s), is a single displacement reaction: the more reactive lead displaces copper; only one element is displaced, not two ions exchanged.
✗ Only 1, 2 and 3 — reaction (1), C + O₂ → CO₂, is a combination (and combustion) reaction, two substances forming one product.
✗ Only 1 and 4 — reaction (4), CaCO₃ → CaO + CO₂, is thermal decomposition, and (1) is combination; neither involves any exchange of ions.
📌 SSC & Railway Exam Facts
• Double displacement: AB + CD → AD + CB; neutralisation is a special case of it.
• BaSO₄ and AgCl are classic white precipitates; PbI₂ gives a yellow precipitate.
• Na₂SO₄ + BaCl₂ and Pb(NO₃)₂ + KI are the two standard NCERT precipitation examples.
• CaCO₃ → CaO + CO₂ is decomposition; CaO is quick lime, used to make cement.
• In a displacement reaction a more reactive metal displaces a less reactive one from its salt solution.

Correct Answer: Option 1 — cotton helps in absorbing water and exposes the sweat to the atmosphere for evaporation
In summer the body cools itself by sweating. Evaporation is a surface phenomenon in which the faster-moving particles of a liquid escape into the air. To do so they take latent heat from the surroundings, so the surface left behind — here, our skin — is cooled. This is called evaporation causing cooling.
Cotton is a good absorber of water. It soaks the sweat from the skin and, being a porous fabric, spreads it over a large surface exposed to the atmosphere. The sweat then evaporates quickly, drawing latent heat of vaporisation from the body and giving a pleasant cooling effect. Synthetic cloth such as nylon or polyester neither absorbs sweat well nor lets it evaporate, so it feels sticky and hot.
❌ Why the other options are incorrect
✗ cotton cannot expose the sweat to the atmosphere for evaporation — the opposite is true; cotton's porous structure is precisely what exposes sweat to air.
✗ cotton does not absorb sweat — cotton is one of the best absorbers of water among common fabrics.
✗ cotton is not helpful in absorbing water and exposes the sweat... — half right only; it wrongly denies cotton's absorbing power, which is the first step of the cooling process.
📌 SSC & Railway Exam Facts
• Evaporation occurs at all temperatures and only at the surface; boiling occurs at a fixed temperature throughout the liquid.
• Evaporation increases with higher temperature, larger surface area, more wind and lower humidity.
• Evaporation causes cooling because of the absorption of latent heat of vaporisation.
• Water kept in an earthen pot (matka) cools by evaporation through its pores.
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg; latent heat of fusion of ice = 3.34 × 10⁵ J/kg.

Correct Answer: Option 1 — sulphur
When a metal is slowly eaten away by the gases and moisture of the air, the process is called corrosion. In the case of silver (Ag) it is given the special name tarnishing.
Air contains small amounts of sulphur compounds, chiefly hydrogen sulphide (H₂S). Silver reacts with this sulphur to form a thin coating of silver sulphide (Ag₂S), which is black. That is why silver ornaments, spoons and coins lose their shine and turn blackish on standing. Cleaning them with a paste or an electrochemical bath removes the Ag₂S layer and restores the shine.
⚗️ The Reaction
2Ag + H₂S → Ag₂S + H₂
• The product Ag₂S (silver sulphide) is the black layer seen on the ornament.
• The same idea explains copper turning green (basic copper carbonate) and iron rusting (hydrated iron oxide).
❌ Why the other options are incorrect
✗ oxygen — silver is a noble, less reactive metal and does not form an oxide layer with atmospheric oxygen at room temperature.
✗ carbon dioxide — CO₂ with moisture turns copper green, not silver black.
✗ nitrogen — atmospheric N₂ is highly inert because of its triple bond and does not react with silver.
📌 SSC & Railway Exam Facts
• Silver tarnishing: Ag₂S, black; copper corrosion: CuCO₃·Cu(OH)₂, green.
• Rusting of iron: Fe₂O₃·xH₂O, reddish-brown; it needs both air (O₂) and water.
• Rusting is prevented by painting, greasing, galvanisation (zinc coating) and electroplating.
• Galvanisation protects iron even if the zinc coating is scratched, because zinc is more reactive.
• Silver is the best conductor of electricity, followed by copper and gold.

Correct Answer: Option 1 — Ductility
Ductility is the property of a metal by which it can be drawn into thin wires without breaking. It arises because the metallic bond allows layers of positive metal ions to slide over one another while the sea of delocalised electrons keeps holding them together.
Gold (Au) is the most ductile metal — one gram of gold can be drawn into a wire about 2 km long — followed by silver. Copper and aluminium are highly ductile too, which is why they are used for electrical wires.
Do not confuse it with malleability, the property of being beaten into thin sheets, in which gold and silver again lead, and by which aluminium foil is made for packing food.
❌ Why the other options are incorrect
✗ Neither malleability nor ductility — wrong, because drawing into wires is exactly the definition of ductility.
✗ Malleability — this is being hammered into sheets or foils, not wires; a very common confusion in exams.
✗ Both malleability and ductility — the question asks for a single specific property, wire-drawing, which is ductility alone.
📌 SSC & Railway Exam Facts
• Ductility → wires; Malleability → sheets/foils. Gold is the best in both.
• Non-metals are neither malleable nor ductile — they are brittle (except graphite, which is soft, and diamond, the hardest).
• Sonorous metals produce a ringing sound; that is why bells are made of metal.
• Mercury (Hg) is the only metal liquid at room temperature; bromine (Br₂) is the only liquid non-metal.
• Sodium (Na) and potassium (K) are so soft they can be cut with a knife.

Correct Answer: Option 4 — 8
In Bohr's model of the atom (1913), electrons revolve around the nucleus only in certain permitted circular paths called orbits, shells or energy levels, named K, L, M, N… for n = 1, 2, 3, 4…. While moving in such a shell an electron does not radiate energy, which is why the atom is stable.
The maximum number of electrons that a shell can hold is given by the Bohr–Bury scheme:
Maximum electrons = 2n²
For the second energy level, n = 2, so 2 × (2)² = 2 × 4 = 8 electrons.
Hence the L shell holds at most 8 electrons. Similarly K = 2, M = 18, N = 32. A further rule says the outermost shell cannot hold more than 8 electrons, which explains the octet rule and the stability of the noble gases.
❌ Why the other options are incorrect
✗ 2 — that is the capacity of the first (K) shell, where n = 1 gives 2(1)² = 2.
✗ 6 — this is the number of p-subshell electrons only, not the whole second shell (2s² 2p⁶ = 8).
✗ 4 — this is merely n² (2² = 4); students forget to multiply by 2.
📌 SSC & Railway Exam Facts
• 2n² rule: K = 2, L = 8, M = 18, N = 32.
• Outermost shell can never have more than 8 electrons (octet rule).
• Bohr's model was proposed by Niels Bohr in 1913; he won the Nobel Prize in 1922.
• Rutherford's alpha-particle scattering experiment discovered the nucleus; Thomson gave the plum pudding model.
• Electrons were discovered by J. J. Thomson, protons by Goldstein and neutrons by James Chadwick.

Correct Answer: Option 3 — Alkyne, alkene, and alkane hydrocarbons, respectively
Hydrocarbons contain only carbon and hydrogen. They are classified by the type of bond between the carbon atoms, and each family has its own general formula:
• Alkanes — CₙH₂ₙ₊₂ — only single C–C bonds, saturated.
• Alkenes — CₙH₂ₙ — at least one double bond, unsaturated.
• Alkynes — CₙH₂ₙ₋₂ — at least one triple bond, unsaturated.
Putting n = 2 in each formula: C₂H₆ (2n+2 = 6) is ethane, an alkane; C₂H₄ (2n = 4) is ethene (ethylene), an alkene; C₂H₂ (2n−2 = 2) is ethyne (acetylene), an alkyne. Reading the question's order C₂H₂, C₂H₄, C₂H₆ therefore gives alkyne, alkene, alkane.
❌ Why the other options are incorrect
✗ Alkane, alkyne, and alkene, respectively — it treats C₂H₂ as an alkane; an alkane with 2 carbons must be C₂H₆.
✗ Alkane, alkene, and alkyne, respectively — this is the answer to the reverse order (C₂H₆, C₂H₄, C₂H₂); the sequence has been read backwards.
✗ Alkene, alkyne, and alkane, respectively — it mixes up C₂H₂ and C₂H₄; the one with fewer hydrogens is the more unsaturated alkyne.
📌 SSC & Railway Exam Facts
• CₙH₂ₙ₊₂ = alkane, CₙH₂ₙ = alkene, CₙH₂ₙ₋₂ = alkyne.
• Ethyne (acetylene, C₂H₂) burns with oxygen to give the oxy-acetylene flame (about 3000 °C) used for welding.
• Ethene (C₂H₄) is used to artificially ripen fruits.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones with a yellow sooty flame.
• Two successive members of a homologous series differ by –CH₂–, i.e. 14 u.

Correct Answer: Option 4 — ((Mass of solute)/(Mass of solution)) × 100
A solution is a homogeneous mixture of a solute (the dissolved substance, present in smaller amount) and a solvent (the dissolving medium, present in larger amount). Concentration tells us how much solute is present in a given amount of solution.
The mass by mass percentage is defined as
Mass % = (Mass of solute / Mass of solution) × 100, where Mass of solution = Mass of solute + Mass of solvent.
For example, dissolving 20 g of salt in 80 g of water gives a solution of mass 100 g, so the concentration is (20/100) × 100 = 20%. Notice that both quantities in the ratio are masses — that is what makes it *mass by mass*. Similarly, volume by volume % = (Volume of solute / Volume of solution) × 100 and mass by volume % = (Mass of solute / Volume of solution) × 100.
❌ Why the other options are incorrect
✗ ((Mass of solute)/(Volume of solution)) × 100 — this is mass by volume percentage, a different expression of concentration.
✗ ((Mass of solvent)/(Mass of solution)) × 100 — it uses the solvent instead of the solute; concentration always refers to the solute.
✗ ((Volume of solute)/(Mass of solution)) × 100 — mixing volume on top with mass below is dimensionally meaningless.
📌 SSC & Railway Exam Facts
• Mass % = (mass of solute / mass of solution) × 100; mass of solution = solute + solvent.
• Solute + Solvent = Solution; in a sugar solution, sugar is the solute and water the solvent.
• Water is called the universal solvent.
• True solution particle size < 1 nm; colloid 1–1000 nm; suspension > 1000 nm.
• Colloids scatter light and show the Tyndall effect; true solutions do not.

Correct Answer: Option 1 — Silver chloride turns grey in sunlight
In a decomposition reaction a single compound breaks down into two or more simpler substances, the general form being AB → A + B. Energy must be supplied, and according to its source the reaction is called thermal decomposition (heat), photolytic decomposition (light) or electrolytic decomposition (electricity).
⚗️ The Reaction
2AgCl(s) → (sunlight) 2Ag(s) + Cl₂(g)
• White silver chloride breaks up into grey metallic silver and chlorine gas, so the solid turns grey.
• Because sunlight supplies the energy, it is a photolytic (photochemical) decomposition — the same reaction that is used in black-and-white photography.
❌ Why the other options are incorrect
✗ Burning of coal — C + O₂ → CO₂ is a combination reaction (and combustion); two reactants give one product, the opposite of decomposition.
✗ Formation of water from H₂(g) and O₂(g) — 2H₂ + O₂ → 2H₂O is again a combination reaction; water being *split* would be decomposition.
✗ Burning of natural gas — CH₄ + 2O₂ → CO₂ + 2H₂O is an oxidation / combustion reaction, giving out heat and light, not a breakdown into simpler elements.
📌 SSC & Railway Exam Facts
• Decomposition: AB → A + B; it is the exact opposite of a combination reaction.
• Thermal: CaCO₃ → CaO + CO₂; 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (brown fumes).
• Photolytic: 2AgCl → 2Ag + Cl₂ (grey) and 2AgBr → 2Ag + Br₂ — used in photography.
• Electrolytic: 2H₂O → 2H₂ + O₂, giving hydrogen at the cathode and oxygen at the anode in a 2 : 1 volume ratio.
• Most decomposition reactions are endothermic; most combination reactions are exothermic.

Correct Answer: Option 4 — 1.01 × 10⁵ Pa
Pressure is the force acting per unit area, P = F/A. Its SI unit is the pascal (Pa), where 1 Pa = 1 N/m².
The atmosphere (atm) is the pressure exerted by the column of air of the earth's atmosphere at sea level. Measured with a mercury barometer, it supports a mercury column of 760 mm (76 cm). Converting that to SI units gives
1 atm = 1.01 × 10⁵ Pa ≈ 101.3 kPa ≈ 1.013 bar.
Because gases are studied at such pressures, 1 atm is the reference pressure used in STP (standard temperature and pressure) along with 273 K.
❌ Why the other options are incorrect
✗ 1.01 × 10⁶ Pa — this is 10 atm, ten times too large; it comes from mis-writing the power as 6 instead of 5.
✗ 1.01 × 10⁻⁶ Pa — a negative power makes the value almost zero, close to a high vacuum, not atmospheric pressure.
✗ 1.01 × 10⁻⁵ Pa — again a near-vacuum value; the correct exponent is positive 5.
📌 SSC & Railway Exam Facts
• 1 atm = 1.01 × 10⁵ Pa = 101.3 kPa = 1.013 bar = 760 mm of Hg.
• 1 bar = 10⁵ Pa; 1 Pa = 1 N/m².
• Pressure is measured by a barometer (atmospheric) and a manometer (gas).
• The mercury barometer was invented by Evangelista Torricelli.
• At STP, temperature = 273 K (0 °C) and pressure = 1 atm; molar volume of a gas = 22.4 L.

Correct Answer: Option 2 — 0.4 m/s²
📖 Given
• Initial velocity, u = 0 (he starts from rest)
• Final velocity, v = 24 m/s
• Time, t = 1 minute = 60 s
✍️ Formula
a = (v − u)/t
🔢 Solution
• a = (24 − 0)/60
• a = 24/60
• a = 0.4 m/s²
This means Raju's speed grows by 0.4 metre per second in every second of pedalling.
❌ Why the other options are incorrect
✗ 0 m/s² — zero acceleration means the velocity never changes; but Raju goes from 0 to 24 m/s, so the acceleration cannot be zero.
✗ 0.6 m/s² — this comes from dividing 24 by 40 instead of by 60 s.
✗ 0.2 m/s² — this comes from taking the time as 2 minutes (120 s) instead of 1 minute.
📌 SSC & Railway Exam Facts
• Acceleration is the rate of change of velocity; SI unit m/s², and it is a vector quantity.
• The three equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• Always convert first — 1 minute = 60 s, 1 hour = 3600 s.
• When the final velocity is less than the initial velocity, the acceleration is negative and is called retardation.
• 1 km/h = 5/18 m/s and 1 m/s = 18/5 km/h.

Correct Answer: Option 2 — 0.2 joule
📖 Given
• Mass, m = 0.1 kg
• Velocity, v = 2 m s⁻¹
✍️ Formula
K.E. = ½mv²
🔢 Solution
• K.E. = ½ × 0.1 × (2)²
• K.E. = ½ × 0.1 × 4
• K.E. = 0.05 × 4
• K.E. = 0.2 joule
So a light 100 g body moving at 2 m s⁻¹ carries only 0.2 J of energy of motion.
❌ Why the other options are incorrect
✗ 0.1 joule — this is ½mv (forgetting to square the velocity).
✗ 0.05 joule — this is just ½m, the velocity has not been used at all.
✗ 0.4 joule — this is mv², i.e. the factor ½ has been left out.
📌 SSC & Railway Exam Facts
• Kinetic energy is the energy of a body due to its motion; SI unit joule (J).
• K.E. is directly proportional to mass and to the square of velocity — doubling v makes K.E. four times.
• Potential energy of a raised body = mgh.
• 1 joule = 1 newton-metre = 10⁷ erg; 1 kWh = 3.6 × 10⁶ J.
• Relation with momentum: K.E. = p²/2m, where p = mv.

Correct Answer: Option 1 — Violet
When white light passes through a glass prism it splits into seven colours — this is called dispersion. The colours appear in the order VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red) from the bottom of the spectrum upward.
The amount of bending depends on the wavelength of the colour. Violet has the shortest wavelength (about 400 nm), so the glass has the highest refractive index for it and it bends (deviates) the most. Red has the longest wavelength (about 700 nm), the lowest refractive index, and therefore deviates the least.
❌ Why the other options are incorrect
✗ Red — it has the longest wavelength in the visible spectrum, so it is bent the least by the prism, not the most.
✗ Yellow — its wavelength lies in the middle of the spectrum, so its deviation is only moderate.
✗ Indigo — it deviates a great deal, but still less than violet, because its wavelength is slightly longer than violet's.
📌 SSC & Railway Exam Facts
• Dispersion of light through a prism was first demonstrated by Sir Isaac Newton.
• The band of seven colours produced is called the spectrum; the sequence is VIBGYOR.
• Order of deviation: Violet > Indigo > Blue > Green > Yellow > Orange > Red.
• Order of wavelength is exactly the reverse: Red > Orange > Yellow > Green > Blue > Indigo > Violet.
• A rainbow is formed by refraction, total internal reflection and dispersion of sunlight in raindrops.

Correct Answer: Option 2 — 5 W
📖 Given
• Work done, W = 50 J
• Time taken, t = 10 s
✍️ Formula
P = W/t
🔢 Solution
• P = 50/10
• P = 5 W
In plain words, the man converts 5 joule of energy every second, which is his rate of doing work.
❌ Why the other options are incorrect
✗ 500 W — this is 50 multiplied by 10 instead of divided; multiplying work by time gives no physical quantity here.
✗ 0.5 W — this is 50 divided by 100; the time has been wrongly taken as 100 s.
✗ 50 W — this is the work done, not the power; power is obtained only after dividing by the time.
📌 SSC & Railway Exam Facts
• Power is the rate of doing work; SI unit watt (W), a scalar quantity.
• 1 watt = 1 joule/second; 1 horsepower (hp) = 746 W.
• 1 kilowatt-hour (kWh) = 3.6 × 10⁶ J — this is the commercial unit of electrical energy (1 'unit').
• The unit watt is named after the Scottish engineer James Watt.
• For electricity, P = VI = I²R = V²/R.

Correct Answer: Option 3 — The kinetic energy is the energy possessed by an object due to its position.
This statement is wrong because energy possessed by a body because of its position or configuration is called potential energy, not kinetic energy. A stone lying on a roof, a stretched spring and water stored in a dam all have potential energy (P.E. = mgh).
Kinetic energy is the energy a body has because it is moving, and is given by K.E. = ½mv². Since m and v² both appear in the formula, the kinetic energy grows when either the mass or the velocity grows.
❌ Why the other options are incorrect
✗ The kinetic energy is the energy possessed by an object due to its motion — this is the correct definition of kinetic energy, so it cannot be the answer.
✗ The kinetic energy of an object increases with the increase of its mass — true, because K.E. ∝ m at a fixed speed.
✗ The kinetic energy of an object increases with the increase of its velocity — true, and in fact K.E. ∝ v², so it rises even faster.
📌 SSC & Railway Exam Facts
• K.E. = ½mv² and P.E. = mgh; both are measured in joule (J).
• If velocity is doubled, kinetic energy becomes 4 times; if it is tripled, it becomes 9 times.
• Mechanical energy = K.E. + P.E., and it stays constant for a freely falling body (law of conservation of energy).
• A body of mass m falling from height h has K.E. = mgh just before hitting the ground.
• Work-energy theorem: work done by the net force = change in kinetic energy.

Correct Answer: Option 2 — Faraday
Electromagnetic induction was discovered by the English scientist Michael Faraday in 1831. He showed that when the magnetic field linked with a coil changes, an electric current is set up in the coil. The current so produced is called an induced current and the voltage an induced emf.
Faraday's law states that the induced emf is proportional to the rate of change of magnetic flux through the circuit. The direction of the induced current is given by Fleming's right-hand rule (or Lenz's law). This single discovery is the basis of the electric generator (dynamo), the transformer and the entire power-supply system.
❌ Why the other options are incorrect
✗ Einstein — Albert Einstein gave the theory of relativity and explained the photoelectric effect, for which he received the Nobel Prize in 1921.
✗ Edison — Thomas Alva Edison invented the practical incandescent electric bulb, the phonograph and the motion-picture camera.
✗ Newton — Isaac Newton gave the three laws of motion and the law of universal gravitation, not electromagnetic induction.
📌 SSC & Railway Exam Facts
• Michael Faraday also gave the laws of electrolysis and discovered benzene.
• Lenz's law — the induced current always opposes the change producing it; it follows from the conservation of energy.
• An electric generator converts mechanical energy into electrical energy; an electric motor does the reverse.
• The magnetic effect of current was discovered by Hans Christian Oersted in 1820.
• SI unit of magnetic flux is weber (Wb) and of magnetic field is tesla (T).

Correct Answer: Option 4 — 24cm
📖 Given
• Focal length of the spherical mirror, f = 12 cm
• To find: radius of curvature, R
✍️ Formula
R = 2f (equivalently f = R/2)
🔢 Solution
• R = 2 × f
• R = 2 × 12
• R = 24 cm
So the mirror is a part of a sphere of radius 24 cm, and its focus lies exactly halfway between the pole and the centre of curvature.
❌ Why the other options are incorrect
✗ 6cm — this is f/2; the focal length has been halved instead of doubled.
✗ 3cm — this is f/4, obtained by dividing by 2 twice.
✗ 2cm — this is just the number 2 from the formula; it has no connection with 12 cm.
📌 SSC & Railway Exam Facts
• For a spherical mirror, f = R/2, i.e. the focal length is half the radius of curvature.
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u = h'/h.
• A concave mirror has a negative focal length and a convex mirror a positive one in the sign convention.
• Concave mirrors are used in torches, headlights, shaving mirrors and solar cookers.
• Convex mirrors are used as rear-view mirrors because they give an erect, diminished image and a wide field of view.

Correct Answer: Option 3 — 10 N
📖 Given
• Weight on the Earth, W(earth) = 60 N
• g(moon) = (1/6) × g(earth)
• Mass m stays the same everywhere
✍️ Formula
W = mg, so W(moon) = m × g(moon) = (1/6) × W(earth)
🔢 Solution
• W(moon) = (1/6) × 60
• W(moon) = 60/6
• W(moon) = 10 N
The object becomes one-sixth as heavy on the moon even though it still contains exactly the same amount of matter.
❌ Why the other options are incorrect
✗ 60 N — this is the Earth weight; it would be right only for mass, which does not change, but weight does change.
✗ 40 N — this is 60 × (2/3); the fraction 1/6 has not been used correctly.
✗ 70 N — weight on the moon must be less than on the Earth, because the moon's gravity is weaker, so a larger value is impossible.
📌 SSC & Railway Exam Facts
• Mass is constant everywhere and is measured in kg; weight = mg is a force measured in newton (N).
• On the Earth g = 9.8 m/s² (often taken as 10 m/s²); on the moon g ≈ 1.63 m/s².
• The moon's gravity is weak because its mass is about 1/81 and its radius about 1/4 of the Earth's.
• A body's weight is zero at the centre of the Earth and maximum at the poles.
• g = GM/R², so g decreases with height and with depth below the surface.

Correct Answer: Option 4 — Only (C)
The equation of motion that links final velocity v, initial velocity u, acceleration a and time t is the first equation of motion:
v = u + at
Taking u to the left-hand side gives (v − u) = at, which is exactly statement (C). For a freely falling body the acceleration is g, so it becomes v = u + gt. Hence only (C) is correct.
❌ Why the other options are incorrect
✗ Statement (A) — s = ut + 2at² — the correct second equation is s = ut + ½at²; the coefficient must be ½, not 2, and in any case it gives distance, not a velocity–time relation.
✗ Statement (B) — v = u + ½at — the extra factor ½ is wrong; the correct form is v = u + at.
✗ (A) and (B) and (B) and (C) — both include a wrong equation, so neither combination can be correct.
📌 SSC & Railway Exam Facts
• The three equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• For free fall, replace a by g = 9.8 m/s² and take u = 0 when the body is simply dropped.
• Distance in the nth second: s(n) = u + (a/2)(2n − 1).
• These equations are valid only for uniform acceleration along a straight line.
• For a body dropped from rest, h = ½gt² and v = √(2gh).

Correct Answer: Option 3 — Both (i) and (iii)
The principal axis of a spherical mirror is the straight line joining the pole (P) of the mirror and its centre of curvature (C). By this very definition the pole always lies on the principal axis, so statement (iii) is true.
The principal focus (F) is the point on the principal axis where rays coming parallel to the principal axis meet (concave) or appear to come from (convex) after reflection. It lies midway between P and C, so (i) is also true. The point of incidence, however, is wherever a particular ray happens to strike the mirror surface — it can be anywhere on the reflecting surface and normally is not on the principal axis, so (ii) is false.
❌ Why the other options are incorrect
✗ Both (ii) and (iii) — it wrongly includes the point of incidence, which changes with every incident ray.
✗ Only (iii) — the pole does lie on the axis, but this option leaves out the principal focus, which also always lies on it.
✗ Only (i) — the principal focus lies on the axis, but the pole, being the centre of the mirror surface, always lies on it too.
📌 SSC & Railway Exam Facts
• Pole (P) — the centre of the reflecting surface; Centre of curvature (C) — the centre of the sphere of which the mirror is a part.
• Aperture is the effective diameter of the reflecting surface; f = R/2.
• Laws of reflection: angle of incidence = angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.
• For a spherical mirror the normal at any point passes through the centre of curvature.
• A concave mirror placed with the object between P and F gives a virtual, erect and enlarged image.

Correct Answer: Option 4 — 40 m/s
📖 Given
• Initial velocity, u = 20 m/s
• Uniform acceleration, a = 4 m/s²
• Time, t = 5 s
✍️ Formula
v = u + at
🔢 Solution
• v = 20 + (4 × 5)
• v = 20 + 20
• v = 40 m/s
The train gains 20 m/s of extra speed in those 5 seconds, so its speed doubles from 20 m/s to 40 m/s.
❌ Why the other options are incorrect
✗ 60 m/s — this comes from multiplying, i.e. 20 + 4 × 10, or from taking t = 10 s instead of 5 s.
✗ 80 m/s — this is 20 × 4, i.e. the initial velocity multiplied by the acceleration; the formula is an addition, not a product.
✗ 16 m/s — this is 20 − 4; the train is accelerating, so the velocity must increase, not fall.
📌 SSC & Railway Exam Facts
• v = u + at is the first equation of motion and gives velocity at any time t.
• Use s = ut + ½at² for distance and v² = u² + 2as when time is not given.
• In a velocity–time graph, the slope gives acceleration and the area under the graph gives displacement.
• Uniform acceleration means the velocity changes by equal amounts in equal intervals of time.
• SI unit of velocity is m/s and of acceleration m/s²; both are vector quantities.

Correct Answer: Option 3 — unbalanced force
An object starts moving, speeds up, slows down or changes direction only when the net force acting on it is not zero. Such a net force is called an unbalanced force. This is the content of Newton's first law of motion, which says a body continues in its state of rest or of uniform motion unless an unbalanced external force acts on it.
On a horizontal table the weight of the object acting downward is exactly cancelled by the normal reaction of the table acting upward. These two are balanced, so they cannot move the object sideways. To slide it, the applied push must be greater than the force of friction, leaving an unbalanced force that produces acceleration according to F = ma.
❌ Why the other options are incorrect
✗ an equal amount of perpendicular forces on the object in both upward and downward directions — equal and opposite forces cancel out; their resultant is zero and the object stays where it is.
✗ only a single force perpendicular to the object — a force perpendicular to the table presses the object down or lifts it; it does not push it along the table.
✗ balanced force — balanced forces give a zero resultant, so they can only change the shape of a body, never its state of motion.
📌 SSC & Railway Exam Facts
• Newton's first law is also called the law of inertia; it defines force.
• Newton's second law: F = ma; SI unit of force is newton (N), where 1 N = 1 kg m/s².
• Newton's third law: every action has an equal and opposite reaction.
• Friction always opposes relative motion; static friction > kinetic friction.
• Balanced forces can change the shape of an object (e.g. squeezing a rubber ball) but not its velocity.

Correct Answer: Option 2 — All in series
📖 Given
• Three resistances, R₁ = 3 Ω, R₂ = 4 Ω, R₃ = 6 Ω
• Required total resistance, R(eq) = 13 Ω
✍️ Formula
Series: R(eq) = R₁ + R₂ + R₃
Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃
🔢 Solution
• Series check: R(eq) = 3 + 4 + 6
• R(eq) = 13 Ω ✓ — this matches the required value
• Parallel check: 1/R(eq) = 1/3 + 1/4 + 1/6 = (4 + 3 + 2)/12 = 9/12
• R(eq) = 12/9 = 1.33 Ω, which is far from 13 Ω
So the only way to obtain 13 Ω is to connect all three resistors in series.
❌ Why the other options are incorrect
✗ Does not depend on any combination — the equivalent resistance depends very strongly on the combination; series and parallel give completely different values.
✗ 3 Ω in series with 6 Ω and 4 Ω in parallel — here 6 Ω ∥ 4 Ω = (6 × 4)/(6 + 4) = 2.4 Ω, so total = 3 + 2.4 = 5.4 Ω, not 13 Ω.
✗ All in parallel — as calculated above this gives only 1.33 Ω, the smallest possible value.
📌 SSC & Railway Exam Facts
• In series the current is the same through each resistor and resistances simply add up.
• In parallel the voltage is the same across each resistor and the equivalent resistance is less than the smallest one.
• For two resistors in parallel, R(eq) = R₁R₂/(R₁ + R₂).
• Ohm's law: V = IR; SI unit of resistance is the ohm (Ω).
• Household appliances are connected in parallel so that each gets the full 220 V and can be switched independently.