RRB Technician Grade III 2024-25 24 December 2024 Shift 3 GK Questions – with answers & solutions

Correct Answer: Option 2 — Variation
A niche is the functional role and position of an organism in its habitat — where it lives, what it eats and how it interacts with other species. If the niche changes drastically (a sudden rise in temperature, a new disease, loss of water), most members of a population may be killed.
Variation means the differences in characters found among individuals of the same species. It is produced mainly by sexual reproduction and by small errors in DNA copying during reproduction. A population carrying many variations will almost always contain a few individuals that can tolerate the new conditions. These survivors reproduce, and the species escapes being completely wiped out. Variation therefore gives a population its survival advantage and is the raw material of natural selection.
❌ Why the other options are incorrect
✗ Isolation — geographical separation of two populations; over long periods it leads to speciation, but it cannot protect a population from a sudden change in its niche.
✗ Nutrition — the process of taking in food and using it for energy and growth; it keeps an individual alive but cannot make a whole population tolerant of new conditions.
✗ Evolution — the slow change in inherited characters over many generations; it is the result of variation and selection, not the immediate saviour of a population.
📌 SSC & Railway Exam Facts
• Asexual reproduction produces very few variations; sexual reproduction produces large variation.
• Charles Darwin gave the theory of natural selection in 'On the Origin of Species' (1859).
• Jean-Baptiste Lamarck proposed the inheritance of acquired characters.
• Niche is the functional role of an organism; habitat is only the place where it lives.
• Variations that give a survival advantage are selected by nature and become common in the population.

Correct Answer: Option 2 — Only B
Substances that are broken down by biological processes — by the action of micro-organisms such as bacteria and fungi — are called biodegradable. Substances that these decomposers cannot break down are non-biodegradable.
Plastic bottles are made of long-chain synthetic polymers for which no natural enzyme exists, so they persist in the soil and water for hundreds of years, choke drains, kill cattle and marine animals, and are the classic example of a non-biodegradable pollutant. Vegetable peels and spoilt food are organic wastes rich in carbohydrates and proteins; decomposers convert them into simple substances within days, which is why they are used to make compost.
❌ Why the other options are incorrect
✗ Only A — vegetable peels are biodegradable kitchen waste, easily rotted by bacteria and fungi.
✗ A, B and C — wrong, because peels and spoilt food are both readily decomposed; only the plastic is not.
✗ Both A and C — these two are exactly the biodegradable items, so they cannot be the answer.
📌 SSC & Railway Exam Facts
• Biodegradable — vegetable and fruit peels, paper, cotton, wool, cow dung, jute, wood.
• Non-biodegradable — plastics, polythene, glass, metals, DDT, aluminium cans, synthetic fibres.
• Non-biodegradable pesticides increase along a food chain — this is biomagnification.
• The 3 Rs of waste management are Reduce, Reuse and Recycle.
• Green dustbins are for biodegradable waste and blue dustbins for non-biodegradable waste.

Correct Answer: Option 2 — 1-b, 2-c, 3-a, 4-d
• Cell wall (1-b) — a rigid outer covering of cellulose found only in plant cells, bacteria and fungi. It gives the plant structural strength, definite shape and protection, and allows the cell to withstand hypotonic surroundings without bursting.
• Ribosomes (2-c) — tiny granules of RNA and protein without any membrane, found free in the cytoplasm or attached to the rough ER. They are the sites of protein manufacture (protein synthesis).
• Lysosomes (3-a) — membrane-bound sacs of powerful hydrolytic (digestive) enzymes that digest foreign material and worn-out cell organelles; they burst during cell injury, hence the name 'suicide bags'.
• Golgi apparatus (4-d) — a stack of membrane-bound cisternae that stores, modifies, packages and dispatches materials; lysosomes are formed from the Golgi apparatus.
❌ Why the other options are incorrect
✗ 1-b, 2-d, 3-c, 4-a — wrongly links ribosomes to lysosome formation and lysosomes to protein synthesis.
✗ 1-c, 2-a, 3-d, 4-b — makes the cell wall a protein factory and gives the Golgi the job of the cell wall.
✗ 1-a, 2-b, 3-c, 4-d — makes the cell wall a digestive organelle, which is entirely wrong.
📌 SSC & Railway Exam Facts
• Lysosome was discovered by Christian de Duve; Golgi body by Camillo Golgi.
• Ribosome was discovered by George Palade; it is the only organelle present in prokaryotes too.
• Cell wall is freely permeable; the plasma membrane is selectively permeable.
• Fungal cell wall is made of chitin, bacterial cell wall of peptidoglycan.
• Golgi apparatus is called the 'dispatching / traffic centre' of the cell.

Correct Answer: Option 2 — regeneration
Regeneration is the ability of a fully differentiated organism to give rise to new individual organisms from its body pieces. If Hydra or Planaria is cut into many pieces, each piece grows into a complete organism.
It is carried out by specialised cells in the body. These cells proliferate by repeated cell division, and the mass of cells then undergoes differentiation — different cells change into different tissues and organs in an organised sequence, until the whole body plan is rebuilt. Regeneration is not the same as reproduction, because most organisms do not depend on being cut into pieces in order to multiply.
❌ Why the other options are incorrect
✗ Fragmentation — the body of a simple multicellular organism breaks up on its own into fragments that grow into new individuals, e.g. Spirogyra; here the cutting is deliberate, not spontaneous.
✗ Binary fission — a unicellular organism splits into two daughter cells, e.g. Amoeba, Leishmania, bacteria; Hydra and Planaria are multicellular.
✗ Budding — a small bud develops on the parent body from regenerative cells and detaches after maturing, e.g. Hydra, yeast; this is a different process from cutting into pieces.
📌 SSC & Railway Exam Facts
• Planaria is a flatworm famous for its very high power of regeneration.
• The starfish can regenerate a lost arm; the lizard can regenerate its tail (autotomy).
• In humans, the liver shows the greatest power of regeneration.
• Hydra shows both budding and regeneration.
• Regeneration is carried out by specialised (stem-like) cells, and the process needs cell division followed by differentiation.

Correct Answer: Option 1 — They detect taste.
Receptors are specialised nerve endings located in our sense organs that detect information from the environment. Gustatory receptors are present in the taste buds of the tongue and they detect taste.
When food dissolves in saliva and touches these receptors, they set up an electrical impulse in the sensory neuron. The impulse travels along the dendrite → cell body → axon, crosses the synapse by releasing chemicals, and finally reaches the fore-brain, where the sensation of taste is recognised. Taste and smell work together, which is why food seems tasteless when we have a cold.
❌ Why the other options are incorrect
✗ They detect smell — that is the work of olfactory receptors, located in the lining of the nose.
✗ They detect sound — sound is detected by phonoreceptors in the inner ear (cochlea), which also maintains balance.
✗ They detect light — light is detected by photoreceptors, the rods and cones in the retina of the eye.
📌 SSC & Railway Exam Facts
• Gustatory → tongue (taste); Olfactory → nose (smell); Photoreceptor → eye (light); Phonoreceptor → ear (sound); Thermoreceptor → skin (heat).
• An adult tongue has about 10,000 taste buds.
• The fore-brain is the main thinking part and contains centres for hearing, smell, sight and taste.
• Rods detect dim light, cones detect bright light and colour; lack of cones causes colour blindness.
• The gap between two neurons is called a synapse; the gap between a neuron and a muscle is a neuromuscular junction.

Correct Answer: Option 3 — Both A and B are correct.
Statement A is correct. Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant — mainly the leaves, through the stomata, and to a small extent through the cuticle and lenticels.
Statement B is also correct. Water lost from the leaf cells creates a suction pressure, and because water molecules stick to one another (cohesion) and to the xylem walls (adhesion), an unbroken water column is pulled up. This transpiration pull is the major driving force for the upward movement of water in the xylem during the day, when the stomata are open. At night, when stomata close, root pressure becomes the main force.
❌ Why the other options are incorrect
✗ A is correct and B is incorrect — wrong, because transpiration pull is indeed the chief force lifting water in daytime.
✗ Both A and B are incorrect — both statements are exactly as given in NCERT, so this cannot be right.
✗ A is incorrect and B is correct — wrong, because statement A is the standard definition of transpiration.
📌 SSC & Railway Exam Facts
• Xylem carries water and minerals upward; phloem carries food (sucrose) in both directions by translocation.
• Transpiration also helps in the absorption and upward movement of minerals and cools the plant.
• Transpiration is called a 'necessary evil' in plants.
• Guttation is the loss of water in liquid form through hydathodes at leaf margins.
• Stomatal opening and closing is controlled by the turgidity of guard cells.

Correct Answer: Option 3 — All plant hormones except cytokinins promote cell division.
This statement is INCORRECT, and in fact says the exact opposite of the truth. Cytokinins are the hormones that most strongly promote cell division; NCERT states that they are present in greater concentration in areas of rapid cell division, such as fruits and seeds.
Plant hormones (phytohormones) are chemical compounds made in one part of the plant that help to coordinate growth, development and responses to the environment. The four main groups are auxins (cell elongation, phototropism), gibberellins (stem elongation), cytokinins (cell division) and abscisic acid (growth inhibitor, closes stomata, causes wilting and dormancy).
❌ Why the other options are incorrect
✗ Different plant hormones help to coordinate responses to the environment — correct; plants have no nervous system, so hormones alone control tropic responses such as bending towards light.
✗ All plant hormones except abscisic acid help in promoting growth — correct; auxin, gibberellin and cytokinin promote growth, while abscisic acid inhibits it and is called the stress hormone.
✗ Different plant hormones help to coordinate growth and development — correct; this is NCERT's own definition of the role of phytohormones.
📌 SSC & Railway Exam Facts
• Auxin was discovered by F. W. Went; it causes phototropism by faster elongation on the shaded side.
• Gibberellin causes bolting and increases stem length; it was isolated from the fungus Gibberella fujikuroi.
• Cytokinin promotes cell division and delays ageing of leaves.
• Abscisic acid (ABA) is the stress hormone; it closes stomata during water shortage.
• Ethylene is the only gaseous plant hormone and is used for the ripening of fruits.

Correct Answer: Option 2 — Hydra
Hydra is a simple multicellular freshwater animal that reproduces asexually by budding. NCERT states that Hydra uses regenerative cells for reproduction in the process of budding.
A small outgrowth called a bud appears on the side of the parent body. It is produced by repeated cell division of these specially set-aside regenerative cells. The bud develops its own mouth and tentacles, remains attached to the parent while it grows, and finally detaches as a new, independent Hydra. Since only one parent is involved and no gametes are formed, the offspring is a genetic copy of the parent.
❌ Why the other options are incorrect
✗ Planaria — a flatworm; it reproduces by regeneration from cut body pieces, not by forming buds.
✗ Spirogyra — a filamentous green alga; it multiplies by fragmentation, in which the filament simply breaks into pieces.
✗ Amoeba — a unicellular protozoan; it reproduces by binary fission, splitting into two daughter cells.
📌 SSC & Railway Exam Facts
• Budding — Hydra and yeast (yeast is a unicellular fungus).
• Binary fission — Amoeba, Leishmania, Paramoecium, bacteria; Multiple fission — Plasmodium.
• Fragmentation — Spirogyra; Regeneration — Hydra and Planaria; Spore formation — Rhizopus.
• Hydra belongs to the phylum Coelenterata (Cnidaria) and shows regeneration as well.
• In asexual reproduction only one parent takes part, and variations are very few.

Correct Answer: Option 3 — Sporangia
On the bread mould Rhizopus, the tiny blob-on-a-stick structures seen on the cottony threads are the sporangia. NCERT describes them exactly in these words while explaining spore formation.
Each sporangium is a swollen knob-like sac raised on an upright hypha called the sporangiophore. Inside it, hundreds of tiny spores are produced. Each spore is covered by a thick protective wall that lets it survive unfavourable conditions such as heat and dryness. When the sporangium bursts, the spores are carried away by air, and on reaching a moist surface such as bread, each spore germinates into a new Rhizopus. This is asexual reproduction by spore formation.
❌ Why the other options are incorrect
✗ Spermatogonia — the germ cells in the testes of animals that divide to form sperms; they have nothing to do with fungi.
✗ Sporophyte — the diploid, spore-producing generation of a plant body in the alternation of generations; it is a whole plant stage, not a blob-on-a-stick.
✗ Gametophyte — the haploid, gamete-producing generation in plants such as mosses and ferns.
📌 SSC & Railway Exam Facts
• Spore formation is seen in Rhizopus, Mucor, ferns, mosses and bacteria.
• Spores have thick walls that protect them until favourable conditions return.
• Rhizopus is the common bread mould; Mucor is another well-known bread fungus.
• Yeast is a unicellular fungus that reproduces by budding.
• Alternation of sporophyte and gametophyte generations is typical of bryophytes and pteridophytes.

Correct Answer: Option 1 — Fallopian tube
The Fallopian tube, also called the oviduct, is a narrow muscular tube that carries the egg (ovum) from the ovary to the uterus (womb). Its funnel-shaped mouth has finger-like fimbriae that sweep the released egg into the tube, and the wall pushes the egg forward by ciliary action and peristalsis.
The Fallopian tube is also the site where fertilisation normally takes place. The sperm, deposited in the vagina, travels up and fuses with the egg in the upper (ampulla) part of the tube, forming a zygote. The zygote then moves down and is implanted in the thick lining of the uterus, where it develops into an embryo.
❌ Why the other options are incorrect
✗ Placenta — a disc-like special tissue formed after implantation; it supplies glucose and oxygen to the embryo and removes its wastes, but it does not transport the egg.
✗ Cervix — the narrow neck of the uterus that opens into the vagina; it dilates during childbirth.
✗ Vagina — the muscular canal that receives the sperm during copulation and acts as the birth canal; it lies below the uterus, not between ovary and uterus.
📌 SSC & Railway Exam Facts
• The ovary produces the ovum and the hormones oestrogen and progesterone.
• A woman releases usually one egg per month (ovulation), and the menstrual cycle is about 28 days.
• Fertilisation occurs in the Fallopian tube; implantation and development occur in the uterus.
• The placenta contains villi that greatly increase the surface area for exchange.
• Human gestation period is about 9 months (280 days); tying the Fallopian tubes (tubectomy) is a contraceptive method.

Correct Answer: Option 1 — Papaya
Vegetative propagation is a type of asexual reproduction in plants in which new plants are produced from the vegetative parts — root, stem, leaf or bud — without the formation of seeds or gametes.
Papaya is normally grown from seeds, which are formed only after pollination and fertilisation, i.e. by sexual reproduction. It is therefore the odd one out. Banana is raised from suckers (and is generally seedless), sugarcane from stem cuttings bearing nodes and buds, and rose from stem cuttings and grafting — all three are classic examples of vegetative propagation.
❌ Why the other options are incorrect
✗ Banana — propagated vegetatively by underground suckers/rhizomes; cultivated bananas are seedless and cannot be grown from seed.
✗ Sugarcane — propagated by planting stem cuttings (setts); the buds at the nodes sprout into new plants.
✗ Rose — propagated by cutting, layering, budding and grafting, which keeps the flower colour and quality identical to the parent.
📌 SSC & Railway Exam Facts
• Advantages — plants bear fruit earlier, seedless varieties can be grown, and all offspring are genetically identical (a clone) to the parent.
• Bryophyllum reproduces from buds on the leaf margins; potato from eyes (buds) of the tuber.
• Layering — jasmine, strawberry; Grafting — mango, apple; Cutting — rose, sugarcane, bougainvillea.
• Tissue culture (micropropagation) grows whole plants from a few cells in a nutrient medium; this ability is called totipotency.
• Onion, ginger, turmeric and dahlia propagate through bulb, rhizome and tuberous roots.

Correct Answer: Option 1 — the medulla in the hind-brain
The hind-brain has three parts — the cerebellum, the pons and the medulla oblongata. The medulla is the part that controls involuntary actions, that is, actions which go on without our thinking about them, such as blood pressure, salivation and vomiting.
Because it also runs heartbeat, breathing, coughing, sneezing and swallowing, the medulla is called the vital centre of the body; an injury to it is usually fatal. These involuntary actions are carried out through the autonomic nervous system, whose sympathetic and parasympathetic nerves connect the medulla to the heart, blood vessels and glands.
❌ Why the other options are incorrect
✗ The cerebellum in the fore-brain — doubly wrong: the cerebellum is part of the hind-brain, not the fore-brain, and its job is posture, balance and precision of voluntary movement, e.g. walking in a straight line or picking up a pencil.
✗ Only the spinal cord — the spinal cord conducts impulses and controls reflex actions, but blood pressure, salivation and vomiting are governed by the medulla.
✗ Only the heart — the heart is a pumping organ; it is itself controlled by the medulla and cannot control salivation or vomiting.
📌 SSC & Railway Exam Facts
• Brain regions — Fore-brain: cerebrum; Mid-brain; Hind-brain: cerebellum, pons, medulla.
• Cerebrum is the largest part and the seat of thinking, memory, intelligence and voluntary action.
• Medulla controls heartbeat, breathing, blood pressure, vomiting, coughing and sneezing.
• The brain is protected by the cranium (skull), three meninges and the shock-absorbing cerebrospinal fluid (CSF).
• Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Correct Answer: Option 3 — Complex tissue includes parenchyma and collenchyma.
This statement is INCORRECT. Parenchyma, collenchyma and sclerenchyma are simple permanent tissues, because each of them is made of only one type of cell.
Complex permanent tissues are made of more than one type of cell which work together as a unit to perform a common function. The two complex tissues of plants are:
• Xylem — made of tracheids, vessels, xylem parenchyma and xylem fibres; it conducts water and minerals upward and gives mechanical strength.
• Phloem — made of sieve tubes, companion cells, phloem parenchyma and phloem fibres; it transports prepared food from the leaves to the rest of the plant.
Xylem and phloem together form the vascular bundles, which is why vascular bundles are typical of complex tissue.
❌ Why the other options are incorrect
✗ Vascular bundles are a characteristic feature of complex tissues — correct; xylem and phloem together make up the vascular bundle.
✗ There is more than one type of cell in a complex tissue — correct; this is the very definition that separates complex tissue from simple tissue.
✗ Cells in a complex tissue work together to perform a common function — correct; different cell types of xylem act together for conduction and support.
📌 SSC & Railway Exam Facts
• Simple permanent tissue — parenchyma, collenchyma, sclerenchyma; Complex permanent tissue — xylem and phloem.
• Parenchyma with chloroplasts is chlorenchyma; with air cavities in aquatic plants it is aerenchyma.
• Collenchyma gives flexibility; sclerenchyma cells are dead and lignified and give hardness and stiffness.
• Phloem is the only plant tissue with a living conducting element — the sieve tube.
• Meristematic tissue divides and causes growth; its cells lack vacuoles and have dense cytoplasm.

Correct Answer: Option 2 — Only (i) and (ii)
Ozone (O₃) is a molecule made of three atoms of oxygen, so statement (i) is correct. It is formed in the stratosphere when high-energy UV radiation splits molecular oxygen (O₂) into free oxygen atoms, which then combine with other O₂ molecules.
Statement (ii) is also correct — ordinary oxygen (O₂) is essential for all aerobic organisms, because they use it in respiration to release energy from glucose as ATP.
Statement (iii) is wrong. NCERT states plainly that ozone is a deadly poison. At ground level it irritates the eyes and lungs and damages crops. Only high up in the stratosphere, far away from living beings, does it act as a protective shield by absorbing harmful UV rays.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — includes statement (iii), but ozone is in fact highly poisonous.
✗ Only (i) and (iii) — again wrongly accepts (iii) and unnecessarily drops the correct statement (ii).
✗ Only (i) — too narrow; statement (ii) about the need for oxygen in aerobic life is perfectly correct.
📌 SSC & Railway Exam Facts
• O₂ and O₃ are allotropes of oxygen; ozone has a pungent smell.
• The ozone layer lies in the stratosphere, roughly 15–35 km above the Earth.
• The ozone hole over Antarctica was reported in 1985; CFCs are the main culprits.
• The Montreal Protocol (1987) regulated CFC production; 16 September is World Ozone Day.
• Ozone thickness is measured in Dobson Units (DU).

Correct Answer: Option 1 — Metal oxide
Air is a mixture in which the reactive component is oxygen (about 21%). When a metal is burnt in air, it combines with this oxygen to form a metal oxide:
Metal + O₂ → Metal oxide
2Mg + O₂ → (burning) 2MgO, 4Na + O₂ → 2Na₂O, 2Cu + O₂ → (heat) 2CuO
Most metal oxides are basic in nature — they dissolve in water to give alkalis (Na₂O + H₂O → 2NaOH) and react with acids to give salt and water. A few, such as Al₂O₃ and ZnO, react with both acids and bases and are called amphoteric oxides.
Highly reactive metals such as K and Na react so fast with the oxygen and moisture of air that they are kept immersed in kerosene.
❌ Why the other options are incorrect
✗ Metal chloride — formed when a metal reacts with chlorine or with hydrochloric acid (2Na + Cl₂ → 2NaCl), not with air.
✗ Metal nitrate — formed when a metal or its oxide reacts with nitric acid (HNO₃); air contains nitrogen as unreactive N₂, not as nitrate.
✗ Metal hydride — formed when a metal reacts with hydrogen (2Na + H₂ → 2NaH); air has almost no free hydrogen.
📌 SSC & Railway Exam Facts
• Air ≈ 78% N₂, 21% O₂, 0.9% Ar, 0.04% CO₂.
• Metal oxides are basic; non-metal oxides are acidic (CO₂, SO₂, NO₂).
• Amphoteric oxides: Al₂O₃, ZnO, PbO, SnO.
• Na and K are stored in kerosene; white phosphorus is stored in water.
• Aluminium forms a thin protective layer of Al₂O₃ that stops further corrosion.

Correct Answer: Option 1 — Combination reaction
Coal is essentially carbon. When it burns, the carbon combines with the oxygen of air to form a single product, carbon dioxide.
⚗️ The Reaction
C(s) + O₂(g) → CO₂(g) + heat
• Two reactants join to form one single product, which is the definition of a combination (synthesis) reaction.
• Heat and light are given out, so it is also combustion, an oxidation and an exothermic reaction.
• Carbon gains oxygen, so carbon is oxidised — it is not a reduction.
A combination reaction can be between two elements (2Mg + O₂ → 2MgO), two compounds (CaO + H₂O → Ca(OH)₂), or an element and a compound (2CO + O₂ → 2CO₂).
❌ Why the other options are incorrect
✗ Decomposition reaction — here a single compound splits into two or more products (CaCO₃ → CaO + CO₂); burning coal is the opposite, two things joining into one.
✗ Double decomposition reaction — another name for a double displacement reaction, in which two compounds exchange ions (AgNO₃ + NaCl → AgCl + NaNO₃); no exchange of ions occurs here.
✗ Reduction reaction — reduction means loss of oxygen or gain of hydrogen; in burning coal, carbon gains oxygen, so it is oxidation, not reduction.
📌 SSC & Railway Exam Facts
• Combination: A + B → AB; Decomposition: AB → A + B.
• CaO + H₂O → Ca(OH)₂ is a combination reaction that is strongly exothermic.
• Burning of coal, natural gas, candle and respiration are all exothermic processes.
• Incomplete combustion of coal gives the poisonous gas carbon monoxide (CO).
• Coal varieties in increasing order of carbon: peat < lignite < bituminous < anthracite.

Correct Answer: Option 4 — H₂
A covalent bond is formed by the mutual sharing of electrons between two atoms. Sharing one pair gives a single bond, two pairs a double bond and three pairs a triple bond.
A hydrogen atom has only 1 electron and needs 1 more to reach the stable duplet of helium. So two hydrogen atoms share one electron pair each way — exactly one shared pair — giving the molecule H–H, a single covalent bond.
Bond strength increases from single to triple, while bond length decreases; that is why N₂ (triple bond) is very unreactive.
❌ Why the other options are incorrect
✗ CO₂ — carbon shares two pairs with each oxygen, giving O=C=O, that is two double bonds (four shared pairs in all).
✗ N₂ — each nitrogen needs 3 electrons, so the two atoms share three pairs, giving a triple bond (N≡N), the strongest of the three.
✗ O₂ — each oxygen needs 2 electrons, so they share two pairs, giving a double bond (O=O).
📌 SSC & Railway Exam Facts
• Single bond = 1 shared pair, double = 2 pairs, triple = 3 pairs.
• H₂, Cl₂, HCl, CH₄, H₂O and NH₃ contain only single covalent bonds.
• Covalent compounds have low melting and boiling points, are poor conductors and are mostly insoluble in water.
• Ionic bond = transfer of electrons (NaCl); covalent bond = sharing of electrons (H₂O).
• Carbon forms four covalent bonds — this tetravalency plus catenation explains the huge number of carbon compounds.

Correct Answer: Option 4 — Gaseous to solid state
Deposition (also called desublimation) is the change of a substance directly from the gaseous state to the solid state, without passing through the liquid state. It is the exact reverse of sublimation, and it releases the latent heat that sublimation absorbs.
Everyday examples: the formation of frost on a cold winter morning, the formation of snow high in the atmosphere, and the white solid ring of ammonium chloride that forms on the cool walls of a tube when NH₃ and HCl vapours meet. In the laboratory, iodine vapour deposits as shining violet crystals on the cold inner surface of an inverted funnel.
❌ Why the other options are incorrect
✗ Solid to liquid state — this is melting (fusion), which takes place at the melting point, e.g. ice at 0 °C.
✗ Solid to gaseous state — this is sublimation, the opposite process; camphor, naphthalene and NH₄Cl sublime.
✗ Liquid to solid state — this is freezing (solidification), e.g. water turning into ice.
📌 SSC & Railway Exam Facts
• Gas → solid = deposition; solid → gas = sublimation.
• Liquid → gas = vaporisation; gas → liquid = condensation; liquid → solid = freezing.
• Deposition is an exothermic change — it releases the latent heat of sublimation.
• Frost and snow are the standard natural examples of deposition.
• Interparticle force: solid > liquid > gas; interparticle space: gas > liquid > solid.

Correct Answer: Option 3 — Gold
Ernest Rutherford (1911) bombarded a very thin gold foil, only about 1000 atoms (≈100 nm) thick, with fast-moving α-particles from a radioactive source. Gold was chosen because it is the most malleable metal, so it can be beaten into the thinnest possible sheet — thin enough for α-particles to pass through.
Observations and conclusions:
• Most α-particles passed straight through → most of the atom is empty space.
• A few were deflected through small angles → there is a concentrated positive charge inside.
• About 1 in 12,000 bounced straight back → the whole positive charge and nearly all the mass are packed into a tiny nucleus.
From this he calculated the nucleus radius ≈ 10⁻¹⁵ m against the atomic radius ≈ 10⁻¹⁰ m, and proposed the nuclear model in which electrons revolve around the nucleus in circular orbits.
❌ Why the other options are incorrect
✗ Platinum — a noble metal, but far less malleable than gold, so an equally thin foil is not possible.
✗ Silver — malleable but it tarnishes in air (forms black Ag₂S), which would spoil the experiment.
✗ Palladium — used as a catalyst and for absorbing hydrogen, not in the α-scattering experiment.
📌 SSC & Railway Exam Facts
• α-particle = helium nucleus (He²⁺), charge +2, mass 4 u.
• Gold is the most malleable and most ductile metal.
• Rutherford's model failed to explain the stability of the atom (Maxwell's theory) — corrected by Bohr.
• Rutherford is called the father of nuclear physics; Nobel Prize in Chemistry, 1908.
• The experiment was performed with his students Hans Geiger and Ernest Marsden.

Correct Answer: Option 4 — neutralisation reaction
The reaction between an acid and a base in which they cancel each other's effect and give a salt and water is called a neutralisation reaction.
⚗️ The Reaction
Acid + Base → Salt + Water
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• In essence, the H⁺ ion of the acid joins the OH⁻ ion of the base to form water: H⁺ + OH⁻ → H₂O.
• Heat is always given out, so neutralisation is an exothermic reaction.
• At the end point the solution becomes neutral, pH ≈ 7.
Everyday uses: an antacid (Mg(OH)₂, milk of magnesia) neutralising excess stomach acid, baking soda on a bee sting, and quick lime or slaked lime added to acidic soil.
❌ Why the other options are incorrect
✗ condensation reaction — an organic reaction in which two molecules join with the elimination of a small molecule like water (esterification); it is not the acid–base reaction meant here.
✗ endothermic reaction — one that absorbs heat; neutralisation actually releases heat, so it is exothermic.
✗ decomposition reaction — a single compound breaks into simpler substances (CaCO₃ → CaO + CO₂); here two compounds react to give two products.
📌 SSC & Railway Exam Facts
• Acid + Base → Salt + Water (neutralisation, exothermic).
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic; pH scale given by Sørensen.
• Litmus: acid turns blue litmus red; base turns red litmus blue.
• Phenolphthalein — colourless in acid, pink in base; methyl orange — red in acid, yellow in base.
• Stomach acid is HCl; antacids such as milk of magnesia, Mg(OH)₂ neutralise it.

Correct Answer: Option 4 — 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
The chlor-alkali process is the electrolysis of a concentrated aqueous solution of sodium chloride, called brine. The name comes from its two main products — "chlor" for chlorine and "alkali" for sodium hydroxide.
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → (electricity) 2NaOH(aq) + Cl₂(g) + H₂(g)
• Chlorine gas (Cl₂) is liberated at the anode.
• Hydrogen gas (H₂) is liberated at the cathode.
• Sodium hydroxide (NaOH) solution is left near the cathode.
• Count the atoms: 2 Na, 2 Cl, 4 H, 2 O on both sides — the equation is balanced.
Uses: NaOH for soaps, detergents and paper; Cl₂ for water treatment, PVC and bleaching powder; H₂ for fuels, margarine and ammonia.
❌ Why the other options are incorrect
✗ 2KCl(aq) + 2H₂O(l) → 2KOH(aq) + Cl₂(g) + H₂(g) — chemically possible, but the chlor-alkali industry uses common salt (NaCl) brine, not potassium chloride; the process is defined for NaCl.
✗ 2MgCl₂(aq) + 2H₂O(l) → 2Mg(OH)₂(aq) + Cl₂(g) + H₂(g) — wrong on two counts: Mg(OH)₂ is insoluble, so it cannot be (aq), and the equation is not balanced (4 Cl on the left, only 2 on the right).
✗ 2CaCl₂(aq) + 2H₂O(l) → 2Ca(OH)₂(aq) + Cl₂(g) + H₂(g) — again unbalanced in chlorine (4 on the left, 2 on the right), and Ca(OH)₂ is only sparingly soluble.
📌 SSC & Railway Exam Facts
• Chlor-alkali = electrolysis of brine; Cl₂ at anode, H₂ at cathode, NaOH near cathode.
• Cl₂ + dry slaked lime Ca(OH)₂ → bleaching powder, CaOCl₂.
• NaOH is caustic soda; Na₂CO₃·10H₂O is washing soda; NaHCO₃ is baking soda.
• NaCl is the raw material for caustic soda, washing soda, baking soda and bleaching powder.
• Baking soda + acid → CO₂, which makes cakes rise; NaHCO₃ is also used in soda-acid fire extinguishers.

Correct Answer: Option 3 — Watermelon
J. J. Thomson (1898) compared an atom to a watermelon. According to his model, the positive charge of the atom is spread uniformly throughout it, like the red edible part of a watermelon, while the negatively charged electrons are studded in it like the black seeds of the watermelon.
His two postulates were:
• An atom is a sphere of positive charge with the electrons embedded in it.
• The negative and positive charges are equal in magnitude, so the atom as a whole is electrically neutral.
Because of this picture, the model is also called the plum-pudding model or the christmas-pudding model. Thomson's model successfully explained the electrical neutrality of the atom, but it could not explain the results of Rutherford's α-particle scattering experiment, which proved that the positive charge is concentrated in a tiny nucleus.
❌ Why the other options are incorrect
✗ Mango — has a single large stone at the centre; no standard atomic model uses a mango comparison.
✗ Banana — has no scattered-seed structure at all, so it cannot illustrate embedded electrons.
✗ Apple — again has seeds only at its core, not spread throughout, so it does not match Thomson's description.
📌 SSC & Railway Exam Facts
• Thomson's model = watermelon / plum-pudding model; he discovered the electron (1897).
• Nobel Prize in Physics 1906 — J. J. Thomson; his student Rutherford won the Nobel in Chemistry 1908.
• Rutherford's model = nuclear model, compared to the solar system.
• Goldstein — proton (canal rays); Chadwick — neutron (1932).
• Bohr gave discrete orbits (shells K, L, M, N) to explain the atom's stability.

Correct Answer: Option 1 — 23u
Sodium (Na) has atomic number 11, which means it has 11 protons and, in a neutral atom, 11 electrons. Its nucleus also contains 12 neutrons.
Atomic mass (mass number) = protons + neutrons = 11 + 12 = 23 u
So the atomic mass of sodium is 23 u, and its gram atomic mass is 23 g/mol. The unit u stands for the unified atomic mass unit, defined as 1/12 of the mass of one carbon-12 atom.
Sodium's electronic configuration is 2, 8, 1, so it has one valence electron, a valency of 1, and it belongs to Group 1 (alkali metals), Period 3.
❌ Why the other options are incorrect
✗ 18u — this is close to the atomic mass of water (18 u) and to the atomic number of argon (18); it is not sodium's mass.
✗ 20u — this is the atomic mass of neon (20 u) and the atomic mass of calcium's atomic number (20); not sodium.
✗ 11u — this is sodium's atomic number (11), a common trap; the atomic number is the proton count, not the mass.
📌 SSC & Railway Exam Facts
• Na: Z = 11, A = 23, neutrons = 12, configuration 2, 8, 1, valency 1.
• 1 u = 1/12 the mass of a carbon-12 atom ≈ 1.66 × 10⁻²⁷ kg.
• Common atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, Ca = 40.
• Sodium is stored in kerosene and gives a golden-yellow flame test.
• Molecular mass of NaCl = 23 + 35.5 = 58.5 u.

Correct Answer: Option 2 — White precipitate of BaSO₄
When solutions of barium chloride and sodium sulphate are mixed, the two compounds exchange their ions — Ba²⁺ pairs with SO₄²⁻ and Na⁺ pairs with Cl⁻. Barium sulphate is insoluble in water, so it separates at once as a thick white precipitate.
⚗️ The Reaction
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
• The (s) marks BaSO₄ as the insoluble white precipitate.
• Both compounds exchange ions, so it is a double displacement reaction; because a precipitate forms it is also called a precipitation reaction.
• Atoms balance: 1 Ba, 2 Cl, 2 Na, 1 S, 4 O on each side.
This reaction is the standard laboratory test for the sulphate ion (SO₄²⁻) — a white precipitate that is insoluble even in dilute HCl.
❌ Why the other options are incorrect
✗ Black precipitate of BaSO₄ — black precipitates are typical of sulphides such as PbS and CuS, not of barium sulphate.
✗ Yellow precipitate of BaSO₄ — yellow precipitates are given by PbI₂ and AgI; BaSO₄ is never yellow.
✗ Brown precipitate of BaSO₄ — brown is the colour of Fe(OH)₃ or rust; barium sulphate is pure white.
📌 SSC & Railway Exam Facts
• BaSO₄ — white; AgCl — white; PbI₂ — yellow; Cu(OH)₂ — blue; Fe(OH)₃ — reddish brown; PbS — black.
• BaSO₄ is used as the "barium meal" in X-ray imaging of the stomach and intestines.
• Double displacement: AB + CD → AD + CB.
• Test for sulphate — white ppt with BaCl₂, insoluble in dilute HCl; test for chloride — white ppt with AgNO₃.
• A precipitation reaction always produces an insoluble salt, shown by (s) in the equation.

Correct Answer: Option 4 — Double displacement reaction
In a double displacement reaction two compounds exchange their ions to form two new compounds: AB + CD → AD + CB.
⚗️ The Reaction
CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)
• Cu²⁺ leaves the sulphate and pairs with OH⁻; Na⁺ pairs with SO₄²⁻ — a clear exchange of ions.
• Copper hydroxide, Cu(OH)₂, is insoluble and separates as a blue precipitate (s), so this is also a precipitation reaction.
• The equation balances: 1 Cu, 1 S, 6 O, 2 Na, 2 H on each side.
The blue colour of copper sulphate solution fades as the blue gelatinous precipitate of copper hydroxide settles out.
❌ Why the other options are incorrect
✗ Combination reaction — two or more reactants would have to give one single product (CaO + H₂O → Ca(OH)₂); here there are two products.
✗ Combustion reaction — burning in oxygen with the release of heat and light; there is no oxygen gas and no burning here.
✗ Neutralisation reaction — needs an acid and a base giving salt + water; CuSO₄ is a salt, not an acid, and no water is formed.
📌 SSC & Railway Exam Facts
• Double displacement: AB + CD → AD + CB; often forms a precipitate.
• Na₂SO₄ + BaCl₂ → BaSO₄(s) + 2NaCl — classic precipitation example.
• Precipitate colours: Cu(OH)₂ blue, Fe(OH)₃ reddish brown, AgCl white, PbI₂ yellow.
• Displacement: Fe + CuSO₄ → FeSO₄ + Cu — only one element is displaced.
• CuSO₄·5H₂O is blue vitriol; on heating it turns white as it loses water of crystallisation.

Correct Answer: Option 3 — The composition of the mixture is non-uniform.
A heterogeneous mixture is one whose composition is not uniform throughout. Different portions of the mixture have different properties, and the components can usually be seen as physically distinct parts, separated by a visible boundary.
Common examples: sand and salt, oil and water, iron filings and sulphur powder, soil, muddy water and a suspension of chalk in water. Such mixtures can normally be separated by simple physical methods like handpicking, sieving, filtration, magnetic separation or a separating funnel.
In contrast, a homogeneous mixture — a true solution such as salt in water, sugar syrup or brass — has a uniform composition throughout and shows no visible boundary between its parts.
❌ Why the other options are incorrect
✗ A clear solution is called a heterogeneous mixture. — a clear solution is homogeneous; its particles are smaller than 1 nm and never settle.
✗ The composition of the mixture is uniform. — uniform composition is the defining property of a homogeneous mixture, the exact opposite of heterogeneous.
✗ Mixtures don't contain physically distinct parts. — heterogeneous mixtures do contain physically distinct parts; that is exactly how they are recognised.
📌 SSC & Railway Exam Facts
• Homogeneous — uniform composition (salt solution, air, brass, alloys); Heterogeneous — non-uniform (oil + water, sand + salt, soil).
• Colloids and suspensions are heterogeneous; a true solution is homogeneous.
• Particle size: solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Mixtures have no fixed composition; compounds have a fixed ratio by mass.
• Components of a mixture keep their own properties and are separated by physical methods.

Correct Answer: Option 1 — Copper
A displacement reaction takes place only when a more reactive metal displaces a less reactive metal from its salt solution. Whether a metal can do this is decided by its position in the reactivity series.
For the four metals given, the order of reactivity is:
Zinc (Zn) > Iron (Fe) > Lead (Pb) > Copper (Cu)
Copper lies lowest, so it cannot displace zinc, iron or lead from their salts — it is the least reactive of the four. On the contrary, all three of them can displace copper from copper sulphate solution.
⚗️ The Reaction
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• The blue colour of CuSO₄ fades and a reddish-brown deposit of copper appears on the zinc.
• The reverse, Cu + ZnSO₄ → no reaction, proves copper is the less reactive metal.
❌ Why the other options are incorrect
✗ Lead — lead stands above copper in the series and can displace copper from CuSO₄, so it is not the least reactive.
✗ Iron and zinc — both are well above lead and copper; iron displaces copper (Fe + CuSO₄ → FeSO₄ + Cu) and zinc displaces both.
✗ Iron — iron is a moderately reactive metal that rusts readily and reacts with dilute acids; it is far more reactive than copper.
📌 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 release H₂ from dilute acids.
• Displacement: A + BC → AC + B, only if A is more reactive than B.
• Copper vessels are used for water because copper is unreactive; it turns green due to basic copper carbonate.
• Thermite reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat, used to weld rail tracks.

Correct Answer: Option 1 — the speed of propagation of light in two different media
When light passes from one transparent medium into another it changes its speed, and this change of speed is what bends the ray. The refractive index is simply the number that measures this change. For light going from medium 1 into medium 2,
n₂₁ = speed of light in medium 1 / speed of light in medium 2 = v₁/v₂
When the first medium is vacuum (or air), we get the absolute refractive index, n = c/v, where c = 3 × 10⁸ m/s. A larger refractive index means light travels more slowly in that medium, so the medium is optically denser and bends the ray towards the normal.
By Snell's law, sin i / sin r = n₂₁, which is exactly the ratio of the two speeds. Refractive index is a ratio of two speeds, so it has no unit.
❌ Why the other options are incorrect
✗ the frequency of light in two different media — the frequency never changes during refraction; only the speed and wavelength change, so frequency cannot define the refractive index.
✗ the magnetic moment of two different media — a magnetic property of matter; it has nothing to do with how fast light travels through a transparent medium.
✗ the dielectric polarizability of two media — an electrical property; it influences the optical behaviour of a material indirectly, but refractive index is defined through speed, not polarizability.
📌 SSC & Railway Exam Facts
• n = c/v; refractive index is a pure number (no unit) and is always ≥ 1 for a real medium.
• n(water) = 1.33, n(glass) = 1.5, n(diamond) = 2.42 — the highest among common materials.
• Snell's law: n₁ sin i = n₂ sin r; it was given by Willebrord Snellius.
• Light travels fastest in vacuum, c = 3 × 10⁸ m/s; a denser medium slows it down.

Correct Answer: Option 3 — negative
Work done by a force is W = F s cos θ, where θ is the angle between the force and the displacement. Here the cart moves forward while the retarding force acts backwards, so θ = 180° and cos 180° = −1. Therefore W = −F s, which is negative work.
Negative work means the force is opposing the motion and is taking energy away from the body. That is exactly what happens: the cart's kinetic energy falls from ½mv² to zero, and the magnitude of the negative work equals the kinetic energy destroyed (work–energy theorem). Braking forces and friction are the standard examples of forces that always do negative work on a moving body.
❌ Why the other options are incorrect
✗ positive — positive work needs the force and displacement to point the same way (θ = 0°), as when an engine drives the cart forward. A retarding force does the opposite.
✗ undetermined — nothing is unknown here; the direction of the force relative to the displacement is clearly stated, so the sign of the work is fixed.
✗ zero — work is zero only when the displacement is zero (like pushing an immovable wall) or when the force is perpendicular to the displacement (θ = 90°). The cart does move, and the force is along the line of motion.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit joule (J), 1 J = 1 N m; work is a scalar.
• θ = 0° ⇒ positive work, θ = 90° ⇒ zero work, θ = 180° ⇒ negative work.
• Work–energy theorem: net work done = change in kinetic energy.
• Friction and air resistance always do negative work on a moving body.
• Kinetic energy K = ½mv²; a body at rest has K = 0.

Correct Answer: Option 1 — 2πR/t
📖 Given
• Radius of the circular track, R
• Time taken for one complete round, t s
• The speed is uniform
✍️ Formula
Speed = Total distance travelled / Time taken, and for one full circle the distance is the circumference = 2πR
🔢 Solution
• Distance covered in one round = 2πR
• Time taken = t s
• Speed, v = 2πR / t
• v = 2πR/t
In plain language, the athlete runs the whole circumference of the track in t seconds, so his speed is that circumference divided by the time.
❌ Why the other options are incorrect
✗ πR/2t — this is a quarter of the correct value; it appears if the circumference is wrongly taken as πR/2.
✗ 3πR/t — uses a distance of 3πR, which is one-and-a-half circles, not one round.
✗ πR/t — this uses πR, the length of only half the circle (a semicircle), so it gives half the true speed.
📌 SSC & Railway Exam Facts
• Circumference of a circle = 2πr; area = πr².
• In one complete round the distance = 2πR but the displacement = 0, so the average velocity is zero while the average speed is 2πR/t.
• Speed is a scalar, velocity is a vector; SI unit of both is m/s.
• In uniform circular motion the speed is constant but the velocity keeps changing because the direction changes — so the motion is accelerated.
• Centripetal acceleration a = v²/R, directed towards the centre.

Correct Answer: Option 4 — 10 N
📖 Given
• Weight on the earth, W(earth) = 60 N
• Acceleration due to gravity on the moon is 1/6 of that on the earth: g(moon) = g(earth)/6
✍️ Formula
W = mg, so W(moon)/W(earth) = g(moon)/g(earth) = 1/6
🔢 Solution
• W(moon) = W(earth) × 1/6
• W(moon) = 60 × 1/6
• W(moon) = 10 N
The mass stays the same everywhere; only the weight falls, because the moon is much smaller and less massive, so it pulls the object with only one-sixth of the earth's force. This is why astronauts can take long, floating leaps on the moon.
❌ Why the other options are incorrect
✗ 30 N — this is half of 60 N, obtained by using a factor of 1/2 instead of the correct 1/6.
✗ 60 N — this assumes the weight does not change; that is true of mass, not of weight.
✗ 39 N — no step of the calculation gives 39; it is a distractor with no physical meaning here.
📌 SSC & Railway Exam Facts
• Weight W = mg; SI unit newton (N) — weight is a force and a vector.
• g(earth) = 9.8 m/s², g(moon) ≈ 1.63 m/s² ≈ g/6.
• Mass is constant everywhere; SI unit kilogram (kg), a scalar.
• g = GM/R² — it depends on the mass and radius of the planet, not on the object.
• Weight is zero at the centre of the earth and maximum at the poles, least at the equator.

Correct Answer: Option 1 — the diameter of the reflecting surface of the mirror
A spherical mirror is a small piece cut out of a hollow sphere and silvered on one side. The aperture of such a mirror is the effective diameter of its reflecting surface — that is, the width of the light-collecting face measured across the rim. A mirror with a large aperture gathers more light and gives a brighter image, which is why the mirrors of reflecting telescopes are made as wide as possible.
In all the standard mirror formulas we assume a small aperture compared with the radius of curvature. This keeps the reflected rays close to the principal axis (paraxial rays) and lets us use f = R/2 and 1/v + 1/u = 1/f without the blurring called spherical aberration.
❌ Why the other options are incorrect
✗ the distance between the centre of curvature and the focus — this distance is equal to the focal length f itself (since PF = FC = R/2); it is not the aperture.
✗ the distance between the object and the mirror — that is the object distance u, measured from the pole, which changes every time the object is moved.
✗ the distance between the image and the mirror — that is the image distance v; it is found from the mirror formula and is not a fixed property of the mirror.
📌 SSC & Railway Exam Facts
• Pole (P) — centre of the reflecting surface; Centre of curvature (C) — centre of the sphere; Radius of curvature R = PC.
• f = R/2; mirror formula 1/v + 1/u = 1/f; magnification m = −v/u.
• A large aperture gives a brighter but more aberrated image.
• Concave mirror — converging, used in solar cookers, headlights, shaving mirrors.
• Convex mirror — diverging, used as a rear-view mirror for its wide field of view.

Correct Answer: Option 1 — 0 J
📖 Given
• Force applied by Ramu, F = 1 N
• The rock does not move, so displacement s = 0 m
✍️ Formula
W = F × s × cos θ
🔢 Solution
• W = 1 × 0 × cos θ
• W = 0 J
In physics, work is done only when the point of application of the force actually moves. Ramu certainly gets tired, because his muscles keep using up chemical energy, but since the rock has no displacement, the work done on the rock is exactly zero. Tiredness is a biological effect, not a measure of physical work.
❌ Why the other options are incorrect
✗ −1 J — this would need a displacement of 1 m against the force; but here there is no displacement at all, so no negative work either.
✗ 1000 J — a large number chosen to match the idea of "working hard"; effort is not work. With s = 0, no amount of force can produce energy transfer.
✗ +1 J — this comes from simply multiplying 1 N × 1 and forgetting that the displacement is zero, not 1 m.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit joule (J), 1 J = 1 N m, CGS unit erg, 1 J = 10⁷ erg.
• Zero work cases: s = 0 (pushing a wall), F = 0, or θ = 90° (a coolie carrying a load on level ground).
• Work is a scalar quantity but can be positive, negative or zero.
• Power = W/t; SI unit watt (W), 1 W = 1 J/s, 1 hp = 746 W.

Correct Answer: Option 1 — Magnetic field
The right-hand thumb rule (also called Maxwell's corkscrew rule) is used to find the direction of the magnetic field around a straight current-carrying conductor. Hold the wire in your right hand so that the stretched thumb points along the direction of the current; then the curled fingers give the direction in which the magnetic field lines encircle the wire.
So in this rule the thumb = current and the curled fingers = magnetic field. The field lines around a straight wire are concentric circles lying in a plane perpendicular to the wire, and they get farther apart — that is, the field gets weaker — as you move away, since B ∝ I/r.
❌ Why the other options are incorrect
✗ Current — the current is shown by the stretched thumb, not by the curled fingers; the two are commonly interchanged in a hurry.
✗ Force — the force on a current-carrying conductor placed in a magnetic field is found by Fleming's left-hand rule, a different rule altogether.
✗ Deflection of needle — the deflection of a compass needle is only the experimental evidence (Oersted's experiment) that a field exists; it is not what the fingers represent.
📌 SSC & Railway Exam Facts
• Right-hand thumb rule: thumb → current, curled fingers → magnetic field.
• Fleming's left-hand rule → motor (force); Fleming's right-hand rule → generator (induced current).
• Hans Christian Oersted (1820) discovered that a current produces a magnetic field.
• For a straight wire, B ∝ I and B ∝ 1/r; SI unit of B is the tesla (T).
• Inside a long solenoid the field is uniform and its pattern is like that of a bar magnet.

Correct Answer: Option 2 — ‘A’ is false and ‘B’ is true.
There are exactly two laws of refraction, and the two statements test them one by one.
Statement (A) is false. The first law of refraction says the incident ray, the refracted ray and the normal to the interface at the point of incidence all lie in the SAME plane. Statement (A) says they do not, so it reverses the law.
Statement (B) is true. This is the second law of refraction, Snell's law: for a given colour (wavelength) of light and a given pair of media,
sin i / sin r = constant = n₂₁
This constant is the refractive index of the second medium with respect to the first. The conditions "given colour" and "given pair of media" matter, because the refractive index changes with wavelength — that is what causes dispersion.
❌ Why the other options are incorrect
✗ Both ‘A’ and ‘B’ are false. — wrong, because (B) is a correct statement of Snell's law.
✗ Both ‘A’ and ‘B’ are true. — wrong, because (A) contradicts the first law of refraction; the three do lie in one plane.
✗ ‘A’ is true and ‘B’ is false. — this reverses both judgements and is the exact opposite of the correct answer.
📌 SSC & Railway Exam Facts
• First law — incident ray, refracted ray and normal are coplanar.
• Second law (Snell's law) — sin i / sin r = n₂₁ = v₁/v₂.
• A ray entering a denser medium bends towards the normal; entering a rarer medium it bends away from the normal.
• A ray falling normally (i = 0°) goes straight through, undeviated.
• Dispersion happens because n is different for different colours — violet bends most, red least.

Correct Answer: Option 2 — bar magnet
A solenoid is a long cylindrical coil made of many circular turns of insulated copper wire wound closely together. When a current flows through it, the fields of all the turns add up, and inside the solenoid the field lines become straight, parallel and equally spaced — a uniform magnetic field. Outside, the lines emerge from one end, curve round and re-enter at the other end.
This whole pattern is exactly like the pattern of a bar magnet. One end of the solenoid behaves as a north pole and the other as a south pole, which is why a solenoid carrying current can be used to magnetise a piece of steel. If a soft iron core is placed inside, we get a strong electromagnet.
❌ Why the other options are incorrect
✗ disc magnet — a flat, thin magnet magnetised across its thickness; its external field pattern is short and spread out, not like the long uniform interior of a solenoid.
✗ horseshoe magnet — a bar magnet bent into a U so that both poles face each other; it gives a strong field only in the narrow gap between the poles.
✗ ring magnet — magnetised along a circle, so its field lines close on themselves and it shows no free poles like a solenoid does.
📌 SSC & Railway Exam Facts
• Field inside a long solenoid is uniform; outside it is like a bar magnet.
• Poles of a solenoid are found by the clock rule or the right-hand thumb rule applied to the turns.
• A soft iron core in a solenoid makes an electromagnet — soft iron is used because it loses magnetism quickly.
• Permanent magnets are made of steel / alnico, which retain magnetism.
• William Sturgeon made the first electromagnet (1824).

Correct Answer: Option 3 — Work is done by the force.
In physics, work is said to be done when a force acts on a body and the body gets displaced along the direction of the force. Both conditions are satisfied here: a force is applied and the object is actually displaced horizontally. Hence work is done by the force, and its amount is W = F × s (with θ = 0°, so cos θ = 1).
Work is the transfer of energy. The energy spent by the agent applying the force reappears as the kinetic energy of the object (and as heat if friction is present). If the object had not moved, the work would have been zero, no matter how large the force.
❌ Why the other options are incorrect
✗ The force increases the mass of the object. — mass is the amount of matter in a body; it does not change when a force is applied (in ordinary, non-relativistic physics).
✗ The force decreases the mass of the object. — equally wrong for the same reason; mass is constant, and a force only changes the body's state of motion.
✗ The force increases the inertia of the object. — inertia is measured by mass. Since the mass is unchanged, the inertia is unchanged too; the force changes velocity, not inertia.
📌 SSC & Railway Exam Facts
• Two conditions for work: a force must act and there must be a displacement along that force.
• W = F s cos θ; SI unit joule (J); 1 J = 1 N m; work is a scalar.
• Work–energy theorem: net work done = change in kinetic energy = ½mv² − ½mu².
• Mass is a measure of inertia and is the same everywhere; weight W = mg changes with place.
• 1 kWh = 3.6 × 10⁶ J; 1 calorie = 4.2 J.

Correct Answer: Option 3 — Gravitation force
Tides — the regular rise and fall of sea water — are caused by the gravitational pull of the Moon and, to a smaller extent, of the Sun on the water of the oceans. By Newton's universal law of gravitation, F = GMm/r², so the side of the Earth facing the Moon is pulled a little more strongly than the Earth's centre, and the far side a little less. This difference in pull across the Earth raises a bulge of water on both sides, producing high tide there and low tide in between.
The Moon's effect is about twice that of the Sun even though the Sun is far more massive, because the Moon is very much nearer and the force falls off as 1/r². When the Sun, Earth and Moon are in a straight line (new moon and full moon) their pulls add and we get the very high spring tides; at quadrature we get the weak neap tides.
❌ Why the other options are incorrect
✗ Nuclear force — the strong force that binds protons and neutrons inside the nucleus; it acts only over about 10⁻¹⁵ m, so it cannot reach across space.
✗ Electromagnetic force — acts between charges and magnets; sea water and the Moon are electrically neutral overall, so this force plays no role in tides.
✗ Electrostatic force — the force between stationary charges, given by Coulomb's law; again, the bodies concerned carry no net charge.
📌 SSC & Railway Exam Facts
• F = GMm/r²; G = 6.67 × 10⁻¹¹ N m² kg⁻², measured by Henry Cavendish.
• Gravity is the weakest of the four fundamental forces but has infinite range and is always attractive.
• Four fundamental forces: gravitational, electromagnetic, strong nuclear, weak nuclear.
• Spring tides — new moon and full moon; neap tides — first and last quarter.
• The Moon is the chief cause of tides; the Sun's tidal effect is roughly half that of the Moon.

Correct Answer: Option 4 — doubled
📖 Given
• Original length l, original resistance R = ρl/A
• New length, l′ = 2l
• Resistivity ρ and area of cross-section A are unchanged
✍️ Formula
R = ρl/A — resistance is directly proportional to length
🔢 Solution
• R′ = ρl′/A = ρ(2l)/A
• R′ = 2 × (ρl/A)
• R′ = 2R, i.e. the resistance is doubled
Making the wire twice as long simply gives the electrons twice as much material to push through, so the opposition to the current becomes twice as large.
❌ Why the other options are incorrect
✗ the same — this would be true only if resistance did not depend on length at all, which contradicts R = ρl/A.
✗ halved — halving happens when the area of cross-section is doubled (a thicker wire), or when the length is halved, not when the length is doubled.
✗ one fourth — this would need R ∝ 1/l²; no such relation exists. A quarter appears if the radius is doubled, since A = πr² then becomes 4 times.
📌 SSC & Railway Exam Facts
• R = ρl/A; R ∝ l and R ∝ 1/A; SI unit of resistance ohm (Ω).
• Resistivity ρ depends only on the material and temperature, not on l or A; SI unit Ω m.
• Ohm's law: V = IR; given by Georg Simon Ohm.
• Series: R(eq) = R₁ + R₂ + …; Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + ….
• Silver is the best conductor; nichrome has high resistivity and is used in heating elements.

Correct Answer: Option 4 — A person can see nearby objects clearly but cannot see distant objects distinctly.
Myopia, or short-sightedness (near-sightedness), is the defect in which a person can see near objects clearly but distant objects appear blurred. It happens because the image of a distant object is formed in front of the retina instead of on it. The two usual causes are excessive curvature of the eye lens (too much converging power) or an elongated eyeball.
In myopia the far point shifts from infinity to some finite distance. The defect is corrected by using a concave (diverging) lens of suitable negative power, which first diverges the incoming parallel rays a little so that the eye lens can then bring them to focus exactly on the retina.
❌ Why the other options are incorrect
✗ A person can see both nearby and distant objects distinctly. — this describes a normal, defect-free eye, not myopia.
✗ A person cannot see both nearby and distant objects distinctly. — this fits presbyopia in old age, where both near and far vision weaken and bifocal lenses are used.
✗ A person can see distant objects clearly but cannot see nearby objects distinctly. — this is the exact opposite defect, hypermetropia (long-sightedness), corrected with a convex lens.
📌 SSC & Railway Exam Facts
• Myopia — concave lens, power is negative; far point comes nearer than infinity.
• Hypermetropia — convex lens, power is positive; near point moves beyond 25 cm.
• Presbyopia — corrected by bifocal lenses; astigmatism — corrected by a cylindrical lens.
• Least distance of distinct vision for a normal eye = 25 cm; far point = infinity.
• The eye focuses by changing the focal length of the eye lens — this is called accommodation, done by the ciliary muscles.