RRB Technician Grade III 2024-25 23 December 2024 Shift 3 GK Questions – with answers & solutions

Correct Answer: Option 4 — Amylase enzymes
Digestion begins in the buccal cavity (mouth). The three pairs of salivary glands pour out saliva, which contains the enzyme salivary amylase, also called ptyalin. This enzyme breaks the complex carbohydrate starch into the simple sugar maltose, so about 30 per cent of starch digestion is finished before the food is even swallowed.
Saliva also moistens and lubricates the food so that the tongue can roll it into a bolus for easy swallowing. Saliva works best in a slightly alkaline to neutral medium (pH ≈ 6.8), and its action stops in the acidic stomach.
❌ Why the other options are incorrect
✗ Pepsin enzymes — secreted by the gastric glands of the stomach; pepsin digests proteins into peptones and works only in the acidic medium created by HCl.
✗ Trypsin enzymes — present in the pancreatic juice; trypsin acts on proteins in the alkaline medium of the small intestine, not on starch in the mouth.
✗ Pancreatic enzymes — these (pancreatic amylase, trypsin, lipase) act in the small intestine, not in the mouth where the question places the action.
📌 SSC & Railway Exam Facts
• Ptyalin is the salivary amylase of humans; starch → maltose.
• Largest gland of the human body — the liver; it secretes bile, which emulsifies fats.
• The pancreas is a mixed gland — it makes digestive juice as well as insulin.
• Most absorption of digested food occurs in the small intestine through finger-like villi.
• Stomach HCl kills germs and gives the acidic pH that pepsin needs; the mucus lining protects the stomach wall.

Correct Answer: Option 1 — It allows the bending of various parts of a plant like tendrils and stems of climbers without breaking.
This statement actually describes collenchyma, not parenchyma. Collenchyma has living cells whose walls are irregularly thickened at the corners with cellulose and pectin; this combination of strength with elasticity gives plant parts flexibility, letting tendrils and the stems of climbers bend without breaking.
Parenchyma is the most common and least specialised simple permanent tissue. Its cells are living, thin-walled, isodiametric and loosely packed with large intercellular spaces. It fills up space, and mainly stores food and water. Chlorophyll-containing parenchyma is called chlorenchyma and performs photosynthesis; parenchyma with large air cavities in aquatic plants is called aerenchyma and gives buoyancy.
❌ Why the other options are incorrect
✗ Living and loosely arranged cells — a correct statement; this loose packing with intercellular spaces is a defining feature of parenchyma.
✗ Large air cavities in aquatic plants — correct; this modification is aerenchyma, which helps water plants float.
✗ Relatively unspecialised cells with thin cell walls — correct; parenchyma is the most unspecialised of all permanent tissues and can even regain the power to divide.
📌 SSC & Railway Exam Facts
• Simple permanent tissues — Parenchyma (packing/storage), Collenchyma (flexibility), Sclerenchyma (strength).
• Collenchyma is found just below the epidermis in the stems and leaf stalks of dicots.
• Sclerenchyma cells are dead and lignified; parenchyma and collenchyma cells are living.
• Aerenchyma in hydrophytes such as the lotus provides buoyancy.
• Complex permanent tissues are xylem (water) and phloem (food).

Correct Answer: Option 4 — Blood platelets
Blood platelets, also called thrombocytes, are tiny, colourless, nucleus-free fragments of large bone-marrow cells. They circulate all over the body and, the moment a vessel is cut, they collect at the injury site and release clotting factors.
These factors convert the soluble plasma protein fibrinogen into insoluble threads of fibrin, which form a mesh over the wound and trap blood cells — this is the clot. The clot seals the leak, so the blood pressure needed by the pumping system is not lost and germs are kept out. Vitamin K and calcium are essential for this process.
❌ Why the other options are incorrect
✗ Red blood corpuscles — they carry oxygen with the help of haemoglobin; they may get trapped in the fibrin mesh but play no active part in clotting.
✗ Lymph — a colourless tissue fluid that carries lymphocytes and digested fats and returns fluid to the blood; it is not a clotting agent.
✗ White blood corpuscles — the defence cells of the body; they engulf germs and make antibodies, protecting against infection, not against bleeding.
📌 SSC & Railway Exam Facts
• Normal platelet count — about 1.5 to 4.5 lakh per cubic millimetre of blood.
• Dengue fever causes a sharp fall in the platelet count.
• Haemophilia is a genetic disease in which blood does not clot; it is a sex-linked disorder.
• Vitamin K is the anti-haemorrhagic vitamin; heparin is a natural anticoagulant.
• RBC ≈ 5 million/mm³, WBC ≈ 5,000–10,000/mm³; the WBC : RBC ratio is about 1 : 600.

Correct Answer: Option 2 — Haemoglobin
Haemoglobin is the red respiratory pigment packed inside the red blood corpuscles (RBCs). It is a conjugated protein made of the protein globin and the iron-containing pigment haem, so iron is its central metal atom — and that is what gives blood its red colour.
Haemoglobin has a very high affinity for oxygen. In the lungs it combines with oxygen to form oxyhaemoglobin, and in the tissues, where oxygen pressure is low, it releases the oxygen. It also carries a small share of carbon dioxide back as carbaminohaemoglobin. Mammalian RBCs are biconcave and lack a nucleus, which makes room for more haemoglobin.
❌ Why the other options are incorrect
✗ Xanthophyll — a yellow plant pigment of the carotenoid group; it gives autumn leaves their yellow colour and is not found in blood.
✗ Chlorophyll — the green plant pigment of the chloroplast that traps sunlight for photosynthesis; its central metal atom is magnesium.
✗ Melanin — the brown-black pigment of the skin, hair and eyes; it protects the skin from ultraviolet rays but has no role in oxygen transport.
📌 SSC & Railway Exam Facts
• Normal haemoglobin — about 14–16 g/dL in men and 12–14 g/dL in women.
• Deficiency of haemoglobin causes anaemia; iron and folic acid help correct it.
• Carbon monoxide binds haemoglobin about 200–250 times more strongly than oxygen, causing suffocation.
• RBCs are formed in the bone marrow and destroyed in the spleen, the graveyard of RBCs.
• Chlorophyll centre — magnesium; haemoglobin centre — iron; a classic PYQ pairing.

Correct Answer: Option 1 — Both contain their own DNA and ribosomes.
Plastids and mitochondria are both double-membrane organelles, and both carry their own circular DNA and their own ribosomes. Because of this they can synthesise some of their own proteins and can even divide on their own, independently of nuclear division. Organelles of this kind are called semi-autonomous organelles.
This shared feature is the basis of the endosymbiotic theory, which proposes that both organelles were once free-living prokaryotes that were engulfed by an early eukaryotic cell and stayed on as permanent partners.
❌ Why the other options are incorrect
✗ Both produce ATPs — chloroplasts do make ATP during the light reaction, but leucoplasts and chromoplasts do not; the powerhouse role of producing ATP for the whole cell belongs to the mitochondrion.
✗ Both are present only in plant cells — plastids are restricted to plant cells, but mitochondria occur in all eukaryotic cells, plant and animal alike.
✗ Both contain chlorophyll — only the chloroplast type of plastid contains chlorophyll; mitochondria never do, and neither do leucoplasts.
📌 SSC & Railway Exam Facts
• Three types of plastids — chloroplast (green, photosynthesis), chromoplast (colour of flowers and fruits), leucoplast (colourless, stores starch, oil and protein).
• Chloroplast — the kitchen of the cell; mitochondrion — the powerhouse of the cell.
• The inner membrane of the mitochondrion is folded into cristae; the chloroplast has stacks of grana in the stroma.
• Mitochondrial DNA is inherited only from the mother.
• Cell organelles with no membrane — ribosomes; with a single membrane — lysosome, vacuole, Golgi body, ER.

Correct Answer: Option 2 — Vacuoles
Meristematic tissue is the dividing tissue of a plant. Its cells are young, actively dividing, closely packed with no intercellular spaces, and each has a thin cell wall, dense cytoplasm and a prominent nucleus. Because the whole cell is filled with dense cytoplasm needed for rapid division, meristematic cells have no vacuoles.
Vacuoles appear only later, as the cells mature and differentiate into permanent tissue; the mature parenchyma cell eventually develops a large central vacuole. Meristem is of three types — apical (at root and shoot tips, increases length), lateral or cambium (increases girth) and intercalary (at the base of leaves and internodes).
❌ Why the other options are incorrect
✗ Cytoplasm — very much present; in fact meristematic cells have dense cytoplasm, which is one of their identifying features.
✗ Nuclei — present and prominent; a dividing cell must have a nucleus, because division begins with the division of the nucleus.
✗ Cell walls — present, though thin and made of cellulose; every plant cell, meristematic or permanent, has a cell wall.
📌 SSC & Railway Exam Facts
• Growth in plants occurs only at certain points — this is called localised growth, and it is done by meristems.
• Apical meristem → primary growth (length); lateral meristem (cambium) → secondary growth (girth).
• Intercalary meristem is why grass keeps growing after being cut or grazed.
• Meristematic cells later differentiate into parenchyma, collenchyma, sclerenchyma, xylem and phloem.
• Annual rings in a tree trunk are produced by the vascular cambium and reveal the tree's age.

Correct Answer: Option 4 — Resistant to heat
This is the standard NCERT illustration of variation and natural selection. A population of bacteria is never uniform — small variations arise during DNA copying at reproduction. If the water is heated by global warming, only those few variants that happen to be heat-resistant can survive the new condition; the rest are killed.
The surviving heat-resistant bacteria then reproduce and multiply, so their proportion in the population rises generation after generation. Nature has, in effect, selected them. This is exactly how variation gives a survival advantage and becomes the raw material of evolution, as proposed by Charles Darwin in his theory of natural selection.
❌ Why the other options are incorrect
✗ Resistant to wind pressure — the bacteria live inside water, where wind pressure is not the selecting factor; the change described is a change in temperature.
✗ Resistant to cold waves — this would help if the water became colder; here the water is getting hotter, so cold resistance gives no advantage.
✗ Resistant to water — meaningless for an aquatic organism, since water is its normal habitat, not a stress.
📌 SSC & Railway Exam Facts
• Variation arises from errors in DNA copying and from sexual reproduction.
• Charles Darwin wrote 'On the Origin of Species' (1859) and gave natural selection; Lamarck gave the theory of inheritance of acquired characters.
• Survival of the fittest was the phrase coined by Herbert Spencer.
• Genetic drift changes gene frequency by chance, without any survival advantage.
• Speciation is the formation of a new species; the founder of modern genetics is Gregor Johann Mendel.

Correct Answer: Option 3 — Urethra – a pair
The urethra is a single, unpaired tube, not a pair. It is the final passage of the excretory system, carrying urine from the urinary bladder to the outside of the body. In the male it is longer (about 20 cm) and also carries semen; in the female it is short (about 4 cm) and carries urine only. So this pair is incorrectly matched.
The complete human excretory system is: a pair of kidneys, a pair of ureters, one urinary bladder and one urethra.
❌ Why the other options are incorrect
✗ Urinary bladder – unpaired — correctly matched; there is only one muscular bladder, which stores urine until it is voluntarily released by the urethral sphincter.
✗ Ureter – a pair — correctly matched; two ureters, one from each kidney, carry urine down into the bladder.
✗ Kidney – a pair — correctly matched; two bean-shaped kidneys lie on either side of the backbone, the right one slightly lower because of the liver.
📌 SSC & Railway Exam Facts
• The nephron is the structural and functional unit of the kidney; each kidney has about one million of them.
• Urine formation has three steps — glomerular filtration, selective reabsorption and tubular secretion.
• Urea is formed in the liver and excreted by the kidneys; humans are ureotelic.
• An adult passes about 1–1.8 litres of urine a day; urine is roughly 95% water.
• Kidney failure is treated by haemodialysis with an artificial kidney, or by a kidney transplant.

Correct Answer: Option 3 — Oestrogen
Oestrogen is the female sex hormone, secreted mainly by the ovary. It brings about the growth and development of the female reproductive organs — uterus, fallopian tubes and vagina — and produces the secondary sexual characters at puberty: enlargement of the breasts, broadening of the hips, a high-pitched voice and growth of hair in the armpits and the pubic region.
Together with progesterone, oestrogen also regulates the menstrual cycle: it rebuilds the uterine lining (endometrium) every month after menstruation and prepares it to receive a fertilised egg. The whole process is under the control of FSH and LH from the pituitary gland.
❌ Why the other options are incorrect
✗ Testosterone — the male sex hormone from the testes; it develops male reproductive organs and male secondary characters such as a deep voice and facial hair.
✗ Thyroxin — from the thyroid gland; it regulates the metabolism of carbohydrates, fats and proteins and needs iodine for its formation.
✗ Adrenaline — the emergency or fight-or-flight hormone from the adrenal gland; it raises heart rate, breathing rate and blood pressure in fear or stress.
📌 SSC & Railway Exam Facts
• Progesterone is the pregnancy hormone; it maintains the uterine lining after fertilisation.
• The menstrual cycle lasts about 28 days; ovulation occurs around the 14th day.
• Deficiency of thyroxin causes goitre; iodised salt prevents it.
• Insulin from the pancreas lowers blood sugar; its deficiency causes diabetes mellitus.
• The pituitary is the master gland, but it is itself controlled by the hypothalamus.

Correct Answer: Option 1 — Plants have low energy needs and use relatively slow transport systems.
Plants do not move about, and a large part of the plant body is made of dead cells such as xylem vessels and sclerenchyma. Both facts mean that a plant spends far less energy than an animal of the same size. Since its energy demand is low, a plant does not need a fast pumping heart — a slow transport system driven mostly by transpiration pull and root pressure is quite enough.
The plant has two conducting tissues: xylem, which carries water and minerals upward from the roots, and phloem, which carries food (sucrose) from the leaves to all other parts by translocation using ATP.
❌ Why the other options are incorrect
✗ Distances very large in herbs and shrubs — the opposite; in small herbs and shrubs the distances are short. Long-distance transport is the problem of tall trees.
✗ Phloem moves water and minerals from the soil — wrong; that is the job of the xylem. Phloem transports prepared food.
✗ Xylem transports products of photosynthesis — wrong; the movement of photosynthetic products from leaves to other parts is translocation, carried out by the phloem.
📌 SSC & Railway Exam Facts
• Xylem — tracheids, vessels, xylem parenchyma, xylem fibres; transport is unidirectional (upward).
• Phloem — sieve tubes, companion cells, phloem parenchyma, phloem fibres; transport is bidirectional.
• Transpiration is loss of water as vapour, mainly through stomata; the suction created is the transpiration pull.
• Transpiration also cools the plant and helps absorb minerals — it is called a necessary evil.
• Translocation in phloem needs energy (ATP); movement in xylem is passive.

Correct Answer: Option 2 — radicle
Inside a seed lies the embryo, the tiny future plant. The embryo has three parts: the plumule, which grows into the shoot (stem and leaves), the radicle, which grows into the root, and one or two cotyledons (seed leaves) that store food for the young seedling.
During germination the radicle emerges first, pushes down into the soil and anchors the seedling while absorbing water and minerals; the plumule then grows upward into the light. Germination needs the right supply of water, oxygen and a suitable temperature.
❌ Why the other options are incorrect
✗ Sepal — the green outermost whorl of a flower; it protects the flower bud and is part of the flower, not of the embryo.
✗ Cotyledon — the seed leaf that stores food for the germinating embryo; it is neither the future root nor the future shoot.
✗ Petal — the coloured, often scented whorl of a flower whose work is to attract insects for pollination.
📌 SSC & Railway Exam Facts
• Monocots have one cotyledon (maize, wheat, rice); dicots have two (gram, pea, bean).
• After fertilisation, the ovule becomes the seed and the ovary becomes the fruit.
• The seed coat has two layers — the outer testa and the inner tegmen.
• A fruit formed without fertilisation is a parthenocarpic fruit, e.g. seedless banana.
• The food store of a maize grain is the endosperm; in gram it is the cotyledons.

Correct Answer: Option 2 — squamous epithelium
Squamous epithelium is made of extremely thin, flat, plate-like cells joined edge to edge like floor tiles. This forms a smooth, delicate lining over surfaces, which is exactly what is needed where food has to slide past without friction.
That is why the lining of the mouth and the oesophagus is squamous epithelium. The same tissue also lines the blood vessels, the lung alveoli and the kidney tubules — places where a very thin barrier is needed. Where wear and tear is heavy, as in the skin, these flat cells are stacked in many layers and the tissue is then called stratified squamous epithelium.
❌ Why the other options are incorrect
✗ Cuboidal epithelium — cube-shaped cells forming the lining of kidney tubules and salivary gland ducts; it provides mechanical support and helps in secretion and absorption.
✗ Ciliated columnar epithelium — tall cells bearing cilia, lining the respiratory tract; the cilia beat to push mucus and dust out of the airway.
✗ Glandular epithelium — epithelium folded inward to form glands that secrete enzymes, hormones, sweat and mucus; it is a secretory, not a simple lining, tissue.
📌 SSC & Railway Exam Facts
• The four basic animal tissues — epithelial, connective, muscular, nervous.
• Epithelium always rests on a basement membrane and has no blood supply of its own.
• Simple squamous is one cell thick, ideal for diffusion, as in the alveoli.
• Columnar epithelium with microvilli in the small intestine aids absorption.
• The oesophagus carries food to the stomach by peristalsis, a wave of muscular contraction.

Correct Answer: Option 4 — Phototropism – movement towards water
Phototropism is movement in response to light, not water. The shoot bends towards light, which is positive phototropism, while the root bends away from light, which is negative phototropism. The bending is caused by the hormone auxin: it accumulates on the shaded side of the shoot and makes those cells elongate more, so the tip curves towards the light. Hence statement 4 is incorrect.
A tropic movement is a directional growth movement of a plant part in response to an external stimulus, and it is always slow because it is brought about by growth, not by muscles.
❌ Why the other options are incorrect
✗ Hydrotropism – movement towards water — a correct match; roots grow towards a source of water, showing positive hydrotropism.
✗ Chemotropism – movement towards chemical — a correct match; the classic example is the growth of the pollen tube towards the ovule through the style.
✗ Geotropism – response to the pull of earth or gravity — a correct match; the root grows downward (positive) and the shoot upward (negative) geotropism.
📌 SSC & Railway Exam Facts
• Thigmotropism — growth movement in response to touch, seen in the tendrils of climbers.
• Nastic movements are non-directional, e.g. the drooping of Mimosa pudica (touch-me-not) leaves — that is thigmonasty, not tropism.
• Auxin is the hormone behind phototropism; it is made at the shoot tip.
• Plants show no nervous system; they coordinate through chemicals (hormones) and electrical-chemical signals.
• Movement in *Mimosa* occurs by a change in the water content of the cells, and it is much faster than a tropic movement.

Correct Answer: Option 4 — Alkenes and alkynes are unsaturated hydrocarbons
A hydrocarbon is saturated when every carbon–carbon linkage is a single bond, so each carbon holds the maximum possible number of hydrogen atoms. It is unsaturated when it contains at least one double or triple bond between carbon atoms, i.e. fewer hydrogens than the maximum.
• Alkanes — CₙH₂ₙ₊₂ — only single bonds → saturated (methane CH₄, ethane C₂H₆, butane C₄H₁₀).
• Alkenes — CₙH₂ₙ — one C=C double bond → unsaturated (ethene C₂H₄).
• Alkynes — CₙH₂ₙ₋₂ — one C≡C triple bond → unsaturated (ethyne C₂H₂).
Unsaturated hydrocarbons are chemically more reactive: they undergo addition reactions, for example adding H₂ over a nickel catalyst to become saturated — the reaction used to make vanaspati ghee from vegetable oils.
❌ Why the other options are incorrect
✗ Methane, ethane and butane are unsaturated hydrocarbons — all three are alkanes with only single bonds, hence saturated.
✗ Alkane, alkenes and alkynes are unsaturated hydrocarbons — it wrongly includes alkanes, the very definition of a saturated family.
✗ Cyclo pentane and hexane are unsaturated hydrocarbons — both contain only single bonds; a ring does not make a compound unsaturated.
📌 SSC & Railway Exam Facts
• Saturated → addition is not possible, only substitution (with Cl₂ in sunlight).
• Unsaturated → addition reactions; they decolourise bromine water, the standard test.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones with a yellow sooty flame.
• Hydrogenation of oils uses a nickel (Ni) catalyst and converts unsaturated oils to saturated fats.
• Ethene (C₂H₄) ripens fruit; ethyne (C₂H₂) gives the oxy-acetylene welding flame.

Correct Answer: Option 1 — Solid
How much a substance changes when pressure is applied depends on its compressibility, and compressibility depends on the space between the particles.
In a solid the particles are packed extremely close together and the force of attraction between them is maximum. There is almost no empty space to squeeze out, so a solid is practically incompressible and keeps its definite shape and definite volume even under large pressure. That rigidity is exactly why a solid shows the least change in shape.
In contrast, gases have huge interparticle spaces and are highly compressible — this is what allows large volumes of LPG and CNG to be stored in small cylinders, and oxygen to be compressed for hospital cylinders.
The order of compressibility is gases > liquids > solids.
❌ Why the other options are incorrect
✗ Gas — a gas is the most compressible state; it has neither fixed shape nor fixed volume, so it changes the most.
✗ Plasma — plasma is an ionised gas (the fourth state of matter); like a gas it has no definite shape at all.
✗ Liquid — a liquid has a fixed volume but no fixed shape; it flows and takes the shape of its container.
📌 SSC & Railway Exam Facts
• Compressibility: gas > liquid > solid; rigidity: solid > liquid > gas.
• Interparticle force: solid > liquid > gas; interparticle space: gas > liquid > solid.
• Plasma is the fourth state of matter — found in the sun, stars, neon signs and fluorescent tubes.
• BEC (Bose–Einstein Condensate) is the fifth state, formed at temperatures near absolute zero (0 K).
• Absolute zero = 0 K = −273.15 °C; K = °C + 273.

Correct Answer: Option 3 — 1 atmosphere
Solid carbon dioxide (CO₂) is stored under high pressure. When the pressure is decreased to 1 atmosphere, the solid changes directly into carbon dioxide gas without ever becoming a liquid. A change of state from solid directly to gas (or gas directly to solid) is called sublimation.
Because it leaves no liquid residue — no wetness at all — solid CO₂ is popularly called dry ice. It sublimes at about −78 °C, which makes it an excellent refrigerant for storing ice cream, frozen food, blood and vaccines, and it is also used to create artificial smoke effects on stage.
This behaviour also shows how state depends on both pressure and temperature: increasing pressure brings particles closer, decreasing pressure lets them fly apart.
❌ Why the other options are incorrect
✗ 10 atmosphere — CO₂ is still held as a solid/liquid at such raised pressure; direct conversion to gas has not yet taken place.
✗ 20 atmosphere — even higher pressure keeps the particles tightly packed; this is closer to the storage pressure, not the subliming pressure.
✗ 5 atmosphere — still above normal atmospheric pressure; the change described happens only when pressure falls to the ordinary 1 atm.
📌 SSC & Railway Exam Facts
• Solid CO₂ = dry ice; it sublimes at about −78 °C at 1 atm.
• Other subliming solids: camphor, naphthalene, ammonium chloride (NH₄Cl) and iodine.
• Sublimation is used to separate a sublimable solid from a non-sublimable one, e.g. NH₄Cl from common salt.
• CO₂ is used in fire extinguishers and in aerated drinks; it is also a major greenhouse gas.
• State of matter depends on temperature and pressure; increasing pressure can turn a gas into a liquid.

Correct Answer: Option 2 — C₂H₂
In the IUPAC system, the number of carbon atoms fixes the prefix (meth-, eth-, prop-, but-…) and the type of carbon–carbon bond fixes the suffix:
• -ane → all single bonds (alkane, CₙH₂ₙ₊₂)
• -ene → a double bond (alkene, CₙH₂ₙ)
• -yne → a triple bond (alkyne, CₙH₂ₙ₋₂)
Ethyne = eth (2 carbon atoms) + yne (triple bond) → an alkyne with n = 2, so CₙH₂ₙ₋₂ = C₂H₂. Its structure is H–C≡C–H, the simplest alkyne. Its common name is acetylene; burnt with oxygen it gives the oxy-acetylene flame at about 3000 °C, used for welding and cutting metals.
❌ Why the other options are incorrect
✗ C₂H₆ — this is ethane, an alkane (suffix -ane) with only a single C–C bond.
✗ CH₄ — this is methane, the first alkane; it has only one carbon, so it cannot be an "eth-" compound.
✗ C₂H₄ — this is ethene (ethylene), an alkene with a double bond; confusing -ene with -yne is the usual error.
📌 SSC & Railway Exam Facts
• Ethyne = acetylene = C₂H₂, H–C≡C–H, the first member of the alkyne series.
• CH₄ methane, C₂H₆ ethane, C₃H₈ propane, C₄H₁₀ butane — the first four alkanes.
• Oxy-acetylene flame ≈ 3000 °C; acetylene alone burns with a sooty flame due to incomplete combustion.
• Suffix guide: -ane (single), -ene (double), -yne (triple).
• Carbon shows catenation and tetravalency, which is why it forms millions of compounds.

Correct Answer: Option 2 — i, ii and iii
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Its particles are large — bigger than 1000 nm (100 nm in some texts) — and can be seen with the naked eye. Chalk powder in water, muddy water and paints are the standard examples. NCERT lists exactly the three properties given here.
(i) is correct — the particles stay as a separate visible phase, so the mixture is heterogeneous.
(ii) is correct — the large particles scatter a beam of light and make its path visible; this is the Tyndall effect, shown by suspensions and colloids but not by true solutions.
(iii) is correct — on standing undisturbed, the heavy particles settle down under gravity, so a suspension is unstable and can be separated by filtration.
❌ Why the other options are incorrect
✗ Only ii and iii — it wrongly rejects (i), yet heterogeneity is the very definition of a suspension.
✗ Only i — it rejects both the Tyndall effect and the settling of particles, which are the two most tested properties.
✗ Only i and ii — it wrongly rejects (iii); unlike a colloid, a suspension is unstable and does settle.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect is shown by colloids and suspensions, not by true solutions.
• A suspension can be separated by filtration; a colloid cannot — it needs centrifugation.
• Colloids are stable and do not settle; suspensions are unstable and settle on standing.
• Everyday Tyndall effect: sunbeam through a dense forest or through a slit into a dusty room.

Correct Answer: Option 2 — Hydrogen
Metals that lie above hydrogen in the reactivity series displace hydrogen from dilute acids, giving a salt and hydrogen gas. The general equation is
Metal + Dilute acid → Salt + Hydrogen gas
⚗️ The Reaction
Zn + 2HCl → ZnCl₂ + H₂↑
Mg + 2HCl → MgCl₂ + H₂↑
• This is a displacement reaction — the metal displaces H⁺ from the acid.
• The gas is confirmed by bringing a burning splinter near it: hydrogen burns with a pop sound.
Metals below hydrogen — such as copper, silver and gold — do not displace hydrogen and so give no reaction with dilute HCl. The vigour of the reaction follows the reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
❌ Why the other options are incorrect
✗ Oxygen — oxygen is released when hydrogen peroxide decomposes or at the anode during electrolysis of water, not from a metal–acid reaction.
✗ Chlorine — chlorine is obtained by electrolysis of brine (NaCl solution); the chloride ion stays in the salt here, it is not released as Cl₂.
✗ Nitrogen — nitrogen comes from the air or from nitrogenous compounds; HCl contains no nitrogen at all.
📌 SSC & Railway Exam Facts
• Metal + dilute acid → salt + H₂; the pop test with a burning splinter confirms hydrogen.
• Cu, Ag and Au are below hydrogen — they do not react with dilute HCl or dilute H₂SO₄.
• Nitric acid (HNO₃) normally does not give H₂, because it is a strong oxidising agent.
• Aqua regia = 3 parts conc. HCl + 1 part conc. HNO₃ — it dissolves gold and platinum.
• The reactivity series order: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.

Correct Answer: Option 3 — Copper sulphate
This is the classic NCERT activity on the displacement reaction. Iron (Fe) lies above copper (Cu) in the reactivity series, so iron is more reactive and displaces copper from a solution of copper sulphate (CuSO₄).
⚗️ The Reaction
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• The displaced copper deposits on the nail as a reddish-brown layer, which is exactly the "brownish" colour observed.
• The solution changes from blue (CuSO₄) to light green (FeSO₄).
• It is also a redox reaction: Fe is oxidised (loses electrons) and Cu²⁺ is reduced.
❌ Why the other options are incorrect
✗ Zinc sulphate — zinc is more reactive than iron, so iron cannot displace zinc; no deposit and no colour change occurs.
✗ Lead sulphate — lead sulphate (PbSO₄) is insoluble in water, so no such solution reaction is possible; besides, lead would deposit greyish, not brown.
✗ Iron sulphate — the nail and the solution contain the same metal, so there is nothing to displace; no reaction at all.
📌 SSC & Railway Exam Facts
• Fe + CuSO₄ → FeSO₄ + Cu: blue solution turns green, nail turns brown.
• A more reactive metal displaces a less reactive metal from its salt solution.
• Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag: colourless solution turns blue, silver deposits.
• CuSO₄·5H₂O is blue vitriol; FeSO₄·7H₂O is green vitriol; ZnSO₄·7H₂O is white vitriol.
• In redox, oxidation = loss of electrons (OIL), reduction = gain of electrons (RIG).

Correct Answer: Option 2 — Only a and b
In classical terms, reduction is the removal of oxygen from a substance or the addition of hydrogen to it. Oxidation is the exact opposite — addition of oxygen or removal of hydrogen. In electronic terms, reduction is the gain of electrons and oxidation is the loss of electrons.
So statements (a) Loss of oxygen and (b) Gain of hydrogen describe reduction correctly.
⚗️ The Reaction
CuO + H₂ → (heat) Cu + H₂O
• CuO loses oxygen → it is reduced to Cu; H₂ here acts as the reducing agent.
• H₂ gains oxygen → it is oxidised to H₂O; CuO acts as the oxidising agent.
• Oxidation and reduction always occur together, so such a reaction is called a redox reaction.
❌ Why the other options are incorrect
✗ Only a and d — (a) is right but (d) loss of hydrogen is oxidation, not reduction.
✗ Only b and c — (b) is right but (c) gain of oxygen is oxidation; the two halves have been swapped.
✗ Only d — loss of hydrogen is oxidation, so this option is entirely wrong.
📌 SSC & Railway Exam Facts
• Reduction = loss of O₂ / gain of H₂ / gain of electrons.
• Oxidation = gain of O₂ / loss of H₂ / loss of electrons. Remember OIL RIG.
• In a redox reaction the oxidising agent is itself reduced and the reducing agent is itself oxidised.
• Corrosion and rancidity are everyday examples of oxidation.
• Rancidity is delayed by adding antioxidants and by packing food in nitrogen gas.

Correct Answer: Option 2 — Thermal decomposition
When a single compound breaks down into two or more simpler substances on being heated, the change is called thermal decomposition. The heat supplies the energy needed to break the bonds, so such reactions are endothermic.
⚗️ The Reaction
CaCO₃(s) → (heat) CaO(s) + CO₂(g)
• Limestone (calcium carbonate, CaCO₃) is heated to about 1000 °C in a lime kiln — the process is called calcination.
• Quick lime (calcium oxide, CaO) is left behind and carbon dioxide escapes as a gas.
• CaO with water gives slaked lime, Ca(OH)₂, an exothermic reaction used for whitewashing.
❌ Why the other options are incorrect
✗ Reduction reaction — nothing here gains hydrogen or loses oxygen; CaCO₃ simply splits into two products.
✗ Photochemical decomposition — that requires light as the energy source, as in 2AgCl → 2Ag + Cl₂ in sunlight; here the energy is heat.
✗ Oxidation reaction — no substance gains oxygen or loses hydrogen; in fact oxygen leaves the compound as part of CO₂.
📌 SSC & Railway Exam Facts
• CaCO₃ → CaO + CO₂ is the classic thermal decomposition; industrially it is calcination.
• CaO = quick lime, Ca(OH)₂ = slaked lime, CaCO₃ = limestone / marble / chalk.
• Whitewash shines because Ca(OH)₂ + CO₂ → CaCO₃ + H₂O forms a hard CaCO₃ layer.
• 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ gives brown fumes of NO₂ on heating.
• Three kinds of decomposition: thermal (heat), photolytic (light) and electrolytic (electricity).

Correct Answer: Option 1 — 24
The molecular formula of glucose is C₆H₁₂O₆. A molecular formula tells us exactly how many atoms of each element are present in one molecule of the compound, so we simply add the subscripts:
• Carbon (C) = 6 atoms
• Hydrogen (H) = 12 atoms
• Oxygen (O) = 6 atoms
Total atoms = 6 + 12 + 6 = 24 atoms in one molecule of glucose.
Glucose is a monosaccharide (simple sugar) produced in photosynthesis and is the body's chief source of instant energy. Its molecular mass is 180 u (6×12 + 12×1 + 6×16), and its empirical formula is CH₂O.
❌ Why the other options are incorrect
✗ 18 — this is the total only of C + H (6 + 12); the 6 oxygen atoms have been left out.
✗ 6 — this is only the number of carbon atoms (or of oxygen atoms), not the total of all atoms.
✗ 12 — this is only the number of hydrogen atoms in the molecule.
📌 SSC & Railway Exam Facts
• Glucose = C₆H₁₂O₆, 24 atoms, molecular mass 180 u, empirical formula CH₂O.
• Sucrose = C₁₂H₂₂O₁₁ (45 atoms); fructose also has the formula C₆H₁₂O₆ — an isomer of glucose.
• Photosynthesis: 6CO₂ + 6H₂O → (sunlight, chlorophyll) C₆H₁₂O₆ + 6O₂.
• Empirical formula gives the simplest whole-number ratio; the molecular formula gives the actual number of atoms.
• One mole of any substance contains 6.022 × 10²³ particles — the Avogadro number.

Correct Answer: Option 2 — Emulsion
A colloid is a heterogeneous mixture whose particles are 1–1000 nm in size — too small to settle, but large enough to scatter light (Tyndall effect). Colloids are classified from the physical state of the dispersed phase and the dispersing medium.
When both the dispersed phase and the dispersing medium are liquids, the colloid is called an emulsion. Face cream, like milk, butter and cold cream, is a liquid dispersed in another liquid, so it is an emulsion. A substance added to keep such a mixture from separating is called an emulsifying agent — soap acts as one when it cleans oily dirt.
❌ Why the other options are incorrect
✗ Gel — a gel is a liquid dispersed in a solid, such as cheese, butter-jelly, curd or table jelly; it is semi-solid and holds its shape.
✗ Solid — "solid" is a state of matter, not a class of colloid; a solid-in-solid colloid would be a solid sol (e.g. coloured gemstone).
✗ Foam — a foam is a gas dispersed in a liquid, such as shaving cream, soap lather or whipped cream.
📌 SSC & Railway Exam Facts
• Emulsion = liquid in liquid — milk, face cream, cold cream.
• Gel = liquid in solid — cheese, butter, jelly; Foam = gas in liquid — shaving cream, soda froth.
• Aerosol = liquid or solid in gas — fog, clouds, mist, smoke; Sol = solid in liquid — paint, ink, blood.
• Milk is an emulsion of liquid fat in water; soap acts as an emulsifying agent.
• Colloid particle size 1–1000 nm; colloids show the Tyndall effect and are separated by centrifugation.

Correct Answer: Option 3 — 6.66%
The mass by mass percentage of a solution is
Mass % = (Mass of solute / Mass of solution) × 100, where Mass of solution = Mass of solute + Mass of solvent.
⚗️ The Calculation
• Mass of solute (common salt) = 20 g; mass of solvent (water) = 280 g.
• Mass of solution = 20 + 280 = 300 g.
• Mass % = (20/300) × 100 = 2000/300 = 6.66%.
So the solution is 6.66% (by mass) common salt. Note that the solvent mass alone is never used in the denominator — the mass of the whole solution must be taken.
❌ Why the other options are incorrect
✗ 66.6% — this is ten times the right value; it comes from a decimal-point slip while dividing 2000 by 300.
✗ 0.66% — again a decimal error, ten times too small; here the ×100 has effectively been applied wrongly.
✗ 11.1% — this is (20/180) × 100 or a similar wrong denominator; the commonest trap is dividing by 280 g of water instead of the 300 g of solution — that would give 7.14%, and using an even smaller denominator gives 11.1%.
📌 SSC & Railway Exam Facts
• Mass % = (mass of solute / mass of solution) × 100; solution = solute + solvent.
• Volume % = (volume of solute / volume of solution) × 100.
• Common salt = sodium chloride = NaCl; obtained from sea water and rock salt.
• Saturated solution: no more solute dissolves at that temperature; solubility usually rises with temperature.
• Water is the universal solvent; a solution of a metal in a metal is an alloy.

Correct Answer: Option 2 — Magnesium (Mg) and manganese (Mn)
Nitric acid (HNO₃) is a strong oxidising agent. When a metal reacts with it, the hydrogen produced is normally oxidised straight to water, while the acid itself is reduced to nitrogen oxides (N₂O, NO, NO₂). That is why metals with HNO₃ usually give no hydrogen gas.
Magnesium (Mg) and manganese (Mn) are the standard exceptions given in NCERT: with very dilute nitric acid these two metals do evolve hydrogen gas, because the acid is too dilute to act effectively as an oxidiser.
⚗️ The Reaction
Mg + 2HNO₃ (very dilute) → Mg(NO₃)₂ + H₂↑
Mn + 2HNO₃ (very dilute) → Mn(NO₃)₂ + H₂↑
• The gas burns with a pop sound, the standard test for hydrogen.
❌ Why the other options are incorrect
✗ Copper (Cu) and cobalt (Co) — copper lies below hydrogen in the reactivity series and cannot displace hydrogen from any acid.
✗ Manganese (Mn) and cobalt (Co) — Mn is right, but cobalt is not one of the two textbook exceptions.
✗ Magnesium (Mg) and copper (Cu) — Mg is right, but copper cannot give H₂; with dilute HNO₃ copper gives NO, and with concentrated HNO₃ it gives NO₂.
📌 SSC & Railway Exam Facts
• Only Mg and Mn give H₂ with very dilute HNO₃ — a classic one-liner.
• HNO₃ is a strong oxidising agent; it normally gives NO₂, NO or N₂O instead of H₂.
• Metals above hydrogen in the reactivity series displace H₂ from dilute HCl and dilute H₂SO₄.
• Aqua regia (3 HCl : 1 HNO₃) dissolves gold and platinum.
• Hydrogen is confirmed by the pop test; it is the lightest element and burns to give only water.

Correct Answer: Option 4 — Potential energy
When a rubber band is stretched, work has to be done against the restoring force of the band. This work is stored inside the band because of the change in its shape (configuration). Energy stored in a body due to its position or configuration is called potential energy, and the form stored in a stretched or compressed elastic body is called elastic potential energy.
The moment the band is released, this stored potential energy changes into kinetic energy and the band — or the stone placed in a catapult — flies off. The same idea explains a wound spring in a clock, a stretched bow and water stored in a dam.
❌ Why the other options are incorrect
✗ Light energy — a stretched rubber band does not glow; light energy comes from luminous sources such as the Sun, a bulb or a flame.
✗ Thermal energy — heat energy is due to the random motion of molecules; only a negligible amount appears here, so it is not the energy acquired by stretching.
✗ Kinetic energy — this is the energy of a body in motion; the stretched band is held at rest, so it has no kinetic energy until it is released.
📌 SSC & Railway Exam Facts
• Potential energy = mgh for a body raised to a height h; SI unit joule (J).
• Elastic potential energy of a spring = ½kx², where k is the spring constant.
• Kinetic energy = ½mv²; a stretched bow, a wound spring and a dam all store potential energy.
• Law of conservation of energy — energy can neither be created nor destroyed, only changed from one form into another.
• 1 joule = 1 N m; the commercial unit of energy is 1 kWh = 3.6 × 10⁶ J.

Correct Answer: Option 2 — Light
In an electric bulb the current passing through the thin tungsten filament heats it to about 2500 °C, and the white-hot filament begins to glow. Thus the electrical energy supplied by the supply line is converted into light energy, together with a large amount of heat.
This is the heating and lighting effect of electric current, governed by Joule's law of heating, H = I²Rt. An ordinary incandescent bulb is very inefficient — only about 5 % of the electrical energy becomes light and the rest is wasted as heat; that is why LED and CFL lamps have replaced it.
❌ Why the other options are incorrect
✗ Sound — a bulb produces no sound; electrical energy becomes sound energy in a loudspeaker, an electric bell or a buzzer.
✗ Wind — moving air carries mechanical energy; a bulb creates no air movement. It is an electric fan that turns electrical energy into mechanical energy.
✗ Potential — potential energy depends on position or configuration; a glowing filament stores no such energy.
📌 SSC & Railway Exam Facts
• The filament of a bulb is made of tungsten (melting point 3380 °C) and the bulb is filled with inert gases such as argon and nitrogen.
• Joule's law of heating: H = I²Rt — the electric fuse works on this principle.
• Energy conversions: loudspeaker electrical → sound; electric motor electrical → mechanical; electric cell chemical → electrical.
• Power of a bulb, P = VI = V²/R; SI unit watt (W).
• 1 unit of electricity = 1 kWh = 3.6 × 10⁶ J.

Correct Answer: Option 4 — 20 N
📖 Given
• Mass of the box, m = 10 kg
• Initial velocity, u = 6 m/s
• Final velocity, v = 10 m/s
• Time for which the force acts, t = 2 s
✍️ Formula
a = (v − u)/t and F = ma
🔢 Solution
• a = (10 − 6)/2
• a = 4/2 = 2 m/s²
• F = m × a = 10 × 2
• F = 20 N
So a steady push of 20 newton is enough to raise the speed of this 10 kg box by 4 m/s in 2 seconds.
❌ Why the other options are incorrect
✗ 40 N — this comes from taking the acceleration as 4 m/s², i.e. using the change in velocity (4 m/s) directly and forgetting to divide by the time 2 s.
✗ 50 N — this is 10 × (10/2); the final velocity has been used in place of the change in velocity.
✗ 25 N — no correct step gives this; it appears if the acceleration is wrongly taken as 2.5 m/s², for example by dividing 10 m/s by 4 s.
📌 SSC & Railway Exam Facts
• Newton's second law: F = ma = (mv − mu)/t — force equals the rate of change of momentum.
• SI unit of force is the newton (N); 1 N = 1 kg m/s², the force that gives a 1 kg mass an acceleration of 1 m/s².
• Impulse = F × t = change in momentum (mv − mu); here impulse = 20 × 2 = 40 kg m/s.
• Newton's first law defines inertia; the third law says action and reaction are equal and opposite.
• 1 kgf = 9.8 N; 1 dyne = 10⁻⁵ N.

Correct Answer: Option 4 — Both (i) and (ii)
A mirror that spreads out (diverges) a beam of light falling on it is a convex mirror. Its reflecting surface bulges outwards, so rays parallel to the principal axis, after reflection, appear to come from a focus behind the mirror. Hence statement (i) is true — a convex mirror is also called a diverging mirror.
In the New Cartesian sign convention the focus of a convex mirror lies behind the mirror, on the same side as the outgoing light, so its focal length is positive — statement (ii) is true. A convex mirror always forms a virtual, erect and diminished image, and a virtual image can never be caught on a screen, so statement (iii) is false.
❌ Why the other options are incorrect
✗ Only (i) — (i) is indeed true, but this option leaves out (ii); a convex mirror also has a positive focal length.
✗ Only(iii) — (iii) is the one statement that is false, because the image formed by a convex mirror is virtual.
✗ Both (ii) and (iii) — it wrongly includes the false statement (iii) and drops (i), the statement that the mirror is convex.
📌 SSC & Railway Exam Facts
• Convex mirror = diverging mirror; concave mirror = converging mirror.
• Sign convention: f is positive for a convex mirror and negative for a concave mirror.
• Convex mirrors are used as rear-view mirrors — erect, diminished image and a wide field of view.
• Real images can be taken on a screen; virtual images cannot.
• Mirror formula 1/v + 1/u = 1/f, magnification m = −v/u, and f = R/2.

Correct Answer: Option 3 — 0
The word "dropped" is the key to this question. When a body is simply dropped, it is released from rest — nobody throws it or pushes it downwards. Therefore its initial speed u = 0, whatever the height may be. The figure 50 m is extra data put there only to distract the candidate.
After release, gravity gives the body a constant downward acceleration g = 9.8 m/s², and its speed keeps increasing. Using v² = u² + 2gh = 0 + 2 × 9.8 × 50 = 980, it would hit the ground at v ≈ 31.3 m/s — but that is the final speed, not the initial one.
❌ Why the other options are incorrect
✗ 20 m/s — a dropped body has no starting speed; this value would apply only if the object were thrown downwards at 20 m/s.
✗ 50 m/s — this is simply the number 50 copied from the height; 50 m is a distance, not a speed, and the two cannot be interchanged.
✗ 25 m/s — again a thrown-down starting speed; nothing in the question says the object was projected with any speed.
📌 SSC & Railway Exam Facts
• "Dropped / released / let fall" ⇒ u = 0; "thrown / projected" ⇒ u ≠ 0.
• Free-fall equations (u = 0): v = gt, h = ½gt², v² = 2gh.
• g = 9.8 m/s² on the Earth (often taken as 10 m/s²) and is independent of the mass of the falling body.
• In a vacuum a feather and a coin fall together — the classic Galileo demonstration.
• Time of fall from height h: t = √(2h/g); here t = √(100/9.8) ≈ 3.2 s.

Correct Answer: Option 2 — The size of the scattering particles.
When a beam of light passes through a medium containing fine suspended particles — smoke, fog, dust or a colloid — the particles scatter the light sideways and the path of the beam becomes visible. This is the Tyndall effect, named after the British scientist John Tyndall.
The colour that is scattered most depends on the size of the scattering particles compared with the wavelength of light. Very fine particles scatter mainly blue (short-wavelength) light, which is why the sky looks blue; larger particles scatter all wavelengths almost equally, so the scattered light looks white, as in clouds, mist and fog. By Rayleigh's law the intensity of scattering is ∝ 1/λ⁴.
❌ Why the other options are incorrect
✗ The angle of refraction of the emerging light — refraction is the bending of light as it enters a new medium; scattering is a different phenomenon, and the colour scattered does not depend on this angle.
✗ The angle of incidence of the incoming light — changing the angle of incidence changes the direction of the beam, not the colour of the light scattered.
✗ The distance between the source of light and the scattering particles — a greater distance only makes the beam fainter; the colour of the scattered light stays the same.
📌 SSC & Railway Exam Facts
• Tyndall effect — scattering of light by colloidal particles that makes the path of a beam visible.
• The blue colour of the sky and the red colour of the Sun at sunrise and sunset are both due to scattering.
• Rayleigh scattering: intensity ∝ 1/λ⁴ — blue light is scattered about 16 times more than red.
• Danger signals are red because red has the longest wavelength and is scattered the least, so it is seen from far away.
• Outside the atmosphere there is nothing to scatter light, so the sky appears black to an astronaut.

Correct Answer: Option 4 — Ohm's law
Ohm's law, given by the German physicist Georg Simon Ohm in 1827, states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) applied across its ends, provided the temperature and other physical conditions remain constant.
So V ∝ I, which gives V = IR, where the constant of proportionality R is the resistance of the conductor, measured in ohm (Ω). For such a conductor a graph of V against I is a straight line passing through the origin, and materials that obey this relation are called ohmic conductors.
❌ Why the other options are incorrect
✗ Faraday's law — Michael Faraday's laws deal with electromagnetic induction (induced emf ∝ rate of change of magnetic flux) and with electrolysis, not with the V–I relation.
✗ Newton's law — Isaac Newton gave the three laws of motion and the law of universal gravitation, which have nothing to do with current and voltage.
✗ Joule's law — Joule's law of heating gives the heat produced by a current, H = I²Rt; it describes the heating effect, not the proportionality of V and I.
📌 SSC & Railway Exam Facts
• Ohm's law: V = IR; SI units — resistance ohm (Ω), current ampere (A), potential difference volt (V).
• 1 ohm = 1 volt/ampere; resistance R = ρl/A, where ρ is the resistivity of the material.
• Resistance increases with length and decreases with area of cross-section; for metals it increases with temperature.
• Semiconductor diodes, gases and filaments at high temperature are non-ohmic — they do not obey Ohm's law.
• Electric power P = VI = I²R = V²/R, and energy = Pt.

Correct Answer: Option 4 — 60 cm in front of the mirror
📖 Given
• Rays from the Sun are effectively parallel to the principal axis and meet 30 cm in front of the mirror
• The point where parallel rays meet is the principal focus, so f = 30 cm
• Required: the object position for which image size = object size
✍️ Formula
R = 2f; for a concave mirror an object placed at the centre of curvature (C) gives an image of the same size (magnification m = −1).
🔢 Solution
• f = 30 cm
• R = 2 × f = 2 × 30
• R = 60 cm
• So the object must be placed at C, i.e. 60 cm in front of the mirror
• Check: u = −60 cm, f = −30 cm → 1/v = 1/f − 1/u = −1/30 + 1/60 = −1/60, so v = −60 cm and m = −v/u = −1
The image is formed at C itself — real, inverted and exactly the same size as the object.
❌ Why the other options are incorrect
✗ 30 cm in front of the mirror — this is the focus F; an object placed at F gives an image at infinity, highly enlarged, not of equal size.
✗ Between 30 and 60 cm in front of the mirror — that is between F and C; the image then forms beyond C and is enlarged, not equal.
✗ More than 30 cm in front of the mirror — too vague; beyond C (more than 60 cm) the image is diminished, so only the exact distance of 60 cm works.
📌 SSC & Railway Exam Facts
• Sun's rays are parallel, so they converge exactly at the principal focus of a concave mirror; f = R/2.
• Object at C → image at C, real, inverted, same size (m = −1).
• Object beyond C → image between F and C, diminished; object between P and F → virtual, erect, enlarged.
• Mirror formula 1/v + 1/u = 1/f; magnification m = −v/u = h'/h.
• Concave mirrors are used in solar cookers, torches, headlights and shaving mirrors.

Correct Answer: Option 1 — The angle of incidence and refraction are equal.
This statement is wrong. When light passes from one transparent medium into another it bends — and bending means precisely that the angle of refraction (r) is different from the angle of incidence (i). The two are related by Snell's law: sin i / sin r = n, where n is the refractive index of the second medium with respect to the first.
The two angles become equal only in the trivial cases of normal incidence (i = r = 0°) or when both media are optically identical. "Angle of incidence = angle of reflection" is a law of reflection, and the question is about refraction, so this statement cannot be correct.
❌ Why the other options are incorrect
✗ The speed of light in the given medium is different for different colours — this is true; violet travels slowest and red fastest in glass, which is why white light splits into seven colours (dispersion) in a prism.
✗ The incident ray, refracted ray and normal to the surface at incidence lie in the same plane — true; this is the first law of refraction.
✗ The speed of light in different media is different — true; light travels at 3 × 10⁸ m/s in vacuum but more slowly in water and in glass.
📌 SSC & Railway Exam Facts
• Snell's law: n = sin i / sin r = c/v; refractive index has no unit.
• Refractive index of water = 1.33, glass = 1.5, diamond = 2.42 (highest among common materials).
• Light bends towards the normal entering a denser medium and away from the normal entering a rarer medium.
• Total internal reflection occurs from denser to rarer medium beyond the critical angle — glass ≈ 42°, water ≈ 48.6°, diamond ≈ 24°.
• Twinkling of stars, the apparent bending of a stick in water and the early sunrise are all effects of refraction.

Correct Answer: Option 2 — thrust
The force acting on a body perpendicular (normal) to a surface is called thrust. Since it is a force, it is a vector quantity and its SI unit is the newton (N). A block resting on a table, for example, presses on the table with a thrust equal to its own weight.
The effect of this thrust depends on the area over which it is spread. Thrust acting on unit area is called pressure: Pressure = Thrust/Area, with SI unit pascal (Pa) = N/m². This is why a sharp knife, a thin nail or a needle — all having a very small area — produce a very large pressure from the same thrust, while a camel's broad feet reduce the pressure on soft sand.
❌ Why the other options are incorrect
✗ pressure — pressure is thrust per unit area (P = F/A) and is measured in pascal; it is the *effect* of the force, not the force itself.
✗ buoyancy — buoyancy, or upthrust, is the upward force exerted by a fluid on a body immersed in it; here the force need not come from a fluid at all.
✗ weight — weight is the force of gravity on a body, W = mg, and always acts vertically downwards; it is perpendicular to a surface only when the surface happens to be horizontal.
📌 SSC & Railway Exam Facts
• Thrust — the force acting perpendicular to a surface; SI unit newton (N).
• Pressure = Thrust/Area; SI unit pascal (Pa), and 1 Pa = 1 N/m².
• Pressure inside a liquid at depth h is P = hρg, and it acts equally in all directions.
• Archimedes' principle — the upthrust on a body equals the weight of the fluid displaced by it.
• Atmospheric pressure ≈ 1.013 × 10⁵ Pa = 76 cm of mercury = 1 atm, measured by a barometer.

Correct Answer: Option 4 — P = 10 W, H = 600 J
📖 Given
• Resistance, R = 10 Ω
• Potential difference of the battery, V = 10 V
• Time, t = 1 minute = 60 s
✍️ Formula
P = V²/R and H = Pt = I²Rt
🔢 Solution
• Current, I = V/R = 10/10 = 1 A
• P = V²/R = (10)²/10 = 100/10
• P = 10 W (check: P = VI = 10 × 1 = 10 W)
• H = P × t = 10 × 60
• H = 600 J
So the resistor converts electrical energy into heat at a steady rate of 10 joule per second, giving 600 J in one minute.
❌ Why the other options are incorrect
✗ P = 100 W, H = 6000 J — here V² = 100 has been taken as the power without dividing by R; every value then comes out ten times too large.
✗ P = 20 W, H = 1200 J — double the correct answer; it appears if the resistance is wrongly taken as 5 Ω instead of 10 Ω.
✗ P = 1 W, H = 60 J — this is the current (V/R = 1 A) mistaken for the power; 1 A is not 1 W.
📌 SSC & Railway Exam Facts
• Electric power: P = VI = I²R = V²/R; SI unit watt (W).
• Joule's law of heating: H = I²Rt = VIt = Pt, measured in joule (J).
• Always convert the time first — 1 minute = 60 s, 1 hour = 3600 s.
• 1 kWh = 3.6 × 10⁶ J is the commercial 'unit' of electrical energy.
• The heating effect of current is used in the electric iron, heater, geyser, toaster and fuse.

Correct Answer: Option 4 — 20 W
📖 Given
• Potential difference of the generator, V = 200 V
• Current drawn by the bulb, I = 0.1 A
• To find: electric power of the bulb, P
✍️ Formula
P = VI
🔢 Solution
• P = 200 × 0.1
• P = 20 W
So this bulb uses 20 joule of electrical energy every second while it is switched on. Its resistance, by Ohm's law, is R = V/I = 200/0.1 = 2000 Ω.
❌ Why the other options are incorrect
✗ 10 W — half the correct value; it comes from taking the current as 0.05 A instead of 0.1 A.
✗ 200 W — this is just the voltage value copied down as the power; the current has not been multiplied at all.
✗ 2 W — a decimal-point slip: 200 × 0.01 = 2 W, using 0.01 A in place of 0.1 A.
📌 SSC & Railway Exam Facts
• Electric power: P = VI = I²R = V²/R; SI unit watt (W), named after James Watt.
• 1 watt = 1 volt × 1 ampere = 1 joule/second; 1 horsepower = 746 W.
• Electrical energy consumed = P × t; the commercial unit is 1 kWh = 1 unit = 3.6 × 10⁶ J.
• Ohm's law V = IR gives the bulb's resistance as 2000 Ω here.
• Household appliances are joined in parallel so that each gets the full supply voltage.

Correct Answer: Option 1 — 6400 km
The Earth is not a perfect sphere — it is an oblate spheroid, slightly flattened at the poles and bulging at the equator. Its mean radius is about 6371 km, which in physics numericals is always rounded to 6400 km, i.e. 6.4 × 10⁶ m.
This is the value of R used in g = GM/R² to get the acceleration due to gravity at the Earth's surface, g = 9.8 m/s². The equatorial radius is about 6378 km while the polar radius is about 6357 km; because R is smaller at the poles, g is maximum at the poles and minimum at the equator.
❌ Why the other options are incorrect
✗ 5400 km — about 1000 km too small; a radius this size would make g noticeably larger than the measured 9.8 m/s².
✗ 8400 km — far too large; the accepted radius lies between the polar 6357 km and the equatorial 6378 km.
✗ 7400 km — also too large by about 1000 km; no standard measurement of the Earth gives this figure.
📌 SSC & Railway Exam Facts
• Radius of the Earth ≈ 6400 km = 6.4 × 10⁶ m; mass of the Earth ≈ 6 × 10²⁴ kg.
• g = GM/R² = 9.8 m/s² at the Earth's surface; G = 6.67 × 10⁻¹¹ N m² kg⁻².
• g is maximum at the poles (≈ 9.83 m/s²) and minimum at the equator (≈ 9.78 m/s²).
• The shape of the Earth is called an oblate spheroid (geoid).
• Escape velocity from the Earth = 11.2 km/s; from the Moon = 2.4 km/s.