RRB Technician Grade III 2024-25 24 December 2024 Shift 1 GK Questions – with answers & solutions

Correct Answer: Option 4 — Leaves and stem
Vegetative propagation is a form of asexual reproduction in plants in which new plants arise from the vegetative parts — the root, stem, leaf or bud — without any fusion of gametes. Because only one parent is involved, the daughter plants are genetically identical to the parent and are together called a clone.
Of the choices given, leaves and stem are both genuine vegetative organs. Bryophyllum produces new plantlets from buds sitting in the notches of its leaf margin, while potato, ginger, sugarcane, rose and mint are propagated from stem tubers, rhizomes, cuttings and eyes.
Flowers and fruits are reproductive structures belonging to sexual reproduction, so they can never be used for vegetative propagation.
❌ Why the other options are incorrect
✗ Root and flowers — roots (sweet potato, dahlia) can indeed propagate, but a flower is the sexual reproductive organ of the plant, so this pair is wrong.
✗ Fruits and leaves — the fruit develops from the ovary after fertilisation; it is a product of sexual reproduction and not a vegetative part.
✗ Fruits and flowers — both are sexual reproductive structures; neither takes any part in vegetative propagation.
📌 SSC & Railway Exam Facts
• Natural methods — runner (grass), tuber (potato), rhizome (ginger), bulb (onion), sucker (banana), leaf buds (Bryophyllum).
• Artificial methods — cutting, grafting, layering and tissue culture (micropropagation).
• Plants such as banana, orange, rose and jasmine have lost the ability to produce viable seeds and are grown only by vegetative propagation.
• Advantage — the plants flower and fruit earlier and every offspring retains the desirable parental characters.
• Disadvantage — there is no variation, so one disease can destroy the entire crop.

Correct Answer: Option 1 — Rings of cartilage
Air drawn in through the nostrils passes into the nasal cavity, then the pharynx (throat) and the larynx (voice box), and finally enters the trachea (wind pipe). The wall of the trachea is supported by C-shaped rings of cartilage. These rings act like a stiff scaffolding and keep the air passage permanently open, so the tube does not collapse when the pressure inside falls or when the food pipe lying behind it bulges during swallowing.
Beyond this the trachea divides into two bronchi, which branch repeatedly into bronchioles ending in balloon-like alveoli, where the actual exchange of gases with the blood takes place.
❌ Why the other options are incorrect
✗ Size of mouth cavity — the buccal cavity only takes part in chewing and swallowing food; it gives no structural support to the respiratory tract.
✗ Nostrils large hairs — hairs in the nostrils, along with mucus, only filter dust and germs from incoming air; they cannot hold the throat open.
✗ Nostrils large pores — wide nasal openings merely allow air to enter; they play no mechanical role in preventing the trachea from collapsing.
📌 SSC & Railway Exam Facts
• The nasal cavity warms, moistens and filters air before it reaches the lungs.
• The epiglottis covers the glottis during swallowing so that food does not slip into the wind pipe.
• Gas exchange occurs across the thin walls of the alveoli; human lungs contain roughly 300 million (30 crore) alveoli.
• The right lung has 3 lobes and the left lung 2 lobes; both are covered by a double membrane, the pleura.
• Breathing movements are produced by the diaphragm and the intercostal muscles; the respiratory pigment is haemoglobin.

Correct Answer: Option 4 — they defend plant cells from a variety of external effects
The cell wall is a rigid, non-living outer covering lying outside the plasma membrane in plants, fungi and bacteria. In plants it is made mainly of cellulose. Being tough yet fully permeable, it gives the cell definite shape, rigidity and mechanical strength and defends it from a variety of external effects — mechanical injury, invasion by pathogens and, most importantly, osmotic stress.
When a plant cell is placed in a hypotonic solution, water rushes in by endosmosis and the cell swells. The wall pushes back with an equal wall pressure against the turgor pressure, so the cell becomes turgid but never bursts. An animal cell, which has no wall, would simply burst under the same conditions.
❌ Why the other options are incorrect
✗ they are required for photosynthesis — photosynthesis occurs in the chloroplast using the green pigment chlorophyll; the cell wall carries no pigment.
✗ they do not have proper genetic material — plant cells are eukaryotic and possess a well-organised nucleus containing DNA on chromosomes.
✗ they do not have cytoplasm — every living cell, plant or animal, contains cytoplasm in which all the organelles are suspended.
📌 SSC & Railway Exam Facts
• Plant cell wall — cellulose; fungal cell wall — chitin; bacterial cell wall — peptidoglycan (murein).
• Robert Hooke (1665) saw and named the cell while examining dead cell walls in a thin slice of cork.
• The cell wall is freely permeable, while the plasma membrane is selectively (semi-) permeable.
• Adjacent plant cells stay connected by fine cytoplasmic bridges called plasmodesmata.
• Loss of water first makes the cell flaccid and then plasmolysed, which is why plants wilt in hot weather.

Correct Answer: Option 2 — voluntary muscles
Muscles that contract and relax under our conscious will are called voluntary muscles. They are also known as striated (striped) muscles because of the alternate light and dark bands seen along their length, and as skeletal muscles because they are attached to the bones of the skeleton by tendons.
Each fibre is long, cylindrical, unbranched and multinucleate. Since the brain sends signals to them only when we decide to move, the muscles of our limbs, face, neck and tongue are all voluntary. They contract powerfully and quickly but also get tired on continuous work.
❌ Why the other options are incorrect
✗ unstriated muscles — smooth, spindle-shaped, unbranched and uninucleate fibres with no bands; found in the alimentary canal, blood vessels, iris and ureters, and they work involuntarily.
✗ cardiac muscles — found only in the heart; they are striated but branched and uninucleate, and are involuntary, beating rhythmically throughout life without fatigue.
✗ involuntary muscles — work without our will under the control of the autonomic nervous system; limb muscles clearly do not belong to this group.
📌 SSC & Railway Exam Facts
• Three types of muscle tissue — striated (skeletal, voluntary), unstriated (smooth, involuntary) and cardiac.
• Muscle cells are called muscle fibres; the contractile proteins are actin and myosin.
• Tendon joins muscle to bone; ligament joins bone to bone.
• Peristalsis in the food pipe and intestine is carried out by smooth (unstriated) muscle.
• Cardiac muscle never fatigues; accumulation of lactic acid in skeletal muscle causes cramps.

Correct Answer: Option 2 — lower temperature than the normal body temperature
In human males the testes are not kept inside the abdomen; they hang outside it in a muscular pouch of skin called the scrotum. This is because spermatogenesis (sperm formation) needs a temperature about 2–3 °C lower than the normal body temperature of 37 °C. The scrotum, being outside the body, acts as a natural cooling chamber and its muscles pull the testes closer to the body in cold weather and let them hang lower in heat, thus maintaining the correct temperature.
The testes also perform an endocrine role — they secrete the hormone testosterone, which controls the male secondary sexual characters.
❌ Why the other options are incorrect
✗ higher temperature than the normal body temperature — a raised temperature damages the germinal epithelium of the seminiferous tubules and stops sperm production.
✗ normal body temperature — the 37 °C of the abdominal cavity is itself too warm for sperm formation; that is exactly why the testes descend into the scrotum.
✗ higher body temperature — the same defect; prolonged fever or very tight clothing is known to lower the sperm count.
📌 SSC & Railway Exam Facts
• Male reproductive organs — testes, epididymis, vas deferens, seminal vesicle, prostate gland, urethra and penis.
• The testis has a dual function — production of sperms and secretion of testosterone.
• Failure of the testes to descend into the scrotum is called cryptorchidism and causes sterility.
• Sperms plus the secretions of the seminal vesicle and prostate together form semen.
• In the female, the ovaries lie inside the abdomen and release one ovum about every 28 days.

Correct Answer: Option 4 — Snails
Sex determination is the process that decides whether a developing individual will be male or female. In most animals this is fixed at the moment of fertilisation by the sex chromosomes inherited from the parents. But in a few animals it is not so rigid.
Snails are the classic NCERT example: an individual snail can change its sex during its life. Since the same individual functions first as one sex and later as the other, its sex clearly cannot be decided by its genes, which never change. This shows that sex determination is not always genetic — in some animals it depends on environmental or physiological factors.
❌ Why the other options are incorrect
✗ Ostriches — like all birds, their sex is fixed genetically by the ZZ (male) / ZW (female) chromosome system; an ostrich cannot switch sex.
✗ Frogs — amphibians have genetically determined sex; an adult frog does not normally change from male to female or the reverse.
✗ Lizards — in many reptiles the incubation temperature of the egg decides the sex, but once the animal is formed that sex stays fixed; lizards do not change sex like snails.
📌 SSC & Railway Exam Facts
• NCERT states that snails can change sex, showing that sex is not always genetically determined.
• In humans the sex chromosomes are XX (female) and XY (male); the father decides the sex of the child.
• Human body cells contain 46 chromosomes (23 pairs) — 22 pairs of autosomes + 1 pair of sex chromosomes.
• In turtles and crocodiles, the temperature at which the fertilised eggs are kept decides the sex of the young.
• The Y chromosome carries the SRY gene, the master switch that triggers male development.

Correct Answer: Option 1 — Round seeds
Gregor Johann Mendel worked on the garden pea (Pisum sativum) and studied seven pairs of contrasting characters. A dominant trait is the one that expresses itself in the F₁ generation and masks the other member of the pair, which is called the recessive trait.
When Mendel crossed a pure round-seeded plant (RR) with a pure wrinkled-seeded plant (rr), all the F₁ plants had round seeds (Rr). Wrinkled seeds reappeared only in the F₂ generation, in a phenotypic ratio of 3 round : 1 wrinkled. Hence round seed shape is the dominant trait and wrinkled is recessive.
❌ Why the other options are incorrect
✗ Wrinkled seeds — the recessive form of seed shape; it disappears in F₁ and reappears in only one-fourth of the F₂ plants.
✗ Short plants — dwarfness is recessive; tallness is the dominant character for plant height in pea.
✗ Green seeds — green colour of the cotyledons is recessive; yellow seed colour is dominant.
📌 SSC & Railway Exam Facts
• Mendel is called the Father of Genetics; he published his work in 1866.
• Dominant characters in pea — round seed, yellow seed, tall plant, violet flower, axial flower, inflated pod, green pod.
• Monohybrid cross — phenotypic ratio 3:1, genotypic ratio 1:2:1; dihybrid cross — 9:3:3:1.
• Mendel's laws — Law of Dominance, Law of Segregation (Law of Purity of Gametes) and Law of Independent Assortment.
• The word gene was coined by W. Johannsen and the word genetics by W. Bateson.

Correct Answer: Option 2 — Reproduction
Reproduction is the life process by which an organism produces new individuals of its own kind. Its most basic event at the molecular level is the creation of a DNA copy, a process called DNA replication. The DNA in the nucleus carries the blueprint of the body design, so before a cell divides it must first make an exact copy of its DNA; only then can each daughter cell receive a complete set of instructions.
DNA copying is remarkably accurate but never perfect. The tiny errors that creep in produce variations, and these inherited variations are the raw material on which natural selection works — which is why reproduction is the starting point of evolution.
❌ Why the other options are incorrect
✗ Circulation — the transport of blood, gases, digested food, wastes and hormones by the heart and blood vessels; no copying of DNA is involved.
✗ Respiration — the breakdown of glucose to release energy as ATP; it supplies the energy for replication but is not itself a DNA-copying process.
✗ Excretion — removal of nitrogenous wastes such as urea and uric acid by the kidneys; it has nothing to do with genetic material.
📌 SSC & Railway Exam Facts
• DNA (Deoxyribonucleic acid) is the hereditary material; its copying is called replication.
• The double helix model of DNA was proposed by Watson and Crick (1953).
• Reproduction is the only life process not necessary for the survival of the individual, but essential for the survival of the species.
• Modes of asexual reproduction — fission, budding, fragmentation, regeneration, spore formation and vegetative propagation.
• Variations produced during DNA copying are the basis of evolution and of biodiversity.

Correct Answer: Option 2 — three-carbon molecule
The first step of respiration is glycolysis, which takes place in the cytoplasm of every cell. Here one molecule of glucose, a six-carbon (C₆) molecule, is broken down into two molecules of pyruvate (pyruvic acid), each of which is a three-carbon (C₃) molecule. A small amount of energy is released as ATP at this stage, and glycolysis is common to both aerobic and anaerobic respiration.
The fate of pyruvate then depends on the availability of oxygen — in mitochondria with oxygen it is completely oxidised to CO₂ and H₂O, releasing a large amount of energy.
❌ Why the other options are incorrect
✗ four-carbon molecule — no four-carbon compound is formed from glucose at this step; oxaloacetic acid (4-C) belongs to the much later Krebs cycle.
✗ five-carbon molecule — glycolysis produces no 5-carbon product; α-ketoglutaric acid (5-C) is again a Krebs cycle intermediate.
✗ two-carbon molecule — ethanol and acetyl CoA are 2-carbon, but they are formed from pyruvate afterwards, not directly from glucose.
📌 SSC & Railway Exam Facts
• Glycolysis occurs in the cytoplasm; the rest of aerobic respiration occurs in the mitochondria.
• In yeast, pyruvate changes to ethanol + CO₂ — this fermentation is used in baking and brewing.
• In our muscles during vigorous exercise, pyruvate forms lactic acid (3-C), causing cramps.
• Complete aerobic respiration of one glucose molecule yields about 38 ATP; anaerobic respiration yields only 2 ATP.
• ATP is called the energy currency of the cell; the term was given by Lipmann.

Correct Answer: Option 4 — Pituitary
The pituitary gland is a tiny, pea-sized endocrine gland attached to the base of the brain in a bony cavity of the skull called the sella turcica. Its anterior lobe secretes Growth Hormone (GH), also called somatotropin.
GH controls the overall growth and development of the body. By increasing protein synthesis and cell multiplication, it stimulates the growth of bones, cartilage, muscles and soft tissues in all organs. Its own release is controlled by GHRH sent down from the hypothalamus, making the whole system a neat example of feedback regulation.
❌ Why the other options are incorrect
✗ Thyroid — secretes thyroxine (T₄), which regulates carbohydrate, protein and fat metabolism; it needs iodine and is stimulated by pituitary TSH.
✗ Adrenal — secretes adrenaline, the fight-or-flight (emergency) hormone, and cortisol; it raises heartbeat and blood pressure but does not control growth.
✗ Pancreas — a mixed gland; its endocrine part secretes insulin and glucagon to regulate blood sugar, and its exocrine part pours pancreatic juice into the intestine.
📌 SSC & Railway Exam Facts
• The pituitary is called the master gland, yet it is itself controlled by the hypothalamus.
• Deficiency of GH in childhood causes dwarfism, excess causes gigantism, and excess in adults causes acromegaly.
• The pituitary also secretes TSH, ACTH, FSH, LH and prolactin; its posterior lobe releases ADH (vasopressin) and oxytocin.
• Deficiency of ADH causes diabetes insipidus — do not confuse it with diabetes mellitus, caused by lack of insulin.
• Hormones are carried by the blood, act on specific target organs, and are needed only in very small amounts.

Correct Answer: Option 1 — Salivary gland – trypsin
The salivary glands in the mouth secrete saliva, whose enzyme is salivary amylase (ptyalin). Ptyalin acts in a slightly alkaline medium and breaks starch down into maltose (sugar), which is why chewed rice or bread slowly begins to taste sweet.
Trypsin, on the other hand, is a protein-digesting (proteolytic) enzyme of the pancreatic juice. It is secreted by the pancreas, poured into the duodenum of the small intestine, and works in an alkaline medium created by bile. Therefore pairing the salivary gland with trypsin is incorrect.
❌ Why the other options are incorrect
✗ Pancreas – pancreatic juice — a correct pair; pancreatic juice contains trypsin (protein), lipase (fat) and pancreatic amylase (starch).
✗ Liver – bile juice — a correct pair; the liver makes bile, which is stored in the gall bladder, makes the medium alkaline and emulsifies fats.
✗ Gastric gland – pepsin — a correct pair; gastric glands of the stomach secrete HCl, pepsin and mucus, and pepsin digests protein in an acidic medium.
📌 SSC & Railway Exam Facts
• There are three pairs of salivary glands — parotid, submandibular and sublingual.
• Bile contains no enzyme; it only emulsifies fat and neutralises the acidic food arriving from the stomach.
• The liver is the largest gland in the human body; the pancreas is the largest mixed gland.
• Final digestion and absorption take place in the small intestine through finger-like projections called villi.
• HCl of the stomach kills germs and keeps the pH at about 1.5–2, which pepsin needs to act.

Correct Answer: Option 3 — The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule.
In flowering plants the male germ-cell is produced inside the pollen grain, which is formed in the anther of the stamen. The female gamete (egg cell) lies inside the ovule, which is enclosed within the ovary of the pistil.
After pollination, the pollen grain lands on a suitable stigma and germinates. It puts out a pollen tube which grows down through the style and reaches the ovule inside the ovary. There the male germ-cell is discharged and fuses with the egg cell — this fusion is fertilisation, and it produces the zygote.
The zygote then divides repeatedly to form the embryo; the ovule becomes the seed and the ovary ripens into the fruit.
❌ Why the other options are incorrect
✗ Option 1 — the ovary never produces the male germ-cell; it only encloses the ovules, which carry the female gamete.
✗ Option 2 — the ovule produces the female gamete, not the male one, and the pollen tube is only a passage, not a place where the egg waits.
✗ Option 4 — the stigma merely receives the pollen grain; the female gamete lies deep inside the ovule, far from the stigma.
📌 SSC & Railway Exam Facts
• Pollination is the transfer of pollen from anther to stigma — self-pollination or cross-pollination (by wind, water, insects or birds).
• After fertilisation, zygote → embryo, ovule → seed, ovary → fruit; the other floral parts wither away.
• Angiosperms show double fertilisation — one male gamete forms the zygote (2n) and the other forms the endosperm (3n).
• Stamen (androecium) = anther + filament; Pistil (gynoecium) = stigma + style + ovary.
• Double fertilisation was discovered by S. G. Nawaschin; the pollen tube was described by G. B. Amici.

Correct Answer: Option 2 — A-i, B-ii, C-iii
A. Placenta – Nutrition. The placenta is a disc-like special tissue embedded in the wall of the uterus. It has finger-like villi on the embryo's side and blood spaces on the mother's side, so a large surface is available for the passage of glucose, oxygen and nutrients from the mother to the developing embryo, and for the removal of the embryo's waste into the mother's blood.
B. Fallopian tube – Fertilization. The fallopian tube (oviduct) carries the released ovum from the ovary. Sperms swim up and meet the ovum here, so fertilisation normally takes place in the fallopian tube, forming the zygote.
C. Uterus – Implantation. The zygote divides into a ball of cells while moving down, and this embryo gets attached to the thick, spongy lining (endometrium) of the uterus — this attachment is implantation, after which development continues in the uterus.
❌ Why the other options are incorrect
✗ A-iii, B-ii, C-i — wrongly links the placenta with implantation and the uterus with nutrition.
✗ A-ii, B-i, C-iii — fertilisation does not occur in the placenta, and the fallopian tube does not nourish the embryo.
✗ A-i, B-iii, C-ii — implantation is not a function of the fallopian tube, and fertilisation does not take place in the uterus.
📌 SSC & Railway Exam Facts
• The human gestation period is about 280 days (nine months).
• The placenta also acts as a temporary endocrine gland, secreting HCG, oestrogen and progesterone.
• If the embryo implants in the fallopian tube instead of the uterus, it is called an ectopic pregnancy.
• If the ovum is not fertilised, the uterine lining breaks down and is shed as menstruation, roughly every 28 days.
• Ovulation — release of an ovum from the ovary — usually occurs around the 14th day of the menstrual cycle.

Correct Answer: Option 3 — Kulhad – non-biodegradable
A kulhad is the traditional Indian cup made of baked clay (earthenware). Clay is a purely natural material; a discarded or broken kulhad simply crumbles and mixes back into the soil through weathering and the action of microorganisms. It is therefore a biodegradable article, and calling it non-biodegradable makes this pair incorrect.
Biodegradable substances are those which are broken down into simple, harmless substances by decomposers such as bacteria and fungi. Non-biodegradable substances resist this breakdown, persist in the environment for a very long time, pollute soil and water, and can enter food chains.
❌ Why the other options are incorrect
✗ Plastic glass – non-biodegradable — a correct pair; plastic is a man-made polymer that decomposers cannot break down, so it survives for hundreds of years.
✗ Vegetable – biodegradable — a correct pair; vegetable peels and leftovers rot quickly and are used to prepare compost and manure.
✗ Paper cup – biodegradable — a correct pair; paper is made of cellulose from wood pulp, which microbes readily decompose.
📌 SSC & Railway Exam Facts
• Biodegradable wastes — food and vegetable waste, paper, cotton, wool, wood, leather, jute, cow dung.
• Non-biodegradable wastes — plastic, polythene, glass, metals, DDT, synthetic fibres, aluminium foil.
• Harmful chemicals such as DDT keep increasing at each higher trophic level — this is biomagnification.
• Follow the 3R principle — Reduce, Reuse, Recycle; wet waste goes in the green bin and dry waste in the blue bin.
• Biodegradable waste rotting in the absence of air gives biogas (mainly methane) and useful manure.

Correct Answer: Option 4 — Evaporation
Evaporation is the change of a liquid into vapour at any temperature and only from the surface of the liquid. The particles that escape are the ones with the highest kinetic energy; to leave, they absorb the latent heat of vaporisation from their surroundings, so the surface left behind becomes cooler. This is why evaporation causes cooling.
In summer the body throws out sweat. Cotton is a good absorber of water and is highly porous, so it soaks up the sweat and exposes it to the air over a large area. The sweat then evaporates rapidly, taking heat from the body and leaving us feeling cool and comfortable. Synthetic fabrics neither absorb nor release sweat easily, so they feel sticky and hot.
❌ Why the other options are incorrect
✗ Sublimation — the change of a solid directly into a gas, as with camphor, naphthalene and dry ice; no solid is involved in sweating.
✗ Condensation — the change of a gas into a liquid, as when dew or fog forms; it releases heat instead of absorbing it.
✗ Fusion — the change of a solid into a liquid (melting), such as ice → water; again irrelevant to cooling by sweat.
📌 SSC & Railway Exam Facts
• Evaporation happens at all temperatures, only at the surface; boiling happens at a fixed boiling point throughout the liquid.
• Rate of evaporation increases with temperature, surface area and wind speed, and decreases with humidity.
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg; latent heat of fusion of ice = 3.34 × 10⁵ J/kg.
• Water in an earthen pot stays cool because water evaporates from its pores.
• Solid → gas = sublimation; gas → solid = deposition; liquid → gas = vaporisation; gas → liquid = condensation.

Correct Answer: Option 1 — addition reaction
Vegetable oils are unsaturated — their carbon chains contain double bonds (C=C). When such an unsaturated compound reacts with hydrogen in the presence of a catalyst, the hydrogen atoms simply add across the double bond and nothing is removed. A reaction in which two reactants combine and no atom leaves the molecule is called an addition reaction.
⚗️ The Reaction
Vegetable oil (unsaturated) + H₂ → (Ni catalyst) Vanaspati ghee (saturated)
• The double bonds are converted into single bonds, so the liquid oil becomes a semi-solid fat.
• Nickel (Ni) acts as the catalyst — it speeds up the reaction without being consumed. Palladium (Pd) and platinum (Pt) can also be used.
• This is exactly how vanaspati ghee and margarine are manufactured from oils.
❌ Why the other options are incorrect
✗ elimination reaction — here a small molecule is removed from a compound, the opposite of what happens in hydrogenation.
✗ rearrangement reaction — atoms of the same molecule reorganise to form an isomer; no hydrogen is added at all.
✗ combustion reaction — burning in oxygen to give CO₂, H₂O, heat and light; no oxygen is involved in hydrogenation.
📌 SSC & Railway Exam Facts
• Unsaturated compounds give addition reactions; saturated ones give substitution reactions.
• Hydrogenation of oils uses a nickel (Ni) catalyst; the product is vanaspati ghee.
• Doctors advise unsaturated (vegetable) oils over saturated fats for a healthy heart.
• Bromine water test: unsaturated compounds decolourise brown bromine water by addition.
• A catalyst changes the rate of a reaction without itself being consumed.

Correct Answer: Option 2 — Both statements I and II are correct
The state of a substance depends on both temperature and pressure. Increasing the pressure pushes the particles closer together; lowering the temperature reduces their kinetic energy. Both changes strengthen the force of attraction between particles.
Statement I is correct — a gas is converted into a liquid by applying high pressure and lowering the temperature. This is exactly how LPG is filled into cylinders and how oxygen and nitrogen are supplied as liquids. Each gas has a critical temperature above which no amount of pressure can liquefy it.
Statement II is correct — solid carbon dioxide (dry ice) is stored under high pressure. When the pressure is reduced to 1 atmosphere, it changes directly into CO₂ gas by sublimation, leaving no liquid behind.
❌ Why the other options are incorrect
✗ Only statement I is correct — it wrongly rejects II; dry ice really is kept under high pressure.
✗ Neither statement I nor II is correct — both are standard NCERT statements about the effect of pressure.
✗ Only statement II is correct — it wrongly rejects I; liquefaction of gases by pressure + cooling is a basic fact.
📌 SSC & Railway Exam Facts
• Gas → liquid by increasing pressure and decreasing temperature.
• Solid CO₂ = dry ice; it sublimes at about −78 °C, used as a refrigerant.
• LPG cylinders contain liquefied butane and propane under pressure.
• Interparticle force: solid > liquid > gas; compressibility: gas > liquid > solid.
• Plasma is the fourth state and BEC the fifth state of matter.

Correct Answer: Option 3 — acidity or basicity
The pH scale measures the concentration of hydrogen ions (H⁺) in a solution and therefore tells us how acidic or how basic that solution is. The letter p stands for the German word "potenz", meaning power, so pH = power of hydrogen. The scale was given by S. P. L. Sørensen and runs from 0 to 14.
• pH < 7 → acidic — the more H⁺ ions, the lower the pH.
• pH = 7 → neutral — as in pure water.
• pH > 7 → basic (alkaline) — more OH⁻ ions.
A universal indicator shows the whole range by colour: red for strongly acidic, green for neutral and violet/blue for strongly basic.
❌ Why the other options are incorrect
✗ density — density is mass per unit volume, measured with a hydrometer, and has nothing to do with H⁺ ions.
✗ pollution — pollution levels are measured by parameters such as AQI, BOD or TDS, not by pH alone.
✗ volume — volume is measured with a measuring cylinder, pipette or burette; it is a physical quantity, not a chemical property.
📌 SSC & Railway Exam Facts
• pH scale 0–14; pH 7 = neutral; term coined by Sørensen; p = potenz (power).
• Gastric juice pH ≈ 1.2, lemon juice ≈ 2.2, pure water = 7, blood ≈ 7.4, milk of magnesia ≈ 10.
• Acid rain has pH below 5.6 and damages crops and monuments.
• Our mouth must stay above pH 5.5 or tooth enamel (calcium phosphate) starts to decay.
• A bee sting is acidic — relieved by baking soda; a wasp sting is basic — relieved by vinegar.

Correct Answer: Option 4 — high pressure and low temperature
In a gas the particles are very far apart, move at high speed and the force of attraction between them is negligible. To convert a gas into a liquid, the particles must be brought closer together and their kinetic energy must be reduced.
• Increasing the pressure squeezes the particles into a smaller volume, so the interparticle distance decreases and the force of attraction becomes effective.
• Decreasing the temperature lowers the kinetic energy of the particles, so they can no longer overcome that attraction and stay bound together.
When both are done together, the gas liquefies. This is exactly how LPG is filled into cylinders and how liquid oxygen and liquid nitrogen are produced. Above a gas's critical temperature no amount of pressure can liquefy it, so cooling is essential.
❌ Why the other options are incorrect
✗ low pressure and high temperature — both changes push the particles further apart and raise their energy; this favours evaporation, the exact opposite of liquefaction.
✗ high pressure and high temperature — high pressure helps, but high temperature gives the particles enough energy to escape again.
✗ low pressure and low temperature — low temperature helps, but low pressure lets the particles spread out; reducing pressure even makes a liquid boil more easily.
📌 SSC & Railway Exam Facts
• Gas → liquid = increase pressure + decrease temperature.
• LPG = butane + propane, stored liquefied under pressure; CNG is stored compressed, not liquefied.
• Liquid nitrogen boils at −196 °C and is used to preserve semen, blood and vaccines.
• Kelvin scale: K = °C + 273; 0 K = −273.15 °C is absolute zero.
• Each gas has a critical temperature above which it cannot be liquefied by pressure alone.

Correct Answer: Option 4 — Zn + CuCl₂ → ZnCl₂ + Cu
The reactivity series arranges metals in decreasing order of their tendency to lose electrons:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au
The rule for a displacement reaction is simple: a more reactive metal displaces a less reactive metal from its salt solution. If the free metal is below the metal in the salt, no reaction takes place.
⚗️ The Reaction
Zn + CuCl₂ → ZnCl₂ + Cu
• Zinc is above copper in the series, so zinc displaces copper — the reaction is possible.
• The blue copper salt solution fades and reddish-brown copper is deposited.
• It is a redox change: Zn is oxidised to Zn²⁺ and Cu²⁺ is reduced to Cu.
❌ Why the other options are incorrect
✗ 3Cu + FeCl₃ → 3CuCl₂ + Fe — copper is below iron, so copper cannot displace iron; no reaction occurs.
✗ 3Ag + AlCl₃ → 3AgCl + Al — silver is far below aluminium; a very unreactive metal can never displace a very reactive one.
✗ Cu + ZnCl₂ → CuCl₂ + Zn — copper is below zinc, so this is impossible; it is the exact reverse of the correct reaction.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• A more reactive metal displaces a less reactive one from its salt solution.
• Fe + CuSO₄ → FeSO₄ + Cu: blue turns green, iron nail turns brown.
• Metals above hydrogen displace H₂ from dilute acids; Cu, Ag, Au do not.
• Potassium is the most reactive and gold the least reactive of the common metals.

Correct Answer: Option 2 — Table salt
An ionic (electrovalent) compound is formed by the complete transfer of electrons from a metal to a non-metal, producing oppositely charged ions held together by a strong electrostatic force.
Table salt is sodium chloride (NaCl). Sodium (Na) has 1 electron in its outermost shell and donates it to chlorine (Cl), which needs 1 electron to complete its octet. This gives Na⁺ and Cl⁻, which attract each other strongly and build a crystal lattice.
Because of this strong lattice, ionic compounds are hard, brittle solids with high melting and boiling points, are generally soluble in water but insoluble in kerosene or petrol, and conduct electricity in molten or aqueous state (not in the solid state, where the ions are not free to move).
❌ Why the other options are incorrect
✗ Sugar — sucrose (C₁₂H₂₂O₁₁) is a covalent compound; its molecules share electrons and its solution does not conduct electricity.
✗ Glucose — C₆H₁₂O₆ is likewise covalent, made of carbon, hydrogen and oxygen — all non-metals, so no electron transfer occurs.
✗ Turmeric — turmeric is not a single compound at all; it is a natural indicator that turns red in a basic solution.
📌 SSC & Railway Exam Facts
• Ionic compound = metal + non-metal, e.g. NaCl, MgCl₂, CaO, KBr.
• Ionic compounds conduct electricity in molten and aqueous state, not as solids.
• They have high melting and boiling points and are soluble in water, insoluble in petrol/kerosene.
• NaCl = common salt; raw material for NaOH, washing soda, baking soda and bleaching powder.
• Turmeric, litmus, red cabbage and China rose (hibiscus) are natural indicators.

Correct Answer: Option 3 — physical, chemical
A physical change alters only the appearance, state or form of a substance; no new substance is produced and the change is usually reversible. A chemical change produces a completely new substance with new properties and is generally irreversible, often accompanied by a change of energy.
When a mixture is made, the substances are only mixed in any proportion; each keeps its own properties and can be recovered by physical methods such as filtration, magnetic separation or distillation. Hence forming a mixture is a physical change.
When a compound is formed, the elements combine chemically in a fixed ratio by mass. A completely new substance appears, heat or light is absorbed or released, and it can be separated only by chemical means. Hence forming a compound is a chemical change.
• Iron filings + sulphur powder mixed → a mixture; iron is still attracted by a magnet.
• Iron filings + sulphur heated → iron sulphide (FeS), a compound; the magnet no longer attracts it.
❌ Why the other options are incorrect
✗ chemical, physical — this is the exact reverse; forming a mixture involves no new substance at all.
✗ physical, physical — forming a compound gives a new substance, so it cannot be physical.
✗ chemical, chemical — mixing needs no chemical reaction, so the first blank cannot be chemical.
📌 SSC & Railway Exam Facts
• Mixture: variable composition, physical change, separable by physical methods.
• Compound: fixed composition, chemical change, separable by chemical methods only.
• Physical change: melting of ice, boiling of water, dissolving sugar, glowing of a bulb filament.
• Chemical change: rusting of iron, burning of paper, curdling of milk, cooking of food.
• Law of Constant Proportions (Proust): a compound always has the same elements in the same ratio by mass.

Correct Answer: Option 1 — Na ions are positively charged, and Cl ions are negatively charged
Sodium chloride (NaCl) is a typical ionic compound formed by the transfer of one electron from the metal to the non-metal.
• Sodium (Na), atomic number 11, has the configuration 2, 8, 1. It loses its single outermost electron to attain the stable octet of neon, becoming Na⁺ — a cation with 11 protons but only 10 electrons, hence positively charged.
• Chlorine (Cl), atomic number 17, has the configuration 2, 8, 7. It gains one electron to complete its octet, becoming Cl⁻ — an anion with 17 protons but 18 electrons, hence negatively charged.
The strong electrostatic attraction between Na⁺ and Cl⁻ builds the crystal lattice of common salt. Remember: a metal loses electrons and forms a cation; a non-metal gains electrons and forms an anion.
❌ Why the other options are incorrect
✗ Ions of both Na and Cl are negatively charged — impossible; two like charges would repel, and a compound must be electrically neutral overall.
✗ Cl ions are positively charged, and Na ions are negatively charged — this reverses the roles; a metal can never gain electrons to become negative here.
✗ Ions of both Na and Cl are positively charged — again impossible; there must be one cation and one anion to form an ionic bond.
📌 SSC & Railway Exam Facts
• Cation = positive (loses electrons, e.g. Na⁺, Mg²⁺, Ca²⁺); anion = negative (gains electrons, e.g. Cl⁻, O²⁻, SO₄²⁻).
• In electrolysis, cations move to the cathode and anions to the anode.
• Na: 2,8,1 and Cl: 2,8,7; after transfer both attain the noble gas configuration.
• NaCl = common salt, used to make NaOH (chlor-alkali process), washing soda, baking soda, bleaching powder.
• Ionic compounds conduct electricity in molten or aqueous state, not as solids.

Correct Answer: Option 3 — Only liquid
Every colloid has two parts: the dispersed phase (the substance present in small particles, like the solute) and the dispersing medium (the substance in which it is spread, like the solvent). Colloids are named from the physical states of these two parts.
Butter is a gel — a liquid dispersed in a solid. Tiny droplets of water (liquid) are trapped inside a continuous network of solid fat. Therefore the dispersed phase of butter is a liquid and its dispersing medium is a solid. Cheese and table jelly belong to the same class.
❌ Why the other options are incorrect
✗ Only solid — a solid dispersed in a liquid gives a sol (paint, ink, blood); butter is not a sol.
✗ Both solid and liquid — a colloid has only one dispersed phase, never two at the same time.
✗ Only gas — a gas dispersed in a liquid gives a foam (shaving cream, soap lather), and a gas in a solid gives a solid foam (sponge, pumice stone).
📌 SSC & Railway Exam Facts
• Butter, cheese, jelly = gel → liquid in solid.
• Milk, face cream = emulsion → liquid in liquid; paint, ink, blood = sol → solid in liquid.
• Shaving cream, whipped cream = foam → gas in liquid; fog, mist, clouds = aerosol → liquid in gas.
• Smoke = aerosol → solid in gas; sponge, pumice = solid foam → gas in solid.
• Colloid particle size 1–1000 nm; they show the Tyndall effect and are separated by centrifugation.

Correct Answer: Option 2 — Conversion of copper oxide into copper
Reduction is the removal of oxygen from a substance, or the addition of hydrogen to it; in electronic terms it is the gain of electrons. Oxidation is the exact opposite.
When copper oxide (CuO) becomes copper (Cu), the compound loses its oxygen — so copper oxide is reduced. This is done in the laboratory by passing hydrogen gas over heated CuO: the black CuO turns into reddish-brown copper.
⚗️ The Reaction
CuO + H₂ → (heat) Cu + H₂O
• CuO loses oxygen → it is reduced; H₂ is the reducing agent.
• H₂ gains oxygen → it is oxidised to water; CuO is the oxidising agent.
• Oxidation and reduction occur together, so this is a redox reaction.
❌ Why the other options are incorrect
✗ Conversion of carbon into carbon dioxide — C + O₂ → CO₂; carbon gains oxygen, so this is oxidation.
✗ Conversion of ethanol into acetic acid — C₂H₅OH → CH₃COOH with alkaline KMnO₄ or acidified K₂Cr₂O₇; oxygen is added, so it is oxidation.
✗ Conversion of copper into copper oxide — 2Cu + O₂ → 2CuO; copper gains oxygen and turns black, which is oxidation, the reverse of the answer.
📌 SSC & Railway Exam Facts
• Reduction = loss of oxygen / gain of hydrogen / gain of electrons; remember OIL RIG.
• CuO + H₂ → Cu + H₂O is the standard NCERT redox example — black to reddish-brown.
• The oxidising agent is itself reduced; the reducing agent is itself oxidised.
• Ethanol → ethanoic (acetic) acid by alkaline KMnO₄ or acidified K₂Cr₂O₇ — called oxidising agents.
• Rusting and rancidity are everyday oxidation processes; rancidity is checked by antioxidants.

Correct Answer: Option 2 — Pascal
Pressure is defined as the force acting perpendicularly per unit area:
P = F/A
Its SI unit is the pascal (Pa), named after the French scientist Blaise Pascal. By definition 1 pascal = 1 newton per square metre (1 N/m²), i.e. the pressure produced when a force of 1 N acts on an area of 1 m².
Because the pascal is a very small unit, larger units such as the bar (1 bar = 10⁵ Pa) and the atmosphere (1 atm = 1.01 × 10⁵ Pa) are used in practice. Gas pressure inside a cylinder, atmospheric pressure and blood pressure are all pressures, though blood pressure is still quoted in mm of Hg.
❌ Why the other options are incorrect
✗ Kelvin — the SI unit of temperature; K = °C + 273, with 0 K as absolute zero.
✗ Cubic metre — the SI unit of volume (m³); 1 m³ = 1000 litres.
✗ Newton — the SI unit of force, not pressure; pressure is force divided by area, so its unit is N/m².
📌 SSC & Railway Exam Facts
• 1 pascal = 1 N/m²; the unit is named after Blaise Pascal.
• 1 atm = 1.01 × 10⁵ Pa = 760 mm of Hg = 1.013 bar.
• SI units: length → metre, mass → kilogram, time → second, temperature → kelvin, current → ampere, amount → mole, luminous intensity → candela.
• Atmospheric pressure is measured by a barometer; gas pressure by a manometer.
• Pascal's law: pressure applied to an enclosed fluid is transmitted equally in all directions — the basis of the hydraulic lift and brakes.

Correct Answer: Option 3 — Pascal
Pressure is the force acting normally per unit area, P = F/A. Substituting the SI units of force (newton, N) and of area (square metre, m²) gives the SI unit of pressure as N/m², which is given the special name pascal (Pa) after Blaise Pascal.
So 1 pascal = 1 N/m² — the pressure exerted when a force of 1 newton is spread over an area of 1 square metre. Since the pascal is very small, the bar (10⁵ Pa) and the atmosphere (1.01 × 10⁵ Pa) are used for larger pressures.
This is also why a sharp knife cuts better (small area → large pressure) and why a camel's broad feet do not sink in sand (large area → small pressure).
❌ Why the other options are incorrect
✗ Newton — the unit of force, not pressure; pressure needs force divided by area.
✗ Newton/meter — this is N/m, the unit of surface tension or of a spring constant; pressure requires N/m².
✗ Angstrom — a unit of length, 1 Å = 10⁻¹⁰ m, used for atomic and molecular sizes.
📌 SSC & Railway Exam Facts
• Pressure = Force / Area; SI unit pascal (Pa) = N/m².
• 1 bar = 10⁵ Pa; 1 atm = 1.01 × 10⁵ Pa = 760 mm of Hg.
• 1 Å = 10⁻¹⁰ m; 1 nm = 10⁻⁹ m.
• N/m is the unit of surface tension; N·m (joule) is the unit of work/energy.
• Pascal's law explains the hydraulic press, hydraulic lift and hydraulic brakes.

Correct Answer: Option 4 — Both Potential and Kinetic
At the top of the tower the ball is at rest, so it has only potential energy, P.E. = mgh. As soon as it is dropped it starts moving, and part of that potential energy is steadily converted into kinetic energy, K.E. = ½mv².
At the midpoint the ball has already fallen a height h/2, so its potential energy is mg(h/2), and the energy lost has appeared as kinetic energy, K.E. = mg(h/2) as well. Hence at the midpoint the ball possesses both potential and kinetic energy, each exactly half of the initial energy. By the law of conservation of energy the total mechanical energy (K.E. + P.E.) stays equal to mgh throughout the fall.
❌ Why the other options are incorrect
✗ Potential — the ball has potential energy only at the top, where it is still at rest; at the midpoint it is already moving, so it must also have kinetic energy.
✗ Kinetic — the ball possesses only kinetic energy just before striking the ground, where its height (and hence its potential energy) becomes zero.
✗ Sound — sound energy is produced only at the moment of impact with the ground; a ball falling freely through air produces no significant sound energy.
📌 SSC & Railway Exam Facts
• P.E. = mgh, K.E. = ½mv²; both are measured in joule (J).
• Law of conservation of energy — energy can neither be created nor destroyed, only converted from one form into another.
• For a freely falling body, K.E. + P.E. = mgh = constant at every point of the fall.
• Just before hitting the ground the whole energy is kinetic, so v = √(2gh).
• Free fall is independent of mass — a heavy and a light ball dropped together reach the ground at the same time in a vacuum.

Correct Answer: Option 1 — Tyndall effect
When a beam of light passes through a colloid — a medium containing extremely fine suspended particles such as smoke, fog, mist or a dusty room — the particles scatter the light sideways. As a result the path of the beam itself becomes visible. This phenomenon is called the Tyndall effect, named after the British scientist John Tyndall.
Everyday examples are a beam of sunlight entering a dark room through a small hole, the visible beam of a cinema projector in a smoky hall, and the headlight beams of a car in fog. The Tyndall effect is also used to distinguish a colloid from a true solution — a true solution, whose particles are far too small, shows no Tyndall effect.
❌ Why the other options are incorrect
✗ delay in sunset — the Sun stays visible for a few minutes after it has actually set because of atmospheric refraction, not scattering.
✗ rainbow — a rainbow is formed inside raindrops by refraction, total internal reflection and dispersion of sunlight, not by scattering off colloidal particles.
✗ twinkling of stars — stars twinkle because of the refraction of starlight through air layers of continuously changing density.
📌 SSC & Railway Exam Facts
• Tyndall effect — scattering of light by colloidal particles, which makes the path of a light beam visible.
• The blue colour of the sky is due to scattering; blue light, having a shorter wavelength, is scattered the most.
• Rayleigh's law: intensity of scattering ∝ 1/λ⁴.
• The Sun looks reddish at sunrise and sunset because blue light is scattered away over the long path through the atmosphere.
• Danger signals are red — red has the longest wavelength and is scattered the least, so it travels farthest.

Correct Answer: Option 1 — South pole to its North pole
Magnetic field lines always form closed continuous loops. Outside a bar magnet they run from the North pole to the South pole, but the loop must be completed, so inside the magnet they run from the South pole to the North pole. This is exactly what makes them different from electric field lines, which start and end on charges.
Other important properties: field lines never intersect one another (if they did, the compass needle would have two directions at one point); they are crowded where the field is strong (near the poles) and far apart where it is weak; and the tangent at any point gives the direction of the magnetic field there.
❌ Why the other options are incorrect
✗ North pole to its South pole — that is the direction of the field lines outside the magnet, in the surrounding space, not inside its body.
✗ always perpendicular to the South pole — field lines leave or enter the pole faces but they do not stay perpendicular; inside the magnet they run along its length.
✗ always perpendicular to the North pole — same mistake; the lines inside a bar magnet are directed along the magnet, from S to N, forming closed loops.
📌 SSC & Railway Exam Facts
• Magnetic field lines are closed curves: N → S outside the magnet and S → N inside it.
• Field lines never cross; the tangent at a point gives the field direction; their crowding shows the field strength.
• SI unit of magnetic field (B) is tesla (T); of magnetic flux it is weber (Wb); 1 T = 1 Wb/m².
• Like poles repel, unlike poles attract; a freely suspended magnet points north–south.
• The magnetic effect of electric current was discovered by Hans Christian Oersted in 1820.

Correct Answer: Option 4 — 0 m/s
📖 Given
• Length of the pool = 60 m
• Total distance covered = 120 m (60 m one way + 60 m back)
• Total time, t = 1 minute = 60 s
• She returns to her starting point, so displacement = 0
✍️ Formula
Average velocity = Total displacement / Total time
(while Average speed = Total distance / Total time)
🔢 Solution
• Displacement = 60 m (forward) − 60 m (backward) = 0 m
• Average velocity = 0/60
• Average velocity = 0 m/s
• (For comparison, average speed = 120/60 = 2 m/s)
Because Uma finishes exactly where she started, her net change of position is zero, so her average velocity is zero even though she has actually swum 120 m.
❌ Why the other options are incorrect
✗ 2 m/s — this is the average speed (120 m ÷ 60 s); speed uses the distance, velocity uses the displacement.
✗ 1 m/s — this comes from using only the one-way length, 60 m ÷ 60 s, but she does not stop at the far end; she comes back.
✗ 3 m/s — no correct step gives this value; it would need a distance of 180 m in one minute, which is not what the question states.
📌 SSC & Railway Exam Facts
• Distance is a scalar and can never decrease; displacement is a vector and can be zero or negative.
• Average speed = total distance/total time; average velocity = total displacement/total time; SI unit of both is m/s.
• For any closed path (round trip) the displacement, and hence the average velocity, is always zero.
• |Displacement| ≤ distance; the two are equal only for motion in a straight line without turning back.
• 1 km/h = 5/18 m/s and 1 m/s = 18/5 km/h.

Correct Answer: Option 2 — 1/3
📖 Given
• Height of the object, h = 30 cm
• Height of the image, h' = 10 cm
• The lens is concave (diverging), so the image is virtual, erect and diminished
✍️ Formula
m = h'/h = v/u
🔢 Solution
• m = h'/h
• m = 10/30
• m = 1/3
A magnification of 1/3 (a positive fraction less than 1) tells us that the image is erect and only one-third the size of the object — exactly what a concave lens always produces.
❌ Why the other options are incorrect
✗ 3 — this is h/h' = 30/10, the ratio taken upside down; magnification is always image height divided by object height.
✗ 20 — this is 30 − 10, a subtraction of the two heights; magnification is a ratio, never a difference.
✗ −20 — this is 10 − 30, the same subtraction mistake with the sign reversed; the value is neither a ratio nor correct in sign, since a concave lens gives a positive magnification.
📌 SSC & Railway Exam Facts
• Magnification of a lens: m = h'/h = v/u; it has no unit.
• m positive ⇒ virtual and erect image; m negative ⇒ real and inverted image.
• |m| > 1 means enlarged, |m| < 1 means diminished, |m| = 1 means the same size.
• A concave (diverging) lens always gives a virtual, erect, diminished image, whatever the object position.
• Lens formula 1/v − 1/u = 1/f; power P = 1/f (in metres), measured in dioptre (D) — negative for a concave lens.

Correct Answer: Option 1 — Dispersion
The splitting of white light into its seven constituent colours on passing through a glass prism is called dispersion of light. It was first demonstrated by Sir Isaac Newton. The band of colours obtained is called the spectrum, and the order from the least to the most deviated is VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red.
Dispersion happens because the refractive index of glass is different for different colours. Violet has the shortest wavelength, so glass bends it the most, while red has the longest wavelength and is bent the least. A second, inverted prism can recombine these colours back into white light.
❌ Why the other options are incorrect
✗ Refraction — refraction is simply the bending of light when it enters a new medium; it is the *cause* of dispersion, but by itself it does not split white light into colours.
✗ Diffraction — diffraction is the bending of light around an obstacle or through a narrow slit, comparable in size to the wavelength; it is a different phenomenon from prism dispersion.
✗ Scattering — scattering is the spreading of light by fine particles; it explains the blue sky and the Tyndall effect, not the prism spectrum.
📌 SSC & Railway Exam Facts
• Dispersion through a prism was first shown by Sir Isaac Newton; the coloured band is the spectrum.
• Order of colours: VIBGYOR; violet deviates the most, red the least.
• Wavelength order is the reverse — red (≈ 700 nm) > ... > violet (≈ 400 nm).
• A rainbow is nature's prism — formed by refraction, total internal reflection and dispersion in raindrops.
• The blue sky and the red Sun at sunset are due to scattering, while twinkling of stars is due to atmospheric refraction.

Correct Answer: Option 3 — at the centre of curvature
For a concave mirror, when the object is placed exactly at the centre of curvature (C), the image is also formed at C and it is real, inverted and exactly the same size as the object. The magnification is then m = −1, where the minus sign shows that the image is inverted and the magnitude 1 shows that the sizes are equal.
This can be checked with the mirror formula. Taking u = −R and f = −R/2, we get 1/v = 1/f − 1/u = −2/R + 1/R = −1/R, so v = −R and m = −v/u = −1. It is the only object position for a concave mirror that gives an image of the same size.
❌ Why the other options are incorrect
✗ at the focus — an object at F gives an image at infinity, highly enlarged; this is the arrangement used in a torch or a searchlight, where a parallel beam is wanted.
✗ far away from the centre of curvature — that is beyond C; the image then forms between F and C and is real, inverted but diminished, not the same size.
✗ between the focus and the centre of curvature — the image then forms beyond C and is real, inverted and enlarged; this is the arrangement used in a projector.
📌 SSC & Railway Exam Facts
• Object at C → image at C, real, inverted, same size, m = −1.
• Object at infinity → image at F, point-sized; object at F → image at infinity.
• Object between P and F → image virtual, erect and enlarged, formed behind the mirror.
• Mirror formula 1/v + 1/u = 1/f; f = R/2; magnification m = −v/u = h'/h.
• Concave mirrors are used in torches, headlights, solar cookers, shaving mirrors and by dentists.

Correct Answer: Option 4 — concave mirror
The front (inner) surface of a spoon curves inwards, exactly like a concave mirror. A concave mirror is a converging mirror: when you hold it at a distance greater than its focal length, it forms a real and inverted image of your face. That is why your reflection in the bowl of a spoon appears upside down.
If you hold the spoon very close to your face — nearer than the focus — the image becomes virtual, erect and enlarged, which is the same principle used in a shaving mirror. The back (outer) surface of the same spoon bulges outwards and behaves as a convex mirror, always giving an erect, diminished image.
❌ Why the other options are incorrect
✗ plane mirror — a plane mirror always forms a virtual, erect image of the same size; it can never give an inverted image.
✗ convex mirror — a convex mirror always forms a virtual, erect and diminished image; that is the back of the spoon, not the front.
✗ concave lens — a spoon is opaque and works by reflection, not by refraction, so it cannot act as a lens; besides, a concave lens gives an erect image.
📌 SSC & Railway Exam Facts
• Inner surface of a spoon = concave mirror; outer surface of a spoon = convex mirror.
• A concave mirror gives a real, inverted image when the object is beyond F, and a virtual, erect, enlarged image when it is between P and F.
• A convex mirror gives a virtual, erect and diminished image for every object position.
• A plane mirror gives a virtual, erect, same-size image with lateral inversion, formed as far behind the mirror as the object is in front.
• f = R/2; mirror formula 1/v + 1/u = 1/f.

Correct Answer: Option 3 — Inertia is proportional to mass.
Inertia is the natural tendency of a body to resist any change in its state of rest or of uniform motion in a straight line. This idea comes from Newton's first law of motion, which is therefore also called the law of inertia.
The measure of inertia is the mass of the body — the heavier a body is, the harder it is to start it moving, to stop it or to turn it. So inertia ∝ mass. A loaded truck is far more difficult to push than an empty cart, and a cricket ball is harder to stop than a table-tennis ball moving at the same speed, purely because of their larger masses.
❌ Why the other options are incorrect
✗ Inertia is inversely proportional to mass — exactly the opposite of the truth; if this were so, a feather would be harder to move than a truck.
✗ Inertia is independent of mass — wrong, because mass is the very measure of inertia; change the mass and the inertia changes with it.
✗ The data provided here is not adequate to draw any conclusion — no extra data is needed; the relation between mass and inertia is a standard result of Newton's first law.
📌 SSC & Railway Exam Facts
• Newton's first law = law of inertia; mass is the measure of inertia, SI unit kilogram (kg).
• Types: inertia of rest, inertia of motion and inertia of direction.
• Dust flying off when a carpet is beaten and a passenger lurching forward when a bus brakes are examples of inertia.
• Newton's second law: F = ma; momentum p = mv, SI unit kg m/s.
• Inertia is not a force — it is a property of matter, and it does not depend on velocity.

Correct Answer: Option 3 — uniformly accelerated motion
A body in free fall moves only under the pull of gravity, and near the Earth's surface this gives it a constant acceleration g = 9.8 m/s², directed vertically downwards. Because the acceleration stays the same in both magnitude and direction, the motion is called uniformly accelerated motion.
In such a motion the velocity increases by an equal amount in equal intervals of time — 9.8 m/s in every second. Its velocity–time graph is a straight sloping line and the distance–time graph is a parabola. Also, g does not depend on the mass of the falling body, so in a vacuum a coin and a feather fall side by side.
❌ Why the other options are incorrect
✗ uniform velocity motion — uniform velocity means the velocity does not change at all, so the acceleration would be zero; but a falling body clearly speeds up.
✗ uniform speed motion — for the same reason, the speed of a falling body keeps increasing every second, so it is not uniform.
✗ non-uniformly accelerated motion — this would require the acceleration itself to change; near the Earth's surface g is practically constant, so the acceleration is uniform.
📌 SSC & Railway Exam Facts
• Free fall — motion under gravity alone; g = 9.8 m/s² on the Earth, 1.63 m/s² on the Moon.
• Free-fall equations (u = 0): v = gt, h = ½gt², v² = 2gh.
• g is independent of the mass of the body — the famous Galileo result.
• g = GM/R²; g is maximum at the poles, minimum at the equator, and zero at the centre of the Earth.
• In uniformly accelerated motion the velocity–time graph is a straight line, and the area under it gives the displacement.

Correct Answer: Option 4 — Only (a)
For a spherical mirror of small aperture the focal length is half the radius of curvature: f = R/2. Since the pole P is at one end and the centre of curvature C at a distance R, the principal focus F lies exactly midway between P and C. Hence statement (a) is correct.
Statement (b) is wrong: the focal length depends only on the radius of curvature (the curvature of the surface), not on how large or small a piece of the sphere the mirror is. Two mirrors cut from the same sphere have the same focal length whatever their sizes. Statement (c) is also wrong, because F lies at R/2, i.e. half way to C, and never at C itself.
❌ Why the other options are incorrect
✗ (a) and (b) — (a) is right, but (b) is wrong; the size (aperture) of a mirror does not shift its principal focus.
✗ Only (c) — (c) is false; the focus lies at R/2, and the point at distance R is the centre of curvature, not the focus.
✗ (b) and (c) — both of these statements are wrong, so this combination cannot be correct.
📌 SSC & Railway Exam Facts
• f = R/2, so R = 2f; the principal focus lies midway between the pole and the centre of curvature.
• Pole (P) — centre of the reflecting surface; aperture — effective diameter of the mirror.
• For a concave mirror F is in front of the mirror (real focus); for a convex mirror F is behind it (virtual focus).
• Mirror formula 1/v + 1/u = 1/f; magnification m = −v/u = h'/h.
• Laws of reflection: angle of incidence = angle of reflection, and the incident ray, reflected ray and normal all lie in the same plane.

Correct Answer: Option 2 — The total resistance is always less than the smallest resistance.
In a parallel combination the reciprocals of the resistances add up:
1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃ + …
Adding more paths gives the current more ways to flow, exactly like opening extra doors in a hall, so the equivalent resistance keeps falling. Because 1/R(eq) is larger than each individual reciprocal, R(eq) must be smaller than every one of the resistances, including the smallest.
For example, 3 Ω and 6 Ω in parallel give R(eq) = (3 × 6)/(3 + 6) = 18/9 = 2 Ω, which is less than 3 Ω, the smaller of the two. In parallel the voltage across each resistor is the same while the current divides between them.
❌ Why the other options are incorrect
✗ The total resistance is equal to the smallest resistance — it is strictly less than the smallest; equality would need only one resistor in the circuit.
✗ The total resistance is always greater than the smallest resistance — that is true of a series combination, where R(eq) = R₁ + R₂ + R₃ and is greater than every resistance.
✗ The total resistance is equal to the highest resistance — in parallel the highest resistance carries the least current and cannot decide the total; the answer is far below it.
📌 SSC & Railway Exam Facts
• Series: R(eq) = R₁ + R₂ + R₃, same current through each, greater than the largest resistance.
• Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃, same voltage across each, less than the smallest resistance.
• For two resistors in parallel, R(eq) = R₁R₂/(R₁ + R₂); n equal resistors R in parallel give R/n.
• Ohm's law V = IR; SI unit of resistance is the ohm (Ω).
• Household appliances are connected in parallel so that each gets the full 220 V and works independently.

Correct Answer: Option 1 — 6.7 × 10⁻¹¹ Nm²kg⁻²
Newton's law of universal gravitation states that two bodies of masses m₁ and m₂ whose centres are a distance r apart attract each other with a force
F = Gm₁m₂/r²
Here G is the universal gravitational constant. Its accepted value is G = 6.67 × 10⁻¹¹ N m² kg⁻², rounded in the option to 6.7 × 10⁻¹¹ Nm²kg⁻². It is called *universal* because it has the same value everywhere in the universe, for every pair of bodies and in every medium. Its value was first measured experimentally by Henry Cavendish using a torsion balance. The unit itself confirms the choice: rearranging the law gives G = Fr²/m₁m₂, whose unit is N m²/kg².
❌ Why the other options are incorrect
✗ 9.1 × 10⁻³¹ kg — this is the mass of an electron; the unit kg shows at once that it is a mass, not a gravitational constant.
✗ 1.6 × 10⁻¹⁹ C — this is the charge on an electron (or proton), measured in coulomb (C), an electrical quantity.
✗ 9.8 ms⁻² — this is g, the acceleration due to gravity on the Earth's surface; g changes from place to place, whereas G never changes.
📌 SSC & Railway Exam Facts
• F = Gm₁m₂/r²; G = 6.67 × 10⁻¹¹ N m² kg⁻², a universal constant measured by Henry Cavendish.
• g = GM/R² = 9.8 m/s² — g is not a constant; it varies with height, depth and latitude.
• Gravitational force follows the inverse-square law: double the distance and the force becomes one-fourth.
• Mass of an electron = 9.1 × 10⁻³¹ kg; charge of an electron = 1.6 × 10⁻¹⁹ C.
• Other standard constants: speed of light c = 3 × 10⁸ m/s, Planck's constant h = 6.63 × 10⁻³⁴ J s.