RRB Technician Grade III 2024-25 29 December 2024 Shift 3 GK Questions – with answers & solutions

Correct Answer: Option 1 — Unidirectional
The flow of energy in a food chain or a food web is always unidirectional — it moves in one direction only, from the Sun → producers (green plants) → herbivores → carnivores. Energy captured by producers during photosynthesis never returns to the Sun, and energy passed on to a herbivore never flows back into the plant.
At every trophic level about 90% of the energy is lost as heat, respiration and other life processes, so only about 10% is passed on to the next level — this is Lindeman's 10% Law. Because of this steady loss, a food chain normally has only 3 or 4 trophic levels.
❌ Why the other options are incorrect
✗ Circular — matter and nutrients (carbon, nitrogen) do move in a circular way through cycles, but energy does not; it escapes as heat and cannot be recycled.
✗ Multidimensional — energy does not spread in many directions at a single feeding step; a food web only looks branched because many separate chains are interlinked.
✗ Bidirectional — energy never flows back from a consumer to a producer; a rabbit cannot return energy to the grass it ate.
📌 SSC & Railway Exam Facts
• The term ecosystem was coined by A.G. Tansley (1935); the food chain concept was given by Charles Elton.
• The 10% Law of energy transfer was given by Raymond Lindeman.
• Producers → primary consumers → secondary consumers → tertiary consumers form trophic levels T1–T4.
• Biomagnification — harmful chemicals such as DDT become more concentrated at higher trophic levels.
• Decomposers (bacteria and fungi) return nutrients to the soil and complete the nutrient cycle.

Correct Answer: Option 1 — Hyphae
Rhizopus (bread mould) is a fungus whose body is built of fine, thread-like hyphae that together form a cottony mycelium. Hyphae are vegetative (somatic) structures — they anchor the mould to the bread, absorb food and help it grow. They take no direct part in reproduction.
Rhizopus reproduces asexually by spore formation. Certain hyphae grow upright as sporangiophores, each ending in a swollen, blob-like sporangium. Hundreds of spores are formed inside the sporangium; when it bursts, the light spores are carried by air and germinate on moist bread into a new mould.
❌ Why the other options are incorrect
✗ Sporangiophores — the erect, stalk-like hyphae that hold up the sporangia; they are definitely reproductive structures.
✗ Spores — the actual reproductive units, each with a thick protective wall that helps it survive unfavourable conditions.
✗ Sporangium — the blob-on-a-stick sac inside which the spores are produced; clearly a reproductive part.
📌 SSC & Railway Exam Facts
• Rhizopus, Mucor, ferns and mosses reproduce by spore formation.
• Yeast and Hydra reproduce by budding; Planaria shows regeneration.
• Amoeba and Leishmania show binary fission; Plasmodium shows multiple fission.
• The study of fungi is Mycology; Rhizopus is commonly called bread mould.
• Spores are light and thick-walled and are spread by wind, which is why moist bread moulds so quickly.

Correct Answer: Option 2 — Adipocyte
An adipocyte is a fat cell of adipose tissue, a kind of connective tissue found below the skin and between the internal organs. Its work is to store fat, act as an insulator against heat loss and cushion the organs. It is not a nerve cell and contains no neuron at all.
A neuron (nerve cell) is the structural and functional unit of the nervous system. It has a cell body (cyton) with a single nucleus, many short branches called dendrites that receive impulses, and one long fibre, the axon, which carries the impulse away. Neurons are the longest cells in the human body.
❌ Why the other options are incorrect
✗ Nerves — bundles of axons of many neurons wrapped in connective tissue; they are made of neurons.
✗ Brain — the chief coordinating centre of the body, built of billions of neurons supported by neuroglia cells.
✗ Spinal cord — a cord of nervous tissue inside the vertebral column that carries impulses and controls reflex actions; it is packed with neurons.
📌 SSC & Railway Exam Facts
• The neuron is the longest cell of the human body; the nerve impulse travels at about 100–120 m/s.
• The gap between two neurons is a synapse; between a neuron and a muscle it is a neuromuscular junction.
• The human brain has three parts — fore-brain, mid-brain and hind-brain; the cerebrum is the largest part.
• The reflex arc is completed in the spinal cord, not in the brain, which makes reflexes very fast.
• Adipose tissue stores fat and works as the body's energy reserve and shock absorber.

Correct Answer: Option 3 — (i)-(d), (ii)-(a), (iii)-(b), (iv)-(c)
In the human female reproductive system each organ has one clear role:
• (i) Ovary — (d) makes the female gametes. The pair of ovaries produce the ova (eggs) and also secrete oestrogen and progesterone.
• (ii) Oviduct — (a) fertilisation. The fallopian tube receives the egg from the ovary; the sperm meets the egg here and fertilisation produces the zygote.
• (iii) Uterus — (b) implantation. The zygote divides into an embryo, which becomes embedded in the thick, spongy lining of the uterus — this is implantation.
• (iv) Placenta — (c) nourishes the growing embryo. A disc-like tissue bearing villi that supplies glucose and oxygen to the foetus and removes its wastes.
❌ Why the other options are incorrect
✗ (i)-(b), (ii)-(a), (iii)-(d), (iv)-(c) — wrong; the ovary does not carry out implantation and the uterus does not make gametes.
✗ (i)-(d), (ii)-(b), (iii)-(a), (iv)-(c) — wrong; implantation happens in the uterus, not the oviduct, and fertilisation occurs in the oviduct, not the uterus.
✗ (i)-(d), (ii)-(c), (iii)-(b), (iv)-(a) — wrong; the oviduct does not nourish the embryo and the placenta is not the site of fertilisation.
📌 SSC & Railway Exam Facts
• Fertilisation normally takes place in the ampulla (upper part) of the fallopian tube.
• The human gestation period is about 9 months (280 days).
• The male gamete is the sperm (made in the testes); the female gamete is the ovum (made in the ovary).
• Tubectomy in women and vasectomy in men are surgical methods of contraception.
• If fertilisation does not occur, the uterine lining breaks down every month — this is menstruation.

Correct Answer: Option 3 — It stimulates growth in all organs.
Growth hormone (GH or somatotropin) is secreted by the pituitary gland, the pea-sized "master gland" lying at the base of the brain. As its name says, it regulates the growth and development of the whole body — it stimulates cell division, protein synthesis, elongation of bones and muscle growth in all organs.
Its secretion has to be finely balanced. A deficiency of growth hormone in childhood causes dwarfism (very short stature with normal body proportions), while over-secretion causes gigantism in children and acromegaly (thickened jaw, hands and feet) in adults.
❌ Why the other options are incorrect
✗ It regulates metabolism for body growth — that is the work of thyroxine from the thyroid gland, which controls carbohydrate, protein and fat metabolism.
✗ It regulates the blood sugar level — this is done by insulin and glucagon from the pancreas; lack of insulin causes diabetes mellitus.
✗ It regulates the menstrual cycle — controlled by oestrogen and progesterone from the ovary, along with FSH and LH.
📌 SSC & Railway Exam Facts
• The pituitary is called the master gland because it controls the other endocrine glands.
• Thyroxine needs iodine; its deficiency causes goitre.
• Adrenaline from the adrenal gland is the emergency (fight-or-flight) hormone.
• Testosterone (testes) and oestrogen (ovary) are the chief sex hormones.
• Hormones are chemical messengers poured directly into the blood by ductless (endocrine) glands.

Correct Answer: Option 3 — Lactic acid – 4 carbon molecules
Lactic acid contains 3 carbon atoms (C₃H₆O₃), not 4, so this pair is incorrect. Lactic acid is formed in our muscles during anaerobic respiration, when the oxygen supply falls short during heavy exercise; its accumulation causes muscle cramps.
Respiration begins with glycolysis in the cytoplasm, where one molecule of glucose (6 carbon) is broken into two molecules of pyruvate (3 carbon). Pyruvate then has three possible fates — in yeast it gives ethanol (2 carbon) + CO₂ (fermentation), in muscle it gives lactic acid (3 carbon), and in the presence of oxygen it is broken down in the mitochondria into CO₂ + H₂O, releasing the maximum energy.
❌ Why the other options are incorrect
✗ Ethanol – 2 carbon molecules — a correct pair; ethanol (C₂H₅OH) is produced along with CO₂ during fermentation in yeast.
✗ Pyruvate – 3 carbon molecules — a correct pair; pyruvate is the end product of glycolysis.
✗ Glucose – 6 carbon molecules — a correct pair; glucose (C₆H₁₂O₆) is the starting substrate of respiration.
📌 SSC & Railway Exam Facts
• Glycolysis takes place in the cytoplasm; aerobic breakdown takes place in the mitochondria.
• The mitochondrion is the powerhouse of the cell; energy is stored as ATP.
• Aerobic respiration yields 38 ATP per glucose molecule, anaerobic respiration only 2 ATP.
• Yeast is used in brewing and baking because of alcoholic fermentation.
• Cramps after running are caused by the build-up of lactic acid in the muscles.

Correct Answer: Option 1 — The cell body of a neuron has many nuclei.
This statement is incorrect. The cell body (cyton or soma) of a neuron contains only one nucleus surrounded by granular cytoplasm. Multinucleated cells are found in striated (skeletal) muscle fibres, not in neurons.
A neuron is the structural and functional unit of nervous tissue. From its cell body arise many short, branched dendrites that receive impulses, and a single long fibre, the axon, that carries the impulse away to the next cell. The impulse crosses the gap between two neurons, the synapse, with the help of chemical neurotransmitters.
❌ Why the other options are incorrect
✗ Neurons typically have one long section called the axon — true; the single long axon transmits the impulse away from the cell body.
✗ Each neuron has multiple short, branching sections called dendrites — true; dendrites receive information from other neurons.
✗ Nervous tissue consists of cells known as neurons — true; nervous tissue is made of neurons supported by neuroglia cells.
📌 SSC & Railway Exam Facts
• The neuron is the longest cell in the human body; nerve impulses travel at about 100–120 m/s.
• Nervous tissue occurs in the brain, spinal cord and nerves.
• Skeletal muscle cells are multinucleated; cardiac muscle cells are uninucleated and involuntary.
• The axons of many neurons bundled together form a nerve.
• The junction between two neurons is the synapse; the chemical that crosses it is a neurotransmitter such as acetylcholine.

Correct Answer: Option 4 — Binary fission
Binary fission is an asexual method of reproduction in which a unicellular organism splits into two equal daughter cells. The nucleus divides first (karyokinesis) and then the cytoplasm divides (cytokinesis); each half grows into a complete new organism identical to the parent.
Common examples are Amoeba, Paramoecium, Euglena, Leishmania and bacteria. In Amoeba the split may take place in any plane, in Leishmania it is longitudinal (along the whip-like flagellum) and in Paramoecium it is transverse. As only one parent is involved, the offspring are genetically identical clones.
❌ Why the other options are incorrect
✗ Regeneration — a cut piece of the body grows into a whole organism, as in Hydra and Planaria; the parent is not split into two equal halves.
✗ Spore formation — thick-walled spores are produced inside a sporangium, as in Rhizopus; the body does not split.
✗ Budding — a small bud grows on the parent body and later detaches, as in Yeast and Hydra; the two parts are clearly unequal.
📌 SSC & Railway Exam Facts
• Multiple fission — the parent splits into many daughter cells at once, as in Plasmodium.
• Amoeba divides in any plane; Leishmania divides in a definite (longitudinal) plane.
• Vegetative propagation examples — potato (eyes/buds), Bryophyllum (leaf notches), ginger (rhizome).
• Asexual reproduction produces clones; variation appears mainly in sexual reproduction.
• Plasmodium causes malaria and is carried by the female Anopheles mosquito.

Correct Answer: Option 1 — unicellular
Organisms whose body is made up of a single cell are called unicellular (single-celled) organisms. In them that one cell carries out all the life processes — nutrition, respiration, excretion, growth, movement and reproduction. Examples are Amoeba, Paramoecium, Euglena, Chlamydomonas, bacteria and yeast.
The cell is the basic structural and functional unit of life. It was discovered by Robert Hooke in 1665 in a thin slice of cork, and the first living cell was seen by Anton van Leeuwenhoek. According to the Cell Theory of Schleiden and Schwann, all organisms are made of cells; Rudolf Virchow added that new cells arise from pre-existing cells.
❌ Why the other options are incorrect
✗ multicellular — organisms made of many cells showing division of labour among tissues and organs, e.g. human beings, the mango tree and Rhizopus.
✗ singular — not a biological term at all; it belongs to grammar and simply means "one in number".
✗ unisexual — describes an organism or flower bearing only one sex (male or female), e.g. papaya; it has nothing to do with the number of cells.
📌 SSC & Railway Exam Facts
• Robert Hooke (1665) coined the term cell while observing cork under his microscope.
• Amoeba moves and feeds with pseudopodia; Paramoecium with cilia; Euglena with a flagellum.
• The ostrich egg is the largest cell, Mycoplasma the smallest, and the neuron the longest.
• Unicellular organisms exchange gases by simple diffusion through the body surface.
• Cell theory — Schleiden (1838) and Schwann (1839), extended by Virchow (1855).

Correct Answer: Option 1 — Throw them openly
Non-biodegradable substances such as plastics, polythene, glass, metal cans, DDT and e-waste cannot be broken down by decomposers (bacteria and fungi). Throwing them openly is therefore not a suitable method — they pile up in the environment for hundreds of years, choke drains, pollute soil and water, kill cattle and marine animals, and enter food chains through biomagnification.
The scientific way to handle such waste is the three R's — Reduce, Reuse and Recycle. Cutting down production, using an article again and again, and reprocessing it into new goods all reduce the load of waste on the ecosystem.
❌ Why the other options are incorrect
✗ Reuse them — a very good method; using a bottle or a bag again and again avoids fresh waste and needs no extra energy.
✗ Recycle them — plastics, glass and metals are reprocessed into new articles, saving raw material and energy.
✗ Reduce their production — cutting down the manufacture and use of such items is the best long-term solution.
📌 SSC & Railway Exam Facts
• Biodegradable wastes — vegetable peels, paper, cow dung, cotton; non-biodegradable — plastic, glass, DDT, aluminium.
• Biomagnification — pesticides such as DDT become more concentrated at each higher trophic level.
• Composting and vermicomposting (using earthworms) turn organic waste into manure.
• The Ganga Action Plan (1985) was launched to reduce river pollution.
• Reduce, Reuse, Recycle are the 3 R's of sustainable waste management.

Correct Answer: Option 4 — Pituitary gland
The pituitary gland is a small, pea-sized endocrine gland attached to the base of the brain below the hypothalamus. It secretes growth hormone (GH), which controls the growth and development of bones, muscles and all the organs of the body. Because it also directs the other endocrine glands, it is called the master gland.
A deficiency of growth hormone in childhood causes dwarfism, while over-secretion causes gigantism in children and acromegaly in adults. The pituitary also produces FSH, LH, TSH, ACTH, prolactin and vasopressin (ADH).
❌ Why the other options are incorrect
✗ Thyroid gland — lies in the neck and secretes thyroxine, which regulates the metabolism of carbohydrates, fats and proteins; it needs iodine.
✗ Testes — the male gonads; they secrete testosterone, responsible for male secondary sexual characters such as a beard and a deep voice.
✗ Adrenal gland — sits above each kidney and secretes adrenaline, the fight-or-flight (emergency) hormone, which raises heart rate and blood pressure.
📌 SSC & Railway Exam Facts
• The pituitary is the master gland, lodged in the sella turcica at the base of the brain.
• Iodine deficiency causes goitre; excess thyroxine causes exophthalmic goitre.
• Insulin from the pancreas lowers blood sugar; its deficiency causes diabetes mellitus.
• The pancreas is a mixed gland — both exocrine and endocrine.
• Hormones are poured by ductless glands directly into the blood.

Correct Answer: Option 4 — Bacterial infections : Gonorrhoea and syphilis; Viral infections : Warts and HIV-AIDS
Diseases are classified according to the kind of pathogen that causes them.
Bacterial sexually transmitted diseases — gonorrhoea, caused by the bacterium *Neisseria gonorrhoeae*, and syphilis, caused by *Treponema pallidum*. Being bacterial, both can be cured with antibiotics.
Viral sexually transmitted diseases — warts, caused by the human papilloma virus (HPV), and AIDS, caused by HIV (human immunodeficiency virus). Viral diseases do not respond to antibiotics; HIV destroys the body's immune system, so the patient finally dies of ordinary infections.
❌ Why the other options are incorrect
✗ Bacterial : Warts and HIV-AIDS; Viral : Gonorrhoea and syphilis — completely reversed; warts and AIDS are viral, gonorrhoea and syphilis bacterial.
✗ Bacterial : Warts and syphilis; Viral : Gonorrhoea and HIV-AIDS — wrong; warts are viral while gonorrhoea is bacterial.
✗ Bacterial : Warts and gonorrhoea; Viral : Syphilis and HIV-AIDS — wrong; warts are viral while syphilis is bacterial.
📌 SSC & Railway Exam Facts
• AIDS is caused by HIV, spread by sexual contact, infected blood, shared needles and from mother to child.
• Using a condom prevents STDs; World AIDS Day is 1 December.
• Bacterial diseases — TB, typhoid, cholera, tetanus, anthrax and plague.
• Viral diseases — common cold, influenza, dengue, polio, rabies, measles and chikungunya.
• Antibiotics such as penicillin (discovered by Alexander Fleming) act on bacteria, not on viruses.

Correct Answer: Option 3 — Plasmodium
Multiple fission is an asexual method in which the nucleus of the parent cell divides many times inside a protective covering, and then the cytoplasm splits so that many daughter individuals are released at once. Plasmodium, the malarial parasite, multiplies exactly like this inside the liver cells and red blood cells of a human being; when the RBCs burst, the released parasites produce the typical bouts of fever with chills.
Multiple fission usually occurs under unfavourable conditions, and it allows a single parent to produce a very large number of offspring in a very short time.
❌ Why the other options are incorrect
✗ Leishmania — reproduces by binary fission in a definite (longitudinal) plane, along its whip-like flagellum; it causes kala-azar.
✗ Yeast — a unicellular fungus that reproduces by budding, producing a small outgrowth that detaches later.
✗ Hydra — a simple multicellular animal that reproduces by budding and also shows regeneration.
📌 SSC & Railway Exam Facts
• Plasmodium causes malaria and is carried by the female Anopheles mosquito.
• Ronald Ross discovered the malarial parasite in the mosquito (Nobel Prize, 1902).
• Amoeba also shows multiple fission by forming a cyst under unfavourable conditions.
• Binary fission — Amoeba, Paramoecium, Leishmania, bacteria; budding — yeast and Hydra.
• Quinine, obtained from the bark of Cinchona, is the classic anti-malarial drug.

Correct Answer: Option 4 — Only (ii) and (iii)
Statement (i) is wrong: UV (ultra-violet) radiation is highly damaging to living organisms. In human beings it causes skin cancer, cataract and damage to the immune system, and it also harms crops and planktons.
Statement (ii) is correct: high up in the atmosphere UV rays split molecular oxygen (O₂) into free oxygen atoms, and these atoms combine with O₂ molecules to form ozone (O₃). So ozone at higher levels is indeed a product of UV radiation acting on oxygen molecules.
Statement (iii) is correct: the amount of ozone in the atmosphere began to drop sharply in 1980, mainly because of CFCs (chlorofluorocarbons) used in refrigerators and fire extinguishers, producing the famous ozone hole over Antarctica.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — wrong because it accepts statement (i); UV radiation is definitely damaging to living organisms.
✗ Only (i) and (ii) — wrong; (i) is false, and this option also leaves out the true statement (iii).
✗ Only (i) — wrong; (i) is in fact the only false statement, while (ii) and (iii) are both true.
📌 SSC & Railway Exam Facts
• Ozone (O₃) occurs in the stratosphere and forms the protective ozone shield.
• The reactions are O₂ → O + O (by UV) and then O + O₂ → O₃.
• The ozone hole over Antarctica was reported in 1985.
• The Montreal Protocol (1987) froze and then phased out CFC production world-wide.
• 16 September is observed as the International Day for the Preservation of the Ozone Layer.

Correct Answer: Option 2 — Carboxylic acid
Soaps are the sodium or potassium salts of long chain carboxylic acids (fatty acids). A carboxylic acid carries the –COOH functional group; when it is neutralised by NaOH or KOH, the acidic hydrogen is replaced by the metal and a –COO⁻Na⁺ salt is formed. Sodium stearate, C₁₇H₃₅COONa, is a typical soap.
This structure is exactly what makes soap clean. The long hydrocarbon chain is hydrophobic and dissolves in grease, while the ionic –COO⁻Na⁺ head is hydrophilic and stays in water. Many such molecules arrange themselves into spherical micelles that trap the dirt and are rinsed away.
Soaps are manufactured by saponification — boiling a vegetable oil or animal fat (an ester of a fatty acid) with sodium hydroxide. Sodium soaps are hard soaps; potassium soaps are soft soaps.
⚗️ The Reaction — Saponification
(C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃
• The fat (an ester) is hydrolysed by alkali into soap and glycerol.
• C₁₇H₃₅COOH is stearic acid, a long chain carboxylic acid — this is the acid the question is asking about.
• Glycerol is the valuable by-product, used in medicines and cosmetics.
❌ Why the other options are incorrect
✗ Nitric acid — a strong mineral acid (HNO₃) used in making fertilisers and explosives; its sodium salt is sodium nitrate, not a soap.
✗ Sulphuric acid — the "king of chemicals" (H₂SO₄); its long chain sulphonate/sulphate salts give detergents, not soaps.
✗ Hydrochloric acid — a strong mineral acid (HCl) present in gastric juice; its sodium salt is simply common salt, NaCl.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of a long chain carboxylic acid; detergent = sodium salt of a long chain benzene sulphonic acid or sulphate.
• Functional group of a carboxylic acid is –COOH; its IUPAC suffix is -oic acid.
• Ethanoic acid, CH₃COOH, is the commonest carboxylic acid — vinegar is its 5–8% solution.
• Soap does not lather in hard water; Ca²⁺/Mg²⁺ ions form an insoluble scum.
• The cleansing action of soap depends on micelle formation, which shows the Tyndall effect.

Correct Answer: Option 2 — It will cause the palm to feel cool.
Acetone (propanone, CH₃COCH₃) is a highly volatile liquid — its boiling point is only about 56 °C and the forces holding its molecules together are weak. Poured on the palm, it therefore evaporates very rapidly.
Evaporation is a cooling process. To change from liquid to vapour, the escaping particles must absorb the latent heat of vaporisation, and the nearest source of that heat is the skin of the palm. As heat is drawn out of the palm, its temperature falls and we feel a distinct coolness.
The same physics explains why petrol or spirit spilt on the hand feels cold, why a nurse rubs spirit on the skin before an injection, and why water kept in an earthen pot (matka) stays cool.
❌ Why the other options are incorrect
✗ It will cause the palm to feel itchy — itching is an allergic or irritant response; the ordinary physical effect of acetone on the skin is cooling by evaporation, not itching.
✗ It will cause no effect on the palm — a clear cooling sensation is always felt, because acetone is far more volatile than water.
✗ It will cause the palm to feel hot — the very opposite; evaporation absorbs heat, it never releases it. Heating would require an exothermic change such as condensation.
📌 SSC & Railway Exam Facts
• Evaporation is a surface phenomenon and always causes cooling; it occurs at all temperatures below the boiling point.
• Evaporation is faster with higher temperature, larger surface area, more wind and lower humidity.
• Volatile liquids — acetone, petrol, ether, spirit — evaporate fastest and give the strongest cooling.
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg; boiling point of acetone ≈ 56 °C.
• Acetone (propanone) is the simplest ketone, used as a nail-polish remover and a laboratory solvent.

Correct Answer: Option 2 — The solute is present in a smaller amount than the solvent.
A solution is a homogeneous mixture of two or more substances. It has two components: the solute, which is the substance present in the smaller amount and which dissolves, and the solvent, the substance present in the larger amount which does the dissolving. In sugar solution, sugar is the solute and water the solvent; water is called the universal solvent.
Because the dissolved particles are smaller than 1 nm, a solution is perfectly uniform throughout, is transparent and stable, does not scatter light (no Tyndall effect) and the solute never settles down on standing.
❌ Why the other options are incorrect
✗ The concentration of a solution can only be expressed in terms of mass per cent — the word *only* makes it wrong; concentration can also be given as volume per cent, mass by volume per cent, molarity, molality, normality or ppm.
✗ A solution is a heterogeneous mixture — a true solution is homogeneous; heterogeneous mixtures are suspensions and colloids.
✗ Solutions can be separated using simple filtration — solute particles are far too small to be retained by filter paper; a solution is separated by evaporation, crystallisation or distillation.
📌 SSC & Railway Exam Facts
• Solution = solute + solvent; the solvent is the larger component.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Mass per cent = (mass of solute ÷ mass of solution) × 100.
• A saturated solution can dissolve no more solute at that temperature; the amount dissolved in 100 g of solvent is its solubility.
• Alloys (brass, bronze, steel) are solid solutions; air is a gaseous solution; soda water is a gas dissolved in a liquid.

Correct Answer: Option 3 — Position of the metals in the reactivity series
A displacement reaction follows the pattern A + BC → AC + B and takes place only when the free element A is more reactive than B. Whether that condition is met is read directly off the reactivity series (activity series) — the list of metals arranged in decreasing order of reactivity:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
A metal higher in the series loses electrons more readily, so it displaces any metal placed below it from that metal's salt solution. A metal lower in the series can never displace one above it, no matter how much the temperature or concentration is changed. So the relative position in the reactivity series is the deciding factor.
⚗️ The Reaction
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Zn is above Cu, so the reaction happens — the blue solution fades and reddish-brown copper is deposited.
• Cu(s) + ZnSO₄(aq) → no reaction, because Cu is below Zn.
• Every displacement reaction is also a redox reaction — the displacing metal is oxidised, the displaced metal ion is reduced.
❌ Why the other options are incorrect
✗ Temperature of the reaction — raising the temperature can make a reaction faster, but it cannot make a less reactive metal displace a more reactive one.
✗ Size of the metal atoms — atomic size influences reactivity indirectly through ionisation energy, but it is not the factor used to predict displacement.
✗ Concentration of the compound — concentration affects the rate of reaction only; a reaction that is impossible by the reactivity series stays impossible at any concentration.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• Potassium is the most reactive and gold the least reactive metal of the series.
• Metals above hydrogen displace H₂ from dilute acids; Cu, Hg, Ag, Au do not.
• Fe + CuSO₄ → FeSO₄ + Cu (blue → pale green) is the classic PYQ.
• The thermite reaction, 2Al + Fe₂O₃ → 2Fe + Al₂O₃, is a displacement used to weld railway tracks.

Correct Answer: Option 3 — Tartaric acid
Baking powder = baking soda (sodium hydrogencarbonate, NaHCO₃) + a mild edible acid, and the acid normally used is tartaric acid.
When baking powder is mixed into the dough and heated, the NaHCO₃ decomposes to give carbon dioxide. The CO₂ bubbles get trapped in the dough and make the cake rise and turn soft and spongy.
If baking soda alone were used, the decomposition would leave behind sodium carbonate (Na₂CO₃), which is strongly alkaline and makes the cake bitter in taste. The tartaric acid neutralises this sodium carbonate, so the bitterness never appears. That is the whole purpose of adding the acid.
⚗️ The Reaction
NaHCO₃ + H⁺ (from tartaric acid) → CO₂↑ + H₂O + sodium salt of the acid
• On heating alone: 2NaHCO₃ → (heat) Na₂CO₃ + H₂O + CO₂↑ — the Na₂CO₃ left behind is what tastes bitter.
• With tartaric acid present, that Na₂CO₃ is neutralised, leaving only sodium tartrate, which is harmless.
• The trapped CO₂ is what makes bread, cake and idli rise.
❌ Why the other options are incorrect
✗ Sodium chloride — common salt (NaCl) is a neutral salt, not an acid; it only adds taste and cannot neutralise sodium carbonate.
✗ Vinegar — vinegar is a dilute (5–8%) liquid solution of ethanoic acid; being a liquid it cannot be packed in a dry powder, though it does react with baking soda in the classic fizzing experiment.
✗ Lactic acid — the acid of curd and sour milk, also formed in muscles during heavy exercise; it is not the acid used in commercial baking powder.
📌 SSC & Railway Exam Facts
• Baking soda = NaHCO₃ (sodium hydrogencarbonate/bicarbonate), mildly alkaline, also used in soda-acid fire extinguishers and as an antacid.
• Washing soda = Na₂CO₃·10H₂O; bleaching powder = CaOCl₂; plaster of Paris = CaSO₄·½H₂O.
• Common acids: citric acid — lemon, lactic acid — curd, acetic acid — vinegar, oxalic acid — tomato, tartaric acid — tamarind, formic acid — ant sting.
• CO₂ turns lime water milky; this is its confirmatory test.
• All three — baking soda, washing soda and bleaching powder — are made from common salt (NaCl).

Correct Answer: Option 4 — Black-coloured copper (II) oxide
When copper is heated strongly in air, the oxygen of the air combines with it and the shiny reddish-brown metal becomes coated with a black layer. This black substance is copper(II) oxide, CuO, in which copper shows the +2 oxidation state. The reaction is a combination reaction and also an oxidation.
The colour is the giveaway in this question: CuO is black, while copper(I) oxide, Cu₂O, is red/reddish-brown. So both the colour (black) and the oxidation state (II) in Option 4 are right.
The change can be reversed — passing hydrogen gas over hot CuO turns the black layer back to the brown colour of copper metal, a standard reduction demonstration.
⚗️ The Reaction
2Cu + O₂ → (heat) 2CuO
• Copper is oxidised; the black coating on the surface is CuO.
• Reverse reduction: CuO + H₂ → (heat) Cu + H₂O — black turns back to brown.
• This is why copper vessels turn dull and blackish when heated in air.
❌ Why the other options are incorrect
✗ Brown-coloured copper(II) oxide — the oxidation state is right but the colour is wrong; CuO is black, and brown is the colour of copper metal itself.
✗ Blue-coloured copper(II) oxide — blue is the colour of hydrated copper sulphate, CuSO₄·5H₂O, not of copper oxide.
✗ Black-coloured copper (I) oxide — the colour is right but the oxidation state is wrong; copper(I) oxide, Cu₂O, is red, not black.
📌 SSC & Railway Exam Facts
• CuO = copper(II) oxide = black; Cu₂O = copper(I) oxide = red.
• 2Cu + O₂ → 2CuO is a combination as well as an oxidation reaction.
• Copper exposed to moist air slowly forms a green coating of basic copper carbonate, CuCO₃·Cu(OH)₂ — the corrosion of copper.
• CuSO₄·5H₂O = blue vitriol (blue); on heating it loses water and turns white.
• Copper is a very good conductor of heat and electricity, so it is used in wires and utensils; symbol Cu from Latin cuprum.

Correct Answer: Option 2 — Nucleus
In 1911 Ernest Rutherford directed a beam of fast-moving alpha (α) particles — doubly-charged helium ions, He²⁺ — at a very thin gold foil and watched where they landed on a fluorescent screen.
Three observations came out of it: most α-particles passed straight through the foil, a few were deflected through noticeable angles, and about 1 in 12,000 bounced straight back.
From these he concluded that an atom is mostly empty space, and that all of its positive charge and nearly its whole mass are concentrated in an extremely small, dense central body which he named the nucleus. The electrons revolve around this nucleus in circular paths called orbits. The nucleus is roughly 10⁻¹⁵ m across against the atom's 10⁻¹⁰ m — the atom is about 100,000 times larger than its nucleus.
❌ Why the other options are incorrect
✗ Neutron — discovered much later, in 1932, by James Chadwick; it is the neutral particle of the nucleus.
✗ Electron — discovered by J. J. Thomson (1897) through his cathode ray (discharge tube) experiments.
✗ Proton — the positive canal rays were found by E. Goldstein (1886), and the proton was later identified and named by Rutherford, but not by the gold foil experiment itself.
📌 SSC & Railway Exam Facts
• Rutherford's α-scattering (gold foil) experiment → discovery of the nucleus; he is the Father of Nuclear Physics.
• Electron — J. J. Thomson, Proton — Goldstein/Rutherford, Neutron — James Chadwick (1932).
• Defect of Rutherford's model: a revolving electron should radiate energy and fall into the nucleus — corrected by Niels Bohr.
• An α-particle is a helium nucleus (He²⁺): 2 protons + 2 neutrons, mass 4 u, charge +2.
• Thomson's plum-pudding (watermelon) model was disproved by this very experiment.

Correct Answer: Option 2 — CH₄
A covalent compound is formed when two atoms share electrons instead of transferring them. Sharing happens between two non-metals, because neither of them can easily give its electrons away.
Methane, CH₄, is the textbook example. Carbon has 4 valence electrons and needs 4 more to complete its octet; instead of losing or gaining 4 electrons (which would need far too much energy), it shares one electron with each of four hydrogen atoms, forming four single covalent bonds. Both carbon and hydrogen are non-metals, so CH₄ is covalent.
An ionic compound, by contrast, forms between a metal and a non-metal by complete transfer of electrons. Simply look for a metal in the formula: if there is one, the compound is almost always ionic.
❌ Why the other options are incorrect
✗ MgO — ionic; magnesium (a metal) gives away 2 electrons to oxygen, producing Mg²⁺ and O²⁻.
✗ MgCl₂ — ionic; magnesium transfers one electron to each of two chlorine atoms, giving Mg²⁺ and 2Cl⁻.
✗ NaCl — the classic ionic compound; sodium loses one electron to chlorine to form Na⁺ and Cl⁻, held together by strong electrostatic force.
📌 SSC & Railway Exam Facts
• Metal + non-metal → ionic bond (transfer); non-metal + non-metal → covalent bond (sharing).
• Covalent compounds have low melting and boiling points, are poor conductors and are generally insoluble in water but soluble in organic solvents.
• Ionic compounds are hard crystalline solids with high melting points, soluble in water, and conduct electricity in molten or aqueous state (not when solid).
• Bond types: single bond (H₂, CH₄, Cl₂), double bond (O₂, C₂H₄, CO₂), triple bond (N₂, C₂H₂).
• Methane (CH₄) is the first member of the alkane series and the main constituent of CNG, biogas and natural gas; it is also a greenhouse gas (marsh gas).

Correct Answer: Option 1 — CₙH₂ₙ₊₂
Alkanes are saturated hydrocarbons — every carbon atom is joined to its neighbours by single covalent bonds (C–C) only, and all the remaining valencies are satisfied by hydrogen. Because each carbon holds the maximum possible number of hydrogen atoms, the general formula works out to CₙH₂ₙ₊₂, where n is the number of carbon atoms.
Putting values of n into it gives the whole homologous series: n = 1 → CH₄ (methane), n = 2 → C₂H₆ (ethane), n = 3 → C₃H₈ (propane), n = 4 → C₄H₁₀ (butane). Each member differs from the next by a –CH₂– unit (14 u), which is the defining property of a homologous series.
Alkanes burn in air with a clean blue flame and are relatively unreactive, which is why they are also called paraffins (Latin *parum affinis* — little affinity).
❌ Why the other options are incorrect
✗ CₙHₙ — this fits nothing in the basic series; it would wrongly suggest one hydrogen per carbon (it is closer to the formula of benzene, C₆H₆, an aromatic compound).
✗ C₂ₙH₂ₙ — on simplifying, the carbon count is doubled for no reason; it is just CₙH₂ₙ written misleadingly and still describes an alkene.
✗ CₙH₂ₙ — this is the general formula of an alkene, an unsaturated hydrocarbon with a C=C double bond (e.g. ethene, C₂H₄).
📌 SSC & Railway Exam Facts
• Alkane CₙH₂ₙ₊₂ (single bond); alkene CₙH₂ₙ (double bond); alkyne CₙH₂ₙ₋₂ (triple bond).
• First members: methane CH₄, ethene C₂H₄, ethyne C₂H₂ (acetylene) — used with oxygen for welding.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones give a yellow sooty flame.
• Successive members of a homologous series differ by –CH₂–, i.e. 14 u, and have similar chemical properties.
• Methane is the main constituent of CNG, biogas and natural gas; it is also called marsh gas.

Correct Answer: Option 1 — All metals are equally reactive.
This is an INCORRECT-type question, so the false statement is the answer. Metals are not equally reactive at all — their reactivities differ so widely that chemists arrange them in a reactivity series:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
Potassium and sodium react violently even with cold water, magnesium needs hot water, iron and zinc need steam, while silver and gold do not react with water or even with dilute acids. Clearly, Option 1 is false and is the required answer.
The remaining three statements are all true. Option 3 is simply the correct opposite of Option 1. Silver lies far below sodium, so it is less reactive. Potassium is the most reactive metal of the series and gold the least reactive, which is why gold is found free (native) in nature and does not tarnish.
❌ Why the other options are incorrect
✗ Silver is less reactive than sodium — true; Ag lies near the bottom of the series while Na is near the top.
✗ All metals are not equally reactive — true; this is precisely why the reactivity series exists.
✗ Potassium is more reactive than gold — true; K is the most reactive and Au the least reactive metal listed.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• With water: cold water — K, Na, Ca; hot water — Mg; steam — Al, Zn, Fe; no reaction — Pb, Cu, Ag, Au.
• Metals above hydrogen displace H₂ from dilute acids; those below it do not.
• Gold, silver and platinum are called noble metals because they resist corrosion and occur native.
• Sodium and potassium are stored in kerosene because they react vigorously with air and moisture.

Correct Answer: Option 1 — Neutralisation reaction
The reaction in which an acid reacts with a base to give a salt and water is called a neutralisation reaction. Its general form is:
Acid + Base → Salt + Water
At the ionic level, the H⁺ ions of the acid combine with the OH⁻ ions of the base to form water, so the acidic and basic characters cancel each other out — the mixture becomes neutral, with pH close to 7. Neutralisation is always an exothermic reaction (heat is released) and is also a form of double displacement.
Everyday uses are everywhere: antacids (Mg(OH)₂, NaHCO₃) neutralise excess stomach acid, baking soda on a bee sting relieves the formic acid, lime is added to acidic soil, and calamine (zinc carbonate) soothes an ant bite.
⚗️ The Reaction
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• The acid's H⁺ and the base's OH⁻ unite: H⁺ + OH⁻ → H₂O.
• The leftover Na⁺ and Cl⁻ form the salt, sodium chloride.
• Another standard example: Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O.
❌ Why the other options are incorrect
✗ Polymerisation reaction — the joining of many small monomers into a giant molecule (ethene → polythene); no acid, base, salt or water is involved.
✗ Photosynthesis reaction — a biological process in green plants: 6CO₂ + 6H₂O → (sunlight, chlorophyll) C₆H₁₂O₆ + 6O₂; it makes glucose, not a salt.
✗ Hydrolysis reaction — here a compound is broken down by water, e.g. a salt of a weak acid dissolving to give an alkaline solution; it is the reverse idea, not acid meeting base.
📌 SSC & Railway Exam Facts
• Acid + base → salt + water is neutralisation, an exothermic double displacement reaction.
• pH scale runs 0–14: < 7 acidic, = 7 neutral, > 7 basic; it was given by S. P. L. Sørensen.
• Acid + metal → salt + H₂; acid + metal carbonate → salt + water + CO₂.
• Strong acid + strong base → neutral salt (NaCl); strong acid + weak base → acidic salt (NH₄Cl); weak acid + strong base → basic salt (Na₂CO₃).
• Tooth decay begins below pH 5.5; toothpastes are basic to neutralise the acid formed in the mouth.

Correct Answer: Option 1 — Homogeneous mixture
Air is a homogeneous mixture of several gases. Clean, dry air contains roughly 78% nitrogen, 21% oxygen, 0.9% argon and 0.03–0.04% carbon dioxide, together with traces of neon, helium, water vapour and dust.
It is called homogeneous because the gases are uniformly mixed right down to the molecular level — a sample of air taken from any part of a room has the same composition, and no boundary can be seen between the different gases. In fact air is often described as a gaseous solution, with nitrogen as the solvent (largest component) and the other gases as solutes.
Air is a mixture and not a compound because its components keep their own properties, are present in variable proportions, and can be separated by physical means — the fractional distillation of liquefied air, with no energy change involved in mixing.
❌ Why the other options are incorrect
✗ Colloids — a colloid needs particles of 1–1000 nm dispersed in a medium and shows the Tyndall effect; the gases of air are individual molecules far smaller than 1 nm. (Smoke and fog *are* colloids, but they are not pure air.)
✗ Suspension — a suspension has particles larger than 1000 nm that settle on standing and can be filtered; the gases of air do neither.
✗ Heterogeneous mixture — air has no visible boundary between its components and is uniform throughout, so it is not heterogeneous.
📌 SSC & Railway Exam Facts
• Composition of air: N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.03–0.04%.
• Air is a mixture, water (H₂O) is a compound — a very common PYQ pairing.
• Air's components are separated by the fractional distillation of liquid air; liquid nitrogen boils at −196 °C.
• Other homogeneous mixtures: sugar solution, salt solution, alloys (brass, bronze, steel), soda water.
• Heterogeneous mixtures: sand and water, oil and water, soil, muddy water.

Correct Answer: Option 4 — Both a and b
Statement (a) is correct. An acid shows its acidic behaviour only in the presence of water. Dry HCl gas does not ionise — it does not turn dry blue litmus red and does not conduct electricity. Water molecules are needed to pull the H⁺ ion away from the chloride, because the separation of ions requires the large energy of hydration supplied by water. That is exactly why dry HCl cannot behave as an acid.
Statement (b) is also correct. A bare H⁺ ion is simply a proton and is far too small and highly charged to exist alone in solution. It immediately attaches itself to a water molecule to form the hydronium ion, H₃O⁺. Hence hydrogen ions in an aqueous solution must always be written as H⁺(aq) or as H₃O⁺.
Both statements are the standard NCERT statements, so the answer is Both a and b.
⚗️ The Reaction
HCl + H₂O → H₃O⁺ + Cl⁻
• The H⁺ released by HCl combines with H₂O to give the hydronium ion, H₃O⁺.
• H⁺ + H₂O → H₃O⁺ is therefore the correct way to show a hydrogen ion in water.
• Dissolving an acid in water is highly exothermic — always add acid slowly to water, never water to acid.
❌ Why the other options are incorrect
✗ Neither a nor b — both statements are true, so this is completely wrong.
✗ Only a — statement (b) is equally true; a free proton cannot exist in water and becomes H₃O⁺.
✗ Only b — statement (a) is equally true; dry HCl gas does not ionise and so shows no acidic character.
📌 SSC & Railway Exam Facts
• Acids produce H⁺(aq)/H₃O⁺ ions in water; bases produce OH⁻ ions — the Arrhenius definition.
• Dry HCl gas does not turn dry blue litmus red; moist litmus is needed.
• The dissolving of an acid or base in water is called dilution and is a highly exothermic process.
• Acids and bases conduct electricity in aqueous solution because they furnish free ions.
• Strong acids (HCl, H₂SO₄, HNO₃) ionise completely; weak acids (CH₃COOH, H₂CO₃) ionise partially.

Correct Answer: Option 2 — 0
📖 Given
• Constant force applied, F = 2 N
• Displacement in the direction of the force, s = 0
✍️ Formula
W = F × s (or W = F s cos θ in general)
🔢 Solution
• W = F × s
• W = 2 × 0
• W = 0 joule
• In physics, work is done only when the point of application of the force actually moves along the force. If that displacement is zero, the work is zero no matter how large the force is.
• The same result follows from W = F s cos θ — with s = 0, the whole product vanishes.
In plain language: a man pushing hard against a stationary wall gets tired, but from the point of view of physics he does no work at all, because the wall does not move.
❌ Why the other options are incorrect
✗ 1 joules — this would need a displacement of 0.5 m; here the displacement along the force is stated to be zero.
✗ 2 joules — this is the mistake of simply copying the force value (2 N) as if s = 1 m; the given s is 0.
✗ 20 joules — this comes from multiplying 2 N by a stray 10 m; no such displacement exists in the question.
📌 SSC & Railway Exam Facts
• W = F s cos θ — work is zero when s = 0, when F = 0, or when θ = 90°.
• SI unit of work is the joule (J); 1 J = 1 N m; CGS unit is the erg, 1 J = 10⁷ erg.
• Work is a scalar quantity even though force and displacement are vectors.
• The centripetal force in uniform circular motion does zero work, since it is always perpendicular to the motion.
• Work–energy theorem: net work done = change in kinetic energy; zero work means no change in speed.

Correct Answer: Option 3 — 300 C
📖 Given
• Current, I = 1 A
• Time, t = 5 minutes
• To find: charge Q flowing through the circuit
✍️ Formula
I = Q/t, therefore Q = I × t
🔢 Solution
• First convert the time into seconds, because the ampere is defined per second: t = 5 × 60 = 300 s.
• Q = I × t
• Q = 1 × 300
• Q = 300 C
• So 300 coulomb of charge passes through the filament in those 5 minutes.
In plain language, a current of 1 ampere means 1 coulomb of charge flows every second; in 300 seconds that is simply 300 coulomb.
❌ Why the other options are incorrect
✗ 5 C — this is the mistake of using t = 5 minutes directly as seconds, forgetting the conversion 1 min = 60 s.
✗ 10 C — no combination of 1 A with the given time gives 10 C; it appears to come from an arbitrary 10 s.
✗ 150 C — this comes from halving the time (using 150 s instead of 300 s), i.e. taking 1 minute as 30 seconds.
📌 SSC & Railway Exam Facts
• Q = It; SI unit of charge is the coulomb (C), of current the ampere (A); 1 A = 1 C/s.
• Charge on one electron = −1.6 × 10⁻¹⁹ C; so 1 coulomb ≈ 6.25 × 10¹⁸ electrons.
• Current is measured by an ammeter, connected in series; potential difference by a voltmeter, connected in parallel.
• Conventional current flows from + to − outside the cell, opposite to the direction of electron flow.
• Ohm's law: V = IR; electric power P = VI = I²R = V²/R.

Correct Answer: Option 4 — +1 J
📖 Given
• Constant force applied, F = 1 N
• Displacement, s = 1 m, and it is in the direction of the force, so θ = 0°
✍️ Formula
W = F s cos θ
🔢 Solution
• Since the displacement is along the force, θ = 0° and cos 0° = +1.
• W = F × s × cos 0°
• W = 1 × 1 × 1
• W = +1 J
• The positive sign shows that the force adds energy to the book — its kinetic energy increases.
In plain language, 1 joule is defined as the work done when a force of 1 newton moves a body through 1 metre in its own direction. This question is simply the definition of the joule.
❌ Why the other options are incorrect
✗ 0 — zero work would need either s = 0 or the displacement perpendicular to the force; here the book clearly moves along the force.
✗ 2 J — this is the error of adding 1 + 1 instead of multiplying, or of assuming a 2 m displacement.
✗ −1 J — the magnitude is right but the sign is wrong; negative work occurs when the displacement is opposite to the force, as with friction.
📌 SSC & Railway Exam Facts
• 1 joule = 1 newton × 1 metre — the definition of the SI unit of work and energy.
• W = F s cos θ: positive for θ = 0°, negative for θ = 180°, zero for θ = 90°.
• Friction and air resistance always do negative work on a moving body.
• 1 J = 10⁷ erg; 1 calorie = 4.2 J; 1 kWh = 3.6 × 10⁶ J.
• Power = W/t, SI unit watt (W); 1 W = 1 J/s, 1 hp = 746 W.

Correct Answer: Option 4 — 1/2 m v²
📖 Given
• Mass of the object = m
• Uniform velocity of the object = v
• To find: the expression for its kinetic energy E
✍️ Formula
E = ½mv²
🔢 Solution
• Suppose a body of mass m starts from rest and is accelerated uniformly to a velocity v over a displacement s by a force F.
• From the third equation of motion, v² = u² + 2as with u = 0, we get s = v²/2a.
• Work done by the force, W = F × s = (ma) × (v²/2a).
• The acceleration a cancels, leaving W = ½mv².
• This work is stored in the body as its kinetic energy, so E = ½mv².
In plain language, the kinetic energy is half the mass times the square of the speed — the factor ½ is not optional, it comes straight out of the derivation.
❌ Why the other options are incorrect
✗ 1/3 m v² — the coefficient is wrong; the derivation gives ½, never ⅓.
✗ m v⁻² — here v is in the denominator, which would mean a faster body has less energy — physically impossible.
✗ m v² — the ½ factor has been dropped; this expression is twice the actual kinetic energy.
📌 SSC & Railway Exam Facts
• KE = ½mv²; also KE = p²/2m and p = √(2m·KE), where p = mv is the momentum.
• KE ∝ v² — doubling the speed makes the kinetic energy four times.
• Potential energy PE = mgh; total mechanical energy = KE + PE is conserved in the absence of friction.
• SI unit of energy: joule (J); 1 J = 1 N m = 10⁷ erg; 1 eV = 1.6 × 10⁻¹⁹ J.
• Kinetic energy is a scalar and is always positive, whatever the direction of motion.

Correct Answer: Option 3 — displacement
Displacement is defined as the shortest distance between the initial and the final position of a moving object, measured in a definite direction. It is therefore a vector quantity: it has both magnitude and direction. Its SI unit is the metre (m).
Displacement must be carefully distinguished from distance. Distance is the total length of the actual path covered, is a scalar, and can never be negative or zero for a moving body. Displacement, on the other hand, can be positive, negative or even zero. If a boy runs once around a circular track and comes back to the starting point, his distance equals the circumference of the track, but his displacement is zero.
Also, displacement ≤ distance always. The two are equal only when the body moves along a straight line in one direction without turning back.
❌ Why the other options are incorrect
✗ acceleration — the rate of change of velocity, a = (v − u)/t, measured in m s⁻², not a length at all.
✗ velocity — displacement per unit time, measured in m/s; it is a rate, whereas displacement is a length.
✗ speed — distance per unit time, a scalar measured in m/s; again a rate, not the shortest length between two positions.
📌 SSC & Railway Exam Facts
• Distance — scalar, total path length; Displacement — vector, shortest straight-line length between the two positions.
• |Displacement| ≤ Distance; they are equal only for motion in a straight line without reversal.
• Speed = distance/time (scalar); Velocity = displacement/time (vector); both in m/s.
• For one complete circular round, displacement = 0 but distance = 2πr.
• In a displacement–time graph, the slope gives velocity; in a velocity–time graph, the slope gives acceleration and the area gives displacement.

Correct Answer: Option 1 — 5 A
📖 Given
• Resistance of the geyser, R = 4 Ω
• Heat produced per second, H/t = 100 J/s, i.e. power P = 100 W
• To find: current I
✍️ Formula
H = I²Rt (Joule's law of heating), so P = H/t = I²R
🔢 Solution
• P = I²R
• 100 = I² × 4
• I² = 100/4
• I² = 25
• I = √25 = 5 A
In plain language, "100 joule of heat per second" is just another way of saying the geyser consumes 100 watt of power; putting that into P = I²R with R = 4 Ω gives a current of 5 ampere.
❌ Why the other options are incorrect
✗ 25 A — this is the value of I², not of I; the final square root step was forgotten.
✗ 2.5 A — this comes from wrongly using P = 2IR or from halving the correct answer; check: 2.5² × 4 = 25 W, not 100 W.
✗ 50 A — far too large; 50² × 4 = 10,000 W, a hundred times the given power.
📌 SSC & Railway Exam Facts
• Joule's law of heating: H = I²Rt — heat ∝ I², ∝ R and ∝ t.
• Electric power: P = VI = I²R = V²/R; SI unit watt (W).
• Heating appliances (geyser, heater, iron, toaster) use nichrome, because of its high resistivity and high melting point, and because it does not oxidise easily.
• A fuse wire works on the same heating effect; it has a low melting point and is connected in series with the live wire.
• 1 unit of electricity = 1 kWh = 3.6 × 10⁶ J.

Correct Answer: Option 2 — equal to 2f
📖 Given
• Mirror is concave, focal length = f
• The image formed is of the same size as the object, i.e. |m| = 1
• To find: the object distance u
✍️ Formula
1/v + 1/u = 1/f and m = −v/u
🔢 Solution
• Same size means |m| = 1. For a concave mirror this image is real and inverted, so m = −1.
• From m = −v/u = −1, we get v = u.
• Substituting in the mirror formula: 1/u + 1/u = 1/f, i.e. 2/u = 1/f.
• Therefore u = 2f — the object is at a distance of twice the focal length.
• Since R = 2f, the object is exactly at the centre of curvature C, and the image is also formed at C.
In plain language: place the object at C of a concave mirror and you get a real, inverted image of the same size, also at C — the only position that gives unit magnification in a concave mirror.
❌ Why the other options are incorrect
✗ equal to f — with the object at the focus F, the reflected rays become parallel and the image is formed at infinity, highly enlarged.
✗ greater than 2f — the object beyond C gives a real, inverted but diminished image, formed between F and C.
✗ less than f — the object between P and F gives a virtual, erect and enlarged image behind the mirror, not a same-size one.
📌 SSC & Railway Exam Facts
• Concave mirror, object at C (u = 2f) → image at C, real, inverted, same size.
• Object beyond C → image between F and C, real, inverted, diminished.
• Object between F and C → image beyond C, real, inverted, enlarged.
• Object between P and F → image behind the mirror, virtual, erect, enlarged.
• f = R/2; mirror formula 1/v + 1/u = 1/f; magnification m = −v/u = h′/h.

Correct Answer: Option 4 — Virtual and diminished
A concave lens is a diverging lens — it is thinner at the middle and thicker at the edges, so it spreads out the rays falling on it instead of bringing them together. Because the refracted rays always diverge, they can never actually meet on a screen. They only appear to come from a point on the same side as the object.
For this reason a concave lens forms, for every position of a real object, an image that is:
• virtual (it cannot be taken on a screen),
• erect (the same way up as the object), and
• diminished (smaller than the object),
• located between the optical centre O and the focus F, on the same side as the object.
So placing the object between F and 2F makes no difference to the nature of the image — unlike a convex lens, a concave lens has no position that gives a real or an enlarged image. The image simply lies somewhere within the focal length, smaller and upright.
❌ Why the other options are incorrect
✗ Virtual and enlarged — this is what a convex lens gives when the object is placed within its focal length (the magnifying-glass case); a concave lens never enlarges.
✗ Real and enlarged — this is a convex lens with the object between F and 2F; the option is a trap for students who read "concave" as "convex".
✗ Real and diminished — a convex lens with the object beyond 2F; a concave lens can never form a real image of a real object.
📌 SSC & Railway Exam Facts
• Concave (diverging) lens — image is always virtual, erect and diminished, formed between O and F.
• Convex lens, object between F and 2F → image beyond 2F, real, inverted, enlarged.
• f is negative for a concave lens, so its power P = 1/f is negative too.
• Lens formula 1/v − 1/u = 1/f; magnification m = v/u; for a concave lens m is positive and less than 1.
• A concave lens corrects myopia (short-sightedness); a convex lens corrects hypermetropia (long-sightedness).

Correct Answer: Option 2 — black
The sky looks blue only because of the scattering of sunlight by the molecules of the atmosphere. According to Rayleigh's law, the amount of scattering is inversely proportional to the fourth power of the wavelength (scattering ∝ 1/λ⁴). Since blue light has a much shorter wavelength than red, it is scattered far more strongly in all directions, and that scattered blue light reaches our eyes from every part of the sky.
If the Earth had no atmosphere, there would be no molecules or dust particles to scatter the sunlight. Sunlight would then travel in straight lines from the Sun to the ground without being spread sideways, so no light would come to the eye from other directions of the sky. The sky would therefore appear completely black, with the Sun and the stars visible at the same time.
This is exactly what astronauts see: from the Moon, which has practically no atmosphere, the sky is pitch black even during the day.
❌ Why the other options are incorrect
✗ blue — blue is the colour we see because of the atmosphere; remove the atmosphere and the scattering that produces blue disappears.
✗ yellow — the Sun's disc may look yellowish because of scattering, but the sky itself would not turn yellow without an atmosphere.
✗ red — red dominates at sunrise and sunset, when sunlight travels a long path through the atmosphere and most of the blue has been scattered away — again an atmospheric effect.
📌 SSC & Railway Exam Facts
• Rayleigh scattering: intensity ∝ 1/λ⁴ — blue scatters most, red scatters least.
• Sky appears blue, sun looks red at sunrise/sunset, and danger signals are red because red is scattered the least and travels farthest.
• On the Moon the sky is black all day, as it has no atmosphere.
• Tyndall effect — scattering of light by colloidal particles, e.g. sunbeams through a forest canopy or through fog.
• Clouds appear white because their large water droplets scatter all wavelengths almost equally.

Correct Answer: Option 3 — focal length
The focal length (f) of a spherical mirror is defined as the distance between the principal focus F and the pole P of the mirror. The pole is the geometrical centre of the reflecting surface, and the principal focus is the point on the principal axis where rays parallel to the axis converge (concave mirror) or from which they appear to diverge (convex mirror) after reflection.
The focal length is related to the radius of curvature R by the simple relation f = R/2, i.e. the focus lies exactly midway between the pole and the centre of curvature.
Under the New Cartesian Sign Convention, f is negative for a concave mirror (the focus is in front of the mirror) and positive for a convex mirror (the focus is behind it). For a plane mirror R = ∞, so f = ∞.
❌ Why the other options are incorrect
✗ centre of curvature — this is a point, not a distance: the centre C of the imaginary sphere of which the mirror is a part.
✗ aperture — the effective diameter of the reflecting surface, i.e. how wide the mirror is; it decides how much light the mirror collects.
✗ radius of curvature — the distance from the pole P to the centre of curvature C, which is twice the focal length (R = 2f), not the focus-to-pole distance.
📌 SSC & Railway Exam Facts
• f = R/2 and R = 2f for every spherical mirror.
• Pole (P) — centre of the mirror surface; Centre of curvature (C) — centre of the sphere; Principal focus (F) — midpoint of PC.
• Concave mirror: f and R are negative; Convex mirror: f and R are positive.
• Mirror formula 1/v + 1/u = 1/f; magnification m = −v/u = h′/h.
• For a plane mirror, R = ∞ and f = ∞, and the image is always virtual, erect, same size and laterally inverted.

Correct Answer: Option 3 — uniform circular motion
When a body moves along a circular path covering equal distances in equal intervals of time, its speed stays constant and the motion is called uniform circular motion.
The important point for exams is that even though the speed is uniform, the motion is accelerated. Velocity is a vector, and at every point of the circle the direction of motion (along the tangent) keeps changing. A changing direction means a changing velocity, and therefore an acceleration.
This acceleration is directed towards the centre of the circle and is called centripetal acceleration, a = v²/r. The force that produces it is the centripetal force, F = mv²/r, supplied by tension in a string, friction on a road, or gravitation for a satellite. If this force suddenly disappears — say the string breaks — the body flies off along the tangent in a straight line, exactly as Newton's first law demands.
The speed in uniform circular motion is v = 2πr/t, where t is the time for one complete revolution.
❌ Why the other options are incorrect
✗ linear motion — motion along a line, in which the direction does not change; here the direction changes continuously.
✗ motion along a straight line — the same idea again; a circular path is by definition not a straight line.
✗ motion along a hemisphere — not a standard term in kinematics, and the question clearly describes a circular path.
📌 SSC & Railway Exam Facts
• Uniform circular motion — speed constant, velocity changing, hence an accelerated motion.
• v = 2πr/t; centripetal acceleration a = v²/r; centripetal force F = mv²/r, directed towards the centre.
• Centripetal force does zero work, because it is always perpendicular to the displacement.
• Examples — a satellite around the Earth, an athlete on a circular track, a stone whirled on a string, the blades of a fan.
• When the string breaks, the stone moves along the tangent — an application of inertia of direction.

Correct Answer: Option 3 — greater in water than in glass
The speed of light in a medium is decided by the refractive index of that medium, through the relation n = c/v, where c = 3 × 10⁸ m/s is the speed of light in vacuum and v is its speed in the medium. Rearranging, v = c/n — so the higher the refractive index, the slower the light.
Standard values are: air ≈ 1.0003 (≈ 1), water = 1.33, glass = 1.5. Since n(water) = 1.33 is less than n(glass) = 1.5, light travels faster in water than in glass.
Working it out: in water v = 3 × 10⁸/1.33 ≈ 2.25 × 10⁸ m/s, while in glass v = 3 × 10⁸/1.5 = 2 × 10⁸ m/s. So the order of speed is air > water > glass, exactly the reverse of the order of refractive index.
This is also why glass is called optically denser than water, and why a ray bends more on entering glass than on entering water.
❌ Why the other options are incorrect
✗ greater in glass than in water — the exact reverse of the truth; glass has the higher refractive index, so light is slower in glass.
✗ greater in water than in air — light is fastest in air (n ≈ 1) and slows down in water (n = 1.33); water can never beat air.
✗ same in air, glass and water — if the speed were the same everywhere, refraction would not occur at all and lenses and prisms would not work.
📌 SSC & Railway Exam Facts
• n = c/v; c = 3 × 10⁸ m/s in vacuum — the maximum possible speed in nature.
• Refractive index: air ≈ 1, water = 1.33, glass = 1.5, diamond = 2.42 (the highest among common media).
• Higher refractive index → optically denser → lower speed of light and greater bending.
• Snell's law: n₁ sin i = n₂ sin r, i.e. n = sin i / sin r for light going from air into a medium.
• Diamond sparkles because of total internal reflection, its critical angle being only about 24°.

Correct Answer: Option 4 — Tides
Tides are the regular rise and fall of sea water, and they are caused mainly by the gravitational pull of the Moon on the Earth's oceans, helped to a smaller extent by the Sun. Water on the side of the Earth facing the Moon is pulled towards it and bulges out, producing a high tide; a second bulge forms on the opposite side because the solid Earth itself is pulled away from the water there. Regions at right angles to these bulges experience a low tide.
Because the Earth rotates once in about 24 hours while these bulges stay lined up with the Moon, most coasts get two high tides and two low tides in roughly 24 hours 50 minutes.
When the Sun, Moon and Earth lie in a straight line (new moon and full moon), the two pulls add and we get the highest tides, called spring tides. When the Sun and Moon are at right angles (first and last quarter), the pulls partly cancel and we get the weakest tides, called neap tides.
❌ Why the other options are incorrect
✗ Blue color of the sky — caused by the scattering of light by air molecules (Rayleigh scattering), in which blue is scattered most because of its short wavelength.
✗ Thunderstorms — a weather phenomenon produced by convection currents, moisture and electrical discharge in clouds, not by lunar gravity.
✗ Rainbow — formed by refraction, total internal reflection and dispersion of sunlight in water droplets; it is an optical, not a gravitational, effect.
📌 SSC & Railway Exam Facts
• Tides are mainly due to the Moon's gravitational pull; the Moon's tidal effect is about twice that of the Sun because it is far closer.
• Spring tides — Sun, Moon and Earth in a line (full moon/new moon); neap tides — Sun and Moon at 90°.
• F = GMm/r²; G = 6.67 × 10⁻¹¹ N m² kg⁻², measured by Henry Cavendish.
• The Moon's gravity is about 1/6 of the Earth's, so weight on the Moon is one-sixth of that on the Earth.
• Gravitation also explains the orbits of planets (Kepler's laws), the revolution of the Moon and the free fall of bodies.