RRB Technician Grade III 2024-25 26 December 2024 Shift 2 GK Questions – with answers & solutions

Correct Answer: Option 1 — Adrenaline
Adrenaline (epinephrine) is secreted by the adrenal glands, which sit like caps on the top of the two kidneys. It is called the emergency hormone or the fight-or-flight hormone, because it is poured into the blood the instant an animal such as a squirrel senses danger.
Adrenaline makes the heartbeat and breathing rate faster, raises blood pressure, diverts blood from the skin and digestive system to the skeletal muscles, makes the liver release stored glucose, and widens the pupils. Together these changes supply the extra oxygen and energy the animal needs either to fight or to run away.
❌ Why the other options are incorrect
✗ Oestrogen — a female sex hormone secreted by the ovary; it develops the female secondary sexual characters and controls the menstrual cycle.
✗ Testosterone — a male sex hormone secreted by the testes; it develops the male secondary sexual characters such as a deep voice and facial hair.
✗ Thyroxin — secreted by the thyroid gland; it regulates carbohydrate, protein and fat metabolism and needs iodine for its synthesis.
📌 SSC & Railway Exam Facts
• The adrenal gland is also called the supra-renal gland; its medulla secretes adrenaline and noradrenaline.
• The adrenal cortex secretes cortisol (the stress hormone) and aldosterone (salt and water balance).
• Adrenaline is used as a life-saving medicine in cardiac arrest and in severe allergy (anaphylaxis).
• The pituitary is the master gland; the pancreas is a mixed gland (both exocrine and endocrine).
• Insulin deficiency causes diabetes mellitus; thyroxin deficiency causes goitre.

Correct Answer: Option 2 — Gibberellins : Help in the growth of the stem; Cytokinins : Promote cell division; Abscisic acid : Inhibits growth
Plant hormones (phytohormones) are chemical compounds made in one part of a plant, carried to another part, and there they control growth, development and the response to stimuli.
• Gibberellins — cause elongation of the stem and of the internodes, break seed dormancy and induce flowering; a sprayed plant becomes unusually tall.
• Cytokinins — promote cell division (cytokinesis); they are present in greatest amount in fruits and seeds and they delay the ageing of leaves.
• Abscisic acid (ABA) — a growth inhibitor; it causes wilting, closing of stomata and falling of leaves, so it is called the stress hormone.
Only Option 2 pairs each of the three hormones with its true function.
❌ Why the other options are incorrect
✗ Option 1 — it makes gibberellins an inhibitor and abscisic acid a stem-growth promoter; these two roles are exactly interchanged.
✗ Option 3 — it gives cell division to gibberellins and stem growth to cytokinins; cell division is the special work of cytokinins.
✗ Option 4 — it wrongly makes cytokinins an inhibitor and abscisic acid a promoter of cell division.
📌 SSC & Railway Exam Facts
• Auxin is made at the shoot tip; it causes cell elongation and is responsible for phototropism.
• Ethylene (ethene) is the only gaseous plant hormone; it ripens fruits.
• Gibberellic acid was first obtained from the fungus Gibberella fujikuroi, which causes the bakanae (foolish seedling) disease of rice.
• Auxin, gibberellin and cytokinin are growth promoters; abscisic acid and ethylene are growth inhibitors.
• The term hormone was coined by Bayliss and Starling; the study of auxin began with the experiments of Charles Darwin on coleoptiles.

Correct Answer: Option 3 — the dividing tissue, also known as meristematic tissue, is located only at these regions
Plant growth is localised, not spread over the whole body, because the cells that are able to divide are confined to a few places. This dividing tissue is called meristem (meristematic tissue). Its cells are small, thin-walled, tightly packed, have a dense cytoplasm, a prominent nucleus and no vacuoles, and they keep dividing to add new cells.
Meristems are of three kinds: apical meristem at the tips of roots and shoots (increase in length), lateral meristem or cambium at the sides (increase in girth), and intercalary meristem at the base of leaves or above the nodes (regrowth of grasses after grazing or mowing).
❌ Why the other options are incorrect
✗ Complex permanent tissue — xylem and phloem; these are made of more than one type of cell and only conduct water, minerals and food; they cannot divide.
✗ Simple permanent tissue — parenchyma, collenchyma and sclerenchyma; they store food, give flexibility and give strength, but they have lost the power of division.
✗ Connective tissue — an animal tissue (bone, blood, cartilage). It does not occur in plants at all.
📌 SSC & Railway Exam Facts
• The word meristem comes from a Greek word meaning 'to divide'.
• Cells of meristem later change into permanent tissue by differentiation.
• Apical meristem → primary growth (length); lateral meristem (cambium) → secondary growth (thickness).
• Annual rings in a tree trunk are formed by the activity of the vascular cambium, and they tell the age of the tree (dendrochronology).
• Intercalary meristem is best seen in grasses and sugarcane.

Correct Answer: Option 4 — Helps the cells to grow longer
Auxin is a plant hormone synthesised at the tip of the shoot. Its basic action is cell elongation — it makes the cell wall loose and stretchable so that the cell takes in water and grows longer. This lengthwise growth of cells is what makes the stem grow taller.
Auxin also explains phototropism. When light falls on one side of a shoot, auxin moves away to the shaded side; the cells there elongate more, so the shoot bends towards the light. In roots auxin acts in the opposite way, which is why roots bend away from light and towards gravity.
❌ Why the other options are incorrect
✗ Increases stem width — increase in girth is secondary growth, done by the lateral meristem (cambium), not by auxin's elongation action.
✗ Induce flowering — flowering is mainly controlled by gibberellins and by the photoperiod (florigen concept); it is not the defining function of auxin.
✗ Make plants insect resistant — resistance to insects comes from alkaloids, resins, tannins and modern Bt genes, not from a growth hormone.
📌 SSC & Railway Exam Facts
• Auxin means 'to grow'; Charles Darwin and Francis Darwin began the work, and F. W. Went isolated it from oat coleoptiles.
• IAA (Indole-3-acetic acid) is the commonest natural auxin.
• Auxin causes apical dominance — the shoot tip suppresses the growth of side buds; that is why gardeners prune the tip.
• 2,4-D is a synthetic auxin used as a weedicide.
• Tropic movements: phototropism (light), geotropism (gravity), hydrotropism (water), chemotropism (chemicals, e.g. pollen tube growth).

Correct Answer: Option 2 — spore formation
Spore formation (sporulation) is an asexual method in which the organism produces hundreds of tiny, single-celled reproductive units called spores inside a sac-like structure. In the bread mould Rhizopus, thread-like hyphae grow upright and their swollen tips, the sporangia (blob-like knobs), are filled with spores.
When the sporangium bursts, the spores are scattered by air. Each spore has a thick protective wall that lets it survive drought and heat; on landing on a moist piece of bread it germinates and grows into a new Rhizopus. A single parent can thus produce a very large number of offspring quickly.
❌ Why the other options are incorrect
✗ Budding — a small outgrowth or bud appears on the parent body and later detaches, as in yeast and Hydra; Rhizopus does not form buds.
✗ Vegetative propagation — new plants arise from a vegetative part (root, stem or leaf) of a higher plant, as in potato, ginger and Bryophyllum.
✗ Regeneration — a cut or broken piece of the body grows into a complete organism, as in Planaria and Hydra; it is not the method used by moulds.
📌 SSC & Railway Exam Facts
• Rhizopus is the bread mould; Mucor, Penicillium and ferns also reproduce by spores.
• The spore-producing sac is the sporangium; the stalk carrying it is the sporangiophore.
• Asexual reproduction needs only one parent and gives offspring that are genetically identical (clones).
• Types of asexual reproduction: fission, budding, fragmentation, regeneration, spore formation and vegetative propagation.
• Bacteria form endospores to survive very harsh conditions.

Correct Answer: Option 3 — Change in the directions that plant parts grow in
Plants have no nervous system, so they respond to the environment by changing the direction of their growth. Such a directional, growth-dependent movement towards or away from a stimulus is called a tropic movement (tropism).
• Phototropism — response to light: the shoot bends towards light (positively phototropic) while the root bends away from it.
• Geotropism (gravitropism) — response to gravity: the root grows downwards (positively geotropic) and the shoot grows upwards.
• These bends occur because the hormone auxin collects unevenly on the two sides, so one side elongates more than the other.
❌ Why the other options are incorrect
✗ Change in the branching pattern of their roots — branching is decided mainly by the genetic plan and by soil water and nutrients, and it is not a directional response to light or gravity.
✗ Change in the size of ovary and anther — the size of floral parts is a genetically fixed feature of the species, not a tropic response.
✗ Change in the shape of the leaves and fruits — shape of leaves and fruits is again largely inherited; light and gravity change the direction of growth, not the basic shape.
📌 SSC & Railway Exam Facts
• Tropic movements are directional; nastic movements are non-directional, e.g. the folding of Mimosa pudica (touch-me-not) leaves on touch.
• Hydrotropism — roots grow towards water; Chemotropism — growth of the pollen tube towards the ovule.
• Thigmotropism — response to touch, seen in the coiling of tendrils.
• Movement in Mimosa is caused by a rapid change of water content in the cells at the leaf base, not by growth.
• The plant hormone responsible for tropic bending is auxin.

Correct Answer: Option 3 — Fragmentation
Spirogyra is a green, thread-like (filamentous) alga found floating in ponds; it is famous for its spiral (ribbon-shaped) chloroplast. On reaching maturity the filament simply breaks into two or more pieces (fragments), and each fragment grows by cell division into a complete new Spirogyra filament. This method is called fragmentation.
Fragmentation is possible only in simple multicellular organisms in which the body is not divided into specialised organs, so every fragment still contains all the cell types needed to build a whole individual.
❌ Why the other options are incorrect
✗ Budding — a bud grows out of the parent body and later separates; seen in yeast (unicellular) and Hydra (multicellular), not in Spirogyra.
✗ Binary fission — a unicellular organism divides into two daughter cells, as in Amoeba, Paramecium and bacteria; Spirogyra is multicellular.
✗ Multiple fission — one parent cell divides into many daughter cells at once inside a cyst, as in the malarial parasite Plasmodium.
📌 SSC & Railway Exam Facts
• Spirogyra is called pond silk or water silk because of its silky, slippery feel.
• Spirogyra also reproduces sexually by conjugation, forming a zygospore.
• Other organisms that reproduce by fragmentation are Sargassum, Ulothrix and many filamentous fungi.
• Planaria shows regeneration; Hydra shows both budding and regeneration.
• Amoeba divides by binary fission and forms a cyst in unfavourable conditions.

Correct Answer: Option 4 — fragmentation
Fragmentation is an asexual mode of reproduction in which a simple multicellular organism, on maturing, breaks up spontaneously into two or more fragments, and every fragment then grows into a complete new individual. No gametes and no fertilisation are involved, and only one parent is needed.
In Spirogyra, a filamentous pond alga, the long green thread breaks at any point and each broken piece continues to divide and elongate until it becomes a full filament. Because the body has a simple, repeated structure with no specialised organs, each fragment is complete in itself.
❌ Why the other options are incorrect
✗ Binary fission — a single cell splits into two daughter cells (Amoeba, bacteria); it applies to unicellular organisms, while Spirogyra is multicellular.
✗ Budding — a bud develops on the parent body, grows and then detaches, as in yeast and Hydra; the parent body itself does not break apart.
✗ Regeneration — an organism rebuilds a lost or cut-off part into a full body, as in Planaria and Hydra; here the breaking is accidental, not a normal reproductive habit.
📌 SSC & Railway Exam Facts
• Fragmentation is seen in Spirogyra, Ulothrix, Sargassum and many filamentous fungi.
• Offspring formed by any asexual method are clones — genetically identical to the parent.
• Regeneration in Planaria was studied through the classic experiments of T. H. Morgan; the cells involved are specialised (differentiated) cells that proliferate.
• Spirogyra has a spiral chloroplast and is commonly called pond silk.
• Yeast reproduces by budding; Plasmodium by multiple fission; Planaria by regeneration.

Correct Answer: Option 3 — vegetative propagation
Vegetative propagation is the asexual production of new plants from a vegetative part of the parent plant — the root, stem or leaf — without any seed, flower or fertilisation. In Bryophyllum (sprout-leaf plant) tiny adventitious buds grow in the notches along the leaf margin; when such a leaf falls on moist soil, every bud strikes root and becomes an independent plant.
Because no gametes are involved, all the new plants are genetically identical to the parent (clones), they flower earlier than seed-grown plants, and seedless varieties such as banana and orange can be multiplied only in this way.
❌ Why the other options are incorrect
✗ Budding — an outgrowth forms on the body of an animal or a fungus and detaches, as in yeast and Hydra; in Bryophyllum the structures arise on a leaf, which makes it vegetative propagation.
✗ Fragmentation — the body breaks into pieces, each growing into a new individual, as in Spirogyra; the Bryophyllum leaf does not break up into fragments.
✗ Regeneration — a lost or cut part is rebuilt into a whole organism, as in Planaria and Hydra; here the buds are formed in the normal course of life, not after injury.
📌 SSC & Railway Exam Facts
• Bryophyllum is the classic example of propagation by leaf; potato and ginger by stem; sweet potato and Dahlia by root.
• Runners in grass, suckers in banana, rhizome in ginger, tuber in potato and bulb in onion are natural vegetative parts.
• Artificial methods are cutting, layering, grafting and tissue culture (micropropagation).
• In tissue culture a small piece of tissue (explant) grows into a mass called callus.
• Vegetative propagation preserves the desired characters of the parent exactly, but reduces genetic variation.

Correct Answer: Option 1 — Only (i)
Excretion is the removal of harmful nitrogenous wastes produced by metabolism. Checking each statement:
• (i) is INCORRECT — the two kidneys lie in the abdominal region, on either side of the backbone, just below the diaphragm. The thoracic (chest) cavity contains the heart and lungs, not the kidneys.
• (ii) is correct — urine made by the kidneys flows through the ureters and is stored in the urinary bladder until it is passed out through the urethra.
• (iii) is correct — carbon dioxide is an excretory product of respiration and is removed from the blood into the alveoli of the lungs and breathed out.
Since only statement (i) is wrong, the answer is Only (i).
❌ Why the other options are incorrect
✗ Only (i) and (ii) — statement (ii) is perfectly true; the bladder is indeed the storage organ for urine.
✗ (i), (ii) and (iii) — it wrongly marks all three as incorrect, although (ii) and (iii) are true.
✗ Only (i) and (iii) — statement (iii) is true, because the lungs really do excrete CO₂.
📌 SSC & Railway Exam Facts
• The human excretory system = 2 kidneys + 2 ureters + 1 urinary bladder + 1 urethra.
• The structural and functional unit of the kidney is the nephron; each kidney has about one million nephrons.
• Filtration occurs in the glomerulus inside the Bowman's capsule, together called the Malpighian body.
• Humans are ureotelic — the main nitrogenous waste is urea, made in the liver.
• In kidney failure, blood is cleaned by dialysis using an artificial kidney.

Correct Answer: Option 4 — Thyroxin hormone
Thyroxin (T₄) is secreted by the thyroid gland, a butterfly-shaped gland lying in front of the trachea (windpipe) in the neck. It controls the basal metabolic rate, that is, the speed at which carbohydrates, proteins and fats are oxidised in the body, and so it decides growth, body heat and general body balance.
Iodine is an essential raw material: the thyroid takes iodine from the blood and attaches it to the amino acid tyrosine to build thyroxin. If the diet is deficient in iodine, thyroxin cannot be made, the gland enlarges and a swelling of the neck called goitre appears. This is why we are advised to use iodised salt.
❌ Why the other options are incorrect
✗ Adrenaline hormone — from the adrenal gland; it is the emergency (fight-or-flight) hormone that raises heartbeat and blood pressure; it needs no iodine.
✗ Insulin hormone — from the beta cells of the pancreas; it lowers blood glucose; its deficiency causes diabetes mellitus.
✗ Testosterone hormone — from the testes; it develops male secondary sexual characters and sperms, and has nothing to do with iodine.
📌 SSC & Railway Exam Facts
• The thyroid is the largest endocrine gland in the human body.
• Deficiency of thyroxin in children causes cretinism (stunted growth, mental retardation); in adults it causes myxoedema.
• Excess thyroxin causes Grave's disease (exophthalmic goitre).
• The thyroid also secretes calcitonin, which lowers blood calcium.
• The thyroid gland is stimulated by TSH (Thyroid Stimulating Hormone) from the anterior pituitary.

Correct Answer: Option 4 — the brain and the spinal cord
The human nervous system has two main divisions. The Central Nervous System (CNS) is made of the brain and the spinal cord; the Peripheral Nervous System (PNS) is made of the cranial and spinal nerves that connect the CNS with the rest of the body.
The CNS is the integrating centre. Information picked up by receptors travels along sensory (afferent) neurons to the brain and the spinal cord, where it is analysed and a decision is taken; the instruction then travels along motor (efferent) neurons to the effectors — muscles and glands. The brain is protected by the cranium (skull) and the spinal cord by the vertebral column, and both are cushioned by cerebrospinal fluid.
❌ Why the other options are incorrect
✗ only the brain — incomplete; the spinal cord is equally a part of the CNS and independently controls reflex actions.
✗ only the heart — the heart is a muscular pumping organ of the circulatory system; it has no role in receiving or integrating nerve impulses.
✗ only the spinal cord — incomplete; thinking, memory, vision, hearing and balance are handled by the brain.
📌 SSC & Railway Exam Facts
• Parts of the brain: fore-brain (cerebrum), mid-brain and hind-brain (cerebellum, pons, medulla).
• Cerebrum — the seat of thinking, memory and intelligence; it is the largest part of the brain.
• Cerebellum — maintains posture and balance; medulla controls blood pressure, salivation and vomiting.
• A reflex arc is completed in the spinal cord, so the response is quick and does not wait for the brain.
• The structural and functional unit of the nervous system is the neuron; the gap between two neurons is the synapse.

Correct Answer: Option 1 — tendrils
Tendrils are slender, thread-like, coiling structures formed by the modification of a leaf, leaflet or stem. In the pea plant the terminal leaflets are modified into tendrils. When a tendril touches a support, the side of the tendril away from the support grows faster than the side touching it, so the tendril coils around the support and the weak stem is held up.
This growth response to touch is called thigmotropism, and it is a good example of a tropic (directional) movement in plants brought about by unequal growth, controlled by the hormone auxin.
❌ Why the other options are incorrect
✗ Prickles — small, sharp outgrowths of the epidermis, as in rose; they give protection from grazing animals and may give a slight hold, but they do not coil or climb.
✗ Sepals — the green outermost whorl of a flower; they only protect the flower bud, and have no role in climbing.
✗ Roots — they absorb water and minerals and fix the plant in soil; climbing roots occur in money plant and betel, but the pea plant climbs by tendrils.
📌 SSC & Railway Exam Facts
• Pea — leaflet tendril; Cucumber, pumpkin and grapevine — stem tendrils.
• Thigmotropism = response to touch; Phototropism = light; Geotropism = gravity; Hydrotropism = water; Chemotropism = chemicals.
• In Mimosa pudica the drooping of leaves on touch is a nastic (non-directional) movement, not tropism.
• Pisum sativum (pea) is the plant used by Gregor Mendel in his classic heredity experiments.
• Tendrils are a modification for climbing; thorns of Bougainvillea are modified stems while spines of cactus are modified leaves.

Correct Answer: Option 4 — Four
Meiosis is the reduction division that takes place only in the reproductive (germ) cells of the gonads. One parent cell passes through two successive divisions — Meiosis I and Meiosis II — but the DNA is copied only once. The result is four daughter cells, each with half (haploid, n) the chromosome number of the parent cell.
This halving is essential: when a haploid sperm (n) fuses with a haploid egg (n) at fertilisation, the diploid (2n) chromosome number of the species is restored, so it stays constant generation after generation. Crossing over during Prophase I also creates genetic variation.
❌ Why the other options are incorrect
✗ Eight — eight cells would need three successive divisions; meiosis has only two, so eight is not possible.
✗ Twenty-four — 24 is not a power of 2 at all and can never be the product of successive equal divisions.
✗ Sixteen — sixteen cells would require four rounds of division, far more than the two rounds of meiosis.
📌 SSC & Railway Exam Facts
• Mitosis gives 2 diploid (2n) daughter cells and occurs in body (somatic) cells for growth and repair.
• Meiosis gives 4 haploid (n) cells and occurs in germ cells for gamete formation.
• Stages of division: Prophase → Metaphase → Anaphase → Telophase, followed by cytokinesis.
• The term mitosis was given by Walther Flemming and meiosis by Farmer and Moore.
• Humans have 46 chromosomes (23 pairs) in body cells and 23 in each gamete.

Correct Answer: Option 4 — Double displacement reaction
In a double displacement reaction two compounds dissolved in solution exchange their ions with one another, so that two completely new compounds are produced. Its general form is AB + CD → AD + CB.
Because the cations and anions simply swap partners, this reaction is also called an exchange reaction. When one of the new compounds is insoluble and settles out as a solid, the same reaction is additionally called a precipitation reaction.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
• Na⁺ pairs with Cl⁻ and Ba²⁺ pairs with SO₄²⁻ — the ions are exchanged between the two reactants.
• The white precipitate of barium sulphate (BaSO₄) is indicated by the state symbol (s).
❌ Why the other options are incorrect
✗ Decomposition — here a single compound breaks down into two or more simpler substances, AB → A + B, as in 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. There is only one reactant, so no exchange is possible.
✗ Addition reaction — an organic reaction in which an unsaturated compound adds atoms across its double or triple bond, e.g. C₂H₄ + H₂ → C₂H₆ (Ni catalyst). Nothing is swapped.
✗ Disproportion reaction — in this the same element is oxidised and reduced at the same time; it involves one species changing oxidation state, not two reactants trading ions.
📌 SSC & Railway Exam Facts
• Neutralisation, NaOH + HCl → NaCl + H₂O, is a double displacement reaction.
• AgNO₃ + NaCl → AgCl(s) + NaNO₃ gives a curdy white precipitate of silver chloride.
• Displacement: Fe + CuSO₄ → FeSO₄ + Cu — a more reactive metal displaces a less reactive one.
• Combination: CaO + H₂O → Ca(OH)₂ — quick lime to slaked lime, a highly exothermic change.
• Every precipitation reaction is a double displacement reaction, but not every double displacement reaction gives a precipitate.

Correct Answer: Option 2 — Calcium
Isobars are atoms of different elements that have different atomic numbers but the same mass number. Because the atomic number differs, isobars are chemically different elements altogether, yet a mass spectrometer records the same mass for them.
Argon has atomic number Z = 18 and mass number A = 40, while calcium has Z = 20 and mass number A = 40. The two therefore hold the same total of 40 nucleons — argon has 18 protons + 22 neutrons and calcium has 20 protons + 20 neutrons. Ar-40 and Ca-40 is the standard NCERT example of an isobaric pair.
Compare this with isotopes, which are atoms of the same element with the same atomic number but different mass numbers, such as ¹H, ²H and ³H.
❌ Why the other options are incorrect
✗ Chlorine — Z = 17 and atomic mass ≈ 35.5 u; it is famous instead for its isotopes Cl-35 and Cl-37 in the ratio 3 : 1.
✗ Potassium — Z = 19 and its most abundant natural form is K-39, so its standard atomic mass is about 39 u, not 40.
✗ Sodium — Z = 11, mass number 23; nowhere near argon's mass number of 40.
📌 SSC & Railway Exam Facts
• Isotopes: same Z, different A — e.g. C-12, C-13, C-14.
• Isobars: same A, different Z — e.g. Ar-40 and Ca-40, Fe-58 and Ni-58.
• Isotones: same number of neutrons — e.g. C-14 and O-16.
• C-14 is used in radio-carbon dating; I-131 in treating goitre; Co-60 in cancer therapy; U-235 as nuclear fuel.
• Mass number (A) = protons + neutrons; atomic number (Z) = number of protons.

Correct Answer: Option 1 — Sodium
Sodium (Na) is the most reactive metal among the four given and sits very high in the reactivity series. It reacts so vigorously with the oxygen and moisture of air that the heat released is enough to set the metal on fire by itself at ordinary temperature.
For this reason sodium and potassium are always stored under kerosene oil, which keeps air and water away from the metal surface. Phosphorus, a non-metal that catches fire in air in the same way, is stored under water.
⚗️ The Reaction
4Na + O₂ → 2Na₂O
2Na + 2H₂O → 2NaOH + H₂ + heat
• The reaction with water is so exothermic that the hydrogen released catches fire immediately.
• Na₂O + H₂O → 2NaOH, a strong base, which is why sodium's oxide is a basic oxide.
❌ Why the other options are incorrect
✗ Copper — a very unreactive metal placed below hydrogen in the reactivity series; in moist air it only slowly forms a green coating of basic copper carbonate, CuCO₃·Cu(OH)₂.
✗ Zinc — moderately reactive; it forms a protective layer of zinc oxide in air and needs strong heating before it burns.
✗ Magnesium — it burns with a dazzling white light, but only after being heated; it does not ignite on its own merely by being left in the open.
📌 SSC & Railway Exam Facts
• Reactivity series (high to low): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au > Pt.
• Sodium and potassium are stored in kerosene; white phosphorus is stored in water.
• Sodium is so soft that it can be cut with a knife, and its density is less than that of water.
• Burning magnesium ribbon gives white magnesium oxide (MgO) ash — a classic combination reaction.
• Gold and platinum are the least reactive metals and occur in the free (native) state.

Correct Answer: Option 4 — Oxygen
The atomicity of an element is the number of atoms present in one molecule of that element. An element whose molecule contains two atoms is called diatomic.
Oxygen exists in air as the molecule O₂, in which two oxygen atoms are joined by a double covalent bond. Its atomicity is therefore 2, making it diatomic. Oxygen is a non-metal with valency 2, and a single molecule of O₂ has a molecular mass of 32 u.
The common diatomic elements to remember are H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
❌ Why the other options are incorrect
✗ Phosphorus — its white form exists as P₄, so phosphorus is tetra-atomic (atomicity 4). Sulphur is likewise S₈, poly-atomic.
✗ Argon — a noble gas of group 18 with a complete octet; it does not bond even with itself, so it is monoatomic (Ar).
✗ Helium — also a noble gas, with a complete duplet of 2 electrons; it exists as single atoms and is monoatomic (He).
📌 SSC & Railway Exam Facts
• Atomicity: monoatomic — He, Ne, Ar, Kr, metals; diatomic — H₂, N₂, O₂, Cl₂; tri-atomic — O₃ (ozone); tetra-atomic — P₄; poly-atomic — S₈.
• Ozone (O₃) is an allotrope of oxygen and is tri-atomic.
• Noble gases are chemically inert because their outermost shell is completely filled.
• Molecular mass of O₂ = 32 u, of O₃ = 48 u, of N₂ = 28 u, of H₂ = 2 u.
• Oxygen makes up about 21% of air by volume and is the most abundant element in the Earth's crust.

Correct Answer: Option 1 — increases
According to the kinetic theory of matter, all matter is made of tiny particles that are in continuous random motion and therefore always possess kinetic energy.
Temperature is a direct measure of the average kinetic energy of these particles. When a substance is heated, the supplied heat energy is converted into the kinetic energy of its particles, so the particles vibrate and move faster, the spaces between them grow, and the force of attraction between them weakens. This is exactly why heating makes solids melt and liquids boil, and why the rate of diffusion and evaporation both rise with temperature.
Mathematically, the average kinetic energy of a particle is directly proportional to the absolute temperature in kelvin, and it becomes zero only at absolute zero, 0 K = −273 °C.
❌ Why the other options are incorrect
✗ decreases — kinetic energy falls only on cooling; that is how a gas is liquefied by lowering the temperature.
✗ remains the same — the kinetic energy stays constant only if the temperature is constant, as during a change of state when the heat supplied acts as latent heat.
✗ becomes zero — particle motion would stop completely only at absolute zero (0 K = −273 °C), a temperature that can never actually be reached.
📌 SSC & Railway Exam Facts
• K = °C + 273; 0 °C = 273 K and 100 °C = 373 K.
• Order of kinetic energy of particles: gas > liquid > solid.
• Latent heat is absorbed during a change of state without any rise in temperature.
• Latent heat of fusion of ice = 3.34 × 10⁵ J/kg; latent heat of vaporisation of water = 22.5 × 10⁵ J/kg.
• Evaporation is a surface phenomenon and it causes cooling, which is why a desert cooler works.

Correct Answer: Option 1 — Gallium and cesium
Most metals have very high melting points because of the strong metallic bonding between their positive ions and the sea of delocalised electrons. Gallium and caesium are the famous exceptions: their metallic bonding is unusually weak, so they melt just above ordinary room temperature.
• Melting point of gallium (Ga) ≈ 29.8 °C — a piece of gallium melts in the palm of the hand.
• Melting point of caesium (Cs) ≈ 28.4 °C, the lowest of any metal that is solid at 25 °C.
This is why NCERT states that gallium and caesium have such low melting points that they will melt if kept on the palm. Mercury (Hg) is the only metal that is already liquid at room temperature, with a melting point of −39 °C.
❌ Why the other options are incorrect
✗ Copper and cobalt — both are hard, high-melting transition metals; Cu melts at 1085 °C and Co at 1495 °C.
✗ Zinc and copper — zinc melts at 420 °C (low for a metal, but far above room temperature) and copper at 1085 °C. This pair forms the alloy brass.
✗ Iron and nickel — Fe melts at 1538 °C and Ni at 1455 °C; both are used precisely because they stay solid at high temperatures.
📌 SSC & Railway Exam Facts
• Mercury — the only metal liquid at room temperature; bromine — the only non-metal liquid at room temperature.
• Tungsten (W) has the highest melting point among metals, 3422 °C, hence its use in bulb filaments.
• Carbon (diamond) is the hardest natural substance; graphite conducts electricity.
• Sodium and potassium are so soft they can be cut with a knife.
• Brass = Cu + Zn, Bronze = Cu + Sn, Stainless steel = Fe + Cr + Ni.

Correct Answer: Option 4 — All (a), (b), (c) and (d)
A chemical reaction takes place when the reactants are converted into new substances with new properties. NCERT lists the observations that tell us a chemical reaction has taken place as: change in state, change in colour, evolution of a gas, and change in temperature, to which the formation of a precipitate is added.
All four statements in the question are among these recognised indicators, so all of them are correct.
⚗️ One example of each
• Gas evolved — Zn + H₂SO₄ → ZnSO₄ + H₂↑, brisk effervescence of hydrogen.
• Precipitate formed — Pb(NO₃)₂ + 2KI → PbI₂(s) + 2KNO₃, the yellow precipitate of lead iodide.
• Change in state — burning of a candle turns solid wax into CO₂ and water vapour.
• Colour change — Fe + CuSO₄ → FeSO₄ + Cu, in which the blue solution turns pale green.
❌ Why the other options are incorrect
✗ Only (a) and (b) — it wrongly leaves out change of state and colour change, both of which are standard textbook indicators.
✗ Only (a), (b) and (c) — colour change is omitted, yet it is the very observation used in the iron–copper sulphate experiment.
✗ Only (c) and (d) — this omits the evolution of a gas and the formation of a precipitate, the two most obvious signs of a reaction in the laboratory.
📌 SSC & Railway Exam Facts
• Signs of a chemical change: change in state, colour, temperature, evolution of gas, formation of a precipitate.
• A chemical change is usually irreversible; a physical change is generally reversible.
• Rusting of iron and souring of milk are chemical changes; melting of ice is a physical change.
• Reactions that release heat are exothermic; those that absorb heat are endothermic.
• Respiration is an exothermic reaction; photosynthesis is endothermic.

Correct Answer: Option 4 — 32
The molecular mass of a substance is the sum of the atomic masses of all the atoms present in one molecule of it. It is expressed in atomic mass units (u) and is a relative quantity, measured against 1/12 of the mass of one carbon-12 atom.
🔢 Calculation
• Oxygen gas exists as the diatomic molecule O₂.
• Atomic mass of one oxygen atom = 16 u.
• Molecular mass of O₂ = 2 × 16
• Molecular mass of O₂ = 32 u
The common error is to write 16, which is the atomic mass of a single oxygen atom, not the molecular mass of the gas. Remember that free oxygen never exists as single atoms under ordinary conditions.
❌ Why the other options are incorrect
✗ 30 — this is the molecular mass of nitric oxide (NO), 14 + 16, and also of ethane C₂H₆.
✗ 26 — this is the molecular mass of acetylene (C₂H₂), (2 × 12) + (2 × 1).
✗ 28 — this is the molecular mass of nitrogen gas (N₂), 2 × 14, and of carbon monoxide (CO), 12 + 16. It is the classic distractor for this question.
📌 SSC & Railway Exam Facts
• 1 u = 1/12 the mass of one C-12 atom.
• Useful molecular masses: H₂ = 2, N₂ = 28, O₂ = 32, O₃ = 48, Cl₂ = 71, CO₂ = 44, H₂O = 18, NH₃ = 17, CH₄ = 16, H₂SO₄ = 98.
• 1 mole of any substance contains 6.022 × 10²³ particles — Avogadro's number.
• The mass of 1 mole of O₂ = 32 g; 1 mole of any gas at STP occupies 22.4 litres.
• Gram molecular mass is the molecular mass expressed in grams.

Correct Answer: Option 4 — Saturated compounds
Carbon compounds in which all the carbon–carbon bonds are single covalent bonds are called saturated compounds. The word "saturated" means each carbon has used all four of its valencies to hold the maximum possible number of hydrogen atoms, so no more atoms can be added to the chain.
The saturated hydrocarbons form the alkane family, whose general formula is CₙH₂ₙ₊₂ — for example CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane) and C₄H₁₀ (butane).
Saturated hydrocarbons are chemically less reactive, they undergo substitution reactions (with chlorine in sunlight), and they burn with a clean blue flame because combustion is complete.
❌ Why the other options are incorrect
✗ Ionic compounds — these are formed by the transfer of electrons between a metal and a non-metal, e.g. NaCl. Carbon compounds are covalent, formed by the sharing of electrons.
✗ Both ionic and saturated compounds — half of the statement is wrong, since carbon does not normally form ionic bonds; carbon has 4 valence electrons and gaining or losing four would need too much energy.
✗ Unsaturated compounds — these contain at least one double bond (alkenes, CₙH₂ₙ) or triple bond (alkynes, CₙH₂ₙ₋₂), such as C₂H₄ (ethene) and C₂H₂ (ethyne); they are more reactive and give addition reactions.
📌 SSC & Railway Exam Facts
• Alkanes CₙH₂ₙ₊₂ (saturated), Alkenes CₙH₂ₙ, Alkynes CₙH₂ₙ₋₂ (both unsaturated).
• Saturated hydrocarbons burn with a blue flame; unsaturated ones burn with a yellow sooty flame.
• Unsaturation is tested by decolourising bromine water or alkaline KMnO₄.
• Catenation — carbon's ability to link with itself in long chains, rings and branches — gives rise to millions of organic compounds.
• The first four alkanes are methane, ethane, propane, butane; CNG and LPG are mainly saturated hydrocarbons.

Correct Answer: Option 3 — in the liquid state, particles move freely and have greater space between each other
Diffusion is the spontaneous intermixing of the particles of two substances on their own. Its rate depends on two things — how freely the particles can move and how much empty space is available between them.
In a solid the particles are very tightly packed, the interparticle space is minimum and the force of attraction is maximum, so the particles can only vibrate about a fixed position. Diffusion in solids is therefore extremely slow.
In a liquid the particles are less tightly packed, the interparticle space is larger and the force of attraction is weaker, so the particles can slide and move freely over one another. Both required conditions are satisfied, which is why liquids diffuse much faster than solids.
Dropping ink or copper sulphate crystals into a beaker of water and watching the colour spread is the standard demonstration.
❌ Why the other options are incorrect
✗ particles do not move freely, but have greater space — only half true; liquid particles do move freely, which is exactly why a liquid flows and takes the shape of its container.
✗ particles do not move freely and do not have greater space — this describes a solid, not a liquid.
✗ particles only move freely and do not have greater space — the increased interparticle space in a liquid is real and is essential for faster diffusion.
📌 SSC & Railway Exam Facts
• Rate of diffusion: gases > liquids > solids.
• Diffusion increases with a rise in temperature because particle kinetic energy increases.
• Order of interparticle space and compressibility: gas > liquid > solid; order of density and force of attraction: solid > liquid > gas.
• Solids do diffuse, but very slowly — the classic example is solid ammonium chloride sublimating, or gold foil diffusing into lead over years.
• Liquids and gases are together called fluids because both can flow.

Correct Answer: Option 1 — By heating sodium bicarbonate
Sodium bicarbonate (NaHCO₃) is baking soda, and washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. On heating, baking soda undergoes thermal decomposition to give sodium carbonate, which is then recrystallised from water to obtain washing soda.
⚗️ The Reaction
2NaHCO₃ → (heat) Na₂CO₃ + H₂O + CO₂
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
• The first step is a thermal decomposition reaction and is endothermic.
• The CO₂ released in the first step is what makes cake and bread rise.
• The 10H₂O in the final product is the water of crystallisation, which gives washing soda its crystalline form.
Washing soda is a basic salt of a strong base (NaOH) and a weak acid (H₂CO₃), so its solution is alkaline with pH above 7. It is used to remove the permanent hardness of water, in the glass, soap and paper industries, and as a domestic cleaning agent.
❌ Why the other options are incorrect
✗ By mixing sodium bicarbonate with water — this merely dissolves the salt; it is a physical change and no sodium carbonate is formed.
✗ By freezing sodium bicarbonate — cooling supplies no energy for decomposition; decomposition needs heat, not the removal of it.
✗ By adding sodium bicarbonate to vinegar — this gives sodium acetate, water and carbon dioxide (NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂), not washing soda.
📌 SSC & Railway Exam Facts
• Washing soda — Na₂CO₃·10H₂O; Baking soda — NaHCO₃; Common salt — NaCl.
• Bleaching powder — CaOCl₂, made by Ca(OH)₂ + Cl₂; Plaster of Paris — CaSO₄·½H₂O.
• Both washing soda and baking soda are made from brine by the Solvay process.
• Baking soda plus a mild edible acid (tartaric acid) makes baking powder.
• Baking soda is used in soda-acid fire extinguishers and as an antacid to neutralise stomach acidity.

Correct Answer: Option 3 — Nitrogen dioxide gas
When lead nitrate, Pb(NO₃)₂, is heated strongly in a test tube it undergoes thermal decomposition. The white crystals first crackle (this is why it is called a decrepitation reaction) and then give off dense brown fumes together with a yellow residue.
⚗️ The Reaction
2Pb(NO₃)₂ → (heat) 2PbO + 4NO₂ + O₂
• The brown fumes are of nitrogen dioxide (NO₂).
• The yellow solid left behind is lead(II) oxide, PbO, also called litharge.
• The colourless gas that escapes along with NO₂ is oxygen, which relights a glowing splinter.
• This is a thermal decomposition and an endothermic reaction.
❌ Why the other options are incorrect
✗ Ozone gas — O₃ is a pale blue gas with a pungent smell; it is formed in the stratosphere from O₂ by UV light, not from lead nitrate.
✗ Nitrogen gas — N₂ is colourless and odourless, makes up 78% of air, and is not a product of this decomposition.
✗ Hydrogen gas — H₂ is colourless; lead nitrate contains no hydrogen at all, so it cannot possibly be released.
📌 SSC & Railway Exam Facts
• Brown gas = NO₂, an acidic oxide; it dissolves in rainwater to give nitric acid and causes acid rain.
• Other classic decomposition reactions: CaCO₃ → CaO + CO₂ (heat), 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (heat), 2H₂O → 2H₂ + O₂ (electricity), 2AgCl → 2Ag + Cl₂ (sunlight).
• Decomposition by heat = thermolysis, by electricity = electrolysis, by light = photolysis.
• Silver salts decompose in sunlight, which is the basis of black-and-white photography.
• Most decomposition reactions are endothermic; the reverse of a combination reaction.

Correct Answer: Option 4 — Emission
A phase transition (change of state) is the conversion of matter from one physical state into another — solid, liquid or gas — brought about by a change of temperature or pressure, while the chemical composition stays exactly the same. It is therefore a physical change and is reversible.
Emission simply means the giving out of energy, light, radiation or particles by a body — for example the emission of light by a bulb or of radiation by a radioactive nucleus. No solid, liquid or gas is converted into another state, so emission is not a phase transition, and it is the required answer.
❌ Why the other options are incorrect (i.e. they ARE phase transitions)
✗ Vaporization — the change from liquid to gas on absorbing heat, as when water boils at 100 °C to give steam.
✗ Sublimation — the direct change from solid to gas without passing through the liquid state, shown by camphor, naphthalene, iodine, ammonium chloride and dry ice.
✗ Condensation — the change from gas to liquid on cooling, seen as dew on a cold glass and in the formation of clouds.
📌 SSC & Railway Exam Facts
• The six phase transitions: melting (fusion), freezing (solidification), vaporisation, condensation, sublimation, deposition.
• Deposition is the reverse of sublimation — gas directly to solid, as in the formation of frost.
• Melting point of ice = 0 °C = 273 K; boiling point of water = 100 °C = 373 K at 1 atm.
• Evaporation occurs at all temperatures and only at the surface; boiling occurs at a fixed temperature throughout the liquid.
• During a change of state the temperature does not rise — the heat taken in is the latent heat.

Correct Answer: Option 1 — parallel straight lines
A solenoid is a long coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder. When current flows, the magnetic field produced by every single turn adds up inside the coil.
Inside the solenoid the magnetic field lines come out as parallel straight lines, all pointing in the same direction along the axis. This means the field inside a solenoid is uniform — it has the same magnitude and direction at every point of that region.
Outside, the pattern is exactly like that of a bar magnet — one end behaves as the north pole and the other as the south pole.
❌ Why the other options are incorrect
✗ curved lines — the lines curve only outside the solenoid, where they spread from one end round to the other.
✗ perpendicular straight lines — the lines run along the axis of the solenoid, never at right angles to it.
✗ concentric circles — that is the pattern of the field around a straight current-carrying wire, not inside a solenoid.
📌 SSC & Railway Exam Facts
• Field inside a solenoid: B = μ₀nI, where n is the number of turns per unit length.
• Putting a soft iron core inside a solenoid makes a powerful electromagnet.
• The direction of the field is found by the right-hand thumb rule (Maxwell's corkscrew rule).
• The SI unit of magnetic field is the tesla (T).
• Hans Christian Oersted (1820) discovered the magnetic effect of electric current.

Correct Answer: Option 2 — Total mechanical energy
The sum of the kinetic energy (KE) and the potential energy (PE) of a body is called its total mechanical energy.
• Kinetic energy is the energy of motion: KE = ½mv².
• Potential energy is the energy of position or configuration: PE = mgh for a body raised to height h.
• Therefore Total mechanical energy, E = ½mv² + mgh.
For a freely falling body, as it comes down its PE keeps decreasing and its KE keeps increasing, but the sum stays constant. This is the law of conservation of mechanical energy, valid when only gravity acts and friction or air resistance is absent.
❌ Why the other options are incorrect
✗ Total electrical energy — energy carried by moving charges, W = VIt; it is not made of KE and PE.
✗ Total chemical energy — energy stored in the bonds of a substance, released in food, fuels and cells.
✗ Total thermal energy — the internal heat energy of a body due to the random motion of its molecules.
📌 SSC & Railway Exam Facts
• SI unit of every form of energy is the joule (J); 1 J = 1 N m.
• 1 kWh (Board of Trade Unit) = 3.6 × 10⁶ J.
• Energy can be transformed but never created or destroyed — law of conservation of energy.
• At the top of its flight a ball has maximum PE and zero KE; just before hitting the ground it has maximum KE.
• Power is the rate of doing work: P = W/t, SI unit watt (W).

Correct Answer: Option 4 — (a) Myopia - near-sightedness, (b) Hypermetropia - far-sightedness
Myopia is also called near-sightedness or short-sightedness. Such an eye can see nearby objects clearly but distant objects appear blurred, because the image of a distant object forms in front of the retina. It happens when the eyeball is too long or the curvature of the lens is too great. It is corrected with a concave (diverging) lens of suitable power.
Hypermetropia is also called far-sightedness or long-sightedness. Such an eye can see distant objects clearly but nearby objects appear blurred, because the image of a near object forms behind the retina. It happens when the eyeball is too short or the focal length of the lens is too long, and it is corrected with a convex (converging) lens.
❌ Why the other options are incorrect
✗ Option 1 — Hypermetropia is far-sightedness, not near-sightedness, so pair (a) is wrong.
✗ Option 2 — Myopia is near-sightedness, not far-sightedness, so pair (b) is wrong.
✗ Option 3 — Presbyopia is the old-age defect of losing the power of accommodation; calling it near-sightedness is wrong.
📌 SSC & Railway Exam Facts
• Myopia → concave lens; Hypermetropia → convex lens; Presbyopia → bifocal lens.
• Normal near point = 25 cm and far point = infinity for a healthy eye.
• Astigmatism is corrected by a cylindrical lens.
• Power of a lens P = 1/f (in metres), SI unit dioptre (D).
• Cataract is the clouding of the eye lens; it is cured by surgery, not by spectacles.

Correct Answer: Option 4 — 1 J/1C
📖 Given
• The quantity to be expressed is potential difference, V
• J = joule (unit of work), C = coulomb (unit of charge), s = second
✍️ Formula
V = W/Q — potential difference = work done per unit charge
🔢 Solution
• Putting the units: V = joule / coulomb
• So 1 volt = 1 J / 1 C
• In words: 1 volt is the potential difference between two points when 1 joule of work is done to move a charge of 1 coulomb between them.
This is why a 12 V battery gives 12 joules of energy to every coulomb of charge it pushes round the circuit.
❌ Why the other options are incorrect
✗ 1 C/s — coulomb per second is the ampere (A), the unit of electric current, not voltage.
✗ 1 Cs — coulomb × second is not a standard electrical unit at all.
✗ 1 Js — joule × second is the unit of Planck's constant, not of potential difference.
📌 SSC & Railway Exam Facts
• 1 A = 1 C/s; 1 Ω = 1 V/A; 1 W = 1 J/s.
• Potential difference is measured with a voltmeter, connected in parallel.
• Current is measured with an ammeter, connected in series.
• Ohm's law: V = IR, given by Georg Simon Ohm.
• Charge on one electron = 1.6 × 10⁻¹⁹ C.

Correct Answer: Option 4 — 10 A
📖 Given
• Heat produced, H = 400 J in time t = 1 s
• So power, P = H/t = 400 J/s = 400 W
• Resistance, R = 4 Ω
✍️ Formula
H = I²Rt (Joule's law of heating), so P = I²R
🔢 Solution
• 400 = I² × 4 × 1
• I² = 400/4
• I² = 100
• I = √100
• I = 10 A
So a current of 10 amperes flowing through the 4 Ω resistor turns 400 joules of electrical energy into heat every second.
❌ Why the other options are incorrect
✗ 20 A — you get this if you wrongly take I² = 400 and forget to divide by R.
✗ 30 A — does not satisfy the equation; 30² × 4 = 3600 W, nine times too large.
✗ 40 A — this is simply 400/10, a random division that has no basis in Joule's law.
📌 SSC & Railway Exam Facts
• Joule's law of heating: H = I²Rt — heat depends on the square of the current.
• Electric power: P = VI = I²R = V²/R.
• Heating effect is used in the electric iron, heater, geyser, toaster and electric bulb.
• The filament of a bulb is made of tungsten, melting point about 3380 °C.
• A fuse wire has high resistance and low melting point, and is always connected in series with the live wire.

Correct Answer: Option 1 — aperture
The aperture of a spherical mirror is the effective diameter of its reflecting surface — that is, the width of the portion of the mirror from which reflection actually takes place.
The aperture decides how much light the mirror can collect. A mirror with a large aperture gathers more light and forms a brighter image, which is why the mirrors of reflecting telescopes are made very wide.
In all the standard mirror formula derivations the mirror is assumed to have a small aperture, so that the relation f = R/2 holds and spherical aberration stays negligible.
❌ Why the other options are incorrect
✗ inverse of the power — the inverse of power is the focal length, f = 1/P; it has nothing to do with the diameter.
✗ radius of curvature — R is the radius of the hollow sphere of which the mirror is a part, measured from the centre of curvature to the pole, not across the rim.
✗ focal length — f is the distance from the pole to the principal focus, and for a spherical mirror f = R/2.
📌 SSC & Railway Exam Facts
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u = h′/h.
• For a spherical mirror, R = 2f.
• Concave mirror — used in torches, headlights, shaving mirrors, solar cookers and dentists' mirrors.
• Convex mirror — used as a rear-view mirror because it gives an erect, diminished image and a wide field of view.
• The pole (P) is the centre of the reflecting surface and lies on the mirror itself.

Correct Answer: Option 3 — Potential energy and kinetic energy
Mechanical energy is the energy a body possesses because of its motion or because of its position. It is therefore made up of exactly two parts:
• Kinetic energy (KE = ½mv²) — energy due to motion. A running train, flowing water and a moving bullet all have it.
• Potential energy (PE = mgh) — energy due to position or configuration. Water stored in a dam, a stretched rubber band and a book on a shelf all have it.
So Mechanical energy = KE + PE. In a swinging pendulum the two keep changing into each other while their sum stays constant — a neat demonstration of the conservation of mechanical energy.
❌ Why the other options are incorrect
✗ Chemical energy and electrical energy — these are separate forms; chemical energy is stored in bonds, electrical energy is carried by moving charges.
✗ Light energy and chemical energy — light is radiant energy; their combination is used in photosynthesis, not in mechanics.
✗ Heat energy and light energy — both are non-mechanical forms released, for example, by a burning candle.
📌 SSC & Railway Exam Facts
• KE = ½mv² — doubling the speed makes the kinetic energy four times.
• PE = mgh depends on mass, g and height, not on the path taken.
• In a hydroelectric power plant, PE of stored water → KE of falling water → electrical energy.
• Work–energy theorem: work done by the net force = change in kinetic energy.
• SI unit of work and energy is the joule (J); the CGS unit is the erg (1 J = 10⁷ erg).

Correct Answer: Option 1 — When the force is opposite to the direction of displacement.
Work done by a force is W = F s cos θ, where θ is the angle between the force and the displacement. The sign of the work depends entirely on cos θ:
• θ = 0° → cos 0° = +1 → W = +Fs, work is positive.
• θ = 180° → cos 180° = −1 → W = −Fs, work is negative.
• θ = 90° → cos 90° = 0 → W = 0, work is zero.
So when the force acts opposite to the displacement, the work done by it is negative. Everyday examples are the work done by friction on a sliding box and the work done by gravity on a ball thrown upwards.
❌ Why the other options are incorrect
✗ When the displacement is zero — then W = F × 0 = zero, not negative. Pushing a wall that does not move is an example.
✗ When the force is in the direction of displacement — this gives maximum positive work, W = +Fs.
✗ When the force is perpendicular to the displacement — cos 90° = 0, so the work is zero; a coolie carrying a load on his head does no work against gravity.
📌 SSC & Railway Exam Facts
• Work, W = F s cos θ; SI unit joule (J), 1 J = 1 N × 1 m.
• Work is a scalar quantity, although both force and displacement are vectors.
• Negative work removes energy from a body; that is how brakes stop a train.
• In uniform circular motion the centripetal force is always perpendicular to motion, so its work is zero.
• 1 kilojoule = 1000 J; 1 calorie = 4.2 J.

Correct Answer: Option 3 — Decreasing its area of cross-section.
📖 Given
• A metallic conductor of length l, area of cross-section A and resistivity ρ
• Temperature is unchanged
✍️ Formula
R = ρl/A
🔢 Solution
• R is directly proportional to l — a longer wire has more resistance.
• R is inversely proportional to A — a thinner wire has more resistance.
• So if A is decreased, R increases.
That is exactly why a thin wire glows in a heater while the thick connecting wires stay cool.
❌ Why the other options are incorrect
✗ Using a material with higher conductivity — conductivity σ = 1/ρ, so higher conductivity means lower resistivity and hence lower resistance.
✗ Decreasing the current flowing through the conductor — at a fixed temperature, resistance is a property of the conductor and does not change with the current.
✗ Decreasing its length — since R ∝ l, a shorter wire has less resistance, not more.
📌 SSC & Railway Exam Facts
• Resistance depends on length, area of cross-section, nature of the material and temperature.
• SI unit of resistance is the ohm (Ω); of resistivity, the ohm metre (Ω m).
• Silver is the best conductor; copper and aluminium are used for wires.
• Nichrome has high resistivity and is used in heating elements.
• For metals, resistance increases with temperature; for semiconductors it decreases.

Correct Answer: Option 3 — a constant greater than 1
According to Snell's law of refraction, for a given colour of light and a given pair of media,
sin i / sin r = n₂₁ = constant
This constant is the refractive index of the second medium with respect to the first.
When light travels from a rarer medium to a denser medium, it bends towards the normal, so the angle of refraction is smaller than the angle of incidence (r < i). Since sin i > sin r, the ratio sin i / sin r is greater than 1. For example, for air → glass the value is about 1.5, and for air → water about 1.33.
❌ Why the other options are incorrect
✗ always 1 — the ratio is 1 only when the two media are optically identical, so that light goes straight without bending.
✗ always zero — sin r can never make the ratio zero; a zero ratio would mean no refracted ray at all.
✗ a constant less than 1 — that happens the other way round, when light goes from a denser to a rarer medium and bends away from the normal.
📌 SSC & Railway Exam Facts
• Snell's law: n = sin i / sin r; the incident ray, refracted ray and normal all lie in one plane.
• Refractive index n = c/v, where c = 3 × 10⁸ m/s is the speed of light in vacuum.
• Diamond has the highest common refractive index, about 2.42.
• Refraction explains a pencil looking bent in water and the apparent raising of a pond's bottom.
• Light going from denser to rarer beyond the critical angle shows total internal reflection, used in optical fibres.

Correct Answer: Option 1 — independent of m
📖 Given
• Mass of the object, m
• Mass of the earth, M, radius of the earth, R
• Universal gravitational constant, G
✍️ Formula
Gravitational force, F = GMm/R², and from Newton's second law F = mg
🔢 Solution
• mg = GMm/R²
• Cancel m from both sides
• g = GM/R²
The mass m of the object has completely disappeared from the expression, so g does not depend on the mass of the falling body. That is why a feather and a stone dropped together in a vacuum tube reach the bottom at the same time.
❌ Why the other options are incorrect
✗ linearly proportional to m — the weight (W = mg) is proportional to m, but g itself is not.
✗ inversely proportional to m — nothing in g = GM/R² contains the object's mass, so no inverse relation exists.
✗ linearly proportional to m² — again, the object's mass cancels out completely; only the earth's mass M matters.
📌 SSC & Railway Exam Facts
• g = GM/R² ≈ 9.8 m/s² on the earth's surface; G = 6.67 × 10⁻¹¹ N m² kg⁻².
• g is maximum at the poles and minimum at the equator, because R is least at the poles.
• g becomes zero at the centre of the earth.
• On the moon, g is about 1/6 of its value on the earth.
• Mass is constant everywhere and measured in kg; weight = mg changes with place and is measured in newton (N).

Correct Answer: Option 4 — v² − u² = 2gh
📖 Given
• Initial velocity = u
• Acceleration = g (the body is falling freely under gravity)
• Distance fallen = h
• Final velocity = v
✍️ Formula
The third equation of motion, v² = u² + 2as
🔢 Solution
• For vertical fall put a = g and s = h
• v² = u² + 2gh
• Shifting u² to the left side, v² − u² = 2gh
This equation connects velocity with distance directly, without needing the time of fall — which is why it is the quickest route whenever time is not given.
❌ Why the other options are incorrect
✗ v² + u² = 2gh — the sign is wrong; u² must be subtracted, since v² = u² + 2gh.
✗ v²/u² = 2gh — dividing the squares is meaningless here and is also dimensionally incorrect.
✗ v²u² = 2gh — multiplying the squares gives units of m⁴/s⁴, which cannot equal 2gh (m²/s²).
📌 SSC & Railway Exam Facts
• The three equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• For free fall from rest (u = 0): v = gt, h = ½gt², v² = 2gh.
• Taking g = 9.8 m/s² downward; for a body thrown upward a = −g.
• At the highest point of an upward throw the velocity is zero but the acceleration is still g.
• Equations of motion apply only for uniform acceleration in a straight line.

Correct Answer: Option 3 — (a) Resistance - ohm, (b) Resistivity - ohm metre
Resistance (R) is the opposition a conductor offers to the flow of current. From Ohm's law, R = V/I, so its unit is volt/ampere, which is given the special name ohm (Ω). Resistance depends on the shape and size of the conductor as well as its material.
Resistivity (ρ) is the resistance of a conductor of unit length and unit area of cross-section. From R = ρl/A we get ρ = RA/l, so its unit is Ω × m²/m = ohm metre (Ω m). Resistivity is a property of the material alone and does not change with the length or thickness of the wire.
❌ Why the other options are incorrect
✗ Option 1 — resistance in volt/ampere is correct, but resistivity is ohm metre, not ohm.
✗ Option 2 — resistance in ohm is correct, but volt/metre is the unit of electric field, not resistivity.
✗ Option 4 — ampere metre is not a unit of resistance at all, and volt/metre is again wrong for resistivity.
📌 SSC & Railway Exam Facts
• R = V/I (ohm) and ρ = RA/l (ohm metre).
• Resistivity of metals ≈ 10⁻⁸ to 10⁻⁶ Ω m; of insulators ≈ 10¹² to 10¹⁷ Ω m.
• Conductance = 1/R, unit siemens (S) or mho.
• Series: R(eq) = R₁ + R₂ + R₃; Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃.
• An alloy like nichrome or manganin has higher resistivity than its constituent metals and does not oxidise easily.