RRB Technician Grade III 2024-25 29 December 2024 Shift 2 GK Questions – with answers & solutions

Correct Answer: Option 1 — Fragmentation
Spirogyra is a simple, filamentous green alga found in ponds and ditches, known for its spiral (helical) chloroplasts. It reproduces asexually by fragmentation.
In fragmentation, the mature filament simply breaks up into two or more pieces (fragments) on maturing or on slight disturbance, and each fragment grows into a new individual. This is possible because Spirogyra has a simple body organisation — there is no division of labour into specialised organs, so every part is capable of growing into a whole organism. Since only one parent is involved and no gametes fuse, the offspring are genetically identical clones.
❌ Why the other options are incorrect
✗ Vegetative propagation — reproduction from a vegetative part (root, stem, leaf or bud) of a higher plant, e.g. potato tuber, ginger rhizome, *Bryophyllum* leaf. Spirogyra has no such organs.
✗ Binary fission — the parent cell splits into two daughter cells; seen in unicellular organisms like Amoeba, Paramecium, Leishmania and bacteria. Spirogyra is multicellular and filamentous.
✗ Budding — a bud grows on the parent body and detaches as a new individual; seen in Hydra and yeast. No bud is produced in Spirogyra.
📌 SSC & Railway Exam Facts
• Fragmentation — Spirogyra; Regeneration — Planaria and Hydra; Spore formation — Rhizopus and Mucor.
• Binary fission in Amoeba is simple, in Leishmania it is longitudinal, and in Plasmodium it is multiple fission.
• Budding — Hydra and yeast; Vegetative propagation — potato (eyes), ginger (rhizome), *Bryophyllum* (leaf notches).
• Spirogyra is called pond silk or water silk because of its slippery, silky filaments.
• All these are asexual methods — a single parent, no gamete fusion, no variation in the offspring.

Correct Answer: Option 4 — collenchyma
Collenchyma is a simple permanent tissue of plants that provides mechanical support along with flexibility. Its cells are living, elongated and irregularly thickened at the corners with cellulose and pectin, and there is very little intercellular space between them.
Because the thickening is only at the corners and the walls are not lignified, collenchyma is strong yet bendable. This lets the stems of climbers and young shoots, and the leaf stalks (petioles), bend in the wind without breaking. It is usually found just below the epidermis of stems and petioles in dicot plants.
❌ Why the other options are incorrect
✗ Sclerenchyma — dead cells with thick, lignified walls and no lumen; it gives hardness and rigidity (e.g. coconut husk, nut shells), not flexibility.
✗ Parenchyma — thin-walled living cells with large intercellular spaces; it performs storage, food manufacture (chlorenchyma) and buoyancy (aerenchyma), providing only slight support.
✗ Cambium — a lateral meristem, not a permanent supporting tissue; its job is to divide and cause secondary growth, increasing the girth of stem and root.
📌 SSC & Railway Exam Facts
• Simple permanent tissues — Parenchyma, Collenchyma, Sclerenchyma; complex permanent tissues — Xylem and Phloem.
• Collenchyma cells thicken at the corners with pectin; sclerenchyma thickens all over with lignin.
• Sclerenchyma fibres of jute, coir and flax are used to make ropes and sacks.
• Aerenchyma (parenchyma with large air cavities) helps hydrophytes like water hyacinth to float.
• Chlorenchyma is parenchyma containing chlorophyll and carries out photosynthesis.

Correct Answer: Option 4 — Dead plant
Substances that can be broken down into simple, harmless materials by the action of micro-organisms such as bacteria and fungi (the decomposers) are called biodegradable substances.
A dead plant is made of cellulose, proteins and other organic compounds, all of which decomposers can digest with their enzymes. Within weeks the plant rots and returns carbon, nitrogen and minerals to the soil, completing the biogeochemical cycles. This is exactly the principle used in making compost and vermicompost.
Substances that decomposers cannot break down are called non-biodegradable; they persist for hundreds of years, choke the soil and water, and cause biomagnification of toxins in food chains.
❌ Why the other options are incorrect
✗ Glass — made of fused silica (sand); no micro-organism has an enzyme to digest it, so it is non-biodegradable, though it is fully recyclable.
✗ Plastic bag — a synthetic polymer (polythene); its long carbon chains resist microbial attack and it may survive for hundreds of years, killing cattle and blocking drains.
✗ Electric wire — made of metal (copper/aluminium) with a plastic or rubber coating; both parts are non-biodegradable and form e-waste.
📌 SSC & Railway Exam Facts
• Biodegradable — food waste, paper, cotton, wool, leaves, cow dung, jute; Non-biodegradable — plastic, glass, metal, DDT, polythene, styrofoam.
• Decomposers (bacteria and fungi) are also called saprophytes; they are nature's cleaners.
• In Indian waste segregation, green bins are for biodegradable (wet) waste and blue bins for non-biodegradable (dry) waste.
• Concentration of harmful chemicals like DDT rising at each trophic level is biomagnification.
• The three R's of waste management are Reduce, Reuse and Recycle.

Correct Answer: Option 1 — buds produced in the notches along the leaf margin
*Bryophyllum* (the 'sprout leaf plant' or *pattharchatta*) is the classic NCERT example of vegetative propagation by leaves. Along the margin of its fleshy leaf there are small depressions called notches, and in these notches adventitious buds develop.
Each bud already carries tiny leaves and roots. When the leaf falls on moist soil — or even while it is still attached to the plant — these buds detach and grow into complete new plants. Since only one parent is involved and no gametes fuse, this is a form of asexual reproduction, and the new plants are genetically identical clones of the parent.
❌ Why the other options are incorrect
✗ Stamen — the male reproductive part of a flower (anther + filament) that produces pollen grains; it takes part in sexual reproduction, not vegetative propagation.
✗ Petal — a coloured accessory part of the flower whose only job is to attract insects for pollination; it cannot grow into a new plant.
✗ Pistil — the female reproductive part (stigma + style + ovary) that bears the ovules; again a sexual reproductive structure.
📌 SSC & Railway Exam Facts
• Vegetative propagation by leaf — *Bryophyllum*; by stem — potato, sugarcane, ginger, rose; by root — sweet potato, dahlia, *Asparagus*.
• Potato propagates through the eyes (buds) of its tuber; ginger and turmeric through rhizomes.
• Artificial methods of vegetative propagation are cutting, layering, grafting and tissue culture (micropropagation).
• A big advantage is that seedless plants like banana, orange, rose and jasmine can still be multiplied, and the plants flower and fruit earlier.
• The main disadvantage is no variation — the whole crop can be wiped out by a single disease.

Correct Answer: Option 3 — Menstrual cycle
The ovary is the female gonad. It is a mixed gland — it produces the ova (eggs) and also secretes the female sex hormones oestrogen and progesterone.
These hormones regulate the menstrual cycle, the roughly 28-day cycle in which the uterine lining (endometrium) thickens to receive an embryo. Oestrogen rebuilds the lining and triggers ovulation around day 14; progesterone, from the corpus luteum, maintains the thickened lining. If fertilisation does not occur, both hormones fall, the lining breaks down and is shed as menstruation. Oestrogen also brings about the female secondary sexual characters.
❌ Why the other options are incorrect
✗ Regulates metabolism — the metabolism of carbohydrates, proteins and fats is regulated by thyroxine from the thyroid gland, which needs iodine.
✗ Stimulates pituitary glands — the pituitary is stimulated by releasing hormones from the hypothalamus, not by the ovary; in fact the pituitary's FSH and LH control the ovary.
✗ Stimulates growth in all organs — that is the work of growth hormone (GH) secreted by the anterior pituitary gland.
📌 SSC & Railway Exam Facts
• Ovary → oestrogen and progesterone; Testis → testosterone; both are mixed glands making gametes and hormones.
• Menarche is the first menstruation; menopause (about 45–50 years) is its permanent stopping.
• Ovulation occurs around the 14th day of a 28-day cycle; the ovum survives about 24 hours.
• Progesterone is called the pregnancy hormone; it is secreted by the corpus luteum and later by the placenta.
• Pituitary is the master gland, but it is itself controlled by the hypothalamus.

Correct Answer: Option 3 — meiosis
Meiosis is the reductional division in which one diploid (2n) parent cell divides twice but the chromosomes replicate only once, producing four haploid (n) daughter cells. The chromosome number is therefore halved.
It takes place in the reproductive (germ) cells — in the testis to form sperms and in the ovary to form ova, and in the anther and ovule of plants to form spores. Halving is essential: when a haploid sperm fuses with a haploid ovum at fertilisation, the diploid number is restored in the zygote, so the species keeps a constant chromosome number generation after generation. Crossing over during prophase I also creates variation.
❌ Why the other options are incorrect
✗ Mitosis — equational division; one cell gives two daughter cells with the same (2n) chromosome number. It occurs in somatic (body) cells for growth and repair.
✗ Amitosis — direct or simple division in which the nucleus and cytoplasm just pinch into two, without spindle or chromosome movements; it does not halve the chromosome number.
✗ Binary fission — an asexual reproduction method of unicellular organisms like Amoeba and bacteria; the parent splits into two identical individuals with the full chromosome set.
📌 SSC & Railway Exam Facts
• Meiosis was discovered by Oscar Hertwig (1876); the term was coined by Farmer and Moore.
• Meiosis has two stages — Meiosis I (reductional) and Meiosis II (equational).
• Crossing over happens at the pachytene stage of prophase I, at points called chiasmata.
• In humans the diploid number is 46 and the gamete's haploid number is 23.
• Mitosis stages: Prophase → Metaphase → Anaphase → Telophase; cytokinesis divides the cytoplasm.

Correct Answer: Option 1 — Protection
Epithelial tissue is the covering or protective tissue of the animal body. Its cells are tightly packed with almost no intercellular space, they form a continuous sheet, and they always rest on a basement membrane.
Because of this continuous, close-packed arrangement it acts as a barrier: the skin epithelium protects the body from injury, germs, chemicals and water loss; the epithelium lining the mouth, oesophagus, stomach, intestine, blood vessels, kidney tubules and lungs protects those organs and also helps in absorption, secretion and exchange of materials.
❌ Why the other options are incorrect
✗ Provides response to stimulus — this is the function of nervous tissue, made of neurons, which receives stimuli and conducts electrical impulses to the brain and spinal cord.
✗ Storage of fats — fat is stored in adipose tissue, a type of connective tissue found below the skin and between internal organs; it also acts as an insulator.
✗ Responsible for involuntary movements — involuntary movement is caused by smooth (unstriated) muscle in the gut, blood vessels and iris, and by cardiac muscle in the heart.
📌 SSC & Railway Exam Facts
• The four types of animal tissue are Epithelial, Connective, Muscular and Nervous.
• Types of epithelium — squamous (lung alveoli), cuboidal (kidney tubules), columnar (intestine), ciliated (respiratory tract) and glandular (glands).
• Stratified squamous epithelium forms the skin, arranged in many layers to stand wear and tear.
• Ciliated epithelium of the windpipe pushes mucus and dust upward and out.
• Tendon joins muscle to bone, and ligament joins bone to bone; both are connective tissues.

Correct Answer: Option 4 — enzymes
Enzymes are biological catalysts (bio-catalysts) — protein molecules made by living cells that greatly speed up biochemical reactions without being used up themselves. In digestion they break complex food into simple, absorbable forms.
Each enzyme is highly specific, fitting its substrate like a lock and key, and each works best at a particular pH and temperature (about 37 °C in humans). Thus salivary amylase converts starch to maltose in the mouth, pepsin breaks proteins to peptones in the acidic stomach, and trypsin, amylase and lipase of the pancreatic juice complete the job in the alkaline small intestine, finally giving amino acids, glucose, fatty acids and glycerol.
❌ Why the other options are incorrect
✗ Oil — a liquid fat, i.e. a nutrient that supplies energy; it is itself digested by lipase, it does not catalyse anything.
✗ Fat — a energy-rich nutrient (about 9 kcal per gram) stored in adipose tissue; a food substance, not a catalyst.
✗ Carbohydrates — the body's chief source of energy (starch, glucose, cellulose); like fats they are the substrates of digestion, not the agents of it.
📌 SSC & Railway Exam Facts
• Almost all enzymes are proteins; a few RNA molecules act as enzymes and are called ribozymes.
• The word enzyme was coined by Wilhelm Kühne; the first enzyme crystallised was urease by James Sumner.
• Enzymes are denatured by high temperature and by extreme pH, which is why high fever disturbs digestion.
• Bile contains no enzyme — it only emulsifies fat and makes the medium alkaline.
• Enzyme actions to remember: Ptyalin (starch), Pepsin (protein), Trypsin (protein), Lipase (fat), Lactase (lactose).

Correct Answer: Option 3 — Plants
Meristematic tissue is found only in plants. It is made of small, thin-walled, actively dividing living cells with dense cytoplasm, a prominent nucleus and no vacuoles or intercellular spaces.
Apical meristem lies at the growing tips of the stem (shoot apex) and the root (root apex). Its division causes primary growth, i.e. increase in the length (height) of the plant. This is why plants show indefinite (open) growth throughout life and can keep growing taller. The two other kinds are intercalary meristem (at the base of leaves and internodes, as in grasses) and lateral meristem / cambium (increases girth).
❌ Why the other options are incorrect
✗ Animals — animal growth is uniform and limited; they have no meristem. Growth occurs by mitosis in ordinary body cells and stops at maturity.
✗ Virus — an acellular particle of nucleic acid in a protein coat; it has no cells at all, so no tissue. It multiplies only inside a host cell.
✗ Bacteria — unicellular prokaryotes; a single cell cannot form a tissue, and they multiply by binary fission, not by meristematic growth.
📌 SSC & Railway Exam Facts
• Apical meristem — increases length; Lateral meristem (cambium) — increases girth; Intercalary meristem — at the base of internodes/leaves in grasses, letting them regrow after mowing.
• Plant tissues are of two types — meristematic (dividing) and permanent (non-dividing).
• Permanent tissues form from meristem by differentiation.
• Cork cambium (phellogen) produces the cork (bark), whose cells contain suberin and are impermeable to water and gases.
• Growth in plants is indefinite and localised, while in animals it is definite and diffused.

Correct Answer: Option 3 — Nerve cell
The nerve cell (neuron) is the longest cell in the human body. A single neuron consists of a cell body (cyton) with dendrites, and one very long fibre called the axon. The axon of a nerve running from the spinal cord to the toe can be nearly one metre long — in tall people even more.
This great length is exactly what a neuron needs: it must carry an electrical impulse from a receptor to the brain or spinal cord, and back to a muscle, in one unbroken stretch without relaying it from cell to cell. Neurons are also amitotic — once formed, they usually do not divide, which is why nerve injuries heal so poorly.
❌ Why the other options are incorrect
✗ Ovum — the human egg is the largest cell by diameter (about 0.1 mm) and can just be seen with the naked eye, but it is round, not long.
✗ Fat cell — an adipocyte of adipose tissue; it is a large, rounded cell swollen with a fat droplet, but it is nowhere near the length of a neuron.
✗ Blood cell — the RBC is a tiny biconcave disc about 7 micrometres wide with no nucleus in mammals; it is one of the smallest cells.
📌 SSC & Railway Exam Facts
• Smallest cell — Mycoplasma (PPLO); smallest human cell — the sperm; largest human cell — the ovum.
• Largest cell in nature — the ostrich egg.
• Parts of a neuron — dendrite → cell body (cyton) → axon → nerve endings; the gap between two neurons is the synapse.
• Myelin sheath insulates the axon and speeds up impulse conduction.
• Longest bone — femur; smallest bone — stapes (in the ear); largest gland — liver; largest organ — skin.

Correct Answer: Option 1 — Muscle cells
Muscular tissue is the only tissue in the body that can contract and relax, and it is this alternate shortening and lengthening that actually moves an organ or a body part. Muscle cells are long and cylindrical, so they are called muscle fibres; they contain special contractile proteins — actin and myosin — that slide over each other when a nerve impulse arrives, using energy from ATP.
There are three types:
• Striated (skeletal / voluntary) muscle — attached to bones, moves the limbs, jaw and tongue; under our will.
• Unstriated (smooth / involuntary) muscle — in the alimentary canal, blood vessels, ureters and iris; produces peristalsis.
• Cardiac muscle — only in the heart; contracts and relaxes rhythmically and tirelessly all life long.
❌ Why the other options are incorrect
✗ Nerve cells — they only receive stimuli and conduct impulses; a neuron carries the command to contract but cannot contract itself.
✗ Cartilages — a solid but flexible connective tissue found in the nose, ear, trachea and at joints; it cushions and supports, it does not contract.
✗ Bones — a hard connective tissue with a matrix of calcium and phosphorus; bones only form the rigid framework and act as levers that muscles pull on.
📌 SSC & Railway Exam Facts
• Voluntary muscle — striated, many nuclei, tires quickly; involuntary muscle — unstriated, single nucleus, does not tire.
• Cardiac muscle is striated but involuntary — a favourite exam point.
• Muscle contraction uses ATP and calcium ions; lack of oxygen causes lactic acid build-up, giving cramps.
• Tendon attaches muscle to bone; ligament attaches bone to bone.
• The diaphragm is a muscle sheet that contracts during inhalation; peristalsis in the food pipe is caused by smooth muscle.

Correct Answer: Option 4 — When a girl is born, the ovaries already contain thousands of immature eggs.
NCERT states that a female child is born with thousands of immature eggs already present in her ovaries. No new eggs are formed after birth. At puberty these begin to mature, and thereafter the ovary releases one mature ovum each month — a process called ovulation, usually around the 14th day of the menstrual cycle.
The released ovum is picked up by the fallopian tube (oviduct), where fertilisation may occur. The two oviducts open into the uterus (womb), a pear-shaped muscular bag in which the embryo develops, nourished through the placenta.
❌ Why the other options are incorrect
✗ Many eggs every month — wrong; normally only one ovum matures and is released each month, alternately by the two ovaries.
✗ One oviduct forms the uterus — wrong; the two oviducts unite to open into the uterus. A single oviduct does not become the uterus; the uterus is a separate organ.
✗ Fallopian tube is also known as womb — wrong; the uterus is the womb. The fallopian tube is only the narrow duct that carries the ovum to the uterus.
📌 SSC & Railway Exam Facts
• Female reproductive parts: Ovary → Fallopian tube (oviduct) → Uterus (womb) → Cervix → Vagina.
• Fertilisation normally takes place in the ampulla of the fallopian tube, not in the uterus.
• The placenta is the disc-like tissue that supplies glucose and oxygen to the embryo and removes its wastes.
• Human gestation (pregnancy) period is about 9 months (280 days).
• Menarche — first menstruation; menopause — permanent stopping of menstruation at about 45–50 years.

Correct Answer: Option 3 — Only (i) and (iii)
Checking each statement against NCERT:
• (i) is correct — Arteries carry blood away from the heart to the organs. (The only exception is the pulmonary artery, which carries deoxygenated blood to the lungs.)
• (ii) is incorrect — arteries have thick, strong and elastic walls, not thin walls. Blood leaves the heart under high pressure, and thick elastic walls are needed to withstand and smooth out that pressure.
• (iii) is correct — Veins have valves along their length. Blood in veins flows at low pressure, often against gravity, so the valves prevent backflow and keep the flow unidirectional.
Hence only (i) and (iii) are correct.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — includes (ii), which wrongly describes arterial walls as thin.
✗ Only (i) and (ii) — again accepts the false statement (ii) and wrongly leaves out the true statement (iii).
✗ Only (i) — too narrow; statement (iii) about valves in veins is also correct.
📌 SSC & Railway Exam Facts
• Artery — thick elastic walls, no valves, carries blood away from the heart, deep-seated.
• Vein — thin walls, valves present, carries blood towards the heart, superficial.
• Capillaries are just one cell thick, so gases and nutrients can be exchanged with tissues.
• The human heart has four chambers; double circulation means blood passes through the heart twice in one full cycle.
• Pulmonary artery carries deoxygenated blood and pulmonary vein carries oxygenated blood — the classic exceptions.

Correct Answer: Option 2 — Cambium
Cambium is the lateral meristem of a plant. It lies between the xylem and the phloem in the vascular bundle of dicot stems and roots, and its cells divide again and again in a radial direction.
The cambium cuts off secondary xylem towards the inside and secondary phloem towards the outside. This continuous addition of tissue on both faces is called secondary growth, and it makes the stem and root increase in girth (diameter). That is why a tree trunk becomes thicker year after year and forms annual rings, from which the age of the tree can be counted.
❌ Why the other options are incorrect
✗ Apical meristem — present at the tips of the stem and root; it causes primary growth, increasing the length (height) of the plant, not its thickness.
✗ Intercalary meristem — present at the base of leaves and internodes, especially in grasses; it lets a mown or grazed grass blade grow back in length.
✗ Underneath meristem — not a recognised category of meristem at all; the three real types are apical, lateral and intercalary.
📌 SSC & Railway Exam Facts
• Vascular cambium — makes secondary xylem and phloem; cork cambium (phellogen) — makes the cork or bark.
• Annual rings are formed because spring wood and autumn wood differ; counting them gives the tree's age — the science is dendrochronology.
• Monocots (like grass, maize, palm) have no cambium, so they generally show no secondary growth.
• Cork cells contain suberin, which makes them impermeable to water and gases.
• Meristematic tissue cells are small, thin-walled, with dense cytoplasm, a large nucleus and no vacuole.

Correct Answer: Option 1 — Sodium Hydroxide
In the chlor-alkali process, electricity is passed through a concentrated aqueous solution of sodium chloride (brine). The process is named chlor-alkali because its two chief products are chlorine and the alkali sodium hydroxide (NaOH).
Sodium chloride here is the raw material, not the product — it is decomposed. Chlorine gas is liberated at the anode, hydrogen gas at the cathode, and sodium hydroxide solution collects near the cathode.
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Cl₂ → anode, H₂ → cathode, NaOH → near the cathode.
• Check the balance — 2 Na, 2 Cl, 4 H and 2 O on both sides.
• Uses: NaOH for soaps, detergents and paper; Cl₂ for bleaching powder, PVC and water treatment; H₂ for fuel and margarine.
❌ Why the other options are incorrect
✗ Baking Soda — sodium hydrogencarbonate, NaHCO₃, is made by Solvay's process from brine, ammonia and CO₂, not by electrolysis.
✗ Washing Soda — Na₂CO₃·10H₂O is obtained by heating baking soda to Na₂CO₃ and then recrystallising it with water.
✗ Sodium Chloride — NaCl is the reactant (brine) that is electrolysed; it is consumed, not formed.
📌 SSC & Railway Exam Facts
• Brine = concentrated NaCl solution; its electrolysis is the chlor-alkali process.
• NaOH = caustic soda; NaHCO₃ = baking soda; Na₂CO₃·10H₂O = washing soda.
• Cl₂ + dry slaked lime Ca(OH)₂ → bleaching powder, CaOCl₂.
• H₂ + Cl₂ → 2HCl, used in the PVC and pharmaceutical industry.
• Common salt is the starting material for NaOH, NaHCO₃, Na₂CO₃ and bleaching powder alike.

Correct Answer: Option 4 — Four
Carbon has the atomic number 6 and the electronic configuration 2, 4. With four valence electrons it can neither lose four electrons (too much energy needed) nor gain four, so it shares all four and forms four covalent bonds. Its valency is therefore always four — a property called tetravalency, and it does not change from one compound to another.
In ethene, C₂H₄ (H₂C=CH₂), each carbon forms one double bond (2 bonds) with the other carbon and one single bond with each of two hydrogen atoms — that is 2 + 1 + 1 = 4 bonds. The valency is still four even though ethene is unsaturated; unsaturation means a double bond, not a reduced valency.
❌ Why the other options are incorrect
✗ Three — no neutral carbon compound leaves carbon with three bonds; a species with three bonds would be an unstable ion or free radical.
✗ Six — six is carbon's atomic number, not its valency; a common trap that confuses the two.
✗ Two — two is the number of hydrogen atoms bonded to each carbon in ethene, or the bond order of the C=C bond, but not carbon's valency.
📌 SSC & Railway Exam Facts
• Tetravalency + catenation give carbon its huge number of compounds (over 10 million).
• Ethane C₂H₆ (C–C), ethene C₂H₄ (C=C), ethyne C₂H₂ (C≡C) — carbon's valency is 4 in all three.
• Carbon's electronic configuration is 2, 4; it forms covalent bonds only.
• Catenation is the self-linking of carbon atoms into chains, branches and rings.
• Valency: H = 1, O = 2, N = 3, C = 4.

Correct Answer: Option 2 — Bromine
Bromine (Br) is the only non-metal that is liquid at room temperature. It is a reddish-brown, dense, fuming liquid with a sharp, choking smell; its melting point is −7 °C and its boiling point is 59 °C, so at 25 °C it is comfortably a liquid. It belongs to Group 17, the halogens, and exists as the diatomic molecule Br₂.
Most non-metals are gases (H₂, O₂, N₂, Cl₂) or brittle solids (carbon, sulphur, phosphorus, iodine) at room temperature. Bromine is the single liquid exception, which is exactly why it is asked so often.
❌ Why the other options are incorrect
✗ Oxygen — a colourless gas at room temperature (O₂); it liquefies only below −183 °C.
✗ Fluorine — a pale-yellow gas and the most reactive non-metal of all; its boiling point is about −188 °C.
✗ Mercury — this is indeed a liquid at room temperature, but it is a metal, not a non-metal, so it does not answer the question. It is the only liquid metal.
📌 SSC & Railway Exam Facts
• Bromine = the only liquid non-metal; Mercury = the only liquid metal.
• Gallium (30 °C) and Caesium (28 °C) are metals that melt in the palm of the hand.
• Iodine is a lustrous solid non-metal that sublimes to a violet vapour.
• Graphite (an allotrope of carbon) is the non-metal that conducts electricity; diamond is the hardest natural substance.
• Halogens (Group 17): F and Cl gases, Br liquid, I and At solids.

Correct Answer: Option 4 — i, ii and iv
A saturated hydrocarbon (alkane) contains only single covalent bonds between its carbon atoms and therefore carries the maximum possible number of hydrogen atoms. Its general formula is CₙH₂ₙ₊₂, so the quickest test in the exam is to put the number of carbons in 2n + 2 and compare with the hydrogens printed.
⚗️ Applying the CₙH₂ₙ₊₂ test
• (i) C₂H₆ — n = 2, 2n + 2 = 6 ✔ → ethane, saturated.
• (ii) C₃H₈ — n = 3, 2n + 2 = 8 ✔ → propane, saturated.
• (iii) C₂H₄ — n = 2 needs 6 H but only 4 are present → a double bond (C=C) is present → ethene, unsaturated.
• (iv) C₄H₁₀ — n = 4, 2n + 2 = 10 ✔ → butane, saturated.
• Hence (i), (ii) and (iv) are saturated.
❌ Why the other options are incorrect
✗ i and ii — correct as far as it goes, but it leaves out C₄H₁₀ (butane), which also fits CₙH₂ₙ₊₂ and is saturated.
✗ i and iv — omits C₃H₈ (propane), a perfectly saturated alkane.
✗ i, ii and iii — wrongly includes C₂H₄ (ethene), which has a double bond and is therefore unsaturated; this is the classic mistake of counting only carbons and ignoring the hydrogen count.
📌 SSC & Railway Exam Facts
• Alkane CₙH₂ₙ₊₂ (single bond), alkene CₙH₂ₙ (double bond), alkyne CₙH₂ₙ₋₂ (triple bond).
• Saturated compounds burn with a clean blue flame; unsaturated ones with a sooty yellow flame.
• Unsaturated hydrocarbons decolourise bromine water; saturated ones do not — the standard laboratory test.
• Alkanes give substitution reactions; alkenes and alkynes give addition reactions.
• C₂H₆ ethane, C₃H₈ propane, C₄H₁₀ butane (in LPG), C₂H₄ ethene, C₂H₂ ethyne.

Correct Answer: Option 2 — Ethene and water
Heating ethanol (C₂H₅OH) at 443 K (170 °C) with excess concentrated sulphuric acid brings about dehydration — the removal of a molecule of water from the alcohol. The –OH group and one hydrogen from the neighbouring carbon are pulled out together, and the two carbons close the gap by forming a double bond, giving the unsaturated hydrocarbon ethene (C₂H₄).
Here concentrated H₂SO₄ acts as a dehydrating agent (not as an oxidising agent), because it has a very strong affinity for water. It is used in excess and is not consumed, so the reaction is written with H₂SO₄ over the arrow.
⚗️ The Reaction
C₂H₅OH → (conc. H₂SO₄, 443 K) C₂H₄ + H₂O
• The alcohol loses H and OH from adjacent carbons — an elimination (dehydration) reaction.
• Atom count: 2 C, 6 H, 1 O on each side, so the equation is balanced.
• The product ethene decolourises bromine water, proving that it is unsaturated.
❌ Why the other options are incorrect
✗ Only ethyne — ethyne (C₂H₂) would need two molecules of water to be removed; simple dehydration of ethanol stops at the double bond, not the triple bond.
✗ Ethyne and hydrogen — no hydrogen gas is liberated here; the hydrogen leaves as part of the water molecule, and the loss of water gives ethene, not ethyne.
✗ Ethane and water — ethane (C₂H₆) is saturated and would require the addition of hydrogen, whereas this reaction removes atoms; dehydration can never make a saturated product.
📌 SSC & Railway Exam Facts
• Ethanol → (conc. H₂SO₄, 443 K) Ethene + Water — the standard dehydration PYQ.
• Conc. H₂SO₄ = dehydrating agent; it also chars sugar to carbon.
• Ethanol + Na → sodium ethoxide + H₂ gas — the test of the –OH group.
• Ethanol + ethanoic acid → (acid catalyst) ester + water is esterification; the reverse with NaOH is saponification.
• Ethene + H₂ → (Ni catalyst) ethane — hydrogenation, used to make vanaspati ghee from oils.

Correct Answer: Option 1 — CuSO₄
In the electrolytic refining of copper, the electrolyte must contain the same metal ion that is being refined, so that copper can dissolve at one electrode and deposit at the other. The electrolyte used is an acidified aqueous solution of copper sulphate (CuSO₄ + a little dilute H₂SO₄).
The thick block of impure copper is made the anode and a thin strip of pure copper the cathode. On passing current, copper of the anode dissolves as Cu²⁺ ions and an equal amount of pure copper is deposited on the cathode. The soluble impurities go into solution, while the insoluble ones — gold, silver and platinum — fall below the anode as anode mud, a valuable by-product.
⚗️ At the Electrodes
Anode: Cu(s) → Cu²⁺(aq) + 2e⁻ | Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)
• The anode gets thinner and the cathode gets thicker.
• The concentration of CuSO₄ stays constant, because as much Cu²⁺ enters the solution as leaves it.
❌ Why the other options are incorrect
✗ CuO — copper(II) oxide is an insoluble solid and cannot serve as an electrolyte solution; it is a reduction raw material in smelting, not in refining.
✗ Na₂SO₄ — it would supply Na⁺ ions, so hydrogen would be released at the cathode instead of copper being deposited.
✗ Cu₂Cl₂ — copper(I) chloride is almost insoluble in water and unstable, so it cannot maintain a steady supply of Cu²⁺ ions.
📌 SSC & Railway Exam Facts
• Impure metal = anode, pure metal = cathode, salt of the same metal = electrolyte.
• Anode mud of copper refining is a source of gold, silver, platinum and selenium.
• CuSO₄·5H₂O = blue vitriol (nila thotha); on heating it turns white, losing water of crystallisation.
• Roasting (sulphide ore + O₂) and calcination (carbonate ore, limited air) both precede reduction to the metal.
• Copper's chief ore is copper pyrites, CuFeS₂.

Correct Answer: Option 4 — Ethanol
Ethanol (C₂H₅OH), commonly called ethyl alcohol or simply "alcohol", is the only member of the alcohol family that is safe in small, controlled doses. It is a colourless liquid with a boiling point of 78 °C, it mixes with water in all proportions, and it is an excellent solvent for many drugs that do not dissolve in water.
For these reasons ethanol is used in cough syrups, tonics, tincture iodine and many other medicinal preparations, where it dissolves the active ingredient and also preserves the mixture.
❌ Why the other options are incorrect
✗ Methanol — CH₃OH (wood spirit) is extremely poisonous. In the body it is oxidised to methanal (formaldehyde), which coagulates protoplasm; even a small quantity causes blindness, and larger amounts cause death. It is added to ethanol to make denatured spirit, deliberately unfit to drink.
✗ Propanol — C₃H₇OH is used as an industrial solvent and in rubbing alcohol/sanitisers, not taken internally in syrups.
✗ Ethanoic acid — CH₃COOH is a carboxylic acid, not an alcohol at all; its 5–8% solution is vinegar, and the pure acid freezes to glacial acetic acid below 16.6 °C.
📌 SSC & Railway Exam Facts
• Ethanol C₂H₅OH — used in cough syrups, tinctures and as a solvent; methanol CH₃OH — poisonous.
• Denatured spirit = ethanol + methanol (+ dye), made unfit for drinking to avoid excise duty.
• Ethanol from sugarcane molasses by fermentation; blended with petrol it makes gasohol.
• Ethanol + Na → sodium ethoxide + H₂; ethanol → (conc. H₂SO₄, 443 K) ethene + water.
• Functional groups: –OH alcohol, –CHO aldehyde, –COOH carboxylic acid, >C=O ketone.

Correct Answer: Option 3 — (NH₄)₂SO₄
A chemical formula is written by balancing the charges of the two radicals so that the compound as a whole is electrically neutral. The ammonium ion is NH₄⁺ (charge +1) and the sulphate ion is SO₄²⁻ (charge −2).
To cancel a charge of −2, we need two ammonium ions, each carrying +1. This is the criss-cross method: the valency of one ion becomes the subscript of the other. The two NH₄ units are enclosed in a bracket before writing the subscript 2, because NH₄ is a polyatomic (compound) radical — writing NH₄₂ would be wrong.
So ammonium sulphate = (NH₄)₂SO₄, containing 2 N, 8 H, 1 S and 4 O atoms and having a molecular mass of 132 u. It is a widely used nitrogenous fertiliser.
❌ Why the other options are incorrect
✗ Na₂SO₄ — this is sodium sulphate (Glauber's salt as the decahydrate); the cation is Na⁺, not ammonium.
✗ (NH₄)SO₄ — charge-wise unbalanced: +1 against −2; one ammonium ion cannot neutralise a sulphate ion.
✗ Ca₂SO₄ — wrong on two counts: the cation is calcium, and Ca²⁺ with SO₄²⁻ gives CaSO₄, never Ca₂SO₄.
📌 SSC & Railway Exam Facts
• Common radicals: NH₄⁺, Na⁺, Ca²⁺, Al³⁺, SO₄²⁻, CO₃²⁻, NO₃⁻, PO₄³⁻.
• In the criss-cross method the valency of one ion becomes the subscript of the other.
• A polyatomic radical taken more than once must be put in brackets: (NH₄)₂SO₄, Ca(OH)₂, Al₂(SO₄)₃.
• (NH₄)₂SO₄ and urea NH₂CONH₂ are nitrogenous fertilisers.
• CaSO₄·2H₂O = gypsum, CaSO₄·½H₂O = Plaster of Paris, Na₂SO₄·10H₂O = Glauber's salt.

Correct Answer: Option 1 — Hydrogen gas
Normally a metal reacting with an acid gives hydrogen, but nitric acid (HNO₃) is a strong oxidising agent: it oxidises the hydrogen produced into water, and is itself reduced to oxides of nitrogen (N₂O, NO, NO₂). That is why HNO₃ generally does not give hydrogen gas with metals.
Magnesium (Mg) and manganese (Mn) are the two standard exceptions. With very dilute nitric acid these two metals are so reactive, and the acid so weakly oxidising at that dilution, that the hydrogen escapes as H₂ gas before it can be oxidised.
⚗️ The Reaction
Mg + 2HNO₃(very dilute) → Mg(NO₃)₂ + H₂↑
Mn + 2HNO₃(very dilute) → Mn(NO₃)₂ + H₂↑
• Both metals lie above hydrogen in the reactivity series, so they displace it from the acid.
• Atom count in the first equation: 1 Mg, 2 H, 2 N, 6 O on each side — balanced.
• The gas burns with a pop sound when a burning splint is brought near it — the test for H₂.
❌ Why the other options are incorrect
✗ Nitrogen gas — N₂ is not liberated here; nitric acid may be reduced to nitrogen oxides, but not to free nitrogen in this reaction.
✗ Helium gas — He is an inert noble gas; it takes part in no chemical reaction and is not present in either reactant.
✗ Oxygen gas — O₂ is not produced by a metal–acid reaction; oxygen from the acid ends up in the salt and in water.
📌 SSC & Railway Exam Facts
• Metal + dilute acid → salt + hydrogen, but HNO₃ is the exception because it is an oxidising acid.
• Only Mg and Mn give H₂ with very dilute HNO₃ — a classic PYQ pair.
• Test for hydrogen: burns with a "pop" sound. Test for CO₂: turns lime water milky.
• Aqua regia = 3 HCl : 1 HNO₃, and it dissolves gold and platinum.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au; metals below H do not displace hydrogen from acids.

Correct Answer: Option 1 — Copper
Whether a metal reacts with a dilute acid is decided by its position in the reactivity series:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
Only a metal placed above hydrogen can displace hydrogen from a dilute acid. Copper lies below hydrogen, so it has a lower tendency to lose electrons than hydrogen and cannot displace H⁺ — that is, copper does not react with dilute HCl or dilute H₂SO₄ and gives no hydrogen gas. Of the four metals given, copper is therefore the least reactive with acids.
Copper does dissolve in hot concentrated H₂SO₄ or in HNO₃, but that is an oxidation by the acid, not a hydrogen-displacement reaction.
⚗️ Compare the Reactions
Zn + 2HCl(dilute) → ZnCl₂ + H₂↑ | Fe + 2HCl(dilute) → FeCl₂ + H₂↑
Cu + HCl(dilute) → no reaction
• Metals above H displace hydrogen; metals below H do not.
• The order of reactivity among the options is Al > Zn > Fe > Cu.
❌ Why the other options are incorrect
✗ Aluminium — the most reactive of the four; 2Al + 6HCl → 2AlCl₃ + 3H₂↑. Its apparent resistance in air is due to a protective oxide layer (Al₂O₃), not low reactivity.
✗ Iron — lies above hydrogen and reacts with dilute acids to give H₂; it also rusts readily in moist air.
✗ Zinc — reacts vigorously with dilute HCl and H₂SO₄; it is the standard metal used in the laboratory preparation of hydrogen gas.
📌 SSC & Railway Exam Facts
• Metals below hydrogen (Cu, Hg, Ag, Au, Pt) do not displace H₂ from dilute acids.
• Gold and platinum dissolve only in aqua regia (3 HCl : 1 HNO₃).
• Galvanisation = coating iron with zinc, which protects the iron because zinc is more reactive.
• Copper vessels form a green coating of basic copper carbonate, CuCO₃·Cu(OH)₂, in moist air.
• Most reactive metal — potassium (K/Cs); least reactive — gold (Au) in the school series.

Correct Answer: Option 2 — They do not have spaces in between them.
The particle nature of matter rests on four established characteristics: particles are extremely small, they have spaces between them, they are continuously moving (they possess kinetic energy), and they attract one another.
The statement in option 2 is the opposite of the truth — particles of matter do have spaces between them. The proof is everyday: when sugar or salt is dissolved in a full glass of water the level does not overflow, because the solute particles slip into the spaces between the water particles. Likewise potassium permanganate spreads through water and two gases mix on their own — both need inter-particle space.
The size of these spaces decides the state: largest in gases, moderate in liquids, smallest in solids — which is why gases are highly compressible and solids are not.
❌ Why the other options are incorrect (they are all TRUE statements)
✗ They are moving constantly — true; particles possess kinetic energy and move ceaselessly, which is shown by diffusion and by Brownian motion. Kinetic energy increases with temperature.
✗ They are very small — true; particles are beyond imagination in smallness — a few crystals of potassium permanganate colour a large bucket of water.
✗ They attract each other — true; this force of attraction holds matter together and is strongest in solids and weakest in gases.
📌 SSC & Railway Exam Facts
• Characteristics of particles: very small, have spaces, always moving, attract each other.
• Inter-particle space and compressibility: gas > liquid > solid; force of attraction: solid > liquid > gas.
• Diffusion increases with temperature, because the kinetic energy of the particles rises.
• Brownian motion — the zig-zag movement of suspended particles — proves that particles are in constant motion.
• Smell of incense spreading, ink diffusing in water and gases mixing are the classic proofs of particle motion and inter-particle space.

Correct Answer: Option 1 — Gypsum
Calcium sulphate hemihydrate, CaSO₄·½H₂O, is the chemical name of Plaster of Paris. It is made by heating gypsum (CaSO₄·2H₂O) at 373 K (100 °C), at which temperature gypsum loses three-quarters of its water of crystallisation, leaving only half a molecule of water per formula unit — hence "hemi" (half) hydrate.
The temperature must be controlled carefully. Above 373 K the whole of the water is driven off, giving anhydrous calcium sulphate (CaSO₄), called dead burnt plaster, which will not set with water and is useless for plastering.
Plaster of Paris is a white powder that, on mixing with water, sets into a hard solid mass in a few minutes by taking the water back and turning into gypsum again. It is used for plaster casts for fractured bones, for making toys and statues, for decorative false ceilings and for smoothing wall surfaces.
⚗️ The Reactions
CaSO₄·2H₂O → (heat, 373 K) CaSO₄·½H₂O + 1½H₂O
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (sets hard)
• Setting is simply the re-formation of gypsum, and it is exothermic.
• Plaster of Paris must be stored in a moisture-proof container, or it sets in the packet.
❌ Why the other options are incorrect
✗ Sodium Carbonate — Na₂CO₃·10H₂O (washing soda) is used in laundry and to remove permanent hardness of water; it contains no calcium sulphate.
✗ Copper Sulphate — CuSO₄·5H₂O (blue vitriol) is a copper compound used in electroplating and as a fungicide; heating it gives white anhydrous CuSO₄, not Plaster of Paris.
✗ Sodium Hydrogen Carbonate — NaHCO₃ (baking soda) is used in baking powder and antacids; on heating it gives Na₂CO₃, H₂O and CO₂.
📌 SSC & Railway Exam Facts
• Gypsum = CaSO₄·2H₂O; Plaster of Paris = CaSO₄·½H₂O; heating temperature 373 K (100 °C).
• Heating above 373 K gives dead burnt plaster (anhydrous CaSO₄), which does not set.
• Plaster of Paris is used for plaster casts, statues, toys and false ceilings.
• Gypsum is added to cement (about 2–3%) to slow down its rate of setting.
• Other hydrated salts: CuSO₄·5H₂O (blue vitriol), FeSO₄·7H₂O (green vitriol), Na₂CO₃·10H₂O (washing soda).

Correct Answer: Option 1 — two triangular bases and three rectangular lateral surfaces
A triangular glass prism is a transparent solid bounded by five plane surfaces in all. Two of them are triangular bases placed parallel to each other at the two ends, and the remaining three are rectangular lateral (side) surfaces that join the corresponding sides of the two triangles. Geometrically it is a triangular prism — a solid with 5 faces, 9 edges and 6 vertices.
When a ray of light enters the prism it refracts twice — once while entering the glass and once while leaving it — and because the two refracting surfaces are not parallel, the emergent ray bends away from the original path. This total bending is the angle of deviation (δ), and the angle between the two refracting surfaces is the angle of the prism (A), usually 60° in a standard laboratory prism.
Since different colours of white light have different refractive indices in glass, each colour bends by a different amount and white light splits into VIBGYOR — this is dispersion of light.
❌ Why the other options are incorrect
✗ three triangular bases and one rectangular lateral surface — a prism has only two bases; three triangular bases is geometrically impossible here.
✗ one triangular base and three rectangular lateral surfaces — with a single base the solid would be open; it would not be a closed prism.
✗ three triangular bases and three rectangular lateral surfaces — again wrong, a triangular prism has exactly 2 triangular + 3 rectangular faces.
📌 SSC & Railway Exam Facts
• Dispersion of white light by a prism was first demonstrated by Sir Isaac Newton.
• Violet deviates the most and red the least, because violet has the shortest wavelength.
• The band of colours obtained is called the spectrum; the order is VIBGYOR.
• A rainbow is a natural spectrum formed by refraction, total internal reflection and dispersion in water droplets.
• Refractive index of ordinary glass ≈ 1.5, of water ≈ 1.33.

Correct Answer: Option 4 — will emerge parallel to the principal axis
This is one of the four standard ray-diagram rules for spherical mirrors. A ray that passes through the principal focus F (in a concave mirror) or appears to pass through F (in a convex mirror, where F lies behind the mirror) always comes back parallel to the principal axis after reflection.
The reason is the reversibility of light. The very first rule says that a ray travelling parallel to the principal axis is reflected through the focus. Reverse the direction of that same ray and you get exactly this rule — a ray directed at the focus returns parallel to the axis. Both are simply the laws of reflection (∠i = ∠r) applied at the mirror surface.
The rule holds for both concave and convex mirrors, which is why the answer carries no extra condition.
❌ Why the other options are incorrect
✗ will emerge perpendicular to the principal axis, only in the case of concave mirror — the emergent ray is parallel, never perpendicular, and no special condition applies.
✗ will emerge perpendicular to the principal axis — wrong direction; a perpendicular emergent ray would never form the images we actually observe.
✗ will emerge perpendicular to the principal axis, only in the case of convex mirror — wrong on both counts: wrong direction and a false restriction to convex mirrors.
📌 SSC & Railway Exam Facts
• Laws of reflection: ∠i = ∠r, and the incident ray, reflected ray and normal all lie in the same plane.
• For a spherical mirror f = R/2, where R is the radius of curvature.
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u = h′/h.
• A ray passing through the centre of curvature C hits the mirror normally and retraces its own path.
• Concave mirrors are used in torches, headlights, shaving mirrors and solar cookers; convex mirrors as rear-view mirrors because they give a wide field of view.

Correct Answer: Option 1 — ρ
📖 Given
• Resistivity of the original resistor = ρ
• The resistor is cut into 3 equal parts
✍️ Formula
R = ρL/A, so ρ = RA/L
🔢 Solution
• Resistivity ρ is a property of the material, not of the size or shape of the conductor. It depends only on the nature of the material and its temperature.
• Cutting the wire changes only L and hence R — for each piece the new length is L/3, so the new resistance is R/3.
• Check with the formula: new ρ = (R/3 × A)/(L/3) = RA/L = ρ.
• Therefore the resistivity of each part remains ρ.
In plain words: breaking a copper wire into pieces does not turn it into a different material — each piece is still copper, so each piece has the same resistivity.
❌ Why the other options are incorrect
✗ 3ρ — this is the mistake of thinking resistivity behaves like resistance when three pieces are joined in series.
✗ 6ρ — has no physical basis; it comes from randomly multiplying the given value.
✗ ρ/3 — this is the value of the resistance of each part (R/3), wrongly written for resistivity.
📌 SSC & Railway Exam Facts
• SI unit of resistivity is ohm metre (Ω m); SI unit of resistance is ohm (Ω).
• R ∝ L and R ∝ 1/A — a thin, long wire has more resistance.
• Silver is the best conductor; copper and aluminium are used for wires because they are cheaper.
• Resistivity of metals increases with temperature but of semiconductors decreases with temperature.
• Nichrome has a very high resistivity, so it is used in heaters, toasters and irons.

Correct Answer: Option 4 — Perpendicular to the plane of the wire
When a current flows through a straight conductor, it produces a magnetic field around itself. The field lines are concentric circles with the wire at their centre, and these circles lie in a plane perpendicular to the wire. So at every point the magnetic field is perpendicular to the wire and tangential to the circle passing through that point.
The sense (clockwise or anticlockwise) is given by Maxwell's Right-Hand Thumb Rule: hold the wire in your right hand with the thumb pointing along the current; the curled fingers then show the direction of the magnetic field lines.
The strength of the field is given by B ∝ I/r — it increases with current I and decreases as the distance r from the wire increases. That is why the circles are drawn crowded close to the wire and far apart away from it.
This effect was discovered accidentally by Hans Christian Oersted (1820), when a compass needle near a current-carrying wire deflected.
❌ Why the other options are incorrect
✗ At an angle 45° to the plane of the wire — the field is never at a fixed slant of 45°; the circular field lines always stay in the perpendicular plane.
✗ Parallel to the plane of the wire — if the field were parallel to the wire, a compass placed near it would not deflect sideways, which contradicts Oersted's experiment.
✗ At an angle 60° to the plane of the wire — an arbitrary angle with no physical meaning for a straight conductor.
📌 SSC & Railway Exam Facts
• Oersted (1820) first showed that electricity produces magnetism; SI unit of magnetic field strength H is named oersted in CGS.
• SI unit of magnetic flux density B is the tesla (T); 1 T = 1 Wb m⁻².
• Right-Hand Thumb Rule (also called Maxwell's corkscrew rule) gives the direction of B around a straight wire.
• Inside a solenoid the field is uniform and parallel to the axis, like that of a bar magnet.
• Fleming's Left-Hand Rule → motor (force on a conductor); Fleming's Right-Hand Rule → generator (induced current).

Correct Answer: Option 4 — is displaced in the direction of the force
Work done by a constant force is defined as W = F s cos θ, where F is the force, s the displacement and θ the angle between them. The sign of the work depends entirely on cos θ.
• When the displacement is along the force, θ = 0° and cos 0° = +1, so W = +Fs — the work is positive. Here the force adds energy to the body and its kinetic energy increases.
• When the displacement is opposite to the force, θ = 180° and cos 180° = −1, so W = −Fs — the work is negative (for example friction, or gravity on a body thrown upward).
• When the displacement is perpendicular to the force, θ = 90° and cos 90° = 0, so W = 0.
So the work done is positive only in the first case.
❌ Why the other options are incorrect
✗ Is displaced opposite to the direction of the force — here θ = 180°, so the work is negative, not positive.
✗ is displaced perpendicular to the force — θ = 90°, cos 90° = 0, so zero work is done, as in uniform circular motion.
✗ is not displaced — with s = 0 the work is zero, however large the force; pushing hard against a fixed wall does no work.
📌 SSC & Railway Exam Facts
• SI unit of work and energy is the joule (J); 1 J = 1 N m. CGS unit is the erg; 1 J = 10⁷ erg.
• W = F s cos θ — work is a scalar quantity though F and s are vectors.
• Work–energy theorem: net work done = change in kinetic energy.
• Power = Work/Time, SI unit watt (W); 1 horsepower = 746 W.
• A coolie carrying a load on his head and walking on level ground does zero work against gravity, since force and displacement are perpendicular.

Correct Answer: Option 2 — Both (iii) and (iv)
Newton's Universal Law of Gravitation states that every body in the universe attracts every other body with a force F = G M m / r², directed along the line joining their centres.
Checking each statement:
• (i) is false — gravitational force is always attractive, never repulsive. It is the only fundamental force that is purely attractive.
• (ii) is false — it is the constant G that has the same value everywhere in the universe, not the force. The force F changes with the masses and with the distance r between the bodies.
• (iii) is true — F ∝ Mm, so doubling either mass doubles the force.
• (iv) is true — the gravitational pull of the Sun supplies the centripetal force that keeps the Earth in its orbit; the same force keeps the Moon revolving around the Earth.
Hence only (iii) and (iv) are correct.
❌ Why the other options are incorrect
✗ Only (iii) — it wrongly leaves out (iv), which is a standard true consequence of gravitation.
✗ Both (i) and (ii) — both statements are false; gravity is attractive, and the force is not the same everywhere.
✗ Both (ii) and (iii) — (iii) is true but (ii) confuses the force F with the constant G.
📌 SSC & Railway Exam Facts
• G = 6.67 × 10⁻¹¹ N m² kg⁻² — a universal constant, same everywhere; measured by Henry Cavendish.
• F ∝ 1/r² — this is called the inverse square law.
• Gravitation is the weakest of the four fundamental forces, but it acts over infinite range.
• g = GM/R²; on the Earth g ≈ 9.8 m s⁻², on the Moon g ≈ 1.63 m s⁻² (about 1/6 of the Earth's).
• Kepler's laws of planetary motion are explained by Newton's law of gravitation.

Correct Answer: Option 3 — +10 cm, convex
📖 Given
• Radius of curvature, R = +20 cm
• To find: focal length f and the nature of the mirror
✍️ Formula
f = R/2
🔢 Solution
• f = R/2
• f = +20/2
• f = +10 cm
• Now the sign tells the nature. Under the New Cartesian Sign Convention, distances measured behind the mirror (in the direction of the incident light) are positive.
• A positive R and f mean the centre of curvature and the focus lie behind the mirror — this happens only for a convex mirror.
• So the mirror has f = +10 cm and is convex.
In short, the magnitude 10 cm comes from halving the radius, and the plus sign identifies it as a convex (diverging) mirror.
❌ Why the other options are incorrect
✗ +40 cm, concave — this is the error of writing f = 2R instead of f = R/2, and concave mirrors have a negative f.
✗ +10 cm, concave — the magnitude is right but the nature is wrong; for a concave mirror f would be −10 cm.
✗ −40 cm, convex — both wrong: the value comes from doubling instead of halving R, and the sign contradicts a convex mirror.
📌 SSC & Railway Exam Facts
• f = R/2 for every spherical mirror; for a plane mirror R = ∞, so f = ∞.
• Concave mirror: f and R are negative; convex mirror: f and R are positive.
• A convex mirror always forms a virtual, erect and diminished image, so it is used as a rear-view mirror.
• A concave mirror is used in torches, headlights, shaving mirrors, dentists' mirrors and solar cookers.
• Mirror formula 1/v + 1/u = 1/f; magnification m = −v/u.

Correct Answer: Option 3 — The total energy before and after the transformation is not constant.
This statement is NOT true because it directly contradicts the Law of Conservation of Energy, one of the most fundamental laws of physics.
The law states that energy can neither be created nor destroyed; it can only be transformed from one form into another, and the total energy of an isolated system always remains constant. So the total energy before a transformation is exactly equal to the total energy after it.
A falling stone is a good example: at the top it has maximum potential energy and zero kinetic energy; as it falls, PE steadily changes into kinetic energy, but at every instant PE + KE = constant. Similarly, an electric bulb converts electrical energy into light and heat, and the total remains unchanged.
Options 1, 2 and 4 are three different ways of stating this same law, so they are all true. Option 3 denies it and is therefore the required answer.
❌ Why the other options are incorrect
✗ Energy can neither be created nor destroyed. — this is the exact statement of the law of conservation of energy, hence true.
✗ The total energy before and after the transformation always remains constant. — true; this is the quantitative form of the same law.
✗ Energy can only be transformed from one form to another. — true; transformation, not creation or destruction, is what actually happens.
📌 SSC & Railway Exam Facts
• Law of conservation of energy — formulated from the work of Julius Robert Mayer, James Prescott Joule and Hermann von Helmholtz.
• For a freely falling body KE + PE = constant at every point (mechanical energy is conserved).
• Common conversions: electric motor — electrical → mechanical; microphone — sound → electrical; solar cell — light → electrical; dynamo/generator — mechanical → electrical.
• Einstein's mass–energy relation E = mc² extends the law to nuclear reactions.
• SI unit of energy is the joule (J); 1 kWh = 3.6 × 10⁶ J (the commercial 'unit' of electricity).

Correct Answer: Option 4 — P = IR
P = IR is not a power relation at all. By Ohm's law, the product IR equals the potential difference V, not the power. So the expression P = IR is dimensionally and physically wrong — it actually gives volts, not watts.
The correct starting point is P = VI (power = potential difference × current). Substituting Ohm's law V = IR into it gives the other two standard forms:
• Put V = IR in P = VI → P = (IR) × I = I²R
• Put I = V/R in P = VI → P = V × (V/R) = V²/R
So P = VI, P = I²R and P = V²/R are all correct, and only P = IR is wrong.
❌ Why the other options are incorrect
✗ P = VI — the basic definition of electric power; correct.
✗ P = V²/R — obtained by substituting I = V/R; correct, and used when V and R are known.
✗ P = I²R — obtained by substituting V = IR; correct, and it is the basis of Joule's law of heating.
📌 SSC & Railway Exam Facts
• SI unit of power is the watt (W); 1 W = 1 J s⁻¹ = 1 V × 1 A.
• Ohm's law: V = IR, valid at constant temperature; given by Georg Simon Ohm.
• Joule's law of heating: H = I²Rt — heat produced is proportional to I², R and t.
• Commercial unit of electrical energy = kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ J.
• In a series circuit the current is the same everywhere; in a parallel circuit the potential difference is the same across each branch.

Correct Answer: Option 4 — heat energy
An electric water heater (geyser / immersion rod) works on the heating effect of electric current, also called Joule heating. When current passes through the heating element, the free electrons collide repeatedly with the atoms of the wire; the electrical energy they lose appears as heat energy, which raises the temperature of the water.
The heat produced is given by Joule's law of heating, H = I²Rt, where I is the current, R the resistance of the element and t the time. Because H is proportional to R, the element is made of an alloy of high resistivity and high melting point — usually nichrome (nickel + chromium), which also does not oxidise readily when red-hot.
So the energy conversion in a water heater is simply electrical energy → heat energy.
❌ Why the other options are incorrect
✗ potential energy — this is stored energy due to position or configuration, as in water in an overhead tank or a stretched spring; a heater does not raise the water's position.
✗ wind energy — the kinetic energy of moving air, used in windmills; nothing in a heater produces wind.
✗ light energy — an electric bulb converts electrical energy mainly into light (and heat), but a water heater produces no useful light.
📌 SSC & Railway Exam Facts
• Joule's law of heating: H = I²Rt = VIt = (V²/R)t.
• Heating element alloy — nichrome; the filament of a bulb is tungsten (melting point ≈ 3380 °C).
• A fuse wire has low melting point and high resistance; it is always connected in series in the live wire.
• Devices based on the heating effect: electric iron, geyser, toaster, room heater, electric kettle.
• In domestic wiring: live — red/brown, neutral — black/blue, earth — green/yellow.

Correct Answer: Option 1 — G
The key word in the question is 'Universal'. G is the Universal Gravitational Constant, and by definition its value is exactly the same everywhere in the universe — on the Earth, on the Moon, on a planet of mass M, on a planet of mass 4M, or in deep space. It does not depend on the masses of the bodies, on the distance between them, or on the medium separating them.
Its value is G = 6.67 × 10⁻¹¹ N m² kg⁻².
What *does* change from planet to planet is g, the acceleration due to gravity, because g = GM/R² depends on the planet's mass M and radius R. Students often confuse g (variable) with G (constant) — that is exactly the trap in this question. So even on a planet of mass 4M, the constant remains G.
❌ Why the other options are incorrect
✗ G/2 — assumes G weakens on a heavier planet; G has no dependence on mass at all.
✗ Zero — G can never be zero; if it were, there would be no gravitational force anywhere in the universe.
✗ 4G — the common mistake of scaling G with the mass ratio 4M/M. It is g, not G, that would change if the radius stayed the same.
📌 SSC & Railway Exam Facts
• G = 6.67 × 10⁻¹¹ N m² kg⁻²; its value was measured experimentally by Henry Cavendish (1798) using a torsion balance.
• G is a scalar and a universal constant; g is a vector and varies with place.
• g = GM/R²; on the Earth g ≈ 9.8 m s⁻², on the Moon about 1/6 of this value.
• g is maximum at the poles and minimum at the equator; it decreases with both altitude and depth, and is zero at the centre of the Earth.
• Mass is constant everywhere, but weight (W = mg) changes from planet to planet.

Correct Answer: Option 1 — potential energy
When a spring is stretched or compressed, we do work against the restoring force of the spring. This work is not lost — it is stored inside the spring because of the change in its configuration (shape). Energy stored in a body due to its position or configuration is called potential energy, and in this particular case it is called elastic potential energy.
For a spring obeying Hooke's law (F = kx), the stored energy is U = ½kx², where k is the spring constant and x the extension or compression. Doubling the stretch therefore stores four times the energy.
The moment the spring is released, this stored potential energy converts back into kinetic energy — which is exactly how a wind-up toy, a spring balance and the mainspring of a watch work.
❌ Why the other options are incorrect
✗ electric energy — arises from the motion or position of charges; stretching a spring involves no charge at all.
✗ kinetic energy — energy due to motion, given by ½mv². A stretched spring that is being held is at rest, so it has no kinetic energy; KE appears only after release.
✗ gravitational energy — the potential energy due to height, U = mgh. Here the energy comes from a change of shape, not from a change of height.
📌 SSC & Railway Exam Facts
• Elastic PE of a spring = ½kx²; Gravitational PE = mgh; Kinetic energy = ½mv².
• Hooke's law: F = kx — within the elastic limit, stress ∝ strain, given by Robert Hooke.
• SI unit of spring constant k is N m⁻¹; a stiffer spring has a larger k.
• Energy in a stretched bow, catapult or rubber band is also elastic potential energy.
• Mechanical energy = KE + PE, and it stays constant when only conservative forces act.

Correct Answer: Option 2 — both plane and spherical surfaces
The laws of reflection are:
• First law — the angle of incidence is equal to the angle of reflection (∠i = ∠r).
• Second law — the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
These laws are applied at the point of incidence, using the normal drawn at that point. For a plane mirror the normal is the same all over the surface. For a spherical mirror the normal at any point is the line joining that point to the centre of curvature, so the normal changes from point to point — but the two laws still hold exactly at every single point.
That is precisely why we can draw ray diagrams for concave and convex mirrors and derive the mirror formula 1/v + 1/u = 1/f. So the laws apply to all reflecting surfaces, plane as well as spherical (and even to rough surfaces, where they give diffuse reflection).
❌ Why the other options are incorrect
✗ neither plane surfaces nor spherical surfaces — if this were true, no mirror could form an image at all.
✗ only to plane surfaces and not to spherical surfaces — spherical mirror ray diagrams are built entirely on ∠i = ∠r, so the laws clearly apply there too.
✗ only to spherical surface and not to plane surfaces — the plane mirror is the simplest demonstration of the laws of reflection.
📌 SSC & Railway Exam Facts
• A plane mirror always forms a virtual, erect image of the same size, at the same distance behind the mirror; it is laterally inverted.
• Regular reflection occurs from a smooth polished surface; diffuse (irregular) reflection from a rough surface — the laws hold in both cases.
• f = R/2 for a spherical mirror; mirror formula 1/v + 1/u = 1/f.
• Magnification m = −v/u = h′/h; a negative m means a real, inverted image.
• A concave mirror gives an erect and magnified image only when the object is placed between the pole and the focus.