RRB Technician Grade III 2024-25 30 December 2024 Shift 2 GK Questions – with answers & solutions

Correct Answer: Option 1 — Phototropism – movement towards water
Tropism (tropic movement) is a directional growth movement of a plant part in response to an external stimulus. Growth towards the stimulus is positive tropism and growth away from it is negative tropism. Such movements are slow and are caused by unequal growth on the two sides of the organ, controlled mainly by the plant hormone auxin.
Phototropism is the movement of a plant part in response to light, not water. Auxin collects on the shaded side of a shoot, so those cells elongate more and the shoot bends towards the light (positive phototropism), while roots bend away from it (negative phototropism). Movement towards water is called hydrotropism, so Option 1 pairs the term with the wrong stimulus and is the incorrect statement.
❌ Why the other options are incorrect
✗ Geotropism – in response to the pull of earth or gravity — this pair is correct; roots show positive geotropism (grow downwards) and shoots show negative geotropism (grow upwards).
✗ Chemotropism – movement towards chemical — correct; the classic example is the growth of the pollen tube through the style towards the ovule.
✗ Hydrotropism – movement towards water — correct; roots bend towards a source of moisture in the soil.
📌 SSC & Railway Exam Facts
• Auxin is the hormone responsible for tropic (growth) movements; it promotes cell elongation.
• Thigmotropism is growth movement in response to touch — seen in the tendrils of pea and grapevine.
• Nastic movements, such as the drooping of Mimosa pudica leaves, are non-directional and are not growth movements.
• Other plant hormones — gibberellin (stem elongation), cytokinin (cell division), abscisic acid (growth inhibitor, wilting), ethylene (fruit ripening).
• Charles Darwin and Francis Darwin were the first to experiment on phototropism, using canary grass coleoptiles.

Correct Answer: Option 4 — It will likely swell
A medium with a higher water concentration (i.e. lower solute concentration) than the cell sap is a hypotonic solution. Water then moves into the cell by osmosis — the movement of water molecules from a region of higher water concentration to lower water concentration through a selectively permeable membrane (the plasma membrane).
As water enters, the vacuole fills up and pushes the cytoplasm against the cell wall, so the cell swells and becomes turgid. The outward push of the cell contents is the turgor pressure, which keeps herbaceous stems and leaves firm and erect. The plant cell does not burst, because the tough cellulose cell wall resists further expansion — this is exactly why a plant cell behaves differently from an animal cell in the same medium.
❌ Why the other options are incorrect
✗ It will burst — only an animal cell (e.g. an RBC) bursts in a hypotonic medium; the plant cell's rigid cell wall prevents bursting.
✗ It will not show any change — no change occurs only in an isotonic medium, where water enters and leaves at equal rates.
✗ It will likely shrink — shrinking (plasmolysis) happens in a hypertonic medium, where water leaves the cell.
📌 SSC & Railway Exam Facts
• Osmosis is the diffusion of water through a semi-permeable membrane; diffusion is the movement of any particles from high to low concentration.
• Plasmolysis — shrinkage of the protoplasm away from the cell wall in a hypertonic solution.
• A turgid cell gives plants mechanical support; wilting occurs when turgor is lost.
• Raisins swell in plain water (endosmosis) and shrink in strong sugar solution (exosmosis).
• Roots absorb water from the soil by osmosis through the root hairs.

Correct Answer: Option 4 — vas deferens
The testes are the male gonads, held outside the abdominal cavity in the scrotum, because sperm formation needs a temperature about 2–3 °C lower than body temperature. Sperms made in the seminiferous tubules first pass into the epididymis, where they mature and are stored.
From the epididymis the sperms are carried by the vas deferens (sperm duct), a muscular tube that unites with a duct from the seminal vesicle and opens into the urethra. Along the way the seminal vesicles and the prostate gland add their secretions, which nourish the sperms and make the mixture easier to transport — sperms plus these secretions form the semen.
❌ Why the other options are incorrect
✗ urinary bladder — it only stores urine made by the kidneys; it plays no part in carrying sperms.
✗ seminal vesicle — it is a gland that adds a fructose-rich fluid to nourish sperms; it does not transport them from the testes.
✗ Urethra — it is the common passage for both urine and semen in the male, but only in the final part of the tract, after the vas deferens.
📌 SSC & Railway Exam Facts
• Testis performs a double role — it makes sperms and secretes the hormone testosterone.
• The vas deferens is the tube cut and tied in vasectomy, a male sterilisation method.
• Sperm pathway (mnemonic SEVEN UP) — Seminiferous tubules → Epididymis → Vas deferens → Ejaculatory duct → Nothing → Urethra → Penis.
• Prostate gland and Cowper's gland also add secretions to semen.
• In females the corresponding duct is the fallopian tube (oviduct), where fertilisation normally occurs.

Correct Answer: Option 1 — Chloroplast
Photosynthesis is the process by which green plants make their own food, converting carbon dioxide and water into carbohydrates using sunlight, and releasing oxygen. It takes place in the chloroplast, a green plastid found in the cells of leaves and other green parts.
The chloroplast is a double-membrane organelle. Its inner space, the stroma, holds stacks of flattened sacs called grana (thylakoids) that carry the green pigment chlorophyll. Chlorophyll traps light energy; the light reaction in the grana splits water and makes ATP and NADPH, while the dark reaction in the stroma fixes CO₂ into sugar. Because chloroplasts hold their own DNA and ribosomes, they are called semi-autonomous organelles and are known as the kitchen of the cell.
❌ Why the other options are incorrect
✗ Nucleus — the control centre of the cell; it stores the DNA (chromatin) and directs cell division and inheritance.
✗ Ribosome — the site of protein synthesis; it has no membrane and no pigment.
✗ Mitochondria — the powerhouse of the cell, where respiration releases energy as ATP; this is the reverse of photosynthesis.
📌 SSC & Railway Exam Facts
• Equation — 6CO₂ + 6H₂O → (light, chlorophyll) → C₆H₁₂O₆ + 6O₂.
• Plastids are of three kinds — chloroplast (green), chromoplast (coloured), leucoplast (colourless, stores starch/oil/protein).
• Chlorophyll contains magnesium (Mg); haemoglobin contains iron (Fe).
• Gas exchange for photosynthesis occurs through stomata, guarded by guard cells.
• Mitochondria and chloroplast are the only two organelles with their own DNA.

Correct Answer: Option 2 — Urea
Urine is formed in the nephron, the functional unit of the kidney, in three steps. In glomerular filtration, blood is filtered under pressure in the Bowman's capsule, and the filtrate contains water, glucose, amino acids, salts and urea. In selective reabsorption, the useful substances are taken back into the blood as the filtrate flows along the tubule. In tubular secretion, extra ions are added to the urine.
Urea is the nitrogenous waste produced in the liver from the breakdown of excess amino acids. Because the whole purpose of excretion is to throw urea out of the body, it is not reabsorbed and stays in the filtrate to become urine. Glucose, amino acids, salts and most of the water are selectively reabsorbed, which is why normal urine contains no glucose or protein.
❌ Why the other options are incorrect
✗ Amino acids — fully reabsorbed in the proximal convoluted tubule; they are the building blocks of proteins and too valuable to lose.
✗ Salts — reabsorbed according to the body's need, to keep osmoregulation and ionic balance.
✗ Glucose — completely reabsorbed in a healthy person; glucose in urine (glycosuria) indicates diabetes mellitus.
📌 SSC & Railway Exam Facts
• A human kidney has about 1 million (10 lakh) nephrons.
• Normal urine is about 95% water, 2.5% urea and 2.5% other salts/wastes.
• Excretory path — kidney → ureter → urinary bladder → urethra.
• The amount of water reabsorbed is controlled by the hormone ADH (vasopressin) from the pituitary.
• Haemodialysis is the artificial kidney technique used in kidney failure.

Correct Answer: Option 3 — Fragmentation
Fragmentation is a mode of asexual reproduction in which the body of a simple multicellular organism breaks up into two or more pieces (fragments) on maturation, and every fragment then grows into a complete new individual. Only a single parent is involved, no gametes are formed, and the offspring are genetically identical to the parent.
The standard NCERT example is the filamentous green alga Spirogyra, which simply breaks into smaller filaments that grow further. Fragmentation is possible only in organisms whose body is simple and not organised into specialised organs — a complex animal cannot survive being cut into pieces because its tissues are highly differentiated.
❌ Why the other options are incorrect
✗ Budding — a small outgrowth (bud) forms on the parent by repeated cell division at one site and detaches later; seen in Hydra and yeast, but the parent body does not break up.
✗ Gamete formation — production of sex cells (sperm and egg); it is a step of sexual, not asexual, reproduction.
✗ Sexual reproduction — needs two parents and the fusion of gametes, producing offspring with variation; fragmentation involves no fusion.
📌 SSC & Railway Exam Facts
• Modes of asexual reproduction — fission, budding, fragmentation, regeneration, vegetative propagation, spore formation.
• Spirogyra shows fragmentation; Planaria shows regeneration; Hydra and yeast show budding.
• Binary fission — Amoeba, Leishmania (definite plane); multiple fission — Plasmodium.
• Regeneration is carried out by specialised cells and is different from true reproduction.
• Rhizopus (bread mould) reproduces by spore formation in sporangia.

Correct Answer: Option 4 — Fungi
A unicellular organism is made of a single cell that performs all the life functions — nutrition, respiration, excretion, growth and reproduction — by itself. A multicellular organism is built of many cells that show division of labour, each group of cells forming a tissue specialised for one job.
Most fungi, such as Rhizopus, Mucor, Penicillium, Aspergillus and mushrooms, are multicellular. Their body is a network of thread-like hyphae collectively called the mycelium, so the group as a whole is treated as multicellular. (Yeast is the well-known exception — a unicellular fungus.) Fungi are eukaryotic, have a chitin cell wall and are heterotrophic, feeding as saprophytes or parasites.
❌ Why the other options are incorrect
✗ Paramecium — a unicellular protozoan with a slipper shape and cilia for locomotion; it has a definite shape and two nuclei.
✗ Chlamydomonas — a unicellular green alga with a cup-shaped chloroplast and two flagella.
✗ Bacteria — unicellular prokaryotes with no true nucleus and no membrane-bound organelles.
📌 SSC & Railway Exam Facts
• Unicellular examples — Amoeba, Paramecium, Euglena, Chlamydomonas, bacteria, yeast.
• Fungi are studied under Mycology; Bacteriology is the study of bacteria.
• Fungal cell wall is made of chitin; plant cell wall of cellulose.
• Penicillin, the first antibiotic, was obtained by Alexander Fleming from the fungus Penicillium notatum.
• Lichen = symbiotic association of a fungus + alga; Mycorrhiza = fungus + plant roots.

Correct Answer: Option 3 — Amino acid
Proteins are large, complex biomolecules built from repeating units called amino acids joined by peptide bonds. Therefore the simplest form — the monomer — of a protein is an amino acid. There are 20 standard amino acids, of which 9 are essential and must come from the diet.
In the human body, protein digestion begins in the stomach, where pepsin (activated by HCl) breaks proteins into peptones. In the small intestine trypsin of the pancreatic juice and erepsin/peptidases of the intestinal juice finish the job, converting proteins completely into amino acids, which are then absorbed by the villi into the blood and used for growth and repair of tissues.
❌ Why the other options are incorrect
✗ Glucose — the simplest form of carbohydrate; starch is digested by amylase into glucose, the body's main energy fuel.
✗ Glycerol — along with fatty acids, an end product of fat (lipid) digestion by lipase, not of protein.
✗ Fatty acid — also a digestion product of fats, formed after bile emulsifies them; it is not a protein unit.
📌 SSC & Railway Exam Facts
• End products — Carbohydrate → glucose; Protein → amino acids; Fat → fatty acids + glycerol.
• Kwashiorkor and marasmus are deficiency diseases caused by lack of protein.
• Proteins are called body-building foods; pulses, milk, eggs, fish and soyabean are rich sources.
• Enzymes and most hormones (e.g. insulin) are proteins; haemoglobin is a protein containing iron.
• Proteins contain C, H, O and N (and often S) — nitrogen is what distinguishes them from carbohydrates and fats.

Correct Answer: Option 2 — Differentiation
Plant tissues are of two broad kinds — meristematic and permanent. Meristematic tissue consists of actively dividing cells with thin walls, dense cytoplasm, a prominent nucleus and no vacuoles; it is found at the growing points (apical, lateral and intercalary meristems).
The cells produced by a meristem soon lose their power to divide and take up a specific shape, size and function. This process of taking a permanent shape, size and function is called differentiation, and it converts meristematic cells into the various permanent tissues. Permanent tissues may be simple (parenchyma, collenchyma, sclerenchyma) or complex (xylem and phloem).
❌ Why the other options are incorrect
✗ Elongation — merely an increase in length of a cell or organ; growth in length alone does not create a new tissue type.
✗ Vegetative Propagation — an asexual reproduction method in which a new plant grows from a root, stem or leaf (e.g. potato, sugarcane).
✗ Germination — the sprouting of a seed into a seedling; it concerns the whole seed, not tissue formation.
📌 SSC & Railway Exam Facts
• Apical meristem increases length; lateral meristem (cambium) increases girth; intercalary meristem is at the base of leaves/internodes (grasses).
• Parenchyma stores food; chlorenchyma contains chlorophyll; aerenchyma gives buoyancy to aquatic plants.
• Collenchyma gives flexibility (leaf stalks); sclerenchyma gives hardness (coconut husk) and is dead with lignin.
• Xylem carries water upward; phloem carries food in both directions.
• Protective permanent tissues — epidermis and cork (phellem); cork cells contain suberin.

Correct Answer: Option 3 — Fruit
In flowering plants, pollination brings pollen to the stigma; a pollen tube then grows down the style and reaches the ovule inside the ovary. The male gamete fuses with the female gamete there — this is fertilisation — and forms the zygote.
After fertilisation, the parts of the flower change: the zygote develops into the embryo, the ovule becomes the seed, and the ovary grows rapidly and ripens to become the fruit. Meanwhile the petals, sepals, stamens, style and stigma usually shrivel and fall off. The fruit wall (pericarp) protects the seeds and helps in their dispersal.
❌ Why the other options are incorrect
✗ Seedling — the young plant that appears only later, when the seed germinates under suitable conditions.
✗ Flower — the reproductive organ itself, present before fertilisation; it does not form from the ovary.
✗ Seed — it develops from the ovule, not the ovary; a seed contains the embryo, cotyledons and a seed coat.
📌 SSC & Railway Exam Facts
• Flower parts — sepal, petal, stamen (androecium) and carpel/pistil (gynoecium).
• Stamen = anther + filament; carpel = stigma + style + ovary.
• Self-pollination occurs within the same flower; cross-pollination between two flowers, aided by wind, water and insects.
• A fruit formed without fertilisation is called parthenocarpic (e.g. seedless banana).
• Double fertilisation — one male gamete forms the zygote, the other forms the nutritive endosperm; it is unique to angiosperms.

Correct Answer: Option 4 — Salivary gland – trypsin
The salivary glands secrete saliva, whose enzyme is salivary amylase (ptyalin). It acts in the mouth on starch and breaks it into maltose (sugar). Trypsin is not a salivary enzyme at all — it is a protein-digesting enzyme of the pancreatic juice, secreted by the pancreas and active in the small intestine in an alkaline medium. So this pair is wrong.
The main digestive glands are the salivary glands, gastric glands, liver, pancreas and intestinal glands. Together their secretions convert carbohydrates to glucose, proteins to amino acids, and fats to fatty acids and glycerol, which are then absorbed by the villi of the small intestine.
❌ Why the other options are incorrect
✗ Pancreas – pancreatic juice — correct; the pancreas pours pancreatic juice containing trypsin, amylase and lipase into the duodenum.
✗ Liver – bile juice — correct; the liver, the largest gland of the body, makes bile (stored in the gall bladder), which emulsifies fat and makes the medium alkaline.
✗ Gastric gland – pepsin — correct; gastric glands of the stomach secrete pepsin, HCl and mucus; pepsin digests protein in an acidic medium.
📌 SSC & Railway Exam Facts
• HCl makes the stomach medium acidic (pH ≈ 1.5–2) and activates pepsinogen into pepsin; mucus protects the stomach lining.
• Bile contains no enzyme; it only emulsifies fats into small droplets.
• Pancreas is a dual (hetero-crine) gland — it also secretes insulin and glucagon.
• Most digestion and absorption occur in the small intestine, the longest part of the alimentary canal.
• Amylase → starch; Pepsin/Trypsin → protein; Lipase → fat.

Correct Answer: Option 3 — Only a and b
Sexually transmitted diseases (STDs) spread mainly through sexual contact, and NCERT divides them by the type of pathogen. Bacterial STDs include gonorrhoea and syphilis, while viral STDs include HIV–AIDS and warts.
HIV (Human Immunodeficiency Virus) attacks the helper T-lymphocytes and destroys the body's immune system, leading to AIDS; it also spreads through infected blood transfusion, shared needles and from an infected mother to her baby. Warts are small skin growths caused by the Human Papilloma Virus (HPV), also a virus. So only statements a and b name viral infections.
❌ Why the other options are incorrect
✗ Only c — gonorrhoea is caused by the bacterium Neisseria gonorrhoeae, not a virus.
✗ Only a — incomplete; warts (b) are also a viral STD caused by HPV.
✗ Only c and d — both gonorrhoea and syphilis (Treponema pallidum) are bacterial, so neither is viral.
📌 SSC & Railway Exam Facts
• AIDS = Acquired Immuno Deficiency Syndrome; World AIDS Day is 1 December.
• Bacterial STDs are curable with antibiotics; viral STDs are not, which is why prevention matters.
• Use of a condom is the standard barrier method that prevents STDs as well as pregnancy.
• HPV vaccine protects against warts and cervical cancer.
• HIV does not spread by touching, sharing food, mosquito bites or working together.

Correct Answer: Option 1 — Repeated cell division at one specific site
Hydra is a simple multicellular animal that reproduces asexually by budding. As NCERT states, in Hydra a bud develops as an outgrowth due to repeated cell division at one specific site. Hydra possesses regenerative cells which, on dividing again and again at one point of the body wall, raise a small swelling — the bud.
The bud then grows, develops its own mouth and tentacles, and finally detaches from the parent body to live as a new independent individual. Because only one parent is involved and no gametes fuse, the young Hydra is genetically identical to the parent — it is a clone.
❌ Why the other options are incorrect
✗ Overgrowth of the whole body — the whole body does not enlarge; division is localised to one specific site where the bud appears.
✗ Mutation — a sudden change in the DNA/gene; it causes variation and is not the mechanism of budding.
✗ Cell cycle — the general sequence of G1, S, G2 and M phases of a cell; it is a routine process in all cells and does not by itself explain bud formation at one site.
📌 SSC & Railway Exam Facts
• Budding is seen in Hydra (animal) and yeast (fungus).
• Hydra also shows a remarkable power of regeneration, like Planaria.
• In yeast, the bud may remain attached and form a chain of buds.
• Asexual reproduction gives no variation; sexual reproduction gives variation and drives evolution.
• Hydra belongs to phylum Coelenterata (Cnidaria) and has stinging cells (cnidoblasts) on its tentacles.

Correct Answer: Option 1 — By changing the amount of water in them
Plants have no nervous system and no muscles, so they cannot move the way animals do. Instead, plant cells change shape by changing the amount of water in them. The classic example is the touch-me-not plant (Mimosa pudica), whose leaves fold and droop immediately when touched.
When the plant is touched, the stimulus travels as an electrochemical signal from cell to cell. In the swollen cushion-like pulvinus at the base of each leaf, some cells lose water and become flaccid, while others gain it. This change in turgor pressure alters the shape of the cells, so the leaflets fold up and the leaf drops. Since it is not caused by growth and has no relation to the direction of the stimulus, it is a nastic (thigmonastic) movement, not a tropic one.
❌ Why the other options are incorrect
✗ By changing the amount of protein in them — proteins act as enzymes and structural material; they do not swell or shrink cells to cause movement.
✗ By changing the amount of nucleic acid in them — DNA and RNA carry and express genetic information; their amount does not change for a quick response.
✗ By changing the amount of lipid in them — lipids are storage and membrane molecules; they play no role in this rapid leaf movement.
📌 SSC & Railway Exam Facts
• Plant movements are of two kinds — growth-dependent (tropic) and growth-independent (nastic).
• Mimosa pudica shows seismonasty/thigmonasty; the folding of leaves at dusk is nyctinasty (sleep movement).
• The swollen leaf base that controls the movement is the pulvinus.
• Tendrils coiling round a support is thigmotropism, which is growth-dependent.
• Plants use chemical (hormonal) coordination — auxin, gibberellin, cytokinin, abscisic acid, ethylene.

Correct Answer: Option 3 — Both Na⁺(aq) and OH⁻(aq) ions
Sodium hydroxide (NaOH), commonly called caustic soda, is a strong base. When it is put into water it dissolves completely and dissociates (ionises) into a sodium ion, Na⁺, and a hydroxide ion, OH⁻. Each ion is surrounded by water molecules, which is shown by writing (aq) after it.
A base that is soluble in water and furnishes OH⁻ ions is called an alkali. It is this large supply of OH⁻(aq) that makes the solution bitter in taste, soapy to touch, gives it a pH greater than 7, and turns red litmus blue.
Notice that no chlorine can appear here — the formula NaOH contains only Na, O and H. Chlorine is released only from a chlorine-bearing compound such as NaCl.
⚗️ The Reaction
NaOH(s) + water → Na⁺(aq) + OH⁻(aq)
• The ionic bond between Na⁺ and OH⁻ is pulled apart by polar water molecules.
• Dissolving NaOH in water is strongly exothermic, so a great deal of heat is released.
❌ Why the other options are incorrect
✗ Both Na⁺(aq) and Cl₂ — this is the common mistake of confusing NaOH with NaCl; NaOH has no chlorine atom, so Cl₂ can never form.
✗ Only Na⁺(aq) ions — a solution must remain electrically neutral, so the negative OH⁻ ions are produced along with Na⁺.
✗ Only OH⁻(aq) ions — the positive Na⁺ ions are set free at the very same moment; both ions always appear together.
📌 SSC & Railway Exam Facts
• NaOH = caustic soda, KOH = caustic potash, Ca(OH)₂ = slaked lime.
• All alkalis are bases, but all bases are not alkalis — an alkali is a water-soluble base.
• Acids give H⁺ (H₃O⁺) ions in water; bases give OH⁻ ions.
• NaOH is made by the electrolysis of brine (chlor-alkali process): 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
• In that process Cl₂ is liberated at the anode and H₂ at the cathode.

Correct Answer: Option 2 — Bleaching powder
When chlorine gas (Cl₂) is passed over dry slaked lime, Ca(OH)₂, the product is bleaching powder, whose chemical name is calcium oxychloride and whose formula is written as CaOCl₂. The word dry in the question is the key — with a solution of slaked lime the reaction would go a different way.
Bleaching powder is a pale yellow powder with a strong smell of chlorine. It is a strong oxidising agent and releases available chlorine, which is what actually does the bleaching and the disinfecting.
Its uses: bleaching cotton, linen and wood pulp in the textile and paper industries, disinfecting drinking water, and making chloroform.
⚗️ The Reaction
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
• Slaked lime (dry) + chlorine → bleaching powder + water.
• Both sides carry 1 Ca, 2 Cl, 2 O and 2 H, so the equation is balanced.
• Bleaching powder bleaches by oxidation, so the whiteness it gives is permanent, unlike the temporary bleaching by SO₂.
❌ Why the other options are incorrect
✗ Calcium chloride (CaCl₂) — obtained when Ca(OH)₂ reacts with hydrochloric acid, not with chlorine gas: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.
✗ Hydrochloric acid (HCl) — formed by dissolving hydrogen chloride gas in water; the reaction of Cl₂ with dry slaked lime gives no acid.
✗ Calcium oxide (CaO) — quick lime, produced by heating limestone: CaCO₃ → (heat) CaO + CO₂; Ca(OH)₂ itself is made by adding water to CaO.
📌 SSC & Railway Exam Facts
• Bleaching powder = CaOCl₂ (calcium oxychloride), made from Cl₂ + dry Ca(OH)₂.
• CaO = quick lime, Ca(OH)₂ = slaked lime, CaCO₃ = limestone/marble/chalk.
• CaO + H₂O → Ca(OH)₂ + heat is a classic exothermic combination reaction.
• Bleaching powder is used to disinfect drinking water and to bleach wood pulp in paper mills.
• Plaster of Paris = CaSO₄·½H₂O, obtained by heating gypsum (CaSO₄·2H₂O) to 373 K.

Correct Answer: Option 2 — Only (b) is correct.
The pH scale runs from 0 to 14: pH < 7 is acidic, pH = 7 is neutral and pH > 7 is basic. It was given by Sørensen, and p stands for 'potenz' (power) — pH measures the concentration of H⁺ ions.
Statement (b) is correct. Our body works within a very narrow pH band of about 7.0 to 7.8; human blood pH is about 7.4. Rainwater or any fluid outside this band upsets the body's chemistry, which is why the range must be so tightly controlled.
Statement (a) is wrong. Tooth enamel — the hardest substance in the body, made of calcium phosphate — starts to corrode when the pH of the mouth falls below 5.5, not when it is above 5.5. Bacteria convert leftover sugar into acid, and toothpastes are basic so that they neutralise this excess acid.
Statement (c) is wrong. Rain is called acid rain when its pH is below 5.6, caused by oxides of sulphur and nitrogen (SO₂, NO₂) dissolving to give H₂SO₄ and HNO₃. A pH above 8 would make the rain basic.
❌ Why the other options are incorrect
✗ Only (a) is correct — decay begins below pH 5.5; the sign in (a) is reversed.
✗ Only (c) is correct — acid rain has pH < 5.6, never greater than 8.
✗ (a), (b) and (c) are correct — only (b) survives; both (a) and (c) state the inequality the wrong way round.
📌 SSC & Railway Exam Facts
• Tooth decay starts when mouth pH < 5.5; toothpaste is basic (alkaline).
• Human blood pH ≈ 7.4; saliva ≈ 6.5–7.5; gastric juice ≈ 1.2–3.0.
• Acid rain: pH < 5.6; normal rain is slightly acidic (pH ≈ 5.6) because of dissolved CO₂.
• Bee sting → methanoic (formic) acid, relieved by baking soda; nettle sting relieved by dock plant leaf.
• Plants grow best in soil of pH about 6.5–7.5; acidic soil is treated with quick lime or slaked lime.

Correct Answer: Option 1 — ZnO
In a redox reaction, oxidation means the gain of oxygen (or loss of hydrogen / loss of electrons) and reduction means the loss of oxygen (or gain of hydrogen / gain of electrons). The two always happen together in the same reaction.
Here zinc oxide, ZnO, loses its oxygen and becomes zinc metal, Zn — so ZnO is reduced, and it acts as the oxidising agent. At the same time carbon, C, gains oxygen and becomes carbon monoxide, CO — so C is oxidised, and it acts as the reducing agent.
This is exactly the smelting step in the extraction of zinc: the ore is first roasted to ZnO and then reduced by coke in a furnace.
⚗️ The Reaction
ZnO + C → Zn + CO
• ZnO → Zn: oxygen is removed ⇒ reduction.
• C → CO: oxygen is added ⇒ oxidation.
• Oxidation number of Zn falls from +2 to 0, while that of C rises from 0 to +2 — the same conclusion by the electron method.
❌ Why the other options are incorrect
✗ CO — carbon monoxide is a product formed by the oxidation of carbon; a product of oxidation cannot be the substance reduced.
✗ Zn — zinc metal is also a product; it is what is obtained after ZnO has been reduced, not the species undergoing reduction.
✗ C — carbon gains oxygen, so carbon is oxidised; it is the reducing agent, the exact opposite of what is asked.
📌 SSC & Railway Exam Facts
• Oxidation = gain of oxygen / loss of hydrogen / loss of electrons (OIL); reduction = loss of oxygen / gain of hydrogen / gain of electrons (RIG).
• The oxidising agent is itself reduced; the reducing agent is itself oxidised.
• Carbon (coke) is the commonest reducing agent used in metallurgy; ZnO + C → Zn + CO.
• Highly reactive metals (K, Na, Ca, Mg, Al) cannot be reduced by carbon — they are obtained by electrolytic reduction.
• Thermite reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat, used for welding rail joints — here Al is the reducing agent.

Correct Answer: Option 2 — Mg
The reactivity series (activity series) arranges metals in decreasing order of their tendency to lose electrons and form positive ions. The standard order is:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
All four metals given here appear near the top of this series, but their order among themselves is K > Na > Ca > Mg. Hence magnesium (Mg) stands lowest and is the least reactive of the four.
The difference shows up clearly in their reaction with water. Potassium and sodium react violently with cold water (potassium catches fire), calcium reacts with cold water more gently, while magnesium does not react with cold water at all — it reacts only with hot water, and burns in steam:
⚗️ The Reaction
Mg + 2H₂O (hot) → Mg(OH)₂ + H₂↑
2K + 2H₂O → 2KOH + H₂↑ + heat (catches fire)
• The more reactive the metal, the more vigorously it displaces hydrogen from water.
• Na and K are stored under kerosene because they react even with the moisture of air.
❌ Why the other options are incorrect
✗ Ca — calcium is above Mg in the series; it reacts with cold water to give Ca(OH)₂ and H₂, so it is more reactive than Mg.
✗ K — potassium is the most reactive metal in this set; it reacts with cold water so violently that the hydrogen evolved catches fire.
✗ Na — sodium also reacts vigorously with cold water and is kept under kerosene; it is far more reactive than magnesium.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• Au (gold) and Pt are the least reactive metals; they are found in the free (native) state.
• Na and K are stored in kerosene; white phosphorus is stored in water.
• Metals above hydrogen displace H₂ from dilute acids; Cu, Ag and Au do not.
• Mg ribbon burns in air with a dazzling white light giving MgO, a white powder — a classic combination reaction.

Correct Answer: Option 2 — Both statements I and II are correct.
Statement I is correct. In a liquid the particles are held by moderate intermolecular forces — strong enough to keep the particles together, so the volume is fixed, but weak enough to let the particles slide over one another, so a liquid flows and takes the shape of its container. One litre of water stays one litre in a bottle, a beaker or a bowl, but its shape changes each time.
Statement II is also correct. Diffusion is the spontaneous intermixing of particles of two substances. It depends on how freely the particles can move. In a solid the particles only vibrate about fixed positions, so diffusion is extremely slow; in a liquid the particles are free to move about, so diffusion is much faster. The standard order is gas > liquid > solid.
An everyday proof: a drop of ink or copper sulphate spreads through a glass of water in minutes, while two metal blocks pressed together take years to diffuse into each other.
❌ Why the other options are incorrect
✗ Only statement I is correct — statement II is also true; liquid particles move far more freely than solid particles, so they diffuse faster.
✗ Only statement II is correct — statement I is the standard definition of the liquid state and is perfectly true.
✗ Neither statement I nor II is correct — both statements are scientifically correct, so this option is rejected.
📌 SSC & Railway Exam Facts
• Solid — fixed shape and volume; liquid — fixed volume, no fixed shape; gas — neither fixed.
• Rate of diffusion: gas > liquid > solid; compressibility: gas > liquid > solid.
• Intermolecular force: solid > liquid > gas; intermolecular space: gas > liquid > solid.
• Fish survive because oxygen diffuses and dissolves in water — diffusion in a liquid.
• Liquids and gases together are called fluids because both can flow.

Correct Answer: Option 1 — It is not used in the paper industry.
Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O — a white crystalline solid that is basic in nature and is obtained from baking soda (NaHCO₃) by heating and then recrystallising.
The statement that it is not used in the paper industry is false. Washing soda is in fact used extensively in the glass, soap and paper industries, which is one of its standard NCERT-listed uses. Since the question asks for the incorrect statement, this is the answer.
The decahydrate form contains 10 molecules of water of crystallisation, which is why the solid looks dry yet the formula carries ·10H₂O. On heating it loses this water and becomes anhydrous sodium carbonate (soda ash).
⚗️ The Reaction
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
Washing soda removing permanent hardness:
Na₂CO₃ + CaCl₂ → CaCO₃↓ + 2NaCl
• The Ca²⁺ and Mg²⁺ ions that cause hardness are thrown out as insoluble carbonates.
❌ Why the other options are incorrect
✗ It is used in the manufacture of sodium compounds such as borax — this is a true statement; washing soda is a raw material for borax and other sodium salts, so it is not the answer.
✗ It is used for removing permanent hardness of water — true; it precipitates the calcium and magnesium salts responsible for hardness.
✗ Sodium carbonate can be used as a cleaning agent for domestic purposes — true; it is a well-known domestic cleaning agent, which is why it is called "washing" soda.
📌 SSC & Railway Exam Facts
• Washing soda = Na₂CO₃·10H₂O; baking soda = NaHCO₃; soda ash = anhydrous Na₂CO₃.
• Washing soda is used in the glass, soap and paper industries and to soften hard water.
• Baking soda + tartaric acid = baking powder; it releases CO₂ that makes the cake rise.
• NaHCO₃ is used in soda-acid fire extinguishers and as an antacid.
• Na₂CO₃ is made by the Solvay (ammonia-soda) process; on heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.

Correct Answer: Option 2 — Heating
Rusting is the corrosion of iron. It happens only when iron is exposed to both oxygen (air) and moisture (water) together, and the reddish-brown flaky rust formed is hydrated iron(III) oxide, Fe₂O₃·xH₂O. Rust is porous, so it keeps flaking off and lets fresh iron below get attacked — which is why an iron object can rust right through.
Every genuine method of rust prevention works by cutting off the contact between iron and air/moisture, or by supplying a metal that corrodes in place of iron. Painting, greasing, galvanising, chrome plating, tin plating, anodising and alloying (stainless steel) all do this.
Heating does none of these. In fact heating speeds up a chemical reaction, and heating iron in moist air makes it corrode faster, so heating is not a method of preventing rust.
⚗️ The Reaction
4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (rust)
• Both air and water must be present — iron does not rust in dry air or in air-free (boiled) water.
• Galvanising coats iron with a layer of zinc; zinc is more reactive, so it corrodes first and saves the iron (sacrificial protection).
❌ Why the other options are incorrect
✗ Painting — a coat of paint forms a physical barrier that keeps oxygen and moisture away from the iron surface; a standard, correct method.
✗ Galvanising — the zinc coating protects iron even if it is scratched, because zinc is above iron in the reactivity series.
✗ Chrome plating — a thin layer of chromium is deposited electrically; it is shiny, hard and does not corrode, so it shields the iron.
📌 SSC & Railway Exam Facts
• Rust = Fe₂O₃·xH₂O (hydrated ferric oxide), reddish-brown and porous.
• Rusting needs oxygen + moisture; it is an oxidation (corrosion) process.
• Galvanisation = coating iron with zinc; tinning = coating with tin (used on food cans).
• Stainless steel = iron + chromium + nickel — an alloy that does not rust.
• Corrosion of copper gives a green coating (basic copper carbonate); silver tarnishes black (Ag₂S).

Correct Answer: Option 2 — Chlorine (Cl)
An anion is a negatively charged ion, formed when an atom gains electrons. Atoms do this to reach the stable octet of the nearest noble gas. It is the non-metals, which have 5, 6 or 7 electrons in their outermost shell, that find it easier to gain a few electrons than to lose many — so non-metals form anions, while metals form cations by losing electrons.
Chlorine (Z = 17) has the electronic configuration 2, 8, 7. It needs just one electron to complete its octet, so it readily gains one and becomes the chloride ion, Cl⁻ with the configuration 2, 8, 8 — the same as argon. Chlorine is therefore the only non-metal in the list and the obvious answer.
⚗️ The Reaction
Cl + e⁻ → Cl⁻ (2, 8, 7 → 2, 8, 8)
Na → Na⁺ + e⁻ (2, 8, 1 → 2, 8)
• In NaCl, sodium donates its single valence electron and chlorine accepts it, giving Na⁺ and Cl⁻ held by an ionic (electrovalent) bond.
• Non-metals also have high electronegativity and high electron affinity, which is why they attract electrons.
❌ Why the other options are incorrect
✗ Sodium (Na) — a metal with configuration 2, 8, 1; it loses 1 electron to form the cation Na⁺, not an anion.
✗ Aluminium (Al) — a metal with configuration 2, 8, 3; it loses 3 electrons to form Al³⁺, a positive ion.
✗ Calcium (Ca) — a metal with configuration 2, 8, 8, 2; it loses 2 electrons to form Ca²⁺, again a cation.
📌 SSC & Railway Exam Facts
• Cation = positive ion (loses electrons), moves to the cathode; anion = negative ion (gains electrons), moves to the anode.
• Metals are electropositive and form cations; non-metals are electronegative and form anions.
• Fluorine is the most electronegative element; caesium is the most electropositive.
• Group 17 (halogens) gain 1 electron → valency 1; group 16 gains 2 → valency 2.
• Ionic compounds have high melting points, are soluble in water and conduct electricity in molten or aqueous state, not in the solid state.

Correct Answer: Option 2 — 273.15 K
The melting point is the fixed temperature at which a solid changes into a liquid at atmospheric pressure. For ice this temperature is 0 °C.
To convert a Celsius temperature to the Kelvin (SI) scale, add 273.15:
K = °C + 273.15
So the melting point of ice = 0 + 273.15 = 273.15 K (often rounded to 273 K).
While the ice is melting, the temperature does not rise even though heat is being supplied. This heat is used only to break the forces of attraction between the particles and is called the latent heat of fusion. For ice it is 3.34 × 10⁵ J/kg, which is why ice at 0 °C cools a drink far better than water at 0 °C.
❌ Why the other options are incorrect
✗ 373.15 K — this is 100 °C, the boiling point of water, not the melting point of ice; the two values are the classic mix-up in this question.
✗ 200.15 K — this works out to −73 °C, far below the freezing point; ice would remain solid at this temperature.
✗ 300.15 K — this is 27 °C, ordinary room temperature; ice has long since melted by then.
📌 SSC & Railway Exam Facts
• Melting point of ice = 0 °C = 273.15 K; boiling point of water = 100 °C = 373.15 K.
• K = °C + 273.15; °F = (9/5)°C + 32; −40 °C = −40 °F.
• Latent heat of fusion of ice = 3.34 × 10⁵ J/kg; latent heat of vaporisation of water = 22.5 × 10⁵ J/kg.
• Temperature stays constant during a change of state — the heat absorbed is the latent heat.
• Kelvin is the SI unit of temperature; 0 K = −273.15 °C is absolute zero.

Correct Answer: Option 2 — 17 u
The molecular mass of a substance is the sum of the atomic masses of all the atoms present in one molecule of it. Its unit is the unified atomic mass unit, u.
The formula NH₃ tells us that one molecule contains 1 atom of nitrogen and 3 atoms of hydrogen — the subscript 3 applies only to the H written just before it.
⚗️ The Calculation
Molecular mass of NH₃ = (1 × mass of N) + (3 × mass of H)
= (1 × 14) + (3 × 1)
= 14 + 3 = 17 u
• Do not multiply 14 by 3 — the subscript belongs to hydrogen only, not to nitrogen.
• Since 1 mole = molecular mass in grams, 1 mole of NH₃ = 17 g.
Ammonia is a colourless gas with a pungent smell, highly soluble in water, and is made industrially by the Haber process from N₂ and H₂. Its aqueous solution is basic, turning red litmus blue.
❌ Why the other options are incorrect
✗ 42 u — obtained by wrongly taking 3 × (14) = 42, i.e. applying the subscript 3 to nitrogen instead of hydrogen.
✗ 21 u — obtained by wrongly adding 14 + 7 or by treating hydrogen's mass as more than 1 u; the hydrogen contribution is only 3 u.
✗ 10 u — obtained by subtracting (14 − 3 − 1) instead of adding; molecular mass is always a sum, never a difference.
📌 SSC & Railway Exam Facts
• Molecular mass = sum of atomic masses of all atoms in the molecule; unit u.
• H₂O = 18 u, CO₂ = 44 u, NH₃ = 17 u, CH₄ = 16 u, H₂SO₄ = 98 u.
• 1 mole = 6.022 × 10²³ particles — Avogadro's number; its mass in grams equals the molecular mass.
• Ammonia (NH₃) is prepared by the Haber process: N₂ + 3H₂ ⇌ 2NH₃.
• Atomic mass unit (u) is defined as 1/12 of the mass of a carbon-12 atom.

Correct Answer: Option 1 — Ionic compounds in solid state conduct electricity.
An ionic (electrovalent) compound is formed by the complete transfer of electrons from a metal to a non-metal, giving oppositely charged ions that are held together by a strong electrostatic force in a rigid crystal lattice — for example NaCl, MgCl₂, CaO.
For a substance to conduct electricity it must contain free, movable charge carriers. In the solid state the ions of an ionic compound are locked in fixed positions in the lattice and cannot move, so a solid ionic compound is a non-conductor. The statement in option 1 is therefore false, and since the question asks for the incorrect statement, it is the answer.
When the compound is melted (molten) or dissolved in water, the lattice breaks and the ions become free to move, so it now conducts electricity — such substances are called electrolytes.
⚗️ The Reaction
Na (2, 8, 1) + Cl (2, 8, 7) → Na⁺ (2, 8) + Cl⁻ (2, 8, 8) → NaCl
• The strong electrostatic attraction between Na⁺ and Cl⁻ needs a lot of energy to break, which is why the melting and boiling points are high.
• Molten or aqueous NaCl conducts; solid NaCl does not.
❌ Why the other options are incorrect
✗ Ionic compounds are generally soluble in water — true; water is a polar solvent and pulls the ions apart, so ionic compounds dissolve in water but are insoluble in kerosene and petrol.
✗ Ionic compounds are solids — true; they are hard, brittle crystalline solids because of the strong forces in the lattice.
✗ Ionic compounds have a high melting point — true; a large amount of heat is needed to overcome the strong interionic attraction (NaCl melts at 1074 K).
📌 SSC & Railway Exam Facts
• Ionic compounds conduct in molten and aqueous state only, never in the solid state.
• They are soluble in water, insoluble in organic solvents like kerosene and petrol.
• They are hard and brittle with high melting and boiling points; NaCl m.p. = 1074 K.
• Metal + non-metal → ionic bond; non-metal + non-metal → covalent bond.
• Covalent compounds have low melting points and are poor conductors of electricity.

Correct Answer: Option 1 — The blue-green colour of copper (II) chloride
Copper oxide (CuO) is a black metal oxide and, like all metal oxides, it is basic in nature. When dilute hydrochloric acid is added to it, a neutralisation takes place: the black powder dissolves and the solution slowly turns a blue-green (bluish-green) colour, which is the colour of copper(II) chloride, CuCl₂.
In CuO the copper has an oxidation state of +2, and this is carried over unchanged into the salt — hence copper(II) chloride, CuCl₂, and not copper(III) chloride. Copper practically never shows a +3 state in ordinary school chemistry.
This is the standard NCERT experiment used to prove that metal oxides are basic and that they react with acids just as bases do.
⚗️ The Reaction
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• Metal oxide + acid → salt + water — the same pattern as base + acid → salt + water.
• Two molecules of HCl are needed because copper is divalent (Cu²⁺), which balances the 2 Cl atoms.
• The black CuO disappears and a blue-green solution of CuCl₂ is left behind.
❌ Why the other options are incorrect
✗ The colourless solution of copper (II) chloride — CuCl₂ solution is not colourless; Cu²⁺ ions are coloured, giving a distinct blue-green tint.
✗ The blue-green colour of copper (III) chloride — the colour is right but the compound is wrong; copper is +2 here, so the salt is CuCl₂, i.e. copper(II) chloride.
✗ The red colour of copper (II) chloride — red is the colour of copper metal, not of its chloride solution; this is the common confusion between the metal and its salt.
📌 SSC & Railway Exam Facts
• Metal oxides are basic; non-metal oxides are acidic (CO₂, SO₂).
• CuO is black; CuCl₂ and CuSO₄ solutions are blue/blue-green; CuSO₄·5H₂O = blue vitriol.
• Al₂O₃ and ZnO are amphoteric — they react with both acids and bases.
• Acid + metal → salt + hydrogen; acid + metal carbonate → salt + water + CO₂.
• FeSO₄·7H₂O = green vitriol, ZnSO₄·7H₂O = white vitriol, FeSO₄ crystals are light green.

Correct Answer: Option 4 — in front of; behind
The centre of curvature (C) is the centre of the imaginary sphere of which the mirror is a part. A concave mirror is silvered on its bulging outer face and reflects from the hollow inner face, so the centre of that sphere lies on the reflecting side — that is, in front of the mirror. C is therefore a real point for a concave mirror.
In a convex mirror the reflecting surface bulges outwards towards the object, so the centre of the sphere falls on the far side — behind the mirror. Both the centre of curvature C and the principal focus F of a convex mirror are virtual points lying behind it. The distance from the pole P to C is the radius of curvature R, and it is always twice the focal length: R = 2f.
❌ Why the other options are incorrect
✗ behind; behind — this would be true only if both mirrors were convex; a concave mirror's C is a real point in front of it.
✗ behind; in front of — this reverses the two mirrors completely.
✗ in front of; in front of — right for the concave mirror, but wrong for the convex mirror, whose C is virtual and behind the surface.
📌 SSC & Railway Exam Facts
• R = 2f, so f = R/2; a mirror of R = 20 cm has f = 10 cm.
• In the New Cartesian sign convention a concave mirror has f negative and a convex mirror f positive.
• A concave mirror is a converging mirror; a convex mirror is a diverging mirror.
• Convex mirrors are used as rear-view mirrors because they give an erect, diminished image with a wide field of view.
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u = h'/h.

Correct Answer: Option 3 — 20 J
📖 Given
• Constant force applied, F = 10 N
• Displacement produced, s = 2 m
• Force and displacement are along the same direction
✍️ Formula
W = F × s
🔢 Solution
• W = 10 × 2
• W = 20 J
So the force transfers 20 joules of energy to the body while pushing it through 2 metres.
❌ Why the other options are incorrect
✗ 10 J — this is just the value of the force in newtons; you get it only if you forget to multiply by the displacement (or take s = 1 m).
✗ 2 J — this is only the displacement in metres; it ignores the force completely.
✗ 0 J — work is zero only when the displacement is zero, or when the force acts at 90° to the displacement. Here the body actually moves 2 m along the force.
📌 SSC & Railway Exam Facts
• Work = Force × Displacement in the direction of the force; in general W = F s cos θ.
• SI unit of work is the joule (J); 1 J = 1 N × 1 m = 1 kg m² s⁻².
• Work is a scalar quantity — it has magnitude but no direction.
• Work done is zero when θ = 90°, e.g. a coolie carrying a load on his head walking on level ground, or the centripetal force in circular motion.
• Work done against gravity in lifting a body of mass m through height h is W = mgh.

Correct Answer: Option 1 — Power
Power is defined as the rate of doing work, or equivalently the rate of transfer of energy. It tells us how *fast* a machine finishes a job, not how much work the job needs.
P = W/t = Energy/Time
Its SI unit is the watt (W), and 1 watt = 1 joule per second. Two machines may do exactly the same 1000 J of work, but the one that finishes in 10 s has a power of 100 W while the one taking 20 s has only 50 W. This is why a motor is rated in watts or horsepower, never in joules.
❌ Why the other options are incorrect
✗ Total force exerted — force alone tells nothing about time or even about distance; a force that moves nothing does zero work.
✗ Total energy consumed — this is the total work done, the whole job. Divided by time it gives power, but by itself it has no "rate" in it.
✗ Rate of change of velocity — that is the definition of acceleration, measured in m/s², a quantity of motion and not of work.
📌 SSC & Railway Exam Facts
• Power P = W/t; also P = F × v for a constant force.
• 1 W = 1 J/s; 1 kW = 1000 W; 1 horsepower (hp) = 746 W.
• Commercial unit of electrical energy is the kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ J.
• Power is a scalar quantity; the watt is named after James Watt.
• Electrical power: P = VI = I²R = V²/R.

Correct Answer: Option 3 — 1
📖 Given
• First resistor, R₁ = 2Ω
• Second resistor, R₂ = 2Ω
• They are joined in series
• Battery voltage, V = 4V
✍️ Formula
Series combination: R(eq) = R₁ + R₂
Ohm's law: I = V/R(eq)
🔢 Solution
• R(eq) = 2 + 2
• R(eq) = 4Ω
• I = 4/4
• I = 1 A
In a series circuit the same current of 1 A flows through both resistors.
❌ Why the other options are incorrect
✗ 4 — this comes from dividing 4V by a single 2Ω resistor and then doubling, or from using R = 1Ω; the total resistance here is 4Ω, not 1Ω.
✗ 1/2 — this is the answer for a parallel combination reversed: it uses R(eq) = 8Ω, which never occurs here.
✗ 2 — this is what you get if you use only one 2Ω resistor (4/2 = 2 A) and forget the second one in series.
📌 SSC & Railway Exam Facts
• Ohm's law: V = IR, valid at constant temperature.
• Series: R(eq) = R₁ + R₂ + R₃… — current is the same, voltage divides.
• Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃… — voltage is the same, current divides.
• Two equal resistances R in parallel give R/2; in series they give 2R.
• SI unit of resistance is the ohm (Ω); of current the ampere (A); 1 A = 1 C/s.

Correct Answer: Option 2 — energy
Work and energy are measured in exactly the same unit — the joule (J). This is not a coincidence: energy is defined as the capacity to do work, so whatever amount of work a body can perform is precisely the amount of energy it carries.
1 joule = 1 newton × 1 metre = 1 kg m² s⁻²
When a force does 20 J of work on a body, the body gains 20 J of energy — this is the work–energy theorem, which states that the net work done equals the change in kinetic energy: W = ½mv² − ½mu². Because of this equality, work, kinetic energy, potential energy and heat are all quoted in joules.
❌ Why the other options are incorrect
✗ force — measured in the newton (N); force is what *causes* work, but N and J differ by one factor of metre (J = N m).
✗ acceleration — measured in m/s²; it describes how quickly velocity changes and is a purely kinematic quantity.
✗ momentum — measured in kg m/s (or N s); it is the product of mass and velocity, a vector, while work is a scalar.
📌 SSC & Railway Exam Facts
• SI unit of work and energy: joule (J); CGS unit: erg; 1 J = 10⁷ erg.
• 1 calorie = 4.18 J (often taken as 4.2 J); 1 kWh = 3.6 × 10⁶ J.
• Kinetic energy = ½mv²; Potential energy = mgh.
• Work–energy theorem: W(net) = ΔKE.
• The joule is named after James Prescott Joule.

Correct Answer: Option 2 — I = Q/t
Electric current is defined as the rate of flow of electric charge — that is, the amount of charge that crosses any cross-section of the conductor in one second.
I = Q/t
So if 6 coulombs of charge pass a point in 2 seconds, the current is I = 6/2 = 3 A. Rearranged, the same relation gives Q = I × t, which is how you find the total charge that has flowed. The SI unit of current is the ampere (A), and 1 ampere = 1 coulomb per second. Since 1 electron carries 1.6 × 10⁻¹⁹ C, one ampere corresponds to about 6.25 × 10¹⁸ electrons crossing a section every second.
❌ Why the other options are incorrect
✗ I = Q t² — dimensionally wrong; this would make the current grow as time passes even for a fixed charge, which is absurd.
✗ I = Qt — this is the product, not the rate; the longer the same charge takes, the *smaller* the current should be, so t must be in the denominator.
✗ I = Q² t — squaring the charge has no physical basis; current is directly (first power) proportional to charge.
📌 SSC & Railway Exam Facts
• I = Q/t, hence Q = It; SI unit of charge is the coulomb (C).
• 1 A = 1 C/s; current is a scalar quantity though it has direction of flow.
• Conventional current flows from + to − outside the cell, opposite to the flow of electrons.
• Current is measured by an ammeter, connected in series and having very low resistance.
• Charge on one electron = −1.6 × 10⁻¹⁹ C; charge is quantised: Q = ne.

Correct Answer: Option 2 — rheostat
A rheostat is a variable resistor — a component whose resistance can be changed smoothly while the circuit is running. It is made of a long nichrome or manganin wire wound on an insulating cylinder, with a sliding contact (jockey). Moving the slider changes the length of wire included in the circuit, and since R = ρl/A, a longer length means more resistance.
Because resistance controls current through I = V/R, a rheostat is used to regulate the current in a circuit without changing the source — exactly how a fan regulator changes fan speed or a dimmer changes lamp brightness. In circuit diagrams it is shown as a rectangle with an arrow across it.
❌ Why the other options are incorrect
✗ ammeter — a measuring instrument for current; connected in series and made with very low resistance so it does not disturb the circuit.
✗ galvanometer — detects the presence and direction of a very small current; its needle deflects either way from the centre zero.
✗ voltmeter — measures potential difference; connected in parallel across a component and made with very high resistance.
📌 SSC & Railway Exam Facts
• Rheostat = variable resistance, used to control current; based on R = ρl/A.
• Ammeter — series, low resistance; Voltmeter — parallel, high resistance.
• An ideal ammeter has zero resistance and an ideal voltmeter has infinite resistance.
• Nichrome is used for heating elements and rheostats: high resistivity, high melting point, does not oxidise readily.
• Ohm's law: V = IR; resistance R = ρl/A, where ρ is resistivity.

Correct Answer: Option 2 — Concave
The sign of the focal length follows the New Cartesian sign convention. In it the pole P of the mirror is the origin, the incident light travels from left to right, and every distance measured against the incident light (i.e. to the left of P, in front of the mirror) is taken as negative.
A concave mirror converges parallel rays to a real focus F lying in front of the mirror, on the same side as the object. That distance is measured opposite to the incident light, so f is negative for a concave mirror.
A convex mirror spreads the rays out; its focus is a virtual point behind the mirror, measured along the direction of the incident light, so its f is positive.
❌ Why the other options are incorrect
✗ Plane mirror — it neither converges nor diverges light; its focus is at infinity, so f = ∞, which is neither positive nor negative in this sense.
✗ Convex — its focus is virtual and behind the mirror, giving a positive focal length; that is why R = +2f for it.
✗ Both convex and concave — cannot be true, since the two mirrors have focal lengths of opposite signs.
📌 SSC & Railway Exam Facts
• Concave mirror: f negative; Convex mirror: f positive; Plane mirror: f = infinity.
• For lenses the rule is the reverse in sign feel: convex (converging) lens f positive, concave (diverging) lens f negative.
• R = 2f for a spherical mirror of small aperture.
• Power of a mirror/lens P = 1/f (f in metres); unit dioptre (D).
• A concave mirror is used as a shaving mirror, dentist's mirror and in headlights and solar cookers.

Correct Answer: Option 4 — both potential energy and kinetic energy
The mechanical energy of a body is the sum of its kinetic energy and its potential energy:
Mechanical energy = KE + PE = ½mv² + mgh
Kinetic energy is the energy a body has because of its motion, and potential energy is the energy it has because of its position or configuration (height, a stretched spring, a bent bow). Together they describe the total energy a body possesses purely on account of mechanics.
The law of conservation of mechanical energy says that in the absence of friction and air resistance this sum stays constant. For a ball falling freely, PE keeps decreasing and KE keeps increasing by exactly the same amount, so KE + PE remains the same at every point of the fall.
❌ Why the other options are incorrect
✗ kinetic energy only — a stone held at the top of a tower is at rest, with zero KE, yet it clearly possesses mechanical energy in the form of PE.
✗ potential energy only — a bullet flying horizontally at constant height has constant PE but enormous KE, which is also mechanical energy.
✗ heat energy only — heat is the energy of random molecular motion; it is a thermal, not a mechanical, form of energy.
📌 SSC & Railway Exam Facts
• KE = ½mv²; Gravitational PE = mgh; both in joules.
• KE = p²/2m, where p = mv is the momentum.
• In free fall, mechanical energy is conserved; at the ground PE = 0 and KE is maximum.
• Doubling the speed makes KE four times larger, since KE ∝ v².
• Other energy forms: heat, light, sound, chemical, electrical, nuclear; energy can be transformed but never created or destroyed.

Correct Answer: Option 3 — 300 W
📖 Given
• Weight of the boy, W = 600 N (this is already the force mg)
• Height climbed, h = 10 m
• Time taken, t = 20 s
✍️ Formula
Work done against gravity: W(done) = Weight × h
Power: P = W(done)/t
🔢 Solution
• W(done) = 600 × 10
• W(done) = 6000 J
• P = 6000/20
• P = 300 W
The boy converts energy at the rate of 300 joules every second while climbing.
❌ Why the other options are incorrect
✗ 150 W — you get this by taking t = 40 s, or by wrongly halving the work done.
✗ 200 W — this comes from dividing 6000 J by 30 s, or from using a height of about 6.7 m.
✗ 100 W — this is 6000/60, i.e. mistaking the time for one minute instead of 20 s.
📌 SSC & Railway Exam Facts
• Power = Work/Time; SI unit watt (W), and 1 W = 1 J/s.
• Work done against gravity: W = mgh; here the weight 600 N is mg itself, so do not multiply by g again.
• If the boy's mass were asked: m = W/g = 600/10 = 60 kg.
• 1 horsepower = 746 W; an average human can sustain roughly 0.1 hp.
• Energy consumed = Power × Time; the commercial unit is the kilowatt-hour (kWh).

Correct Answer: Option 3 — All the appliances are connected in parallel
In every domestic wiring circuit each appliance is connected in parallel across the live and neutral wires. There are three solid reasons for this.
First, a parallel connection gives every appliance the same full voltage of 220 V, which is the voltage each one is designed for. In a series circuit the supply voltage would be shared out, and no device would get its rated voltage.
Second, each appliance can be switched on or off independently, and if one of them fuses the others keep working, because the other paths are still complete.
Third, parallel connection lets each appliance draw the current it needs according to I = V/R, so a 2000 W geyser and a 10 W LED can run side by side on the same line.
❌ Why the other options are incorrect
✗ Heavy appliances in series, light appliances in parallel — putting an AC or geyser in series would drop its voltage below 220 V and it would not work properly.
✗ Heavy appliances in parallel, light appliances in series — bulbs in series would glow dim, and a single fused bulb would switch off the whole lighting line.
✗ All the appliances are connected in series — the same current would flow through every device, the total resistance would rise, and one failure would kill the entire house circuit.
📌 SSC & Railway Exam Facts
• Domestic supply in India: 220 V, 50 Hz AC.
• Wire colours: live — red/brown, neutral — black/blue, earth — green.
• The fuse is always connected in the live wire, in series with the circuit.
• Earth wire carries leakage current safely to the ground and prevents electric shock from metallic bodies.
• Separate circuits are used for lights (5 A) and heavy appliances (15 A).

Correct Answer: Option 2 — Voltmeter
A voltmeter is the instrument that measures the potential difference (voltage) between two points of a circuit, in volts (V). Potential difference is defined as the work done in moving a unit charge from one point to the other:
V = W/Q, so 1 volt = 1 joule per coulomb
A voltmeter is always connected in parallel across the component whose voltage is to be measured, and it is built with a very high resistance so that it draws almost no current and leaves the circuit undisturbed. An ideal voltmeter has infinite resistance.
❌ Why the other options are incorrect
✗ Battery — a battery is a source of potential difference, not a measuring device; it supplies the emf that drives the current.
✗ Galvanometer — it only detects a small current and shows its direction; by adding a high resistance in series it can be *converted into* a voltmeter, but by itself it is not one.
✗ Ammeter — it measures current in amperes, is connected in series, and has a very low resistance.
📌 SSC & Railway Exam Facts
• Voltmeter — parallel, high resistance; Ammeter — series, low resistance.
• 1 volt = 1 joule/coulomb; the volt is named after Alessandro Volta.
• A galvanometer + high series resistance = voltmeter; a galvanometer + low shunt resistance = ammeter.
• Ohm's law: V = IR, where V is potential difference, I current and R resistance.
• Current always flows from higher to lower potential in the external circuit.

Correct Answer: Option 4 — 90°
The force on a straight conductor of length L carrying current I placed in a magnetic field B is
F = BIL sin θ
where θ is the angle between the direction of the current and the direction of the magnetic field. The force therefore depends on sin θ, and sin θ reaches its greatest value of 1 at θ = 90°. So the force is maximum, F = BIL, when the current flows perpendicular to the magnetic field.
Conversely, when the conductor lies parallel or antiparallel to the field, θ = 0° or 180°, sin θ = 0 and the force is zero. The direction of this force is given by Fleming's left-hand rule, and it is the principle behind the electric motor.
❌ Why the other options are incorrect
✗ 60° — sin 60° = 0.866, so the force is 86.6% of the maximum, large but not the largest.
✗ 0° — sin 0° = 0; here the conductor is along the field and experiences no force at all. This is the minimum, the exact opposite of what is asked.
✗ 45° — sin 45° = 0.707, giving only about 71% of the maximum force.
📌 SSC & Railway Exam Facts
• F = BIL sin θ; maximum at θ = 90° (F = BIL), zero at θ = 0°.
• Fleming's left-hand rule gives the direction of force on a current-carrying conductor — used in the electric motor.
• Fleming's right-hand rule gives the direction of induced current — used in the generator.
• SI unit of magnetic field B is the tesla (T); 1 T = 1 N A⁻¹ m⁻¹.
• Force on a moving charge: F = qvB sin θ; a charge moving along B feels no magnetic force.