RRB Technician Grade III 2024-25 27 December 2024 Shift 1 GK Questions – with answers & solutions

Correct Answer: Option 3 — Only apical meristem
Apical meristem is the dividing tissue located exactly at the growing tips of the stem and the root. By adding new cells at the tip it increases the length of the axis, a process called primary growth or apical growth.
Cells of meristematic tissue divide continuously. They are small, thin-walled (cellulose), have dense cytoplasm, a prominent nucleus and no vacuoles. On the basis of position, meristem is classified as apical, lateral and intercalary.
❌ Why the other options are incorrect
✗ Only lateral meristem — the cambium lies along the sides of stem and root and increases girth (thickness) by secondary growth; it is not at the tip.
✗ Only intercalary meristem — occurs at the base of leaves or at the internodes (as in grasses); it elongates internodes but does not sit at the growing tip.
✗ Lateral meristem and intercalary meristem — a wrong pair, because neither of the two occupies the apex of the stem or root.
📌 SSC & Railway Exam Facts
• Apical meristem → primary growth (length); lateral meristem (cambium) → secondary growth (girth).
• Intercalary meristem explains why grass grows back quickly after mowing or grazing.
• Meristematic cells lack vacuoles and are packed with protoplasm.
• The root apical meristem is protected by a root cap.
• Meristem later matures into permanent tissues — parenchyma, collenchyma, sclerenchyma, xylem and phloem.

Correct Answer: Option 2 — old xylem
Plants have no specialised excretory organ, so they handle waste in several simple ways. Gaseous wastes such as oxygen (from photosynthesis) and carbon dioxide (from respiration) leave through the stomata; excess water is removed by transpiration.
Solid and semi-solid wastes are simply stored rather than thrown out. Many of them collect in the cellular vacuoles, some are stored in leaves that later fall off, and other wastes are stored as resins and gums, especially in old xylem — the xylem that has already become non-functional for conduction. Plants also excrete some waste into the surrounding soil.
❌ Why the other options are incorrect
✗ cellular vacuoles — they do store many plant wastes, but the specific storage form of resins and gums named in NCERT is old xylem, not the vacuole.
✗ tissues with dead cells — a vague description; resins and gums are stored in a definite tissue, the old (secondary) xylem.
✗ leaves — leaves store wastes that are removed when the leaf falls (abscission), but resins and gums are not stored there.
📌 SSC & Railway Exam Facts
• Plants remove gaseous wastes through stomata and lenticels.
• Resins and gums are typical waste products of the old xylem (heartwood).
• Latex, rubber, tannins and alkaloids are also plant waste/secondary products.
• Excess water is lost as vapour by transpiration, and in liquid form by guttation.
• In humans the excretory unit is the nephron; plants have no such organ.

Correct Answer: Option 3 — Only (i) and (ii)
Adrenaline is the emergency hormone (the "fight or flight" hormone) secreted by the adrenal medulla of the adrenal glands, which sit on top of the kidneys. It is poured directly into the blood and reaches the heart, lungs, muscles and blood vessels.
Its effects prepare the body for sudden action — the heart beats faster so that more oxygen reaches the muscles, the breathing rate increases because the diaphragm and rib muscles contract faster, blood is diverted away from the skin and digestive system to the skeletal muscles, and the blood pressure rises. Statement (iii) is therefore false; adrenaline raises, never lowers, blood pressure.
❌ Why the other options are incorrect
✗ Only (ii) and (iii) — includes the false statement (iii); adrenaline increases blood pressure.
✗ Only (i) and (iii) — again wrongly accepts (iii) and drops the true statement (ii).
✗ (i), (ii) and (iii) — cannot be correct because (iii) contradicts the actual action of adrenaline.
📌 SSC & Railway Exam Facts
• Adrenaline (epinephrine) is secreted by the adrenal medulla; the cortex secretes cortisol and aldosterone.
• It is called the 3F hormone — fight, flight and fright.
• Thyroxine (thyroid) needs iodine; its deficiency causes goitre.
• Insulin from the pancreas lowers blood sugar; its deficiency causes diabetes mellitus.
• Growth hormone comes from the pituitary, the "master gland".

Correct Answer: Option 4 — Tendon
A tendon is a fibrous connective tissue that joins a muscle to a bone. It is made of tightly packed bundles of collagen fibres, which makes it very strong but only limited in flexibility, so the pull of the contracting muscle is transmitted efficiently to the bone and movement takes place.
A ligament is also a connective tissue but it joins bone to bone at a joint. It contains elastic fibres, so it is very elastic yet has considerable strength, and it keeps the joint stable.
❌ Why the other options are incorrect
✗ Cartilage — a solid but flexible connective tissue with a matrix of proteins and sugars; it smoothens bone surfaces at joints and forms the nose, ear, trachea and larynx, but joins nothing to bone.
✗ Fibroblast — a cell, not a tissue; fibroblasts secrete the collagen fibres and matrix of connective tissues such as areolar tissue.
✗ Blood — a fluid connective tissue with the liquid matrix plasma; it transports gases, food, wastes and hormones.
📌 SSC & Railway Exam Facts
• Tendon → muscle to bone; Ligament → bone to bone — a classic one-line PYQ.
• Both are dense connective tissues; tendon is rich in collagen, ligament in elastin.
• Achilles tendon at the heel is the largest tendon in the human body.
• Areolar tissue fills space inside organs; adipose tissue stores fat.
• Connective tissues include bone, cartilage, blood, tendon, ligament, areolar and adipose tissue.

Correct Answer: Option 3 — smell
Olfactory receptors are the specialised sensory receptors for smell. They lie in the olfactory epithelium in the roof of the nasal cavity. Odour molecules dissolve in the mucus, stimulate these receptors, and the impulse travels through the olfactory nerve to the brain, where the smell is identified.
When the nose is blocked (as in a cold), air carrying the odour molecules cannot reach the olfactory epithelium, so the receptors are not stimulated and smell cannot be detected. Because flavour depends largely on smell, food also seems tasteless at that time — but the taste receptors themselves are still working.
❌ Why the other options are incorrect
✗ walking — walking is a voluntary movement controlled by the cerebellum and skeletal muscles; no receptor "detects" it as a stimulus.
✗ hearing — sound is detected by the phonoreceptors in the cochlea of the ear, not by olfactory receptors.
✗ taste — taste is detected by gustatory receptors in the taste buds on the tongue.
📌 SSC & Railway Exam Facts
• Receptors: olfactory (smell), gustatory (taste), photoreceptors (light), phonoreceptors (sound), thermoreceptors (heat).
• The olfactory nerve is the first cranial nerve.
• The cerebellum maintains posture and balance; the medulla controls heartbeat, breathing and blood pressure.
• The four basic tastes sensed by the tongue are sweet, sour, salty and bitter.
• The neuron is the structural and functional unit of the nervous system.

Correct Answer: Option 1 — Unisexual flower - Watermelon
A unisexual flower carries only one kind of reproductive organ — either the stamens (male) or the pistil/carpel (female), but not both. Watermelon, along with papaya, cucumber, pumpkin and maize, bears such flowers, so NCERT cites it as a standard example of a unisexual flower.
A bisexual (perfect) flower has both stamens and pistil in the same flower. Mustard, hibiscus, rose and pea are the standard examples. Thus the pairing "Unisexual flower – Watermelon" is the only correct match.
❌ Why the other options are incorrect
✗ Bisexual flower - Papaya — papaya is unisexual and also dioecious, the male and female flowers being borne on separate plants.
✗ Unisexual flower - Mustard — mustard is a classic bisexual flower; a single flower carries stamens as well as the pistil.
✗ Bisexual flower - Watermelon — watermelon is unisexual and monoecious; male and female flowers occur on the same plant but separately.
📌 SSC & Railway Exam Facts
• Unisexual: papaya, watermelon, cucumber, maize, coconut. Bisexual: mustard, hibiscus, rose, pea, lily.
• Monoecious = both sexes on one plant (maize, watermelon); dioecious = on separate plants (papaya, date palm).
• Male part = stamen (anther + filament); female part = pistil (stigma, style, ovary).
• After fertilisation, the ovary → fruit and the ovule → seed.
• Pollination before fertilisation may be self or cross; cross-pollination needs an agent such as wind, water or insects.

Correct Answer: Option 4 — Leeuwenhoek in 1674
Anton van Leeuwenhoek, using his own improved microscope, observed free living cells in pond water for the first time in 1674. The tiny moving organisms he saw he called "animalcules"; today we know them as protozoa and bacteria, which is why he is called the Father of Microbiology.
This observation was a milestone in the history of the cell. Cells had first been seen in dead cork by Robert Hooke, but Leeuwenhoek was the first to see living cells, proving that life exists in single-celled form as well.
❌ Why the other options are incorrect
✗ Virchow in 1855 — Rudolf Virchow stated that "all cells arise from pre-existing cells", completing the cell theory; he did not observe pond-water cells.
✗ Robert Brown in 1831 — he discovered the nucleus in the cell of an orchid leaf.
✗ Robert Hooke in 1665 — he examined a thin slice of cork under his own microscope, saw tiny compartments and coined the word "cell" (from Latin *cellula*, a little room); these were dead cells.
📌 SSC & Railway Exam Facts
• Robert Hooke (1665) — coined "cell", from cork; wrote Micrographia.
• Leeuwenhoek (1674) — first saw free living cells in pond water.
• Robert Brown (1831) — discovered the nucleus.
• Purkinje (1839) — coined the term protoplasm.
• Cell theory: Schleiden (1838) and Schwann (1839), extended by Virchow (1855).

Correct Answer: Option 3 — Roots
Roots always grow downwards, into the soil and towards the pull of gravity. This directional growth movement in response to gravity is called geotropism (gravitropism). Because the root grows towards gravity it shows positive geotropism, while the shoot grows away from gravity and shows negative geotropism.
Such tropic movements are growth-dependent and are controlled by the plant hormone auxin. Auxin collects on the lower side of a horizontally placed root and there it inhibits root growth, so the upper side grows faster and the root bends downward. Roots are also positively hydrotropic, bending towards water.
❌ Why the other options are incorrect
✗ Branches — branches are part of the shoot system and grow upwards and outwards, showing negative geotropism and positive phototropism.
✗ Buds — buds develop on the stem into shoots, flowers or leaves and grow upwards/outwards, towards light.
✗ Leaves — leaves are borne on the stem, spread outwards and turn towards light (positive phototropism) to trap maximum sunlight.
📌 SSC & Railway Exam Facts
• Geotropism — response to gravity; root = positive, shoot = negative.
• Phototropism — response to light; shoot = positive, root = negative.
• Hydrotropism — response to water; roots are positively hydrotropic.
• Chemotropism — growth of the pollen tube towards the ovule.
• Auxin promotes shoot elongation but inhibits root growth; other plant hormones are gibberellin, cytokinin, abscisic acid (ABA) and ethylene.

Correct Answer: Option 1 — Cilia
Paramoecium is a unicellular organism that shows holozoic nutrition. Its whole body surface is covered with fine hair-like structures called cilia. The cell has a definite shape and a fixed opening — the oral groove — so food cannot be taken in at any random point.
The rhythmic beating of the cilia creates a water current that sweeps food particles to that specific spot in the cell, where they are engulfed to form a food vacuole. Inside the vacuole, enzymes break the food into simpler substances (digestion); the useful part is absorbed into the cytoplasm and the undigested residue is thrown out.
❌ Why the other options are incorrect
✗ Pseudopodia — the temporary "false feet" of Amoeba, formed by flowing cytoplasm; they are used for movement and to engulf food, but Amoeba has no fixed spot for ingestion.
✗ Parapodia — paired fleshy lateral appendages used for swimming and crawling in annelids such as Nereis; not found in unicellular organisms.
✗ Flagella — long whip-like structures that give locomotion to Euglena and to the sperm; they are far fewer and much longer than cilia.
📌 SSC & Railway Exam Facts
• Nutrition in Amoeba = phagocytosis, using pseudopodia and a food vacuole.
• Paramoecium is called the slipper animalcule; cilia serve both feeding and locomotion.
• Cilia are short and many, flagella are long and few.
• Ingestion → digestion → absorption → assimilation → egestion are the five steps of holozoic nutrition.
• Human trachea is also lined with ciliated epithelium that pushes dust and mucus out.

Correct Answer: Option 1 — Muscular tissue
In animals the nervous system only detects the stimulus and carries the message, but the final job of actually producing the action or movement is done by the muscular tissue. Muscle cells are long and are therefore called muscle fibres; they contain special contractile proteins whose contraction and relaxation shorten or lengthen the muscle and bring about the movement.
There are three kinds — striated (voluntary/skeletal) muscle attached to bones, smooth (unstriated/involuntary) muscle in the alimentary canal and blood vessels, and cardiac muscle found only in the heart, which is involuntary yet rhythmically contracting throughout life.
❌ Why the other options are incorrect
✗ Connective tissue — binds, supports and packs organs and transports materials (bone, cartilage, blood, tendon, ligament); it does not contract.
✗ Meristematic tissue — a plant tissue of dividing cells found at growing points; it does not occur in animals at all.
✗ Epithelial tissue — the covering and lining tissue of the body; it protects, absorbs and secretes, but has no power of contraction.
📌 SSC & Railway Exam Facts
• Striated muscle — voluntary, cylindrical, multinucleate, attached to bone.
• Smooth muscle — involuntary, spindle-shaped, uninucleate, in the gut, iris and ureters.
• Cardiac muscle — involuntary but striated and branched; never tires.
• Contractile proteins of muscle are actin and myosin.
• The four animal tissues are epithelial, connective, muscular and nervous.

Correct Answer: Option 4 — two cells
Binary fission is a mode of asexual reproduction in which a single parent cell divides into two daughter cells of nearly equal size. The nucleus divides first, followed by the cytoplasm, and the parent cell as such ceases to exist. Amoeba, Paramoecium, Leishmania and bacteria reproduce in this way.
In multiple fission the nucleus of the parent cell divides repeatedly inside a protective cyst, producing many daughter cells at one time, which are released when conditions become favourable — as in Plasmodium, the malarial parasite. Hence, multiple fission gives many cells and binary fission gives exactly two.
❌ Why the other options are incorrect
✗ one cell — a single cell would mean no reproduction at all; the parent must give rise to more than one individual.
✗ four cells — division into four is not binary fission; four cells arise after two successive binary fissions or in meiosis.
✗ three cells — binary fission is always into two equal halves, never three; "bi" itself means two.
📌 SSC & Railway Exam Facts
• Binary fission — Amoeba, Paramoecium, bacteria; multiple fission — Plasmodium.
• In Leishmania (kala-azar parasite) binary fission occurs in a definite plane, along the whip-like flagellum.
• In Amoeba fission can take place in any plane.
• Other asexual modes: budding (Yeast, Hydra), fragmentation (Spirogyra), regeneration (Planaria), spore formation (Rhizopus), vegetative propagation.
• Asexual reproduction involves a single parent and gives genetically identical offspring (clones).

Correct Answer: Option 3 — Castor and Balsam
Seed dispersal by the bursting (explosion) of fruits is a case of dispersal by the plant itself. As the fruit dries, tension builds up in its wall; it suddenly splits with a jerk and flings the seeds far away from the parent plant. Castor and balsam are the standard NCERT examples of this method.
Dispersal is necessary so that the seeds do not all fall at one place. Crowded seedlings would compete for sunlight, water, minerals and space, so scattering them helps the species spread to new areas and prevents overcrowding.
❌ Why the other options are incorrect
✗ Drumstick and Maple — their seeds have wings and are dispersed by wind, not by bursting.
✗ Xanthium and Urena — these fruits bear hooks/spines that cling to the body of animals and are dispersed by animals.
✗ Sunflower and Madar — sunflower and madar (aak) seeds are light and hairy, and are also carried away by wind.
📌 SSC & Railway Exam Facts
• Agents of seed dispersal: wind, water, animals and explosion (the plant itself).
• Wind — maple, drumstick, madar, sunflower, dandelion (light, winged or hairy seeds).
• Water — coconut, which has a fibrous, buoyant husk.
• Animals — Xanthium, Urena (hooks) and fleshy fruits eaten by birds.
• After fertilisation the ovule becomes the seed and the ovary becomes the fruit.

Correct Answer: Option 2 — By changing the amount of water in them
The folding of the leaves of the touch-me-not plant (Mimosa pudica) is a nastic movement — it is independent of growth and does not depend on the direction of the stimulus. The plant has no nerves and no muscles, so it must move the leaf in some other way.
At the base of each leaflet there is a swollen cushion called the pulvinus. When the plant is touched, an electrochemical signal passes through the cells; water moves out of the pulvinus cells on one side, they lose their turgidity (turgor pressure) and shrink, while the cells on the other side stay swollen. This change in the amount of water makes the leaf fold and droop. Growth-independent movements of this kind are reversible — the leaf opens again when water returns.
❌ Why the other options are incorrect
✗ lipid — lipids are stored fat and membrane material; their quantity does not change quickly enough to bend a leaf.
✗ nucleic acid — DNA and RNA store and express genetic information; they play no part in this rapid mechanical movement.
✗ protein — proteins build enzymes and structures, and animals use contractile proteins for movement, but plant cells have no such contractile machinery.
📌 SSC & Railway Exam Facts
• Nastic movement — non-directional and growth-independent (Mimosa leaves).
• Tropic movement — directional and growth-dependent (phototropism, geotropism).
• Mimosa pudica is the touch-me-not (chhui-mui) plant; the response is thigmonasty (seismonasty).
• Electrical-chemical signals travel from cell to cell in plants; plants have no nervous tissue.
• Plant movement is controlled by hormones — auxin, gibberellin, cytokinin, abscisic acid, ethylene.

Correct Answer: Option 2 — Chlorofluorocarbons
Ozone (O₃) forms a protective layer in the upper atmosphere (the stratosphere) and absorbs the harmful ultraviolet (UV) radiation of the Sun, which otherwise causes skin cancer, cataract and damage to crops.
Chlorofluorocarbons (CFCs) are man-made chemicals used as refrigerants and in fire extinguishers and aerosol sprays. In the stratosphere UV light splits a CFC molecule and frees a chlorine atom, which then destroys ozone molecules in a chain reaction — a single chlorine atom can break down thousands of O₃ molecules. NCERT records that the amount of ozone began to drop sharply in the 1980s, producing the ozone hole over Antarctica.
❌ Why the other options are incorrect
✗ Insecticides — chemicals that kill insects; they pollute soil and water and enter the food chain as biomagnification, but do not destroy ozone.
✗ Pesticides — a broader group of crop-protection chemicals; again a soil-and-water pollutant, not an ozone-depleting substance.
✗ Carbon monoxide — a poisonous gas from incomplete combustion that binds to haemoglobin; it is an air pollutant, not an ozone destroyer.
📌 SSC & Railway Exam Facts
• Ozone = O₃, formed when UV splits O₂ into free oxygen atoms which then combine with O₂.
• The Montreal Protocol (1987) froze and phased out CFC production.
• 16 September is celebrated as the World Ozone Day.
• The ozone hole was detected over Antarctica.
• Ozone is harmful at ground level (a pollutant) but essential in the stratosphere.

Correct Answer: Option 1 — The new product formed has totally different properties.
A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass. Because a genuine chemical bond is made, the compound is a brand-new substance whose properties are completely different from those of the elements that formed it.
The classic illustration is iron + sulphur. A mere mixture of iron filings and sulphur powder still shows both sets of properties — the iron is still attracted by a magnet and still dissolves in dilute acid. But on heating the two combine to give iron sulphide (FeS), a compound that is not attracted by a magnet at all and behaves quite differently.
Compound formation is therefore a chemical change, it is accompanied by a change in energy (heat or light), and its constituents can be separated only by chemical means (electrolysis, heating) — never by filtration or by using a magnet.
❌ Why the other options are incorrect
✗ The new product formed has similar properties to the constituent substance — that is true of a mixture, not a compound; in a mixture every component keeps its own properties.
✗ It is a physical change — the formation of a compound is a chemical change, because new bonds are made and a new substance appears.
✗ The constituents can be separated by physical process — only a mixture can be separated physically, by filtration, distillation, magnet, sublimation or chromatography.
📌 SSC & Railway Exam Facts
• Elements — made of one kind of atom; Compounds — elements combined in a fixed ratio; Mixtures — any ratio, no chemical bond.
• Water (H₂O) is a compound of H and O in a fixed 1 : 8 ratio by mass, yet neither H nor O puts out a fire the way water does.
• Air is a mixture, brass is a mixture (alloy), but common salt and water are compounds.
• A mixture is either homogeneous (solution) or heterogeneous (suspension, colloid).
• Law of constant proportions (Proust) — a compound always contains the same elements in the same fixed ratio by mass.

Correct Answer: Option 1 — a saturated solution
A saturated solution is a solution in which no more solute can be dissolved at a given temperature. Once this point is reached, any further solute added simply lies at the bottom as undissolved solid.
The quantity of solute needed to saturate 100 g of solvent at a stated temperature is called the solubility of that solute. Solubility must always be quoted at a particular temperature, because heating a solution generally lets it hold more solute.
For most solids the solubility increases with temperature, so a saturated solution becomes unsaturated on heating; when it is cooled again, the surplus solute separates out as crystals — the principle behind the purification technique of crystallisation. For gases, in contrast, solubility decreases as temperature rises and increases with pressure.
❌ Why the other options are incorrect
✗ an unsaturated solution — exactly the opposite; it is a solution that can still dissolve more solute at that temperature.
✗ a suspension — a heterogeneous mixture whose solid particles never dissolve at all; they stay visible and settle down on standing.
✗ a colloidal solution — a heterogeneous mixture with particles of 1–100 nm that show the Tyndall effect; it is not a question of dissolving a solute to its limit.
📌 SSC & Railway Exam Facts
• Solubility = mass of solute in grams that saturates 100 g of solvent at a given temperature.
• Solution = solute + solvent; the solvent is the component present in the greater amount.
• Concentration = (mass of solute / mass of solution) × 100 for mass percentage.
• Gases dissolve better at low temperature and high pressure — Henry's law; that is why a cold soft drink fizzes when opened.
• Crystallisation is used to purify alum, copper sulphate and salt obtained from sea water.

Correct Answer: Option 4 — Iron stick
The force of attraction between particles is greatest in solids, smaller in liquids and least in gases. That single rule decides this question, because only one of the four options is a solid.
An iron stick is a solid. Its particles are very tightly and regularly packed, the interparticle space is minimum and the force of attraction is maximum. This is why a solid has a definite shape and definite volume, is almost incompressible, has a high density and shows a very high rate of rigidity.
• Solid — maximum force of attraction, particles only vibrate about fixed positions.
• Liquid — moderate force, particles slide over one another, definite volume but no definite shape.
• Gas — negligible force, particles move freely at high speed and fill the whole container.
❌ Why the other options are incorrect
✗ Perfume — perfume vapour is a gas, which is precisely why its smell spreads across a room by diffusion; gases have the weakest interparticle force.
✗ Ink of pen — a liquid; its particles move much more freely than those of a solid, so the force between them is far smaller.
✗ Water — also a liquid. It takes the shape of its container and flows, which would be impossible if the attraction were as strong as in a solid.
📌 SSC & Railway Exam Facts
• Force of attraction: solid > liquid > gas; interparticle space and compressibility: gas > liquid > solid.
• Rate of diffusion: gas > liquid > solid; kinetic energy of particles: gas > liquid > solid.
• Solids have definite shape and volume, liquids have definite volume only, gases have neither.
• Gases are highly compressible, which is why LPG and CNG are stored in cylinders and oxygen is carried in hospitals.
• Liquids and gases together are called fluids, because both can flow.

Correct Answer: Option 4 — Combination reaction
A combination reaction is one in which two or more reactants join together to form a single product, in the general form A + B → AB.
Here calcium oxide (CaO), the quick lime of commerce, combines with water to give a single product, calcium hydroxide, Ca(OH)₂, known as slaked lime. Since two substances become one, it is a textbook combination reaction. As a large amount of heat is given out, it is also exothermic.
⚗️ The Reaction
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
• Two reactants → one product, so it is a combination reaction.
• Heat is released, so it is exothermic — the vessel becomes hot and the mixture hisses.
• A solution of Ca(OH)₂ is lime water; blowing CO₂ into it turns it milky, forming CaCO₃.
❌ Why the other options are incorrect
✗ Decomposition reaction — the opposite process, in which one compound splits into two or more, as in CaCO₃ → CaO + CO₂ (heat). Here, nothing splits.
✗ Double decomposition reaction — two compounds exchange their ions, AB + CD → AD + CB; here there is only one product, so no exchange happens.
✗ Precipitation reaction — this must give an insoluble solid that settles out; Ca(OH)₂ is only sparingly soluble but is formed as a solution/suspension of slaked lime, and the reaction is defined by the joining of two reactants.
📌 SSC & Railway Exam Facts
• Slaked lime is used to whitewash walls; Ca(OH)₂ + CO₂ → CaCO₃ + H₂O gives the shiny finish.
• Other combination reactions: 2Mg + O₂ → 2MgO, C + O₂ → CO₂, N₂ + 3H₂ → 2NH₃, SO₃ + H₂O → H₂SO₄.
• Quick lime — CaO, Slaked lime — Ca(OH)₂, Lime stone/Marble/Chalk — CaCO₃, Gypsum — CaSO₄·2H₂O.
• Exothermic — respiration, burning, combination reactions; Endothermic — photosynthesis, most decompositions.
• Decomposition is the exact reverse of a combination reaction.

Correct Answer: Option 2 — Metal salt
Electrolytic refining is the final step of metallurgy, used to obtain a metal in a very pure form. The arrangement is always the same:
• Anode (+) — a thick block of the impure metal.
• Cathode (−) — a thin strip of the pure metal.
• Electrolyte — a solution of a salt of the same metal, i.e. a metal salt solution.
When current is passed, the impure metal at the anode dissolves into the electrolyte as metal ions, and an equivalent amount of pure metal is deposited on the cathode. The soluble impurities stay in solution while the insoluble ones fall below the anode as anode mud, from which valuable metals like gold, silver, platinum and selenium are recovered.
For the refining of copper, the electrolyte used is acidified copper sulphate solution (CuSO₄ + dilute H₂SO₄) — a metal salt.
❌ Why the other options are incorrect
✗ Impure metal — the impure metal is the anode, a solid electrode; it is not the electrolyte.
✗ Combination of pure metal and metal salt — the pure metal is the cathode, again a solid electrode; only the salt solution serves as the electrolyte.
✗ Pure metal — this is the cathode on which the refined metal is deposited, not the conducting medium between the electrodes.
📌 SSC & Railway Exam Facts
• Metallurgy steps: concentration → roasting/calcination → reduction → refining.
• Roasting — heating a sulphide ore in excess air; Calcination — heating a carbonate ore in limited air.
• In electrolysis, oxidation occurs at the anode and reduction at the cathode.
• Anode mud is the valuable source of Au, Ag, Pt and Se.
• Highly reactive metals such as Na, K, Ca, Mg and Al are obtained by the electrolytic reduction of their molten chlorides or oxides.

Correct Answer: Option 1 — C₅H₁₂
Pentane ends in -ane, so it belongs to the alkane family — the saturated hydrocarbons, in which every carbon–carbon bond is a single bond. The root pent- tells us there are 5 carbon atoms.
🔢 Calculation
• General formula of an alkane: CₙH₂ₙ₊₂
• Here n = 5
• Hydrogen atoms = (2 × 5) + 2 = 12
• Formula of pentane = C₅H₁₂
Pentane is a colourless, volatile liquid and is one of the components of petrol. It has three structural isomers — n-pentane, iso-pentane and neo-pentane — the first member of the alkane series to show branched-chain isomerism along with butane.
❌ Why the other options are incorrect
✗ C₅H₉ — this does not fit any of the three general formulae; CₙH₂ₙ₊₂ = C₅H₁₂, CₙH₂ₙ = C₅H₁₀, CₙH₂ₙ₋₂ = C₅H₈. C₅H₉ would be a free radical, not a stable hydrocarbon.
✗ C₅H₁₄ — carbon's valency is 4, and a 5-carbon chain cannot hold 14 hydrogen atoms; the maximum possible is 12.
✗ C₅H₁₀ — this is CₙH₂ₙ, which is pentene, an unsaturated alkene with one double bond, not pentane.
📌 SSC & Railway Exam Facts
• Alkanes: CH₄ methane, C₂H₆ ethane, C₃H₈ propane, C₄H₁₀ butane, C₅H₁₂ pentane, C₆H₁₄ hexane.
• Alkanes CₙH₂ₙ₊₂, Alkenes CₙH₂ₙ, Alkynes CₙH₂ₙ₋₂.
• Successive members of a homologous series differ by −CH₂− and by 14 u in molecular mass.
• Butane (C₄H₁₀) is the first alkane to show chain isomerism; pentane has 3 isomers.
• LPG is mainly butane, CNG is mainly methane and petrol is a mixture of C₅–C₁₂ hydrocarbons.

Correct Answer: Option 1 — Sulphuric acid
Ferrous sulphate crystals, FeSO₄·7H₂O, are green in colour. On heating, they first lose their water of crystallisation and turn white, and on stronger heating the anhydrous salt undergoes thermal decomposition.
⚗️ The Reaction
FeSO₄·7H₂O → (heat) FeSO₄ + 7H₂O
2FeSO₄ → (heat) Fe₂O₃ + SO₂ + SO₃
• The products are the reddish-brown solid Fe₂O₃ (ferric oxide) plus two gases, SO₂ and SO₃, which have a characteristic burning-sulphur smell.
• No sulphuric acid (H₂SO₄) is formed, because the decomposition takes place in the dry state with no water present to combine with SO₃.
• This is a thermal decomposition and an endothermic reaction.
Sulphuric acid would appear only if the SO₃ gas were later dissolved in water, SO₃ + H₂O → H₂SO₄ — a separate step that is not part of this heating.
❌ Why the other options are incorrect (they ARE formed)
✗ Sulphur trioxide — SO₃ is one of the two gases released; it is the anhydride of sulphuric acid.
✗ Sulphur dioxide — SO₂ is the other gas released; it has a pungent smell of burning sulphur and turns acidified KMnO₄ colourless.
✗ Ferric oxide — Fe₂O₃ is the reddish-brown solid residue left in the test tube, the colour change from green to brown being the visible sign of the reaction.
📌 SSC & Railway Exam Facts
• FeSO₄·7H₂O — green vitriol; CuSO₄·5H₂O — blue vitriol; ZnSO₄·7H₂O — white vitriol.
• H₂SO₄ is the "king of chemicals", made industrially by the Contact process.
• SO₂ and NO₂ dissolve in rainwater and cause acid rain.
• Other classic decompositions: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂, CaCO₃ → CaO + CO₂.
• Heating CuSO₄·5H₂O turns it from blue to white; adding water turns it blue again — a test for water.

Correct Answer: Option 1 — Particles of the solution can be seen by the naked eye.
A true solution is a homogeneous mixture of two or more substances, in which the solute particles are smaller than 1 nm (10⁻⁹ m). Particles that small are far too tiny to be seen with the naked eye, or even with a microscope. That is why the statement in Option 1 is not true, and it is the required answer.
Because of this extremely small particle size, a true solution also does not scatter a beam of light — it shows no Tyndall effect — and it is completely stable.
Sugar or salt dissolved in water, air, brass and tincture iodine are common examples of true solutions.
❌ Why the other options are incorrect (they ARE true)
✗ The solute particles cannot be separated by filtration — true; the particles are so small that they pass straight through filter paper. A solution is separated by evaporation, distillation or crystallisation.
✗ A solution is a homogeneous mixture — true; its composition is uniform throughout, and no boundary can be seen between solute and solvent.
✗ The solute particles do not settle down when left undisturbed — true; a true solution is stable and never shows sedimentation, unlike a suspension.
📌 SSC & Railway Exam Facts
• Particle size: solution < 1 nm, colloid 1–100 nm, suspension > 100 nm.
• Solution — homogeneous, stable, no Tyndall effect, passes through filter paper.
• Colloid — appears homogeneous but is heterogeneous, stable, shows Tyndall effect, separated by centrifugation.
• Suspension — heterogeneous, unstable, shows Tyndall effect, separated by filtration.
• Alloys such as brass (Cu + Zn) are solid solutions, and air is a gaseous solution.

Correct Answer: Option 4 — 1.2
Gastric juice is the digestive fluid secreted by the glands in the wall of the stomach. It contains hydrochloric acid (HCl), the enzyme pepsin and mucus. Because of the HCl, gastric juice is strongly acidic, and the value learnt from NCERT is pH ≈ 1.2.
This strong acidity serves two purposes: it kills the bacteria swallowed with food, and it provides the acidic medium that pepsin needs to digest proteins. The stomach wall is protected from this acid by a thick layer of mucus.
When too much acid is secreted, it causes acidity and indigestion, treated with antacids — mild bases such as milk of magnesia, Mg(OH)₂, or sodium bicarbonate, NaHCO₃ — which neutralise the excess HCl.
❌ Why the other options are incorrect
✗ 10.0 — a distinctly basic value; nothing in the stomach is alkaline. A pH near 10 belongs to solutions such as milk of magnesia.
✗ 7.4 — this is the pH of human blood (7.35–7.45), which is slightly basic and very tightly controlled. It is the standard distractor here.
✗ 2.2 — acidic, but not the textbook figure; NCERT gives the pH of gastric juice as 1.2, so this value is not the one asked for.
📌 SSC & Railway Exam Facts
• pH = −log[H⁺]; the scale runs from 0 to 14 and was given by S. P. L. Sørensen.
• Standard pH values: gastric juice 1.2, lemon juice 2.2, vinegar 3.0, milk 6.6, pure water 7, blood 7.4, baking soda solution 9, milk of magnesia 10, NaOH 14.
• Tooth decay starts below pH 5.5; toothpastes are basic to neutralise mouth acids.
• Bee sting is acidic — treated with baking soda; wasp sting is basic — treated with vinegar.
• Acid rain has pH below 5.6, caused by SO₂ and NO₂ dissolving in rainwater.

Correct Answer: Option 4 — Solid phase
Every colloid has two parts: the dispersed phase, which is the substance present in small quantity and spread through the mixture, and the dispersion medium, which is the substance in which it is spread.
Automobile exhaust is a form of smoke. Smoke consists of fine solid particles — carbon (soot), unburnt hydrocarbons and metallic particulates — suspended in air. Therefore:
• Dispersed phase = solid (the carbon and particulate matter)
• Dispersion medium = gas (the air / exhaust gases)
• This type of colloid is called an aerosol.
That is why the answer is the solid phase. These suspended solid particles are the particulate matter (PM₂.₅ and PM₁₀) that causes serious respiratory disease and smog in cities.
❌ Why the other options are incorrect
✗ Liquid phase — a liquid dispersed in a gas gives fog, mist and clouds, not the sooty smoke of an exhaust pipe.
✗ Plasma phase — plasma is the ionised fourth state of matter found in stars and fluorescent tubes; it is never the dispersed phase of a colloid.
✗ Gaseous phase — a gas dispersed in a gas would simply form a homogeneous mixture like air, not a colloid at all, since gases mix completely.
📌 SSC & Railway Exam Facts
• Aerosol — solid or liquid in gas: smoke (solid in gas), fog, mist, clouds (liquid in gas).
• Foam — gas in liquid (shaving cream); solid foam — gas in solid (sponge, pumice).
• Emulsion — liquid in liquid (milk, cream); Gel — liquid in solid (cheese, butter, jelly).
• Sol — solid in liquid (ink, paint, muddy water, starch solution).
• Smoke is removed from factory chimneys by an electrostatic precipitator (Cottrell precipitator).

Correct Answer: Option 2 — H⁺ ions and OH⁻ ions, respectively
According to the Arrhenius theory, an acid is a substance that gives hydrogen ions, H⁺, in aqueous solution, and a base is one that gives hydroxide ions, OH⁻, in aqueous solution.
⚗️ The Ionisation
HCl(aq) → H⁺(aq) + Cl⁻(aq) — an acid
NaOH(aq) → Na⁺(aq) + OH⁻(aq) — a base
• The free H⁺ ion never exists alone in water; it attaches to a water molecule to form the hydronium ion, H₃O⁺.
• Acids and bases conduct electricity in aqueous solution precisely because of these free ions.
• Dry HCl gas does not turn dry blue litmus red — it must have water to release H⁺, which proves that ionisation needs water.
When the two are mixed, H⁺ + OH⁻ → H₂O, which is the essence of a neutralisation reaction.
❌ Why the other options are incorrect
✗ K⁺ ions and CO₃²⁻ ions — K⁺ comes from a potassium salt or from KOH, and CO₃²⁻ from a carbonate; neither ion defines an acid or a base.
✗ Li⁺ ions and CO₃²⁻ ions — lithium and carbonate ions are found in lithium carbonate, an ordinary salt, and have nothing to do with the definition.
✗ Na⁺ ions and CO₃²⁻ ions — these two make up sodium carbonate (washing soda), which is a salt, even though its solution happens to be basic.
📌 SSC & Railway Exam Facts
• Arrhenius — acid gives H⁺, base gives OH⁻ in water.
• Bronsted–Lowry — acid is a proton donor, base a proton acceptor; Lewis — acid is an electron-pair acceptor, base a donor.
• pH = −log[H⁺]; pH < 7 acid, = 7 neutral, > 7 base.
• Strong acids: HCl, H₂SO₄, HNO₃; weak acids: CH₃COOH, H₂CO₃. Strong bases: NaOH, KOH; weak base: NH₄OH.
• Always add acid to water, never water to acid — the dilution is highly exothermic and can cause the mixture to splash.

Correct Answer: Option 2 — 25 degree Celsius
📖 Given
• Temperature on the Kelvin scale = 298 K
• To find: the same temperature in degree Celsius (°C)
✍️ Formula
K = °C + 273, and therefore °C = K − 273
🔢 Solution
• °C = 298 − 273
• °C = 25 °C
So water at 298 K is at 25 °C, which is ordinary room temperature — exactly the temperature at which most laboratory values, such as standard pH and solubility data, are quoted.
Note that the kelvin (K) is the SI unit of temperature, and by convention the degree symbol is never written with it — we write 298 K, not 298 °K. The Kelvin scale is also called the absolute scale, because its zero is absolute zero, the lowest temperature possible.
❌ Why the other options are incorrect
✗ 28 degree Celsius — this would correspond to 301 K; it comes from subtracting 270 instead of 273.
✗ 30 degree Celsius — this corresponds to 303 K, five kelvin higher than the value given.
✗ 32 degree Celsius — this corresponds to 305 K; 32 is also the Fahrenheit value of 0 °C, which is a common source of confusion.
📌 SSC & Railway Exam Facts
• K = °C + 273; °C = K − 273; °F = (9/5 × °C) + 32.
• 0 °C = 273 K = 32 °F; 100 °C = 373 K = 212 °F; −40 °C = −40 °F.
• Absolute zero = 0 K = −273.15 °C, at which particle motion theoretically ceases.
• Normal human body temperature = 37 °C = 98.6 °F = 310 K.
• The kelvin is one of the seven SI base units, and the degree sign is never used with it.

Correct Answer: Option 2 — 24 V
📖 Given
• Current, I = 2 A
• Resistance, R = 12 Ω
• Temperature is constant, so the conductor is ohmic
✍️ Formula
Ohm's law: V = IR
🔢 Solution
• V = I × R
• V = 2 × 12
• V = 24 V
So a potential difference of 24 volts must be maintained across this conductor to push a steady current of 2 amperes through it.
❌ Why the other options are incorrect
✗ 35 V — does not come from any correct step; 35 is neither 2 × 12 nor 12/2.
✗ 12 V — this is just the resistance value written as a voltage, forgetting to multiply by the current.
✗ 6 V — this is R/I = 12/2, the division taken the wrong way round; Ohm's law needs I × R, not R ÷ I.
📌 SSC & Railway Exam Facts
• Ohm's law: V = IR, given by Georg Simon Ohm in 1827.
• 1 Ω = 1 V/A; 1 A = 1 C/s; 1 V = 1 J/C.
• A V–I graph of an ohmic conductor is a straight line through the origin.
• Voltmeter is connected in parallel; ammeter in series.
• Electric power P = VI = I²R = V²/R; SI unit watt (W).

Correct Answer: Option 3 — 0 m
📖 Given
• Distance between A and B = 20 m
• Path followed: A → B → A
• The object finally comes back to its starting point A
✍️ Formula
Displacement = shortest distance from the initial position to the final position (a vector)
🔢 Solution
• Initial position = A
• Final position = A (the object returns)
• Since the initial and final positions are the same point, the straight-line gap between them is 0
• Net displacement = 0 m
Note that the total distance travelled is 20 + 20 = 40 m, but distance and displacement are not the same thing.
❌ Why the other options are incorrect
✗ 10 m — this would be half of AB; the object does not stop midway, it returns to A.
✗ 20 m — this is the displacement only for the one-way trip A → B, not for the full journey.
✗ 40 m — this is the total distance covered (20 m going + 20 m returning), not the displacement.
📌 SSC & Railway Exam Facts
• Distance is a scalar and can never be negative or zero for a moving body; displacement is a vector and can be zero.
• Displacement ≤ distance, always; they are equal only for straight-line motion in one direction.
• For a body completing one full circle of radius r: distance = 2πr, displacement = 0.
• Average speed = total distance/total time; average velocity = net displacement/total time.
• Here the average velocity for the whole trip is also zero.

Correct Answer: Option 1 — Ellipsoidal and intersects with each other
Inside a current-carrying solenoid the magnetic field lines are straight, parallel to the axis, equally spaced and all in the same direction. Such a pattern means the field is uniform — the same in magnitude and direction at every interior point.
The statement that the lines are ellipsoidal and intersect with each other is therefore not true on two counts. First, the interior lines are straight, not ellipsoidal. Second, and more importantly, two magnetic field lines can never intersect. If they did, the compass needle placed at the crossing point would have to point in two different directions at the same time, which is impossible.
So option 1 is the false statement and is the required answer.
❌ Why the other options are incorrect
✗ Uniform inside the solenoid — this is true; that is the very reason solenoids are used to produce uniform fields.
✗ Along the axis and parallel to each other — true; the fields of all the turns add up along the axis.
✗ The same at all points inside the solenoid — true, and it is simply another way of saying the field is uniform.
📌 SSC & Railway Exam Facts
• Magnetic field lines never intersect — a basic property of field lines.
• Field inside a solenoid: B = μ₀nI, where n = turns per unit length.
• Field lines come out of the north pole and go into the south pole outside a magnet.
• A solenoid behaves like a bar magnet; with a soft iron core it becomes an electromagnet.
• Closer field lines mean a stronger magnetic field.

Correct Answer: Option 1 — 2.02 × 10²⁰ N
📖 Given
• Force exerted by the earth on the moon, F = 2.02 × 10²⁰ N
• Both bodies attract each other gravitationally
✍️ Formula
F = Gm₁m₂/r² — the same expression holds for both bodies, and by Newton's third law action and reaction are equal in magnitude and opposite in direction.
🔢 Solution
• Force on the moon due to the earth = Gm(earth)m(moon)/r²
• Force on the earth due to the moon = Gm(moon)m(earth)/r²
• The two expressions are identical, so the magnitudes are equal
• Force exerted by the moon on the earth = 2.02 × 10²⁰ N
The moon pulls the earth exactly as hard as the earth pulls the moon; the moon simply moves much more because its mass is far smaller.
❌ Why the other options are incorrect
✗ 6.06 × 10²⁰ N — this is 3 times the given force; no rule of gravitation multiplies the reaction by 3.
✗ 1.01 × 10²⁰ N — this is half the given force, wrongly assuming the smaller body feels a smaller force.
✗ 4.04 × 10²⁰ N — this is double the given force; the reaction is equal, not doubled.
📌 SSC & Railway Exam Facts
• Newton's third law: to every action there is an equal and opposite reaction, acting on two different bodies.
• Universal law of gravitation: F = Gm₁m₂/r², with G = 6.67 × 10⁻¹¹ N m² kg⁻².
• Gravitational force is always attractive and acts along the line joining the two centres.
• F ∝ 1/r² — doubling the distance makes the force one-fourth.
• The moon's gravitational pull on the earth's water causes tides.

Correct Answer: Option 1 — 2 m s⁻¹
📖 Given
• First part: distance s₁ = 60 m in time t₁ = 30 s
• Second part: distance s₂ = 80 m in time t₂ = 40 s
✍️ Formula
Average speed = total distance / total time
🔢 Solution
• Total distance = 60 + 80 = 140 m
• Total time = 30 + 40 = 70 s
• Average speed = 140/70
• Average speed = 2 m s⁻¹
So over the whole journey the bus covers 2 metres every second, even though its speed in each separate part was exactly 2 m s⁻¹ as well.
❌ Why the other options are incorrect
✗ 1 m s⁻¹ — half the correct value; you get it by dividing 70 s by 140 m the wrong way, or by using only one part of the trip.
✗ 0.15 m s⁻¹ — this is 70/(60 × 80)-type nonsense; time must be divided into distance, not the other way round.
✗ 2.28 m s⁻¹ — this is 160/70, obtained by wrongly adding the distances as 80 + 80 instead of 60 + 80.
📌 SSC & Railway Exam Facts
• Average speed = total distance/total time; it is not the plain average of the two speeds unless the times are equal.
• Average velocity = net displacement/total time, a vector.
• SI unit of speed is m s⁻¹; 1 km/h = 5/18 m s⁻¹.
• Uniform speed means equal distances in equal intervals of time.
• Slope of a distance–time graph gives the speed.

Correct Answer: Option 1 — 1.5 m
📖 Given
• Force applied, F = 2 N
• Work done, W = 3 J
• The force is parallel to the ground, i.e. along the displacement, so θ = 0° and cos θ = 1
✍️ Formula
W = F s cos θ, so for θ = 0°, s = W/F
🔢 Solution
• s = W/F
• s = 3/2
• s = 1.5 m
So the brick slides forward through one and a half metres while the person spends 3 joules of energy on it.
❌ Why the other options are incorrect
✗ 5 m — this comes from adding 2 and 3 instead of dividing; work and force cannot be added.
✗ 6 m — this is W × F = 3 × 2; multiplying gives a quantity with units J N, which is meaningless here.
✗ 0.66 m — this is F/W = 2/3, the ratio taken upside down; displacement is work divided by force.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit of work is the joule (J), and 1 J = 1 N × 1 m.
• Work is done only when the force produces a displacement in its own direction.
• θ = 0° → positive work, θ = 90° → zero work, θ = 180° → negative work.
• Work and energy are scalar quantities.
• Power = work/time, SI unit watt (W); 1 W = 1 J/s.

Correct Answer: Option 2 — −2.0 m, concave lens
📖 Given
• Power of the corrective lens, P = −0.5 D
• Focal length is required in metres
✍️ Formula
P = 1/f, so f = 1/P (with f in metres and P in dioptres)
🔢 Solution
• f = 1/P
• f = 1/(−0.5)
• f = −2.0 m
• A negative focal length always belongs to a concave (diverging) lens
• So she needs a concave lens of focal length −2.0 m
A negative power means the defect is myopia (near-sightedness), in which distant objects look blurred; a concave lens diverges the incoming rays so that the image is pushed back onto the retina.
❌ Why the other options are incorrect
✗ 2.5 m, convex lens — a convex lens has positive power; it is used for hypermetropia, not for a −0.5 D defect.
✗ −2.5 m, concave lens — the lens type is right, but −2.5 m would correspond to a power of −0.4 D, not −0.5 D.
✗ 2.0 m, convex lens — the magnitude is right but the sign and the lens type are wrong; +2.0 m corresponds to +0.5 D.
📌 SSC & Railway Exam Facts
• Power P = 1/f (f in metres); SI unit dioptre (D), and 1 D = 1 m⁻¹.
• Convex lens → positive power; concave lens → negative power.
• Myopia → concave lens; Hypermetropia → convex lens; Presbyopia → bifocal lens.
• Lens formula: 1/v − 1/u = 1/f; magnification m = v/u.
• Power of lenses in contact adds up: P = P₁ + P₂.

Correct Answer: Option 2 — v/u
The magnification of a lens tells us how many times larger (or smaller) the image is compared with the object. It is defined as
m = height of image / height of object = h′/h
and for a lens it is related to the distances by
m = v/u
where v is the image distance and u the object distance, both measured from the optical centre of the lens with the usual sign convention.
If |m| > 1 the image is enlarged, if |m| < 1 it is diminished, and if m = 1 it is the same size as the object. A positive m means a virtual and erect image, a negative m means a real and inverted image.
❌ Why the other options are incorrect
✗ u/v — this is the ratio turned upside down; it is the reciprocal of the magnification.
✗ u − v — subtracting two distances gives a length in metres, not a pure ratio; magnification has no unit.
✗ v − u — same error as above: a difference of distances can never measure how many times the image is enlarged.
📌 SSC & Railway Exam Facts
• For a lens: m = v/u; for a mirror: m = −v/u.
• Lens formula: 1/v − 1/u = 1/f; mirror formula: 1/v + 1/u = 1/f.
• Magnification is a pure number with no unit.
• Power of a lens P = 1/f (metres), unit dioptre (D).
• A convex lens can form both real and virtual images; a concave lens always forms a virtual, erect, diminished image.

Correct Answer: Option 4 — 15 J
📖 Given
• Force applied, F = 5 N
• Displacement produced, s = 3 m
• The displacement is along the direction of the force, so θ = 0° and cos θ = 1
✍️ Formula
W = F s cos θ = F × s (when θ = 0°)
🔢 Solution
• W = 5 × 3 × cos 0°
• W = 5 × 3 × 1
• W = 15 J
So the force transfers 15 joules of energy to the object while moving it through 3 metres. The work is positive, because force and displacement point the same way.
❌ Why the other options are incorrect
✗ 50 J — this treats the displacement as 10 m instead of 3 m; 5 × 10 = 50.
✗ 20 J — this comes from using a force of 5 N over 4 m, or from a slip such as 5 + 15; neither matches the given data.
✗ 5/3 J — this is F/s = 5/3, obtained by dividing instead of multiplying; work is force times displacement.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit joule (J), 1 J = 1 N m; CGS unit erg, 1 J = 10⁷ erg.
• 1 joule of work is done when a force of 1 newton moves a body 1 metre in its own direction.
• Work is a scalar quantity.
• Work–energy theorem: net work done = change in kinetic energy.
• 1 calorie = 4.2 J; 1 kWh = 3.6 × 10⁶ J.

Correct Answer: Option 4 — πr metre/second
📖 Given
• Radius of the circular path = r
• Time for one complete round, t = 2 s
✍️ Formula
v = distance/time, and the distance in one round is the circumference = 2πr, so v = 2πr/t
🔢 Solution
• v = 2πr/t
• v = 2πr/2
• v = πr metre/second
So an object going once round a circle of radius r in 2 seconds moves with a speed of πr metres every second. Note that although the speed is constant, the velocity keeps changing because the direction of motion changes at every point.
❌ Why the other options are incorrect
✗ πr/2 metre/second — this comes from dividing the circumference by 2 twice, i.e. using (2πr/2)/2.
✗ πr/4 metre/second — this divides the circumference by 8 in effect; no step of the problem gives a factor of 4 in the denominator.
✗ 2πr metre/second — this is the circumference itself, i.e. the distance covered; it would be the speed only if the time taken were 1 second.
📌 SSC & Railway Exam Facts
• For uniform circular motion, v = 2πr/t, where t is the time period.
• Circumference of a circle = 2πr; area = πr².
• In circular motion speed may be constant but velocity is always changing, so the motion is accelerated.
• The force that keeps a body on a circular path is the centripetal force, F = mv²/r, directed towards the centre.
• Work done by the centripetal force in one full circle is zero.

Correct Answer: Option 1 — Undiminished in all directions.
This is the statement of Pascal's law, given by Blaise Pascal. It says that the pressure applied to an enclosed (confined) fluid is transmitted equally and undiminished in all directions, and it acts at right angles to every surface the fluid touches.
A liquid has no fixed shape, so its molecules can pass a push on to their neighbours in every direction. That is why squeezing one end of a sealed tube of toothpaste makes the paste come out at the other end with the same pressure.
Because pressure is force per unit area (P = F/A), a small force on a small piston can be turned into a very large force on a large piston. This is the working principle of the hydraulic press, hydraulic lift and hydraulic brakes.
❌ Why the other options are incorrect
✗ In one direction only. — this describes a solid, where a push is passed along only the direction it is applied.
✗ In two directions only. — a fluid has no such restriction; pressure acts sideways, upwards and downwards alike.
✗ In three directions only. — pressure is transmitted in every direction, not in some fixed count of directions.
📌 SSC & Railway Exam Facts
• Pascal's law — pressure in a confined fluid is transmitted equally and undiminished in all directions.
• Pressure P = F/A; SI unit pascal (Pa), and 1 Pa = 1 N/m².
• Pressure due to a liquid column: P = hρg — it depends on depth, not on the area of the base.
• Applications: hydraulic lift, hydraulic press, hydraulic brakes.
• Atmospheric pressure at sea level ≈ 1.013 × 10⁵ Pa, measured with a barometer.

Correct Answer: Option 2 — kg, N
Mass is the quantity of matter contained in a body. It is a scalar quantity, it is the measure of a body's inertia, and it stays the same everywhere — on the earth, on the moon or in space. Its SI unit is the kilogram (kg).
Weight is the force with which the earth attracts a body, W = mg. Being a force, it is a vector quantity and its SI unit is the newton (N). Weight changes from place to place because the value of g changes — a body weighs about one-sixth as much on the moon as on the earth.
For example, a body of mass 10 kg has a weight of 10 × 9.8 = 98 N on the earth.
❌ Why the other options are incorrect
✗ kg, s — the second (s) is the SI unit of time, not of weight.
✗ kg, kg — the kilogram is right for mass but cannot be the unit of weight; weight is a force, measured in newton.
✗ gm, N — the unit of weight is correct here, but gram (g) is a CGS unit; the SI unit of mass is the kilogram.
📌 SSC & Railway Exam Facts
• Mass → kilogram (kg), a scalar, measured with a beam balance.
• Weight → newton (N), a vector, measured with a spring balance.
• W = mg, with g = 9.8 m s⁻² on the earth.
• Weight is zero at the centre of the earth and maximum at the poles.
• 1 kgf = 9.8 N; 1 N = 10⁵ dyne.

Correct Answer: Option 4 — Will increase
When a soft iron bar is placed inside a current-carrying solenoid, the magnetic field inside the coil increases greatly, and the arrangement becomes an electromagnet.
Soft iron is a ferromagnetic material. The field of the solenoid lines up the tiny magnetic domains inside the iron, so the iron itself becomes strongly magnetised and adds its own field to that of the solenoid. Since the relative permeability of soft iron is several hundred to a few thousand times that of air, the resultant field becomes hundreds of times stronger.
In formula terms the field changes from B = μ₀nI (air core) to B = μ₀μ(r)nI (iron core), where μ(r) ≫ 1.
Soft iron is chosen as the core because it is magnetised strongly but loses its magnetism as soon as the current is switched off — exactly what a controllable electromagnet needs.
❌ Why the other options are incorrect
✗ Will remain same — this would be true only for a non-magnetic core such as wood, plastic or copper.
✗ Will become zero — nothing can cancel the field of the current; the current keeps flowing and so the field remains.
✗ Will decrease — the field would weaken only for a diamagnetic material, and even then the change is extremely small; soft iron does the opposite.
📌 SSC & Railway Exam Facts
• Field inside a solenoid with a core: B = μ₀μ(r)nI.
• Soft iron is used for electromagnet cores (temporary magnets); steel is used for permanent magnets.
• Electromagnets are used in electric bells, cranes lifting scrap iron, loudspeakers, relays and MRI machines.
• Hans Christian Oersted (1820) discovered the magnetic effect of current; William Sturgeon made the first electromagnet.
• The strength of an electromagnet increases with the current, the number of turns and a soft iron core.