RRB Technician Grade III 2024-25 26 December 2024 Shift 1 GK Questions – with answers & solutions

Correct Answer: Option 2 — United Nations Environment Programme
UNEP stands for the United Nations Environment Programme. It is the environmental authority of the United Nations, set up in 1972 after the Stockholm Conference, with its headquarters at Nairobi, Kenya.
As NCERT records, in 1987 the UNEP succeeded in forging an agreement — the Montreal Protocol — to freeze CFC production at 1986 levels. This became necessary because Chlorofluorocarbons (CFCs), used as refrigerants and aerosol propellants, rise to the stratosphere, are split by UV rays into chlorine free radicals, and a single chlorine atom then destroys thousands of ozone molecules. The thinning of the ozone layer lets harmful ultraviolet radiation reach the Earth, causing skin cancer, cataract and damage to crops.
❌ Why the other options are incorrect
✗ United Nations Environment Policy — 'Policy' is not the correct expansion; UNEP is a Programme, an organisation, not a written policy.
✗ United Nations Ecological Programme — the letter E in UNEP stands for Environment, not Ecological.
✗ United Nations Ecological Policy — wrong on both counts, it changes Environment to Ecological and Programme to Policy.
📌 SSC & Railway Exam Facts
• UNEP was established in 1972; headquarters Nairobi (Kenya).
• The Montreal Protocol was signed in 1987 and came into force in 1989.
• 16 September is observed as the International Day for the Preservation of the Ozone Layer (World Ozone Day).
• The ozone hole over Antarctica was reported in 1985; ozone thickness is measured in Dobson Units.
• Ozone (O₃) in the stratosphere is a protective shield, but at ground level it is a harmful pollutant.

Correct Answer: Option 2 — A, B and C
Plants have no special excretory organs, so they get rid of waste by several simple methods — and all three statements are correct.
• A — correct. Plants remove excess water as water vapour through the stomata of leaves by transpiration.
• B — correct. Many waste products are stored in leaves; when these leaves become old and fall off, the wastes are thrown out of the plant body along with them.
• C — correct. Plants also store wastes as resins and gums, especially in old xylem, which is no longer used for conduction. Lac, rubber and gums are such products.
In addition, plants release oxygen as a by-product of photosynthesis and carbon dioxide during respiration, and excrete some wastes into the soil through roots.
❌ Why the other options are incorrect
✗ Both A and B — leaves out C, but storage of wastes as resins and gums in old xylem is an NCERT-stated method.
✗ Both A and C — leaves out B, but shedding of waste-laden leaves is a genuine method of plant excretion.
✗ Both B and C — leaves out A, but removal of excess water by transpiration is also correct.
📌 SSC & Railway Exam Facts
• Plant excretory products include resins, gums, latex, tannins, rubber and alkaloids.
• Opium comes from poppy latex, quinine from cinchona bark, rubber from the latex of *Hevea*.
• Plants can afford to keep wastes because many of their tissues are dead (e.g. old xylem).
• In humans, excretion is done by the kidney, lungs, skin and liver; the nephron is the functional unit of the kidney.
• Plants lose water in liquid form by guttation through hydathodes.

Correct Answer: Option 4 — Rhizopus
Rhizopus, the common bread mould, reproduces asexually by spore formation. Its white cottony body is made of thread-like hyphae. Some hyphae grow upright and bear tiny blob-on-a-stick structures called sporangia.
Inside each sporangium, hundreds of minute spores are formed. Every spore is protected by a thick wall that allows it to survive unfavourable conditions such as heat and dryness. When the sporangium bursts, the spores are dispersed by air, water or insects; on landing on a moist, warm surface like damp bread, each spore germinates into a new Rhizopus plant.
❌ Why the other options are incorrect
✗ Plasmodium — the malarial parasite; it reproduces by multiple fission (schizogony), splitting into many daughter cells at one time.
✗ Amoeba — a unicellular protozoan that reproduces by binary fission, dividing into two daughter amoebae.
✗ Paramecium — a slipper-shaped ciliate; it also multiplies by binary fission (transverse fission).
📌 SSC & Railway Exam Facts
• Spore formation — Rhizopus, Mucor, ferns, mosses, bacteria.
• Binary fission — Amoeba, Paramecium, Leishmania, bacteria; Multiple fission — Plasmodium.
• Budding — yeast and Hydra; Fragmentation — Spirogyra; Regeneration — Hydra and Planaria.
• Fungi are heterotrophic and saprophytic; their cell wall is made of chitin.
• Leishmania divides by longitudinal binary fission; Amoeba divides in any plane.

Correct Answer: Option 2 — The nuclear membrane is absent in prokaryotic cells.
Cells are of two kinds. In prokaryotic cells the genetic material is not enclosed by a nuclear membrane; the naked DNA lies free in the cytoplasm in a region called the nucleoid. In eukaryotic cells the DNA is enclosed in a well-defined nucleus bounded by a double-layered nuclear membrane with nuclear pores.
Prokaryotic cells also lack membrane-bound organelles — there is no mitochondrion, plastid, endoplasmic reticulum, Golgi apparatus or lysosome. The only organelle they possess is the ribosome (the smaller 70S type). Bacteria and blue-green algae (cyanobacteria) are prokaryotes; all plants, animals, fungi and protists are eukaryotes.
❌ Why the other options are incorrect
✗ The nuclear membrane is present in prokaryotic cells — false; the very meaning of *pro-karyon* is 'before the nucleus', i.e. no true nucleus.
✗ All cytoplasmic organelles are present in prokaryotic cells — false; membrane-bound organelles are absent, only ribosomes are present.
✗ The nuclear membrane is absent in eukaryotic cells — false; *eu-karyon* means 'true nucleus', so the nuclear membrane is definitely present.
📌 SSC & Railway Exam Facts
• The term cell was given by Robert Hooke (1665); Leeuwenhoek first saw living cells.
• Robert Brown discovered the nucleus (1831); Purkinje coined the term protoplasm.
• The Cell Theory was given by Schleiden and Schwann, and Virchow added that new cells arise from pre-existing cells.
• Prokaryotic ribosomes are 70S; eukaryotic ribosomes are 80S.
• Mycoplasma is the smallest cell; the ostrich egg is the largest cell, and the nerve cell the longest.

Correct Answer: Option 3 — Auxin, Cytokinins and Gibberellins
Plant hormones (phytohormones) are chemicals produced in one part of a plant that control its growth, development and responses to the environment. They are of two functional kinds — growth promoters and growth inhibitors.
The three growth-promoting hormones are:
• Auxin — made at the shoot tip; it causes cell elongation and is responsible for phototropism, as the cells on the shaded side elongate faster and the shoot bends towards light.
• Cytokinins — promote cell division; they are present in high concentration in fruits and seeds and delay the ageing of leaves.
• Gibberellins — cause stem elongation, seed germination and bolting.
Abscisic acid (ABA) is the exception; it inhibits growth, closes stomata, and causes wilting and dormancy of buds and seeds.
❌ Why the other options are incorrect
✗ Gibberellins, Cytokinins and Abscisic acid — includes abscisic acid, which is an inhibitor, not a promoter.
✗ Auxin, Abscisic acid and Gibberellins — again wrongly places abscisic acid among the growth promoters.
✗ Auxin, Cytokinins and Abscisic acid — same mistake; ABA is the growth-inhibiting stress hormone.
📌 SSC & Railway Exam Facts
• Auxin was discovered by F. W. Went; IAA (Indole-3-Acetic Acid) is the natural auxin.
• Gibberellin was isolated from the fungus Gibberella fujikuroi.
• Ethylene is the only gaseous hormone and ripens fruits; ABA is the stress hormone.
• Growth towards light is phototropism, towards gravity is geotropism, towards water is hydrotropism, towards chemicals is chemotropism.
• Plants have no nervous system, so all coordination is done by hormones.

Correct Answer: Option 1 — Bryophyllum – Spore formation
This pair is wrongly matched. Bryophyllum does not reproduce by spore formation; it reproduces by vegetative propagation. Its thick leaves carry buds (adventitious buds) in the notches of the leaf margins. When such a leaf falls on moist soil, each bud grows into a new independent plant.
Spore formation is a mode of asexual reproduction seen in fungi (Rhizopus, Mucor), ferns, mosses and bacteria, where thick-walled spores are produced inside a sporangium and germinate when conditions become favourable.
❌ Why the other options are incorrect
✗ Spirogyra – Fragmentation — correctly matched; this filamentous green alga simply breaks into fragments, and each fragment grows into a new filament.
✗ Hydra – Budding — correctly matched; a bud made of regenerative cells grows on the parent, develops a mouth and tentacles, and then detaches.
✗ Amoeba – Binary fission — correctly matched; this unicellular organism splits into two daughter cells after its nucleus divides.
📌 SSC & Railway Exam Facts
• Bryophyllum is the classic example of vegetative propagation by leaf; potato by stem tuber, sweet potato by root.
• Vegetative propagation methods include cutting, layering, grafting and tissue culture.
• Plants raised by vegetative propagation are genetically identical clones and bear fruit earlier.
• Multiple fission is seen in Plasmodium; regeneration in Planaria and Hydra.
• Yeast, a unicellular fungus, reproduces by budding, not by fission.

Correct Answer: Option 2 — Regeneration
Regeneration is the ability of a fully differentiated organism to rebuild a complete body from a body piece. Planaria, a flatworm, is the classic example — if it is cut into many pieces, each piece grows into a complete organism.
Regeneration is carried out by specialised cells present in the body. These cells multiply by rapid cell division, and the resulting mass of cells then undergoes differentiation, changing into the various tissues and organs in an organised sequence until the whole body plan is restored. NCERT points out that regeneration is not the same as reproduction, since most such organisms do not normally depend on being cut into pieces for multiplying.
❌ Why the other options are incorrect
✗ Binary fission — a unicellular organism divides into two daughter cells, e.g. Amoeba and Leishmania; Planaria is a multicellular animal.
✗ Fragmentation — the body breaks spontaneously into fragments that grow into new individuals, e.g. Spirogyra; in the question the cutting is done externally.
✗ Multiple fission — the parent cell divides simultaneously into many daughter cells inside a cyst, e.g. Plasmodium.
📌 SSC & Railway Exam Facts
• Planaria and Hydra both show a very high power of regeneration.
• Starfish regenerates lost arms; lizard regrows its tail after autotomy.
• In the human body, the liver has the greatest regenerative capacity.
• Regeneration is possible because of specialised (stem-like) cells and needs division followed by differentiation.
• Asexual reproduction involves one parent and gives offspring nearly identical to it.

Correct Answer: Option 4 — Embryo
After pollination, the pollen grain germinates on the stigma and grows a pollen tube through the style to reach the ovule inside the ovary. There the male gamete fuses with the egg cell — this fusion is fertilisation — and a zygote is formed.
The zygote then divides several times within the ovule and develops into the embryo, the tiny future plant made of the plumule (future shoot), radicle (future root) and cotyledons. Meanwhile the ovule develops a tough coat and is gradually converted into the seed, and the ovary grows into the fruit. When the seed germinates, the embryo grows into a new plant.
❌ Why the other options are incorrect
✗ Pistil — the female reproductive part of the flower (stigma + style + ovary); it exists before fertilisation, it is not formed from the zygote.
✗ Stamen — the male reproductive part (anther + filament) that produces pollen grains; it too is present before fertilisation.
✗ Pollen grain — produced in the anther and carries the male gamete; it is a participant in fertilisation, not its product.
📌 SSC & Railway Exam Facts
• After fertilisation — ovule → seed, ovary → fruit, zygote → embryo, and the other floral parts usually wither and fall.
• Self-pollination is within the same flower; cross-pollination is between two flowers, aided by wind, water, insects and birds.
• In angiosperms double fertilisation occurs — one male gamete forms the zygote, the other forms the endosperm (3n).
• Cotyledons store food; pea and gram are dicots, maize and wheat are monocots.
• Germination of the seed needs water, air and a suitable temperature.

Correct Answer: Option 1 — Plasmodium
Plasmodium, the malarial parasite, reproduces by multiple fission. In this process the nucleus of the parent cell divides repeatedly while the cell is enclosed in a protective cyst, producing many nuclei. Each nucleus then gets a bit of cytoplasm around it, and the cyst finally bursts to release many daughter cells at the same time.
This rapid multiplication inside human red blood cells is why the RBCs rupture in cycles and the patient of malaria gets recurring bouts of fever with chills. Multiple fission also helps the parasite survive unfavourable conditions inside the cyst.
❌ Why the other options are incorrect
✗ Amoeba — reproduces by binary fission; the parent divides into two daughter amoebae and the plane of division can be any plane.
✗ Leishmania — the parasite of kala-azar; it too divides by binary fission, but in a definite longitudinal plane starting from the base of the flagellum.
✗ Yeast — a unicellular fungus that reproduces by budding, in which a small bud pinches off from the parent cell.
📌 SSC & Railway Exam Facts
• Malaria is caused by Plasmodium and spread by the bite of the female Anopheles mosquito.
• Ronald Ross discovered the malarial parasite in the mosquito; World Malaria Day is 25 April.
• Kala-azar is caused by Leishmania, spread by the sandfly; sleeping sickness by Trypanosoma.
• Multiple fission — Plasmodium; Binary fission — Amoeba, Leishmania, Paramecium, bacteria.
• Quinine, obtained from the bark of cinchona, is the classical anti-malarial drug.

Correct Answer: Option 1 — Pulling hand from the flame
A reflex action is a sudden, automatic and involuntary response to a stimulus, made without the conscious thinking of the brain. Withdrawing the hand on touching a flame is the standard example.
The pathway followed is called the reflex arc:
• Receptor in the skin detects the heat (stimulus).
• Sensory neuron carries the impulse to the spinal cord.
• Relay (inter) neuron in the spinal cord immediately passes it on.
• Motor neuron carries the impulse to the effector, the arm muscle, which contracts and pulls the hand away.
Because the decision is taken in the spinal cord and not in the brain, the response is extremely fast and prevents injury; the brain only comes to know of it afterwards.
❌ Why the other options are incorrect
✗ Walking on the street — a voluntary action planned and controlled by the cerebrum, with balance maintained by the cerebellum.
✗ Talking to a child — a voluntary action involving thinking, memory and speech centres of the fore-brain.
✗ Dancing on the stage — a learned voluntary activity needing precise control of posture and balance by the cerebellum.
📌 SSC & Railway Exam Facts
• Other reflexes — blinking, sneezing, coughing, knee-jerk, salivation on smelling food, and the pupil narrowing in bright light.
• The reflex arc is completed in the spinal cord; reflex arcs evolved because thinking is slower.
• Neuron is the structural and functional unit of the nervous system; the gap between two neurons is the synapse.
• Voluntary actions — controlled by the cerebrum; involuntary actions such as heartbeat and breathing — by the medulla.
• Humans have 31 pairs of spinal nerves and 12 pairs of cranial nerves.

Correct Answer: Option 3 — One pair
A human body cell contains 46 chromosomes, that is 23 pairs. Of these, 22 pairs are autosomes, which carry genes for ordinary body characters, and the remaining one pair is the sex chromosomes, which decide the sex of the individual.
A female has the sex chromosome pair XX, and a male has XY. All eggs made by the mother therefore carry an X chromosome, while the father's sperms are of two kinds — half carrying X and half carrying Y. If an X-bearing sperm fertilises the egg the child is a girl (XX); if a Y-bearing sperm does so the child is a boy (XY). Hence the father's sperm decides the sex of the child, not the mother.
❌ Why the other options are incorrect
✗ Two pairs — humans have only one pair of sex chromosomes; there is no second pair.
✗ Twenty two pairs — 22 pairs are the autosomes, not the sex chromosomes.
✗ Forty six pairs — 46 is the total number of chromosomes, not the number of pairs; 46 pairs would mean 92 chromosomes.
📌 SSC & Railway Exam Facts
• Human somatic cells are diploid (2n = 46); gametes are haploid (n = 23).
• Female = XX, Male = XY; the Y chromosome is the male-determining chromosome.
• Gregor Johann Mendel is the Father of Genetics; he worked on the garden pea (Pisum sativum).
• Haemophilia and colour blindness are X-linked (sex-linked) recessive disorders, commoner in males.
• The term gene was coined by Johannsen; DNA was shown to be the genetic material and its double helix described by Watson and Crick (1953).

Correct Answer: Option 3 — Both cranial and spinal nerves
The nervous system is divided into the Central Nervous System (CNS) — the brain and spinal cord — and the Peripheral Nervous System (PNS), which is made of the nerves arising from the CNS.
The PNS consists of both cranial nerves and spinal nerves:
• Cranial nerves — 12 pairs, arising from the brain, supplying the sense organs and muscles of the head and neck (e.g. the optic nerve for sight, the auditory nerve for hearing).
• Spinal nerves — 31 pairs, arising from the spinal cord, supplying the trunk and limbs.
These nerves carry sensory impulses from receptors to the CNS and motor impulses from the CNS to muscles and glands, so the PNS is the communication link between the body and the CNS.
❌ Why the other options are incorrect
✗ Neither cranial nor spinal nerves — completely wrong; these are exactly the two sets of nerves forming the PNS.
✗ Only cranial nerves — incomplete; the 31 pairs of spinal nerves are equally a part of the PNS.
✗ Only spinal nerves — incomplete; the 12 pairs of cranial nerves also belong to the PNS.
📌 SSC & Railway Exam Facts
• CNS = brain + spinal cord; PNS = cranial nerves + spinal nerves.
• The autonomic nervous system (sympathetic + parasympathetic) controls involuntary organs such as the heart and gut.
• 12 pairs of cranial nerves and 31 pairs of spinal nerves are present in humans.
• The vagus nerve is the longest cranial nerve; the sciatic nerve is the longest and thickest nerve in the body.
• The spinal cord is protected by the vertebral column, and the brain by the cranium and cerebrospinal fluid.

Correct Answer: Option 1 — Rhizopus
NCERT states that a single cell may constitute a whole organism, as in Amoeba, Chlamydomonas, Paramoecium and bacteria. Such organisms are called unicellular, and that one cell performs all the life processes — nutrition, respiration, excretion, movement and reproduction — by itself.
Rhizopus, the bread mould, is the exception. Its body is made of many thread-like hyphae that together form a mycelium, with coenocytic (multinucleate) filaments and separate sporangia for reproduction. It is therefore a multicellular (filamentous) fungus, not a single-celled organism.
❌ Why the other options are incorrect
✗ Amoeba — a unicellular protozoan with an irregular changing shape; it moves and feeds using pseudopodia and divides by binary fission.
✗ Bacteria — unicellular prokaryotes without a nuclear membrane or membrane-bound organelles; they reproduce by binary fission.
✗ Chlamydomonas — a unicellular green alga with a cup-shaped chloroplast and two flagella; it makes its own food by photosynthesis.
📌 SSC & Railway Exam Facts
• Unicellular — Amoeba, Paramoecium, Chlamydomonas, bacteria, yeast; Multicellular — Rhizopus, mushroom, plants and animals.
• In multicellular organisms there is division of labour, with different cells forming different tissues and organs.
• Yeast is a unicellular fungus, while Rhizopus and mushroom are multicellular fungi.
• Fungal cell walls are made of chitin; fungi are saprophytes or parasites.
• Mycoplasma is the smallest cell, and the nerve cell is the longest cell in the human body.

Correct Answer: Option 4 — Sodium hydroxide
Basicity is measured on the pH scale, which runs from 0 to 14. A pH below 7 is acidic, pH 7 is neutral and a pH above 7 is basic — the higher the pH, the stronger the base.
Sodium hydroxide (NaOH), also called caustic soda, is a strong base. It ionises completely in water to give a very high concentration of OH⁻ ions, and a concentrated solution has a pH of about 13–14 — the highest on the whole scale. It is so strongly alkaline that it is corrosive to skin.
Of the remaining choices, blood has the highest pH (7.35–7.45), but that is only very mildly basic compared with NaOH.
❌ Why the other options are incorrect
✗ Tap water — essentially neutral, with a pH close to 7; it is neither a base nor an acid.
✗ Saliva — its pH is about 6.5 to 7.5, so it is nearly neutral and even slightly acidic before a meal.
✗ Blood — buffered at pH 7.35–7.45, only slightly basic; the body keeps this narrow range strictly constant.
📌 SSC & Railway Exam Facts
• pH scale 0–14 was given by S. P. L. Sørensen; p stands for potenz (power) of hydrogen.
• Strong bases: NaOH, KOH, Ca(OH)₂; weak bases: NH₄OH, Mg(OH)₂.
• Gastric juice pH ≈ 1.2–3, lemon juice ≈ 2.2, milk ≈ 6.4, pure water = 7, blood ≈ 7.4.
• NaOH = caustic soda, KOH = caustic potash, Ca(OH)₂ = slaked lime.
• Tooth decay begins when the mouth pH falls below 5.5; toothpastes are mildly basic.

Correct Answer: Option 1 — Heterogeneous mixture
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Its particles are larger than 1000 nm (1 μm), big enough to be seen with the naked eye.
Key properties of a suspension:
• It is heterogeneous — the particles and the medium form two visible phases.
• The particles settle down when left undisturbed, so it is unstable.
• The particles are retained by filter paper, so a suspension can be separated by simple filtration.
• It scatters a beam of light, showing the Tyndall effect.
Common examples: muddy water, chalk powder in water, paints, slaked lime in water and a flour–water mixture.
❌ Why the other options are incorrect
✗ Solid mixture — a suspension always needs a liquid or gaseous medium; a mixture of two solids is not a suspension.
✗ Plasma mixture — plasma is the fourth state of matter, a super-energetic ionised gas found in stars and fluorescent tubes; it has nothing to do with suspensions.
✗ Homogenous mixture — a homogeneous mixture is a true solution, whose particles are smaller than 1 nm and never settle; a suspension is the opposite.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Solution — homogeneous, stable, no Tyndall effect; colloid and suspension — heterogeneous, show Tyndall effect.
• A suspension is separated by filtration; a colloid needs centrifugation.
• States of matter: solid, liquid, gas, plasma and Bose–Einstein condensate (BEC).
• Milk, blood, fog, smoke are colloids, not suspensions.

Correct Answer: Option 3 — an oxidising agent
Bleaching powder (calcium oxychloride, CaOCl₂) is made by passing chlorine over dry slaked lime:
⚗️ The Reaction
Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)
• In moist air it slowly releases chlorine, which with water gives nascent oxygen [O].
• It is this nascent oxygen that oxidises dyes, germs and stains — so bleaching powder acts as a powerful oxidising agent in many chemical industries.
Uses: bleaching cotton, linen and wood pulp in the textile and paper industries, disinfecting drinking water, and as an oxidising agent in chemical manufacture.
❌ Why the other options are incorrect
✗ a dehydrating agent — a dehydrating agent removes water from a substance; that is the job of concentrated H₂SO₄ or P₂O₅, not of bleaching powder.
✗ a reducing agent — a reducing agent donates electrons / removes oxygen (H₂, C, CO); bleaching powder does exactly the opposite, it supplies oxygen.
✗ a fire extinguisher — fire extinguishers use CO₂ or sodium bicarbonate; a chlorine-releasing oxidiser would feed a fire, not put it out.
📌 SSC & Railway Exam Facts
• Bleaching powder = CaOCl₂, made from Ca(OH)₂ + Cl₂; it smells strongly of chlorine.
• NaOH = caustic soda, Na₂CO₃·10H₂O = washing soda, NaHCO₃ = baking soda, CaSO₄·½H₂O = plaster of Paris.
• Chlorine for bleaching powder comes from the chlor-alkali process.
• SO₂ bleaches by reduction (temporary); bleaching powder and Cl₂ bleach by oxidation (permanent).
• Bleaching powder is used to make drinking water germ-free (chlorination).

Correct Answer: Option 1 — Applying pressure and reducing temperature
In a gas the particles are very far apart, move at high speed and have the weakest forces of attraction. To turn a gas into a liquid, these particles must be brought close together and made to move slowly, so that the forces of attraction can hold them.
• Applying high pressure squeezes the particles into a much smaller volume, greatly reducing the space between them.
• Lowering the temperature reduces the kinetic energy of the particles, so they can no longer overcome the attractive forces.
Applying both together is exactly how gases are liquefied industrially — LPG in cylinders, CNG, and liquid oxygen, nitrogen and ammonia. Above a certain temperature, called the critical temperature, a gas cannot be liquefied by pressure alone, which is why cooling is essential.
❌ Why the other options are incorrect
✗ Reducing pressure and reducing temperature — reducing pressure lets the gas expand and its particles move further apart, which works against liquefaction.
✗ Applying pressure and increasing temperature — raising the temperature gives particles more kinetic energy to escape, exactly the opposite of what liquefaction needs.
✗ Applying temperature and reducing pressure — both changes push the substance towards the gaseous state; this is how a liquid is made to vaporise, not condense.
📌 SSC & Railway Exam Facts
• Interparticle force: solid > liquid > gas; interparticle distance: gas > liquid > solid.
• Compressibility: gases are highly compressible, liquids very slightly, solids almost none.
• LPG = Liquefied Petroleum Gas (mainly butane + propane); CNG = Compressed Natural Gas (mainly methane).
• Dry ice is solid CO₂ kept under high pressure; on release it sublimes.
• Above the critical temperature no amount of pressure can liquefy a gas.

Correct Answer: Option 3 — Both protons and neutrons
The particles present inside the nucleus of an atom are collectively called nucleons. The nucleus contains exactly two kinds of particle — the positively charged protons and the electrically neutral neutrons — so both protons and neutrons together are the nucleons.
• Proton — charge +1, mass ≈ 1 u, discovered by Goldstein.
• Neutron — charge 0, mass ≈ 1 u, discovered by James Chadwick (1932).
• Total number of nucleons = mass number (A) = Z + n.
Since a proton and a neutron each weigh about 1 u while an electron weighs only 1/1836 u, virtually the entire mass of an atom lies in its nucleons. For example, an aluminium atom (Z = 13, A = 27) has 13 protons + 14 neutrons = 27 nucleons.
❌ Why the other options are incorrect
✗ Only protons — protons alone give the atomic number (Z), not the nucleon count; the neutrons are also nucleons.
✗ Only neutrons — neutrons are nucleons, but so are protons; taking only neutrons gives n = A − Z, not A.
✗ Only electrons — electrons revolve outside the nucleus in shells; they are extranuclear particles and are never called nucleons.
📌 SSC & Railway Exam Facts
• Nucleons = protons + neutrons; their total = mass number (A).
• Atomic number Z = number of protons; neutrons n = A − Z.
• Discoveries: electron — J. J. Thomson (1897), proton — Goldstein, neutron — Chadwick (1932).
• The nucleus was discovered by Rutherford in the α-particle scattering experiment.
• Radius of the nucleus ≈ 10⁻¹⁵ m; radius of the atom ≈ 10⁻¹⁰ m.

Correct Answer: Option 3 — a homologous series
A homologous series is a group of organic compounds having the same functional group, the same general formula, and in which each successive member differs from the previous one by a −CH₂− unit (14 u in mass).
All four compounds given — CH₃OH, C₂H₅OH, C₃H₇OH, C₄H₉OH — contain the alcohol group −OH and fit the general formula CₙH₂ₙ₊₁OH:
• CH₃OH (methanol), C₂H₅OH (ethanol), C₃H₇OH (propanol), C₄H₉OH (butanol).
• Consecutive members differ by CH₂, i.e. by 14 u in molecular mass.
Because chemical properties are decided by the functional group, every member of the series shows almost identical chemical behaviour. Their physical properties, however, such as boiling point and density, change gradually as the chain grows longer.
❌ Why the other options are incorrect
✗ a heterologous series — not a recognised term in organic chemistry; it is coined only to look like the right answer.
✗ an electrochemical series — this is the arrangement of metals/electrodes by their standard electrode potential, used in electrochemistry, not for organic compounds.
✗ a periodic series — the periodic table arranges elements by atomic number; it does not classify compounds such as alcohols.
📌 SSC & Railway Exam Facts
• Alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂, alcohols CₙH₂ₙ₊₁OH.
• Successive homologues differ by −CH₂−, i.e. 14 u.
• Functional groups: alcohol −OH, aldehyde −CHO, ketone >C=O, carboxylic acid −COOH.
• CH₃OH = methanol (wood spirit, poisonous); C₂H₅OH = ethanol (drinking alcohol).
• Members of a homologous series have similar chemical but gradually changing physical properties.

Correct Answer: Option 4 — In an ionic compound, the transfer of electrons takes place from a metal to a non-metal.
An ionic (electrovalent) compound is formed by the complete transfer of electrons from one atom to another. Metals have few valence electrons and low ionisation energy, so they lose electrons easily and become cations. Non-metals have nearly complete outer shells and high electron affinity, so they gain those electrons and become anions.
⚗️ The Reaction
Na → Na⁺ + e⁻ and Cl + e⁻ → Cl⁻, giving Na⁺Cl⁻
• Sodium (2, 8, 1) loses its single valence electron to attain the stable configuration of neon.
• Chlorine (2, 8, 7) gains that electron to attain the stable configuration of argon.
• The oppositely charged ions are held by a strong electrostatic force of attraction — the ionic bond.
❌ Why the other options are incorrect
✗ from a metal to a metal — two metals both want to lose electrons; neither will accept them, so they form an alloy or metallic bond, not an ionic bond.
✗ from a non-metal to a non-metal — two non-metals both want to gain electrons, so they share electrons instead and form a covalent bond (H₂O, CO₂, CH₄).
✗ from a non-metal to a metal — this reverses the actual direction; a non-metal is electronegative and does not donate electrons to a metal.
📌 SSC & Railway Exam Facts
• Metal loses e⁻ → cation (+); non-metal gains e⁻ → anion (−); e.g. NaCl, MgCl₂, CaO, MgO.
• Ionic compounds have high melting and boiling points and are usually solid and brittle.
• They conduct electricity in molten or aqueous state, but not in solid state.
• They are generally soluble in water and insoluble in kerosene or petrol.
• Ionic bond = transfer of electrons; covalent bond = sharing of electrons.

Correct Answer: Option 4 — 32
Sulphur (S) has atomic number 16, so it has 16 protons and, in a neutral atom, 16 electrons. Its most abundant isotope also has 16 neutrons.
Mass number (A) = number of protons + number of neutrons = 16 + 16 = 32
So the mass number — and the atomic mass — of sulphur is 32 u. Its electronic configuration is 2, 8, 6, so it needs 2 electrons to complete its octet, giving it a valency of 2 and the ion S²⁻. It lies in Group 16, Period 3 of the periodic table.
Sulphur is a non-metal, a yellow brittle solid, and its most familiar compounds are H₂SO₄ (sulphuric acid, the king of chemicals) and H₂S (rotten-egg smell).
❌ Why the other options are incorrect
✗ 35.5 — this is the atomic mass of chlorine, which is fractional because chlorine is a 3 : 1 mixture of Cl-35 and Cl-37.
✗ 37 — this is the mass number of the heavier chlorine isotope, Cl-37, not of sulphur.
✗ 31 — this is the mass number of phosphorus (P, Z = 15), the element just before sulphur in the same period.
📌 SSC & Railway Exam Facts
• S: Z = 16, A = 32, neutrons = 16, configuration 2, 8, 6, valency 2.
• Common masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, P = 31, S = 32, Cl = 35.5, Ca = 40.
• H₂SO₄ is called the king of chemicals; molecular mass = 98 u.
• Sulphur dioxide (SO₂) causes acid rain and bleaches by reduction (temporary bleaching).
• Sulphur is used in vulcanisation of rubber, in gunpowder and in sulpha drugs.

Correct Answer: Option 1 — Molecule
Subatomic particles are the particles that make up an atom — they are smaller than an atom. The three fundamental subatomic particles are the electron, proton and neutron.
A molecule, on the other hand, is the smallest particle of a substance that can exist independently and is formed when two or more atoms combine chemically. A molecule is therefore bigger than an atom, not a part of it — so it is not a subatomic particle.
• Molecule of an element: O₂, H₂, N₂, O₃, P₄, S₈.
• Molecule of a compound: H₂O, CO₂, NH₃, NaCl.
❌ Why the other options are incorrect
✗ Electron — a genuine subatomic particle with charge −1 and mass 9.1 × 10⁻³¹ kg; discovered by J. J. Thomson (1897). It revolves outside the nucleus.
✗ Neutron — a subatomic particle with no charge and mass ≈ 1 u; discovered by James Chadwick (1932). It lies inside the nucleus.
✗ Proton — a subatomic particle with charge +1 and mass ≈ 1 u; discovered by Goldstein through canal rays. It lies inside the nucleus.
📌 SSC & Railway Exam Facts
• Subatomic particles = electron, proton, neutron; protons and neutrons together = nucleons.
• An atom is the smallest particle that takes part in a chemical reaction; a molecule can exist independently.
• Atomicity: He = 1 (monoatomic), O₂ = 2 (diatomic), O₃ = 3, P₄ = 4, S₈ = 8.
• The electron is the lightest of the three; the neutron is very slightly heavier than the proton.
• Modern physics finds protons and neutrons are themselves made of quarks.

Correct Answer: Option 3 — 25 g
The law of conservation of mass, given by Antoine Lavoisier (1774), states that mass can neither be created nor destroyed in a chemical reaction. Therefore the total mass of the reactants is always equal to the total mass of the products.
⚗️ The Calculation
Total mass of reactants = mass of A + mass of B = 10 g + 15 g = 25 g
Total mass of products = 25 g
• Atoms are only rearranged during a reaction; no atom is created or lost.
• This is exactly why every chemical equation must be balanced — the number of atoms of each element must be equal on both sides.
Example: 2H₂ + O₂ → 2H₂O, where 4 g + 32 g = 36 g on each side.
❌ Why the other options are incorrect
✗ 10 g — this is only the mass of substance A; the mass of B has been left out entirely.
✗ 11 g — no chemical logic gives this figure; mass cannot fall from 25 g, as nothing escapes in a closed system.
✗ 16 g — again less than the total reactant mass; mass can never simply disappear in a chemical change.
📌 SSC & Railway Exam Facts
• Law of conservation of mass — Antoine Lavoisier, the father of modern chemistry.
• Law of constant (definite) proportions — Joseph Proust: a compound always has the same elements in the same mass ratio (water always H : O = 1 : 8).
• Dalton's atomic theory explained both these laws.
• A balanced equation is a direct statement of the law of conservation of mass.
• In reactions that give off a gas, the vessel must be closed, or the mass will seem to decrease.

Correct Answer: Option 2 — On heating, diffusion becomes faster.
Diffusion is the spontaneous intermixing of the particles of two substances on their own, because the particles of matter are continuously moving and possess kinetic energy.
On heating, the particles absorb heat energy, so their kinetic energy and speed increase. Faster-moving particles spread into the other substance more quickly, and therefore the rate of diffusion increases with a rise in temperature.
Everyday proof: a crystal of potassium permanganate colours hot water far faster than cold water; incense or perfume spreads much more quickly in a warm room; tea leaves release colour faster in hot water than in cold.
The rate of diffusion is fastest in gases, slower in liquids and negligible in solids, because the interparticle space and freedom of movement decrease in that order.
❌ Why the other options are incorrect
✗ There is no effect of heating on diffusion. — wrong; temperature is the main factor controlling the rate of diffusion.
✗ On heating, diffusion becomes slower. — the opposite is true; heat increases particle speed, so diffusion speeds up.
✗ Diffusion depends on volume of matter. — diffusion depends mainly on temperature, state of matter and particle size (mass), not on volume.
📌 SSC & Railway Exam Facts
• Rate of diffusion: gases > liquids > solids.
• Rate of diffusion increases with temperature and decreases with larger particle mass.
• Diffusion proves that matter is made of tiny particles that are constantly moving.
• Smelling food or perfume from a distance, and the spread of ink in water, are classic examples.
• Diffusion in solids is extremely slow but real — e.g. two metal blocks pressed together for years.

Correct Answer: Option 3 — gypsum is heated at 373 K and it loses water molecules
Plaster of Paris (POP) is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is made by heating gypsum (calcium sulphate dihydrate, CaSO₄·2H₂O) to 373 K (100 °C), at which temperature gypsum loses three-fourths of its water of crystallisation.
⚗️ The Reaction
CaSO₄·2H₂O → (373 K) CaSO₄·½H₂O + 1½H₂O
• Only ½ molecule of water stays behind per formula unit, hence the name hemihydrate.
• On adding water back, POP sets into a hard solid mass of gypsum again — that is why it is used for plaster casts for fractured bones, and for making toys, statues, false ceilings and smooth wall surfaces.
• It must be stored in a moisture-proof container, or it sets on its own.
Heating above 373 K drives out all the water and gives dead burnt plaster, which will no longer set.
❌ Why the other options are incorrect
✗ brine — a concentrated solution of NaCl in water; heating it gives common salt, and it is the raw material of the chlor-alkali process, not of POP.
✗ ammonia — NH₃ is a gas containing no water of crystallisation at all, so it cannot lose water molecules.
✗ soda lime — a mixture of NaOH and CaO, used as a drying agent and in decarboxylation reactions; it does not yield plaster of Paris.
📌 SSC & Railway Exam Facts
• POP = CaSO₄·½H₂O; gypsum = CaSO₄·2H₂O; 373 K = 100 °C.
• Other common salts: NaHCO₃ baking soda, Na₂CO₃·10H₂O washing soda, CaOCl₂ bleaching powder, CuSO₄·5H₂O blue vitriol.
• POP is used in surgical bandages, moulds, statues and as a fire-proofing material.
• Gypsum is also added to cement to slow down its setting.
• Plaster of Paris gets its name from the large gypsum deposits at Montmartre, Paris.

Correct Answer: Option 3 — Sugar and sponge
A solid has a definite shape, distinct boundaries and a fixed volume. Its particles are very closely packed, the force of attraction between them is the strongest, and they can only vibrate about fixed positions. Solids are almost incompressible and rigid.
Both members of this pair are solids:
• Sugar is a crystalline solid with a definite shape and fixed volume.
• A sponge is a solid too. It can be compressed only because it is full of tiny air-filled pores — the sponge material itself does not shrink, the trapped air is squeezed out. So a sponge is the classic exam example of a compressible solid.
❌ Why the other options are incorrect
✗ Juice and milk — both are liquids; they take the shape of the container and have a fixed volume but no fixed shape.
✗ Oil and sugar — oil is a liquid, so the pair is mixed; only sugar is solid.
✗ Water and oxygen — water is a liquid and oxygen is a gas; a gas has neither a fixed shape nor a fixed volume.
📌 SSC & Railway Exam Facts
• Solid — definite shape and volume; liquid — definite volume, no definite shape; gas — neither.
• Interparticle force: solid > liquid > gas; compressibility: gas > liquid > solid.
• Rate of diffusion: gas > liquid > solid.
• Sponge is the standard example of a solid that can be compressed (because of trapped air).
• Rubber band shows that a solid can change shape under force yet remain a solid.

Correct Answer: Option 4 — there is an equal and opposite reaction and they act on two different bodies
Newton's third law of motion states: "To every action there is an equal and opposite reaction." The two words that decide this question are equal, opposite — and the crucial rider that the action and the reaction always act on two different bodies.
That last point is what makes the law work. If the two forces acted on the same body they would cancel each other and nothing could ever be accelerated. Because they act on different bodies, each body feels only one of them. When you push a wall, the wall pushes you back; when a gun is fired, the bullet goes forward and the gun recoils backward; when a rocket throws hot gases down, the gases push the rocket up; and a swimmer pushes the water back to move forward.
❌ Why the other options are incorrect
✗ there is an equal and perpendicular reaction and they act on two different bodies — the reaction is opposite (180°) to the action, never perpendicular (90°).
✗ there is an unequal and opposite reaction and they act on the same body — wrong twice over: the reaction is exactly equal in magnitude, and the forces do not act on the same body.
✗ there is an equal and opposite reaction and they act on the same body — the classic trap. If both acted on one body, their resultant would be zero and no motion could ever begin.
📌 SSC & Railway Exam Facts
• Third law: action and reaction are equal, opposite and act on two different bodies; they never cancel.
• Conservation of linear momentum follows from the third law: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂.
• Recoil of a gun, rocket propulsion and walking are its standard examples.
• First law defines force and inertia; second law measures it, F = ma.
• Newton's laws appear in Principia Mathematica (1687).

Correct Answer: Option 4 — Newton
The SI unit of force is the newton (N), named after Sir Isaac Newton. It is defined from Newton's second law, F = ma: one newton is the force that gives a body of mass 1 kilogram an acceleration of 1 metre per second squared. In base units, 1 N = 1 kg m s⁻².
Force is a vector quantity — it has both magnitude and direction — and its effect is to change a body's state of rest or of uniform motion, or to change its shape. The weight of a 1 kg body on the earth is W = mg = 1 × 9.8 = 9.8 N, which is why a newton is roughly the weight of a 100 g object.
❌ Why the other options are incorrect
✗ rad/s — the unit of angular velocity (ω), telling how fast an object turns, not how hard it is pushed.
✗ Dyne — this is a unit of force, but it belongs to the CGS system, not SI: 1 dyne = 1 g cm s⁻² = 10⁻⁵ N.
✗ m/s — the unit of speed or velocity; it measures how fast a body moves, not the force acting on it.
📌 SSC & Railway Exam Facts
• F = ma; 1 N = 1 kg m s⁻² = 10⁵ dyne.
• 1 kgf (kilogram-force) = 9.8 N; weight W = mg.
• Other derived SI units: work/energy → joule (J), power → watt (W), pressure → pascal (Pa).
• Momentum p = mv in kg m/s; impulse = F × t in N s.
• The seven SI base units are metre, kilogram, second, ampere, kelvin, mole and candela.

Correct Answer: Option 3 — It will be doubled.
📖 Given
• Original resistance R, original current I = V/R
• New resistance, R′ = R/2
• Voltage V across the resistor is kept constant
✍️ Formula
Ohm's law: V = IR, so I = V/R — at constant voltage the current is inversely proportional to resistance
🔢 Solution
• I′ = V/R′
• I′ = V/(R/2)
• I′ = 2 × (V/R)
• I′ = 2I, i.e. the current is doubled
In plain words, halving the resistance removes half of the opposition to the flow of charge, so with the same push (voltage) twice as much current flows.
❌ Why the other options are incorrect
✗ It will be halved. — this would be right only if current were directly proportional to resistance; Ohm's law says the opposite, I ∝ 1/R.
✗ It will remain the same. — the current could stay the same only if both V and R were unchanged, or if both were halved together.
✗ It will become zero. — current becomes zero only for an open circuit (R → ∞) or when V = 0. Reducing the resistance actually increases the current.
📌 SSC & Railway Exam Facts
• Ohm's law: V = IR, valid at constant temperature; given by Georg Simon Ohm.
• I ∝ 1/R at constant V; I ∝ V at constant R.
• R = ρl/A; SI unit of resistance ohm (Ω), of current ampere (A), of voltage volt (V).
• 1 A = 1 C/s; current is measured by an ammeter in series, voltage by a voltmeter in parallel.
• Electric power P = VI = I²R = V²/R; SI unit watt (W).

Correct Answer: Option 3 — (i) Work and (ii) Energy
Work and energy are measured in the same unit, the joule (J), because energy is defined as the capacity to do work. When a force of 1 N moves a body through 1 m along its own direction, the work done is 1 J, and exactly 1 J of energy is transferred to the body. So 1 J = 1 N m.
This is also why the work–energy theorem can be written at all: net work done = change in kinetic energy. Both are scalar quantities, and their CGS unit is the erg, with 1 J = 10⁷ erg.
❌ Why the other options are incorrect
✗ (i) Force and (ii) Work — force is measured in newton (N) and work in joule (J); they differ by a factor of length, since J = N m.
✗ (i) Force and (ii) Inertia — inertia is measured by mass, so its unit is the kilogram (kg), while force is in newton (N).
✗ (i) Energy and (ii) Inertia — energy is in joule (J) and inertia (mass) in kilogram (kg); two completely different physical quantities.
📌 SSC & Railway Exam Facts
• Work = Energy → joule (J); 1 J = 1 N m = 10⁷ erg.
• Work–energy theorem: W(net) = ½mv² − ½mu².
• Other quantities sharing units: work, energy, heat and torque all reduce to N m (torque is written as N m, never as joule).
• Inertia ∝ mass, unit kilogram (kg); force unit newton (N).
• 1 kWh = 3.6 × 10⁶ J, 1 calorie = 4.2 J, 1 eV = 1.6 × 10⁻¹⁹ J.

Correct Answer: Option 2 — Only (A) and (B)
An electric oven and an electric laundry iron both work on the heating effect of electric current (Joule heating). A current I passing through a coil of high resistance R for time t produces heat H = I²Rt. The coils are made of nichrome, an alloy chosen because it has a high resistivity, a high melting point and does not oxidise easily when red hot. Both are therefore genuine practical applications of electricity.
A solar water heater, however, does not run on electricity at all. It absorbs solar (radiant) energy directly in a black collector plate and converts it into heat in the water. The energy conversion is solar energy → heat energy, with no electric current involved, so (C) does not belong with the other two.
❌ Why the other options are incorrect
✗ (A), (B) and (C) — wrong because it wrongly includes the solar water heater, which uses sunlight, not electric current.
✗ Only (B) and (C) — it drops the electric oven, a clear example of the heating effect of current, and keeps the solar heater instead.
✗ Only (A) and (C) — it drops the electric laundry iron, which is one of the most familiar electrical heating appliances of all.
📌 SSC & Railway Exam Facts
• Joule's law of heating: H = I²Rt; used in electric iron, oven, heater, toaster, geyser and the electric bulb filament.
• Nichrome is used in heating coils; tungsten (melting point 3380 °C) is used in bulb filaments.
• An electric fuse works on the heating effect and is connected in the live wire in series.
• A solar water heater and a solar cooker use solar thermal energy; a solar cell converts sunlight directly into electricity.
• Electric power P = VI = I²R = V²/R; commercial unit of energy 1 kWh = 3.6 × 10⁶ J.

Correct Answer: Option 1 — 6 × 10¹⁸
📖 Given
• Total charge, Q = 1 C
• Charge on one electron, e = 1.6 × 10⁻¹⁹ C
• Number of electrons required, n = ?
✍️ Formula
Q = n × e, so n = Q/e
🔢 Solution
• n = 1 / (1.6 × 10⁻¹⁹)
• n = (1/1.6) × 10¹⁹
• n = 0.625 × 10¹⁹
• n = 6.25 × 10¹⁸ ≈ 6 × 10¹⁸ electrons
So one coulomb — the charge that flows in one second when the current is 1 ampere — is made up of about six billion billion electrons. That shows how extremely small the charge on a single electron is.
❌ Why the other options are incorrect
✗ 9.1 × 10³¹ — this figure comes from the mass of an electron, 9.1 × 10⁻³¹ kg, with the sign of the power changed; it is a mass, not a count.
✗ 4.2 × 10¹⁶ — no step of the calculation gives this; 4.2 is the mechanical equivalent of heat (1 cal = 4.2 J), placed here as a distractor.
✗ 1.6 × 10¹⁹ — this is the value of 1/e written with the wrong digits; it repeats the 1.6 of the electronic charge instead of dividing by it.
📌 SSC & Railway Exam Facts
• Q = ne; e = 1.6 × 10⁻¹⁹ C, measured by R. A. Millikan's oil-drop experiment.
• 1 C ≈ 6.25 × 10¹⁸ electrons; charge is quantised — it comes only in whole multiples of e.
• I = Q/t; 1 ampere = 1 coulomb/second.
• Mass of an electron = 9.1 × 10⁻³¹ kg; the electron was discovered by J. J. Thomson (1897).
• SI unit of charge is the coulomb (C), named after Charles-Augustin de Coulomb.

Correct Answer: Option 2 — Volt
Despite its name, the electromotive force (EMF) is not a force at all — it is the energy supplied by a cell or battery to each unit charge that it drives round the circuit. Since it is energy per unit charge, its unit is joule per coulomb, which is given the special name volt (V):
EMF = Work done / Charge, so 1 V = 1 J/C
A cell of EMF 1.5 V therefore gives 1.5 joule of energy to every 1 coulomb of charge passing through it. EMF is measured when no current is being drawn; once current flows, the terminal voltage is a little less because of the cell's internal resistance r: V = EMF − Ir.
❌ Why the other options are incorrect
✗ Ampere — the SI unit of electric current, the rate of flow of charge; 1 A = 1 C/s, a different quantity altogether.
✗ Newton — the SI unit of mechanical force. The word "force" in EMF is only historical; EMF is energy per charge, not a push.
✗ Sec — the second is the SI unit of time; it has no connection with the energy given to a charge.
📌 SSC & Railway Exam Facts
• EMF and potential difference are both measured in volt (V); 1 V = 1 J/C.
• V = EMF − Ir, where r is the internal resistance of the cell.
• EMF is measured accurately by a potentiometer, potential difference by a voltmeter (connected in parallel).
• A dry cell gives about 1.5 V; a lead-acid car battery about 12 V.
• Ohm's law V = IR; P = VI; SI unit of charge coulomb (C), of current ampere (A).

Correct Answer: Option 3 — less than the density of the liquid
When a body is placed in a liquid, two forces act on it: its weight acting downwards and the upthrust (buoyant force) acting upwards. By Archimedes' principle, the upthrust equals the weight of the liquid displaced.
If the density of the object is less than the density of the liquid, then even when only a part of the body is submerged, the liquid displaced already weighs as much as the whole body. The upthrust therefore balances the weight, and the body floats with a part of it above the surface. If the densities were equal, the body would just float fully submerged, and if the object were denser, the upthrust could never match its weight and it would sink. This is why a ship of iron floats — its hollow shape makes its average density less than that of water — while a solid iron nail sinks.
❌ Why the other options are incorrect
✗ equal to the density of the liquid — then the body neither rises nor sinks; it stays completely submerged at rest anywhere in the liquid, which is not ordinary floating.
✗ exactly twice the density of the liquid — a body twice as dense as the liquid is far heavier than the upthrust it can get, so it sinks straight to the bottom.
✗ greater than the density of the liquid — the weight exceeds the maximum possible upthrust, so the body sinks; this is the opposite of floating.
📌 SSC & Railway Exam Facts
• Archimedes' principle: upthrust = weight of the fluid displaced.
• Floats if ρ(object) < ρ(liquid); fully submerged & stationary if equal; sinks if greater.
• Density ρ = mass/volume; SI unit kg/m³; density of water = 1000 kg/m³ = 1 g/cm³.
• Relative density has no unit; it is measured with a hydrometer.
• Ice floats on water because ice (920 kg/m³) is less dense than water; about 1/10 of an iceberg stays above the surface.

Correct Answer: Option 4 — Atmospheric refraction
The twinkling (scintillation) of stars is caused by atmospheric refraction. The earth's atmosphere is not uniform: its density and hence its refractive index keep changing with height and with time as air layers move. Starlight entering this restless atmosphere is refracted continuously, and because a star is so far away it acts as a point source. So the apparent position of the star goes on shifting slightly, and the amount of light reaching the eye keeps varying — the star appears to twinkle.
Planets do not twinkle. A planet is much nearer and behaves as a collection of a large number of point sources; the variations from all those points average out, giving a steady light. Atmospheric refraction also makes the Sun appear about 2 minutes earlier at sunrise and 2 minutes later at sunset, and makes stars appear a little higher than they really are.
❌ Why the other options are incorrect
✗ Scattering — scattering of light by air molecules explains the blue sky, the red sun at sunrise and sunset and the Tyndall effect, not twinkling.
✗ Reflection — bouncing back of light from a surface; it produces images in mirrors and does not occur in the open atmosphere in this way.
✗ Total internal reflection — happens only when light goes from a denser to a rarer medium beyond the critical angle; it explains optical fibres, mirages and the sparkle of a diamond.
📌 SSC & Railway Exam Facts
• Twinkling of stars, advance sunrise and delayed sunset → atmospheric refraction.
• Blue colour of the sky, red sun at sunrise/sunset, white clouds → scattering of light.
• Rayleigh scattering: amount of scattering ∝ 1/λ⁴ — blue scatters most, red least.
• Mirage and optical fibre → total internal reflection; rainbow → refraction + dispersion + total internal reflection in raindrops.
• Stars twinkle but planets do not, because planets are extended sources.

Correct Answer: Option 1 — Same at all points
A solenoid is a long coil of many closely wound circular turns of insulated copper wire. When a current flows, the magnetic fields of the neighbouring turns add up along the axis and cancel sideways, so that inside the solenoid the field lines come out straight, parallel and equally spaced.
Parallel, equally spaced field lines are the signature of a uniform magnetic field — a field having the same magnitude and the same direction at every point. Hence the field strength inside a long current-carrying solenoid is the same at all points. Its value is B = μ₀nI, where n is the number of turns per unit length; note that it does not depend on the position inside or on the radius of the coil. Placing a soft iron core inside makes a powerful electromagnet.
❌ Why the other options are incorrect
✗ Minimum in the middle — exactly wrong; the middle region is where the field is most perfectly uniform and strong. The field weakens only near and beyond the ends.
✗ Found to increase from one end to the other — this would mean the field lines get steadily closer together, which is not the pattern seen inside a solenoid.
✗ More at the ends than at the centre — the opposite is true; at the very ends the field falls to about half its central value because the lines begin to spread out.
📌 SSC & Railway Exam Facts
• Field inside a long solenoid: B = μ₀nI, uniform; outside, the pattern resembles a bar magnet.
• μ₀ = 4π × 10⁻⁷ T m A⁻¹ (permeability of free space); SI unit of B is the tesla (T).
• Uniform field ⇒ parallel, equidistant field lines; crowded lines ⇒ stronger field.
• A soft iron core turns a solenoid into an electromagnet; William Sturgeon made the first one in 1824.
• Right-hand thumb rule — thumb → current, curled fingers → magnetic field.

Correct Answer: Option 4 — The object moves with a constant speed.
An object is said to be in uniform motion when it covers equal distances in equal intervals of time, however small those intervals are taken. Covering equal distances in equal times is exactly the same as saying that the speed stays constant.
Because the speed does not change, a body in uniform motion (moving along a straight line) has zero acceleration. Its distance–time graph is a straight line, whose slope gives the speed, and its speed–time graph is a straight line parallel to the time axis. In non-uniform motion, unequal distances are covered in equal times, the speed keeps changing, and the acceleration is not zero.
❌ Why the other options are incorrect
✗ The object travels with different acceleration. — in uniform motion along a straight line the acceleration is zero and constant; a changing acceleration means the motion is definitely non-uniform.
✗ The object moves with varying speeds. — a varying speed is the very definition of non-uniform motion, the opposite of what is asked.
✗ The object does not travel equal distances with respect to equal time intervals. — this again describes non-uniform motion; uniform motion requires equal distances in equal times.
📌 SSC & Railway Exam Facts
• Uniform motion — equal distances in equal time intervals; speed constant, acceleration = 0.
• Distance–time graph: straight line for uniform motion; its slope = speed.
• Velocity–time graph: its slope = acceleration, and the area under it = distance travelled.
• Speed is a scalar, velocity a vector; SI unit m/s; 1 km/h = 5/18 m/s.
• In uniform circular motion the speed is constant but the velocity changes, so the motion is accelerated.

Correct Answer: Option 1 — Kinetic energy
The energy possessed by a body by virtue of its motion is called kinetic energy. A bullet fired from a gun is moving very fast, so it carries kinetic energy, given by
K = ½mv²
Notice that kinetic energy depends on the square of the speed. That is why a bullet of small mass but very high speed carries enough energy to pierce a thick plank — doubling the speed makes the energy four times larger. When the bullet strikes a target, this kinetic energy is spent as work done against friction, appearing as heat and deformation.
❌ Why the other options are incorrect
✗ Light energy — the energy carried by electromagnetic radiation; a moving bullet does not shine, and light plays no part in its motion.
✗ Electrical energy — the energy associated with the flow of charge through a circuit; a bullet carries no current.
✗ Potential energy — the energy stored because of position or configuration, such as gravitational PE = mgh or the elastic PE of a stretched spring. It is the stored form, not the form due to motion.
📌 SSC & Railway Exam Facts
• Kinetic energy K = ½mv²; K ∝ v²; SI unit joule (J); it is a scalar and always positive.
• Relation with momentum: K = p²/2m and p = √(2mK).
• Potential energy (gravitational) = mgh; a stretched bow, a wound spring and water in a dam store PE.
• Law of conservation of energy: energy can neither be created nor destroyed, only transformed.
• A falling body converts PE → KE; total mechanical energy stays constant when friction is absent.

Correct Answer: Option 1 — Inversely proportional to A
The resistance of a uniform conductor is given by
R = ρl/A
where ρ is the resistivity of the material, l the length and A the area of cross-section. Since A sits in the denominator, the resistance is inversely proportional to the area of cross-section: R ∝ 1/A.
Physically, a thicker wire offers a wider path for the electrons, so more of them can move through side by side and the opposition to the current falls. Doubling the area halves the resistance; making the wire four times thicker in area reduces the resistance to one-fourth. This is exactly why thick wires are used for heavy-current appliances such as an air conditioner or a geyser, while thin wires are used in heating coils where high resistance is wanted.
❌ Why the other options are incorrect
✗ Proportional to A — this would mean a thicker wire resists current more, which contradicts both R = ρl/A and everyday wiring practice.
✗ Does not depend on A — resistance clearly changes with thickness; only resistivity ρ is independent of l and A.
✗ Proportional to A² — no such relation exists. A square appears only when the radius changes, because A = πr², so R ∝ 1/r².
📌 SSC & Railway Exam Facts
• R = ρl/A; R ∝ l, R ∝ 1/A; SI unit of resistance ohm (Ω).
• Resistivity ρ depends only on material and temperature; SI unit ohm metre (Ω m).
• If a wire is stretched to n times its length, its resistance becomes n²R (volume stays the same).
• Silver has the lowest resistivity; nichrome has a high resistivity and is used in heating elements.
• Ohm's law V = IR; series R(eq) = R₁ + R₂, parallel 1/R(eq) = 1/R₁ + 1/R₂.