RRB Technician Grade III 2024-25 28 December 2024 Shift 1 GK Questions – with answers & solutions

Correct Answer: Option 3 — Oxygen
Terrestrial organisms take oxygen from the atmospheric air for aerobic respiration. Inside the cell, oxygen completes the breakdown of glucose in the mitochondria, releasing carbon dioxide, water and a large amount of energy stored as ATP.
Air contains about 21% oxygen, which is far more than the amount dissolved in water, so land animals get oxygen easily and can afford a high rate of respiration. Aquatic organisms, which depend on dissolved oxygen, must breathe much faster. The overall equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
❌ Why the other options are incorrect
✗ Neon — a chemically inert noble gas present in traces; it takes part in no biological reaction and is used in glow signs.
✗ Hydrogen — almost absent from the atmosphere as free gas; it is highly inflammable and is not used in breathing.
✗ Nitrogen — makes up about 78% of air, but animals cannot use it directly; only nitrogen-fixing bacteria such as *Rhizobium* can fix it.
📌 SSC & Railway Exam Facts
• Composition of air: N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.04%.
• Oxygen was discovered by Carl Wilhelm Scheele and Joseph Priestley.
• Haemoglobin in RBCs carries oxygen as oxyhaemoglobin.
• Aerobic respiration gives 38 ATP per glucose; anaerobic gives only 2 ATP.
• Breakdown of glucose to pyruvate (glycolysis) occurs in the cytoplasm.

Correct Answer: Option 1 — Snail
In some animals the sex of the individual is not fixed by chromosomes at all. Snails are such animals — they can change their sex, and the sex is decided by the individual's own condition and surroundings rather than by a genetically predetermined pattern.
In most other animals sex determination is genetic. Human beings have 23 pairs of chromosomes; the 23rd pair is the sex chromosome pair — XX in females and XY in males. Since the mother always gives an X, it is the father's sperm (X or Y) that decides the sex of the child. In some reptiles like turtles and lizards, the temperature of incubation decides the sex.
❌ Why the other options are incorrect
✗ Human — sex is strictly genetic; XX = girl, XY = boy, decided by the father's gamete.
✗ Dogs — mammals with the same XX / XY chromosomal sex determination as humans.
✗ Pigeon — birds use the ZZ / ZW system; the female (ZW) decides the sex of the chick.
📌 SSC & Railway Exam Facts
• Humans have 46 chromosomes (23 pairs) — 22 pairs autosomes + 1 pair sex chromosomes.
• Y chromosome carries the SRY gene that makes an embryo male.
• In Drosophila and humans the male is heterogametic (XY).
• Gregor Johann Mendel is the Father of Genetics; he worked on garden pea (Pisum sativum).
• The temperature-dependent sex determination is seen in turtles and crocodiles.

Correct Answer: Option 4 — Sieve tubes and companion cells
Phloem is the food-conducting complex tissue of plants. It is made of four elements — sieve tubes, companion cells, phloem parenchyma and phloem fibres (bast fibres). Except the fibres, phloem cells are living.
Sieve tubes are long tube-like cells placed end to end, their end walls perforated like a sieve (sieve plate); they carry prepared food. Being enucleate, each sieve tube is controlled by an adjoining companion cell, which is a living cell with a nucleus and supplies energy for the transport. This movement of food from leaves to the rest of the plant is called translocation and needs ATP.
❌ Why the other options are incorrect
✗ Tracheids and companion cells — tracheids belong to the xylem, not to the phloem, so the pair is mixed.
✗ Tracheids and vessels — both are xylem elements; dead, lignified and meant for water conduction.
✗ Sieve cells and vessels — vessels are again xylem elements, so this pair is also wrong.
📌 SSC & Railway Exam Facts
• Xylem conducts water and minerals upward; phloem conducts food in both directions.
• Xylem elements: tracheids, vessels, xylem parenchyma and xylem fibres.
• Phloem fibres (bast fibres) of jute and flax are used to make ropes and sacks.
• Translocation of food is explained by the pressure flow (mass flow) hypothesis.
• Xylem and phloem together form the vascular bundle, a complex permanent tissue.

Correct Answer: Option 1 — Both A and C
Sexually transmitted diseases (STDs) spread through sexual contact with an infected person. They are caused by different kinds of germs, so we must separate the bacterial ones from the viral ones.
Gonorrhoea is caused by the bacterium *Neisseria gonorrhoeae* and Syphilis by the bacterium *Treponema pallidum* — both are therefore bacterial STDs and are curable with antibiotics. AIDS, on the other hand, is caused by the HIV (Human Immunodeficiency Virus), which destroys the helper T-cells of the immune system; antibiotics have no effect on it. So only A and C are bacterial.
❌ Why the other options are incorrect
✗ Both A and B — AIDS is viral, not bacterial, so B cannot be included.
✗ Both B and C — again includes the viral disease AIDS along with the bacterial syphilis.
✗ A, B and C — wrong because B (AIDS) is caused by a virus.
📌 SSC & Railway Exam Facts
• HIV also spreads through infected blood transfusion, shared needles and from mother to child.
• Genital warts are caused by a virus and trichomoniasis by a protozoan.
• Condoms give protection against STDs as well as unwanted pregnancy.
• 1 December is observed as World AIDS Day.
• Antibiotics act on bacteria only; they do not work on viruses.

Correct Answer: Option 4 — Cuscuta
Cuscuta (Amarbel / dodder) is a total stem parasite. It has no chlorophyll and cannot make its own food, so it twines around a host plant and pushes special sucking roots called haustoria into the host's vascular bundles. Through these it draws water, minerals and readymade food, weakening and often killing the host.
This is parasitic nutrition, a type of heterotrophic nutrition in which an organism lives on or inside another living organism and takes its food from it, harming the host in the process.
❌ Why the other options are incorrect
✗ Orchids — most are epiphytes; they only grow *on* a tree for support and make their own food, so they are not parasites.
✗ Mosses — small green bryophytes with chlorophyll; they are autotrophs that photosynthesise their own food.
✗ Acacia (babool) — a normal green tree with leaves and chlorophyll; a complete autotroph.
📌 SSC & Railway Exam Facts
• Heterotrophic nutrition is of three kinds — holozoic, saprophytic and parasitic.
• Rafflesia is a total root parasite and bears the largest flower in the world.
• Mistletoe (Viscum) is a partial stem parasite as it has chlorophyll.
• Pitcher plant (Nepenthes) and Venus flytrap are insectivorous plants that trap insects for nitrogen.
• Lichen shows symbiosis between an alga and a fungus.

Correct Answer: Option 1 — One
A human female has a pair of ovaries, and at birth they already contain thousands of immature eggs. On reaching puberty, one egg (ovum) matures and is released every month by one of the two ovaries — this release is called ovulation, and the two ovaries usually take turns.
The released egg travels into the fallopian tube (oviduct), where fertilisation can occur. Meanwhile the uterus prepares a thick, spongy lining to receive a fertilised egg. If the egg is not fertilised, this lining breaks down and is shed with blood as menstruation, a cycle of about 28 days.
❌ Why the other options are incorrect
✗ Four — no; only a single mature ovum is normally released in a cycle, not four.
✗ Three — incorrect; release of more than one egg is a rare exception that leads to fraternal twins.
✗ Two — incorrect; the two ovaries release one egg between them, not one each.
📌 SSC & Railway Exam Facts
• The ovary makes both the ovum and the hormones oestrogen and progesterone.
• Fertilisation takes place in the fallopian tube; implantation happens in the uterus.
• The placenta supplies nutrition and oxygen to the developing embryo.
• Human gestation period is about 9 months (280 days).
• Stoppage of the menstrual cycle at about 45–50 years is called menopause.

Correct Answer: Option 2 — Only (i) and (iii)
Respiration always begins with glycolysis in the cytoplasm, where one glucose molecule is broken into two molecules of the 3-carbon compound pyruvate. What happens to pyruvate next depends on the availability of oxygen.
(i) is correct — in yeast, in the absence of oxygen, pyruvate is converted into ethanol + CO₂; this anaerobic process is fermentation and is used in brewing and baking.
(ii) is wrong — the oxygen-using (aerobic) break-down of pyruvate into CO₂ and water occurs in the mitochondria, not in the chloroplast. The chloroplast is the site of photosynthesis.
(iii) is correct — in our muscle cells during sudden activity, the lack of oxygen turns pyruvate into lactic acid, which causes cramps.
❌ Why the other options are incorrect
✗ Only (i) and (ii) — statement (ii) wrongly places aerobic breakdown in the chloroplast.
✗ Only (i) — it leaves out (iii), which is a correct statement about muscle cells.
✗ (i), (ii) and (iii) — cannot be right because (ii) itself is false.
📌 SSC & Railway Exam Facts
• Glycolysis = cytoplasm; Krebs cycle = mitochondrial matrix.
• Mitochondrion is the powerhouse of the cell; the energy currency is ATP.
• Aerobic respiration gives 38 ATP; fermentation gives only 2 ATP.
• Lack of oxygen in muscles builds up oxygen debt, relieved by deep breathing.
• Yeast is used in bread making, beer and wine because of fermentation.

Correct Answer: Option 3 — membrane
Every living cell — plant, animal or bacterial — is surrounded by a cell membrane (plasma membrane). It is the outermost covering of an animal cell and lies just inside the cell wall in a plant cell, and it keeps the contents of the cell separate from the external environment.
The membrane is made of lipids and proteins and is selectively permeable: it allows some substances to pass and blocks others. Movement across it occurs by diffusion (gases like CO₂ and O₂), by osmosis (movement of water), and by active transport, which uses energy. Being flexible, it also allows Amoeba to form pseudopodia, and this flexibility explains endocytosis.
❌ Why the other options are incorrect
✗ Cell wall — a rigid outer layer of cellulose, present only in plant cells, fungi and bacteria; it is fully permeable, not a boundary in animal cells.
✗ Nucleus — the control centre of the cell containing chromosomes and DNA; it lies inside, not around, the cell.
✗ Mitochondria — the powerhouse organelle that releases energy as ATP; it is an organelle, not a covering.
📌 SSC & Railway Exam Facts
• The fluid mosaic model of the plasma membrane was given by Singer and Nicolson (1972).
• The plasma membrane is selectively (semi-) permeable; the cell wall is fully permeable.
• A cell placed in a hypotonic solution swells (endosmosis) and in a hypertonic solution shrinks (plasmolysis).
• The cell was discovered by Robert Hooke (1665) in cork.
• Cell theory was given by Schleiden and Schwann; Virchow added *Omnis cellula-e cellula*.

Correct Answer: Option 1 — Hydra
Budding is a method of asexual reproduction in which a small outgrowth called a bud appears on the parent body, grows by repeated mitotic divisions, and later separates to live independently.
Hydra, a simple multicellular animal of fresh water, reproduces in exactly this way. Its regenerative cells divide at one spot to make a bud, which develops its own mouth and tentacles while still attached to the parent, and then detaches as a new Hydra. As only one parent is involved, the young one is a clone, genetically identical to the parent.
❌ Why the other options are incorrect
✗ Mosquito — reproduces sexually; the female lays eggs in water that pass through larva and pupa stages.
✗ Liver fluke — a parasitic flatworm that reproduces sexually and has a complicated life cycle with a snail host.
✗ Algae — most algae multiply by fragmentation (e.g. Spirogyra) or by spores, not by budding.
📌 SSC & Railway Exam Facts
• Yeast, a unicellular fungus, also reproduces by budding.
• Binary fission — Amoeba, Leishmania; multiple fission — Plasmodium.
• Spore formation — Rhizopus; regeneration — Hydra and Planaria.
• Hydra belongs to phylum Coelenterata (Cnidaria) and has stinging cells.
• Offspring formed asexually are clones, showing very little variation.

Correct Answer: Option 3 — DNA
Inside the nucleus, the hereditary material DNA (deoxyribonucleic acid) does not stay as neat rod-like chromosomes all the time. In a non-dividing (interphase) cell it lies as a fine, entangled, thread-like network called chromatin material, made of DNA and histone proteins.
When the cell is about to divide, this chromatin condenses and coils up to become visible chromosomes. Thus chromatin and chromosomes are the same material in two different states. DNA carries the genes, the functional units of heredity, which pass information from parents to offspring and also direct the making of proteins.
❌ Why the other options are incorrect
✗ RNA — a single-stranded nucleic acid made from DNA; it works mainly in the cytoplasm for protein synthesis, not as chromatin.
✗ Ribosome — the granular organelle, the site of protein synthesis, found on the rough ER and freely in the cytoplasm.
✗ Nucleus — the organelle that *contains* chromatin; chromatin is the material inside it, not the nucleus itself.
📌 SSC & Railway Exam Facts
• The double-helix model of DNA was given by Watson and Crick (1953).
• The nucleus was discovered by Robert Brown (1831).
• A human cell has 46 chromosomes (23 pairs); the gamete has 23.
• Gene is a segment of DNA that carries the information for one character.
• The nucleus is bounded by a porous nuclear membrane and contains the nucleolus.

Correct Answer: Option 2 — Cuscuta
Cuscuta, known in India as Amarbel, is a yellow, thread-like climbing plant that has no chlorophyll and almost no leaves. Being unable to photosynthesise, it lives as a total parasite: it coils around the stem of a host plant and sends haustoria — special sucking roots — into the host's xylem and phloem to draw water, minerals and prepared food.
This is parasitic nutrition, a form of heterotrophic nutrition. The host plant gradually becomes weak, its growth is stunted and it may finally die, which is why Cuscuta is treated as a serious weed.
❌ Why the other options are incorrect
✗ Cucumber — a green autotrophic climber (creeper) that makes its own food by photosynthesis.
✗ Cabbage — a green leafy vegetable full of chlorophyll; a complete autotroph.
✗ Cactus — a xerophyte of deserts; its green stem (phylloclade) does the photosynthesis while leaves are reduced to spines.
📌 SSC & Railway Exam Facts
• Cuscuta is a total stem parasite; Rafflesia is a total root parasite.
• Mistletoe (Viscum) and Sandalwood (Santalum) are partial parasites.
• Pitcher plant (Nepenthes) is insectivorous, getting nitrogen from insects.
• Fungi growing on bread and pickles are saprophytes.
• In lichen an alga and a fungus live together in symbiosis; Rhizobium in root nodules of legumes is also symbiotic.

Correct Answer: Option 3 — Leishmania
Leishmania, the protozoan that causes kala-azar, has a whip-like structure (flagellum) at one end of its body. Because the cell has this definite structure, it also has a definite body organisation, and so its binary fission takes place in a definite orientation in relation to the flagellum — it divides longitudinally, along the length of the cell.
In contrast, Amoeba has no fixed shape, so the plane of its division can be any plane. This comparison is a standard NCERT point used to show that even in simple organisms, division may be oriented when the body plan demands it.
❌ Why the other options are incorrect
✗ Amoeba — divides by binary fission in any plane because it has no definite shape or flagellum.
✗ Yeast — a unicellular fungus that reproduces by budding, not by oriented binary fission.
✗ Plasmodium — the malarial parasite; it divides by multiple fission, producing many daughter cells at once.
📌 SSC & Railway Exam Facts
• Leishmania causes kala-azar (visceral leishmaniasis), spread by the sandfly.
• Plasmodium causes malaria, spread by the female Anopheles mosquito.
• Binary fission — Amoeba, Leishmania, Paramecium, bacteria.
• Multiple fission — Plasmodium; budding — Hydra and yeast.
• Whip-like flagella help in locomotion; hair-like cilia are seen in Paramecium.

Correct Answer: Option 3 — Xanthium and Urena
Seed dispersal is the scattering of seeds away from the parent plant so that the young plants get enough light, water, minerals and space and do not compete with the parent.
Seeds of Xanthium (cocklebur) and Urena are covered with spines and hooks. When an animal or a person brushes past the plant, these seeds cling to the fur, hair or clothes and are carried far away before falling off. This is dispersal by animals (zoochory), and such hooked fruits are the classic NCERT example of it.
❌ Why the other options are incorrect
✗ Castor and Balsam — dispersed by the bursting (explosion) of fruits, which flings the seeds out.
✗ Drumstick and Maple — have wings; they are light and are dispersed by wind.
✗ Sunflower and Madar — have hairy outgrowths / pappus; these too are carried by wind.
📌 SSC & Railway Exam Facts
• Agents of dispersal: wind, water, animals and explosion of fruits.
• Coconut is dispersed by water because of its light fibrous husk.
• Aak (Madar) and Drumstick are wind-dispersed; cocklebur (Xanthium) is animal-dispersed.
• Edible fleshy fruits like guava, mango and tomato are dispersed by animals eating them.
• Dispersal prevents overcrowding and competition and helps plants colonise new areas.

Correct Answer: Option 4 — Bryophyllum
Bryophyllum, also called the sprout leaf plant (Patharchatta), shows vegetative propagation by leaves. Tiny adventitious buds develop in the notches along its leaf margin. Each bud already has a small shoot and roots, and when the leaf falls on moist soil, these buds grow into new independent plants.
This is a natural method of asexual reproduction: only one parent is involved and no gametes fuse, so all the new plants are clones, genetically identical to the parent. The leaf simply detaches and every notch can give rise to a plantlet.
❌ Why the other options are incorrect
✗ Jasmine — propagated artificially by layering and by stem cuttings, not by leaf buds.
✗ Rose — grown from stem cuttings and by grafting/budding, not from leaf notches.
✗ Grapes — propagated commercially from stem cuttings, again not from leaves.
📌 SSC & Railway Exam Facts
• Vegetative propagation by leaf — Bryophyllum; by stem (tuber) — potato; by root — sweet potato and dahlia.
• Artificial methods: cutting, layering, grafting and tissue culture.
• Banana, orange, rose and jasmine are raised vegetatively as they rarely form viable seeds.
• The eye of a potato is a bud that sprouts into a new plant.
• Tissue culture (micropropagation) raises many plants from a few cells in a nutrient medium.

Correct Answer: Option 4 — large latent heat of vaporisation of water helps to cool the hot surface
Latent heat of vaporisation is the heat that 1 kg of a liquid must absorb to change completely into vapour at its boiling point, without any rise in temperature. For water this value is unusually large — 22.5 × 10⁵ J/kg (2260 kJ/kg) — the highest among common liquids.
When water is sprinkled on a hot roof, it begins to evaporate. To escape as vapour, every gram of that water must draw its large latent heat from the roof and from the air just above it. This heat is taken out of the surface, so the roof loses energy and its temperature falls. The surface therefore feels distinctly cooler after sprinkling.
The same principle explains why water in an earthen pot (matka) stays cool, why sweating cools the body, and why a desert cooler works well only in dry weather.
❌ Why the other options are incorrect
✗ less latent heat of vaporisation of water helps to cool the hot surface — water's latent heat of vaporisation is large, not small; and a small latent heat would remove less heat, giving poor cooling.
✗ large latent heat of vaporisation of water helps to heat the cold surface — evaporation absorbs heat from the surface, it never gives heat to it; the surface is cooled, not heated.
✗ large latent heat of vaporisation of water does not help to cool the hot surface — the exact opposite of the truth; the large latent heat is precisely the reason the cooling works.
📌 SSC & Railway Exam Facts
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg; latent heat of fusion of ice = 3.34 × 10⁵ J/kg.
• Evaporation is a surface phenomenon and a cooling process; it occurs at all temperatures below the boiling point.
• Evaporation is faster with higher temperature, larger surface area, greater wind speed and lower humidity.
• During a change of state the temperature stays constant, because the heat supplied is used as latent heat.
• Steam at 100 °C causes a worse burn than water at 100 °C, as it carries this extra latent heat.

Correct Answer: Option 2 — Scattering of light
The Tyndall effect is the scattering of a beam of light by the particles of a colloid or a suspension, which makes the path of the beam become visible. It was described by the British scientist John Tyndall.
Scattering occurs because the colloidal particles (1–1000 nm) are comparable in size to the wavelength of visible light (about 400–700 nm). Each particle intercepts the light and re-radiates it in all directions, so an observer standing at the side can see the beam. In a true solution the particles are smaller than 1 nm, far too small to scatter light, so a true solution shows no Tyndall effect — this is the standard laboratory test to tell the two apart.
Everyday examples: a sunbeam entering a dusty room, light from a cinema projector in a smoky hall, head-light beams in fog, and sunlight streaming through the canopy of a dense forest (mist).
❌ Why the other options are incorrect
✗ Dispersion of light — the splitting of white light into its seven colours by a prism; it produces the spectrum and the rainbow, not the visible path of a beam.
✗ Reflection of light — the bouncing back of light from a polished surface such as a mirror; it obeys the laws of reflection and makes images, not a glowing beam through a medium.
✗ Refraction of light — the bending of light when it passes from one medium to another because of a change in speed (e.g. a pencil looking bent in water); it explains lenses, not the Tyndall effect.
📌 SSC & Railway Exam Facts
• Tyndall effect is shown by colloids and suspensions; a true solution never shows it.
• Scattering also makes the sky look blue and the Sun red at sunrise and sunset (Rayleigh scattering, ∝ 1/λ⁴).
• Dispersion → prism/rainbow, Refraction → lens/mirage, Reflection → mirror/periscope.
• Danger signals are red because red has the longest wavelength and is scattered the least.
• C. V. Raman won the Nobel Prize (1930) for the Raman effect, another scattering phenomenon.

Correct Answer: Option 3 — a concave lens of suitable power
Myopia, also called near-sightedness or short-sightedness, is the defect in which a person can see nearby objects clearly but distant objects appear blurred. It happens because the image of a far object is formed in front of the retina instead of on it.
The two causes are (a) excessive curvature of the eye lens, which makes its converging power too high, and (b) elongation of the eyeball, which makes the retina sit too far back.
The remedy is to make the rays diverge a little before they reach the eye lens. A concave (diverging) lens of suitable power does exactly this, so the image is pushed back and falls sharply on the retina. The far point of a myopic eye is at a finite distance, not at infinity, and the corrective lens must have its focal length equal to that far-point distance.
❌ Why the other options are incorrect
✗ a convex lens of suitable power — a convex lens converges light further and is used to correct hypermetropia (long-sightedness), where the image falls behind the retina.
✗ a plain glass plate — a plane plate has zero power; it only shifts the ray sideways and cannot change where the image forms.
✗ a convex mirror of suitable focal length — eye defects are corrected with lenses, not mirrors; a convex mirror is used as a rear-view mirror.
📌 SSC & Railway Exam Facts
• Myopia → concave lens; Hypermetropia → convex lens; Presbyopia → bifocal lens.
• Astigmatism is corrected using a cylindrical lens.
• Power of a lens P = 1/f (in metre); SI unit dioptre (D); a concave lens has negative power.
• The least distance of distinct vision for a normal eye is 25 cm; its far point is at infinity.
• The eye focuses by changing the focal length of its lens — this is called accommodation, done by the ciliary muscles.

Correct Answer: Option 1 — Refraction of light
When air is heated — above a fire, above a hot road or above the exhaust of a jet — the hot air becomes less dense (optically rarer) than the cool air around it. These hot and cold layers keep mixing and moving, so the refractive index of the air keeps changing from point to point and from moment to moment.
Light from an object behind this turbulent air therefore passes through layers of continually changing refractive index and gets refracted by continually changing amounts. The path of the light keeps wobbling, so the object appears to shimmer and wave instead of standing still.
The same physics — atmospheric refraction through moving air of varying density — explains the twinkling of stars and the mirage seen on a hot road in summer.
❌ Why the other options are incorrect
✗ Interference of light — interference needs two coherent sources and produces alternate bright and dark fringes, as in a soap film; it does not make objects wobble.
✗ Absorption of light — absorption only makes an object look dimmer or coloured; it cannot change the apparent shape or position.
✗ Diffraction of light — diffraction is the bending of light around an obstacle or slit whose size is comparable to the wavelength; there is no such obstacle in open hot air.
📌 SSC & Railway Exam Facts
• Refraction is caused by the change in speed of light between two media; n = c/v.
• Twinkling of stars and the early sunrise / delayed sunset (by about 2 minutes) are due to atmospheric refraction.
• Planets do not twinkle — they are extended sources, so the flickering averages out.
• A mirage in a desert is due to refraction followed by total internal reflection.
• Scattering (not refraction) explains the blue sky and the red Sun at sunrise and sunset.

Correct Answer: Option 1 — Scattering of light
The blue colour of the sky is caused by the scattering of sunlight by the molecules of air and other very fine particles in the atmosphere.
According to Rayleigh's law of scattering, the amount of light scattered is inversely proportional to the fourth power of the wavelength:
Scattering ∝ 1/λ⁴
Blue light has a short wavelength (about 450 nm) while red light has a long wavelength (about 700 nm). Because of the 1/λ⁴ rule, blue is scattered roughly ten times more strongly than red. This scattered blue light reaches our eyes from every direction of the sky, so the sky looks blue.
The same law explains why the Sun looks red at sunrise and sunset — the light then travels through a much thicker layer of air, all the blue is scattered away and only the red reaches us. For an astronaut, where there is no atmosphere, the sky appears black.
❌ Why the other options are incorrect
✗ Atmospheric refraction — this explains the twinkling of stars and the apparent early sunrise, not the colour of the sky.
✗ Reflection — reflection would simply return sunlight unchanged; it cannot separate the colours of white light.
✗ Total internal reflection — this occurs 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
• Rayleigh scattering ∝ 1/λ⁴ — shorter wavelength means stronger scattering.
• Danger signals are red because red has the longest wavelength and is scattered the least, so it is seen from far away.
• Tyndall effect is the scattering of light by a colloid, which makes the beam path visible.
• Clouds look white because large water droplets scatter all wavelengths equally.
• Sequence of visible spectrum: VIBGYOR; violet has the least wavelength, red the most.

Correct Answer: Option 3 — Atmospheric refraction of starlight towards the normal
The Earth's atmosphere is not uniform — its density, and therefore its refractive index, increases steadily as we come down towards the surface. So starlight entering the atmosphere travels continuously from an optically rarer to an optically denser medium.
Whenever light goes from a rarer to a denser medium it bends towards the normal. Because the change is gradual, the ray follows a slightly curved path and bends a little more at every layer.
We always see a star along the straight line in which the light finally enters the eye. That final direction is tilted, so the star appears to be higher up (elevated) than its true position. The effect is largest near the horizon, where the light has to cross the greatest thickness of atmosphere.
The same effect makes the Sun visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset.
❌ Why the other options are incorrect
✗ The gravitational force of Earth on starlight — the bending of light by the Earth's gravity is far too small to be noticed; it is a general-relativity effect seen only near very massive bodies like the Sun.
✗ Atmospheric absorption of starlight — absorption only makes the star look fainter; it cannot shift its apparent position.
✗ Reflection of starlight away from the normal — reflection sends light back into the same medium and in any case reflection cannot bend light away from the normal in this sense; the phenomenon here is refraction.
📌 SSC & Railway Exam Facts
• Rarer to denser → bends towards the normal; denser to rarer → bends away from the normal.
• Atmospheric refraction causes the twinkling of stars, the advanced sunrise and delayed sunset, and the oval-shaped Sun at the horizon.
• The day is lengthened by about 4 minutes in total because of atmospheric refraction.
• Refractive index of air ≈ 1.0003; n = c/v, with c = 3 × 10⁸ m/s.
• Stars twinkle but planets do not, because planets subtend a larger angle at the eye.

Correct Answer: Option 2 — water
📖 Given
• Refractive index of water, n = 1.33
• Refractive index of oil, n = 1.46
• Refractive index of glycerine, n = 1.5
• Refractive index of glass-slab, n = 1.65
✍️ Formula
n = c / v, so v = c / n
(c = speed of light in vacuum = 3 × 10⁸ m/s)
🔢 Solution
• Since c is the same for all four media, v is inversely proportional to n.
• The smallest refractive index gives the largest speed.
• Comparing: 1.33 < 1.46 < 1.5 < 1.65, so the smallest n is 1.33 (water).
• v in water = 3 × 10⁸ / 1.33 ≈ 2.26 × 10⁸ m/s — the highest of the four.
So light travels fastest in water, because water is optically the rarest of the four given media.
❌ Why the other options are incorrect
✗ glass-slab — n = 1.65 is the largest value, so light is slowest here: v ≈ 1.82 × 10⁸ m/s. Choosing it comes from wrongly assuming higher n means higher speed.
✗ oil — n = 1.46 gives v ≈ 2.05 × 10⁸ m/s, slower than in water.
✗ glycerine — n = 1.5 gives v ≈ 2.0 × 10⁸ m/s, again slower than in water.
📌 SSC & Railway Exam Facts
• Refractive index n = c/v; it has no unit and is always greater than 1 for a material medium.
• Higher n → optically denser → light slower → ray bends more.
• Standard values: air = 1.0003, water = 1.33, glass ≈ 1.5, diamond = 2.42.
• Diamond has the highest refractive index among common materials and the smallest critical angle (24.4°).
• Snell's Law: n₁ sin i = n₂ sin r.

Correct Answer: Option 4 — Newton's third law of motion
Newton's Third Law of Motion states that to every action there is an equal and opposite reaction, and the two forces act on two different bodies.
When the sailor jumps forward out of the boat, his feet push the boat backwards — that is the action. The boat in turn pushes the sailor forward with an equal force — that is the reaction. Since the boat is free to move on water, the backward push makes it move backwards while the sailor moves forward.
The same situation can also be read through the law of conservation of momentum: the total momentum of (sailor + boat) was zero before the jump, so after the jump the forward momentum of the sailor must be balanced by an equal backward momentum of the boat, m₁v₁ = m₂v₂.
Other everyday examples: the recoil of a gun, the motion of a rocket, and walking on the ground.
❌ Why the other options are incorrect
✗ Law of inertia — this is another name for the first law; it explains why a body resists a change in its state, not why the boat is pushed back.
✗ Newton's second law of motion — the second law, F = ma, tells how much acceleration a given force produces; it does not say that forces occur in pairs.
✗ Newton's first law of motion — the first law deals with a body at rest or in uniform motion when no unbalanced force acts; here a definite force does act.
📌 SSC & Railway Exam Facts
• Third law: action = −reaction; they are equal in magnitude, opposite in direction and act on different bodies, so they never cancel out.
• Second law: F = ma = rate of change of momentum (F = Δp/Δt).
• First law = Law of Inertia; mass is the measure of inertia.
• Rocket propulsion and the recoil of a gun are standard third-law PYQ examples.
• Law of conservation of momentum follows directly from the third law.

Correct Answer: Option 3 — 40g J
📖 Given
• Mass of the object, m = 8 kg
• Height above the ground, h = 5 m
• Acceleration due to gravity = g (in SI unit, m/s²)
✍️ Formula
Gravitational potential energy, PE = mgh
🔢 Solution
• PE = m × g × h
• PE = 8 × g × 5
• PE = 40g J
So the object stores 40g joule of energy purely because of its position. Taking g = 10 m/s², this is 400 J; taking g = 9.8 m/s², it is 392 J. This whole amount would be converted into kinetic energy by the time the object reached the ground.
❌ Why the other options are incorrect
✗ 20g J — this is what you get if you use half the product, i.e. wrongly applying ½mgh (the ½ belongs to kinetic energy ½mv², not to potential energy).
✗ 100g J — this comes from wrongly using the mass and height as 8 + 5 = 13 or from a stray multiplication; it does not follow from mgh.
✗ 200g J — this is 5 times the correct value; it comes from multiplying 8 × 5 and then again by 5.
📌 SSC & Railway Exam Facts
• PE = mgh; KE = ½mv²; both are measured in joule (J).
• 1 joule = 1 N m = 1 kg m² s⁻²; the CGS unit is the erg, 1 J = 10⁷ erg.
• At the ground (h = 0) the potential energy is zero; PE is measured with respect to a chosen reference level.
• Total mechanical energy = KE + PE stays constant for a freely falling body.
• g = 9.8 m/s² on Earth; PE does not depend on the path taken, only on the height.

Correct Answer: Option 4 — P = V²/R
📖 Given
• Electric power = P
• Potential difference (voltage) = V
• Resistance = R
✍️ Formula
P = VI (power = voltage × current)
V = IR (Ohm's law), so I = V/R
🔢 Solution
• Start with P = V × I
• Substitute I = V/R from Ohm's law
• P = V × (V/R)
• P = V²/R
So for a fixed voltage, the power consumed is inversely proportional to the resistance — a lower resistance appliance draws more power. Using V = IR the other way round also gives the companion form P = I²R.
❌ Why the other options are incorrect
✗ P = R²/V — this has resistance and voltage interchanged; its units work out to Ω²/V, which is not the watt.
✗ P = V²R — this comes from multiplying by R instead of dividing by it; it would wrongly mean a higher resistance gives more power at fixed voltage.
✗ P = R/2V² — an invented expression; there is no factor of 2 anywhere in the derivation of electric power.
📌 SSC & Railway Exam Facts
• The three forms of electric power: P = VI = I²R = V²/R.
• SI unit of power is the watt (W); 1 W = 1 J/s and 1 horsepower = 746 W.
• Ohm's law: V = IR; SI unit of resistance is the ohm (Ω).
• Commercial unit of electrical energy is the kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ J.
• Heat produced H = I²Rt — this is Joule's law of heating, used in heaters, irons and fuses.

Correct Answer: Option 2 — A solenoid of finite length carrying current.
A solenoid is a long coil made of many closely wound circular turns of insulated copper wire. When a current flows through it, the fields of all the turns add up.
The resulting pattern is exactly like that of a bar magnet: the field lines come out of one end and go into the other, so one end behaves as the north pole and the other as the south pole. Inside the solenoid the field lines are straight, parallel and equally spaced, which means the field there is strong and uniform, given by B = μ₀nI.
This is why a current-carrying solenoid is used to make an electromagnet — put a soft iron core inside it and the field becomes many times stronger. The polarity is found by the right-hand thumb rule or the clock rule.
❌ Why the other options are incorrect
✗ A rectangular loop carrying current. — a single loop of any shape gives a weak field that is not uniform and does not resemble the extended two-pole field of a bar magnet.
✗ A circular loop carrying current. — near its centre a single circular loop behaves somewhat like a tiny magnetic dipole, but its field is far weaker and much less uniform than a solenoid's.
✗ An infinitely long wire carrying current. — a straight wire produces concentric circular field lines around it, a pattern completely unlike a bar magnet; here B = μ₀I/2πr.
📌 SSC & Railway Exam Facts
• Field inside a long solenoid: B = μ₀nI, where n = number of turns per unit length.
• Field at the centre of a circular coil: B = μ₀nI/2r; around a straight wire: B = μ₀I/2πr.
• Magnetic field lines never intersect, and they always form closed loops.
• Oersted (1820) discovered the magnetic effect of electric current.
• An electromagnet uses a soft iron core (temporary magnet); a permanent magnet uses steel.

Correct Answer: Option 4 — Earth's gravitational force
Every object near the Earth is pulled towards its centre by the Earth's gravitational force, which we call the weight of the body, W = mg. It is this force, and only this force, that gives a falling body its downward acceleration.
By Newton's Law of Universal Gravitation, the force between the Earth (mass M) and a body (mass m) at distance R is
F = GMm/R²
Putting F = ma gives a = GM/R² = g, the acceleration due to gravity, whose value near the Earth's surface is g = 9.8 m/s².
Notice that m cancels out — that is why a heavy stone and a light stone fall together in the absence of air resistance, exactly as Galileo demonstrated.
❌ Why the other options are incorrect
✗ Electromagnetic force — this acts only between charged or magnetic bodies; a falling stone is electrically neutral, so it plays no part here.
✗ Earth's repulsive force — gravitation is always attractive; there is no such thing as a gravitational repulsion.
✗ Nuclear force — the strong nuclear force acts only inside the nucleus, over a range of about 10⁻¹⁵ m; it has no effect on everyday falling bodies.
📌 SSC & Railway Exam Facts
• F = GMm/R²; G = 6.67 × 10⁻¹¹ N m² kg⁻² is the universal gravitational constant.
• g = GM/R² = 9.8 m/s² on Earth; on the Moon it is about 1/6 of this value.
• g is maximum at the poles, minimum at the equator, and zero at the centre of the Earth.
• Mass is constant everywhere; weight W = mg changes from place to place.
• The four fundamental forces: gravitational, electromagnetic, strong nuclear and weak nuclear — gravitation is the weakest but has infinite range.

Correct Answer: Option 2 — The sum of the individual resistances.
In a series combination the resistors are joined end to end, so there is only one path for the current. Therefore the same current I flows through every resistor, while the total voltage is shared among them:
V = V₁ + V₂ + V₃
Using Ohm's law (V = IR) for each part:
IR(eq) = IR₁ + IR₂ + IR₃
Cancelling the common I gives
R(eq) = R₁ + R₂ + R₃
So the equivalent resistance of a series combination is simply the sum of the individual resistances, and it is always larger than the largest single resistance in the set. Connecting resistors in series is therefore the way to increase the total resistance of a circuit.
❌ Why the other options are incorrect
✗ The product of the individual resistances. — multiplying resistances gives units of Ω², which is not a resistance at all. The product appears only in the two-resistor parallel shortcut R = R₁R₂/(R₁+R₂).
✗ The difference between the individual resistances. — resistances always add; a difference could even give a negative value, which is impossible.
✗ The reciprocal of the sum of the individual resistances. — this is a confusion with the parallel rule, where 1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃ (the reciprocal of the sum of the reciprocals).
📌 SSC & Railway Exam Facts
• Series: R(eq) = R₁ + R₂ + R₃, current is the same, voltage divides.
• Parallel: 1/R(eq) = 1/R₁ + 1/R₂ + 1/R₃, voltage is the same, current divides.
• In parallel, R(eq) is less than the smallest resistance in the combination.
• Domestic appliances are connected in parallel so that each gets the full 220 V and works independently.
• Ohm's law V = IR; resistance R = ρl/A, where ρ is the resistivity in Ω m.

Correct Answer: Option 4 — 'A' is correct and 'B' is incorrect.
In a parallel combination, both ends of every resistor are joined to the same two points of the circuit. Since the two points have fixed potentials, every resistor gets exactly the same potential difference V. So statement (A) is correct.
The current, however, has more than one path to choose from. The main current divides at the junction and, by Ohm's law, each branch carries
I = V/R
Because the two resistors are different, their R values are different, so the currents through them must also be different — the smaller resistance carries the larger current. So statement (B) is incorrect.
The total current is I = I₁ + I₂, and the equivalent resistance follows 1/R(eq) = 1/R₁ + 1/R₂.
❌ Why the other options are incorrect
✗ 'A' is incorrect and 'B' is correct. — exactly the reverse of the truth; the equal quantity in parallel is the voltage, not the current.
✗ Both 'A' and 'B' are incorrect. — statement (A) is definitely correct, since all branches lie between the same two nodes.
✗ Both 'A' and 'B' are correct. — (B) would be true only if the two resistors were equal; the question clearly says they are different.
📌 SSC & Railway Exam Facts
• Parallel: V same, current divides, 1/R(eq) = 1/R₁ + 1/R₂; for two resistors R(eq) = R₁R₂/(R₁+R₂).
• Series: I same, voltage divides, R(eq) = R₁ + R₂.
• In parallel the equivalent resistance is smaller than the smallest branch resistance.
• House wiring uses parallel connection so that each appliance gets the full 220 V and can be switched separately.
• In parallel, current is inversely proportional to resistance: I₁/I₂ = R₂/R₁.

Correct Answer: Option 4 — all types of reflecting surfaces including spherical surfaces
The laws of reflection are:
• (1) The angle of incidence is equal to the angle of reflection, i.e. ∠i = ∠r.
• (2) The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
These laws are universal — they hold for every reflecting surface, whether it is plane, concave, convex, cylindrical or irregular. The reason is that reflection is a point-by-point phenomenon: at each tiny point of the surface we draw the normal to the surface at that point, and the two laws are applied there.
For a spherical mirror the normal at any point is simply the line joining that point to the centre of curvature (C). Because the normal changes direction from point to point, a curved mirror converges or diverges the beam — yet at every single point ∠i = ∠r is still obeyed.
❌ Why the other options are incorrect
✗ only curved reflecting surface — the laws were first stated and are most easily demonstrated on a plane mirror, so they certainly apply there too.
✗ only plane reflecting surface — if this were true, ray diagrams for concave and convex mirrors, which are drawn using ∠i = ∠r, would be impossible.
✗ all types of reflecting surfaces excluding spherical surfaces — spherical mirrors are not an exception; the laws apply at each point using the normal through the centre of curvature.
📌 SSC & Railway Exam Facts
• Laws of reflection: ∠i = ∠r, and incident ray, reflected ray and normal are coplanar.
• They apply to regular (specular) as well as irregular (diffuse) reflection — a rough wall obeys them at every point.
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u; R = 2f.
• A plane mirror forms a virtual, erect, same-sized image at an equal distance behind the mirror, with lateral inversion.
• Reflection does not change the wavelength, frequency or speed of light, since the medium is unchanged.

Correct Answer: Option 4 — Formation of iron sulphate from copper sulphate and iron
A combination reaction is one in which two or more substances join together to form a single product: A + B → AB. Its test is simple — count the products; a combination reaction has exactly one.
When iron is dropped into copper sulphate solution, iron does not combine with it. Being more reactive than copper, iron pushes copper out of its salt and takes its place, giving iron sulphate and free copper. Two products are formed and a free element is set free, so this is a displacement reaction, not a combination — hence it is the odd one out.
⚗️ The Reaction
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• Iron lies above copper in the reactivity series, so Fe displaces Cu²⁺.
• The blue colour of the copper sulphate solution fades to pale green (FeSO₄), and a reddish-brown deposit of copper forms on the iron nail.
• Two products and one free element → displacement, the classic PYQ example.
❌ Why the other options are incorrect
✗ Formation of slaked lime from quick lime — CaO(s) + H₂O(l) → Ca(OH)₂(aq); two reactants give one product, so it is a combination reaction (and a strongly exothermic one).
✗ Formation of water — 2H₂(g) + O₂(g) → 2H₂O(l); hydrogen and oxygen combine into the single product water, a textbook combination reaction.
✗ Burning of coal — C(s) + O₂(g) → CO₂(g); carbon and oxygen give the single product carbon dioxide, so it is a combination reaction (also a combustion/oxidation reaction).
📌 SSC & Railway Exam Facts
• Combination: A + B → AB; Decomposition: AB → A + B (the exact reverse).
• Quick lime = CaO, slaked lime = Ca(OH)₂, limestone/marble/chalk = CaCO₃.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• A reaction can belong to two classes at once — burning of coal is both combination and combustion.
• Ca(OH)₂ solution (lime water) turns milky with CO₂ because insoluble CaCO₃ is formed.

Correct Answer: Option 1 — Colloid is a homogeneous mixture.
A colloid is a mixture in which the dispersed particles measure 1–1000 nm. Although the mixture looks uniform to the naked eye, the particles are in fact big enough to form a separate phase, so a colloid is scientifically classified as a heterogeneous mixture. Calling it homogeneous is therefore the incorrect statement, and that is what the question asks for.
Every colloid has two parts — the dispersed phase (the particles) and the dispersion medium (the substance they float in). Milk, for example, is an emulsion: tiny fat droplets dispersed in water.
The other three statements are all true of colloids: they scatter light (Tyndall effect), they are stable and do not settle on standing, and they are indeed heterogeneous.
❌ Why the other options are incorrect
✗ Colloidal particles scatter a beam of light passing through it — this is true; it is the Tyndall effect, the standard test used to tell a colloid from a true solution.
✗ They do not settle down when left undisturbed — true; colloidal particles are kept suspended by Brownian motion, which is why a colloid is a stable mixture (unlike a suspension).
✗ Colloid is a heterogeneous mixture — true, and the very reason Option 1 is false; only one of these two opposite statements can be correct.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect is shown by colloids and suspensions, never by a true solution.
• Colloids cannot be separated by filtration, but can be separated by centrifugation.
• Types of colloid: sol (solid in liquid), gel (liquid in solid), emulsion (liquid in liquid), aerosol (liquid/solid in gas), foam (gas in liquid).
• Examples — milk, blood, ink, smoke, fog, cheese, butter, shaving cream are all colloids.

Correct Answer: Option 4 — Gas to Liquid: Sublimation
The change of a gas into a liquid is called condensation (liquefaction), not sublimation. It happens on cooling, when the particles lose kinetic energy and the forces of attraction pull them close enough to form a liquid, and it releases the latent heat of vaporisation. Dew on grass, water droplets on a cold glass and clouds turning into rain are all condensation. This pairing is therefore wrong, which is exactly what the question asks for.
Sublimation is something quite different — the direct change of a solid into a gas without passing through the liquid state. Camphor, naphthalene balls, ammonium chloride, iodine and dry ice (solid CO₂) are the standard sublimating substances.
❌ Why the other options are incorrect
✗ Liquid to Gas: Evaporation — correctly matched; a liquid changing into vapour at any temperature below its boiling point is evaporation (the same change at the boiling point is boiling/vaporisation).
✗ Gas to Solid: Deposition — correctly matched; deposition (desublimation) is the direct change of a gas into a solid, as when frost forms on a cold window.
✗ Solid to Gas: Sublimation — correctly matched; this is the definition of sublimation, shown by camphor and naphthalene.
📌 SSC & Railway Exam Facts
• Solid → Liquid = melting (fusion); Liquid → Solid = freezing (solidification).
• Liquid → Gas = vaporisation/evaporation; Gas → Liquid = condensation.
• Solid → Gas = sublimation; Gas → Solid = deposition.
• Dry ice is solid CO₂; it sublimes directly and is used to keep ice-cream and food cold.
• Melting point of ice = 0 °C (273 K); boiling point of water = 100 °C (373 K) at 1 atm.

Correct Answer: Option 3 — It has a small amount of solute dissolved in a large amount of solvent.
A solution is a homogeneous mixture of two or more substances. The component present in the smaller quantity is the solute and the one present in the larger quantity is the solvent — in a sugar solution, sugar is the solute and water the solvent.
Solutions are classified by how much solute they contain. A dilute solution holds only a small quantity of solute in a large quantity of solvent, while a concentrated solution holds a large quantity of solute. This comparison is the whole meaning of the word *dilute*, so Option 3 is the true statement.
The amount of solute is measured by concentration, commonly as mass percentage = (mass of solute ÷ mass of solution) × 100. A solution that holds the maximum solute it can dissolve at a given temperature is called saturated; its solubility rises with temperature for most solids.
❌ Why the other options are incorrect
✗ It contains a large amount of solute dissolved in a small amount of solvent — that describes a concentrated solution, the exact opposite of a dilute one; this is the most common trap in this question.
✗ It exhibits a heterogeneous appearance — every true solution is homogeneous and transparent; a heterogeneous appearance belongs to a suspension or a colloid.
✗ It has an equal ratio of solute to solvent by mass — no such definition exists; "dilute" and "concentrated" are relative comparisons, not fixed 1 : 1 ratios.
📌 SSC & Railway Exam Facts
• Solution = solute + solvent; the solvent is always the component in the greater amount.
• Dilute → less solute; Concentrated → more solute; Saturated → maximum solute at that temperature.
• Mass % of solute = (mass of solute ÷ mass of solution) × 100.
• A true solution is homogeneous, its particles are < 1 nm, and it shows no Tyndall effect.
• Alloys (brass, bronze, steel) are solid solutions; air is a gaseous solution.

Correct Answer: Option 2 — Freezing point
The freezing point is the fixed temperature at which a liquid changes into a solid on cooling at a given pressure. As the liquid is cooled its particles lose kinetic energy, the intermolecular forces of attraction take over, and the particles settle into the fixed, ordered positions of a solid lattice. The change is called freezing or solidification.
During freezing the temperature does not fall even though heat is still being removed, because the escaping heat is the latent heat of fusion being given out by the particles as they lock into place. For water this value is 3.34 × 10⁵ J/kg.
For a pure substance the freezing point and the melting point are numerically the same — water freezes at 0 °C and ice melts at 0 °C. The two names simply describe the direction of the change: melting is solid → liquid, freezing is liquid → solid.
❌ Why the other options are incorrect
✗ Melting point — the temperature at which a solid turns into a liquid on heating; the value is the same for a pure substance, but the direction of the change is opposite to what the question describes.
✗ Boiling point — the temperature at which a liquid turns into vapour throughout its bulk (water: 100 °C); no solid is formed at all.
✗ Condensation point — the temperature at which a gas turns into a liquid; it is the reverse of boiling, not of melting.
📌 SSC & Railway Exam Facts
• For a pure substance, freezing point = melting point (water/ice = 0 °C = 273 K).
• Latent heat of fusion of ice = 3.34 × 10⁵ J/kg; latent heat of vaporisation of water = 22.5 × 10⁵ J/kg.
• Temperature remains constant during any change of state, as the heat is used as latent heat.
• Water expands on freezing, which is why ice floats and water pipes burst in winter.
• Salt lowers the freezing point of water — the reason salt is spread on icy roads and used in ice-cream making.

Correct Answer: Option 1 — evaporation
Evaporation is the change of a liquid into vapour at any temperature below its boiling point. Unlike boiling, which takes place throughout the liquid at a fixed temperature, evaporation is a surface phenomenon — only the particles at the free surface escape.
The particles of a liquid have different kinetic energies. Some surface particles gain enough kinetic energy to overcome the intermolecular force of attraction and fly off as vapour. Because the fastest particles leave, the average kinetic energy of the liquid left behind falls, and the liquid is cooled — this is why evaporation is a cooling process, absorbing the latent heat of vaporisation from its surroundings.
The rate of evaporation increases with temperature, surface area and wind speed, and decreases with humidity. This is why wet clothes dry faster when spread out on a hot, windy day, and slowly in humid weather.
❌ Why the other options are incorrect
✗ sublimation — the direct change of a solid into a gas without becoming a liquid, shown by camphor, naphthalene, iodine, ammonium chloride and dry ice; no liquid is involved.
✗ fusion — another name for melting, the change of a solid into a liquid on heating (ice → water at 0 °C); it is not a liquid-to-vapour change.
✗ deposition — the direct change of a gas into a solid (frost on a cold window); it is the reverse of sublimation.
📌 SSC & Railway Exam Facts
• Evaporation is a surface phenomenon at all temperatures; boiling occurs throughout the liquid at a fixed temperature.
• Evaporation causes cooling — the reason for sweating, earthen pots (matka) and desert coolers.
• Evaporation ↑ with temperature, surface area, wind speed; ↓ with humidity.
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg (2260 kJ/kg).
• Fusion = solid → liquid, Sublimation = solid → gas, Deposition = gas → solid, Condensation = gas → liquid.

Correct Answer: Option 4 — It has a fixed shape.
A gas has neither a fixed shape nor a fixed volume. Its particles are separated by very large intermolecular spaces and the forces of attraction between them are negligible, so the particles move about randomly at high speed and spread out to completely fill whatever container they are put in. Having a fixed shape is therefore not a property of a gas — it is a property of a solid, so Option 4 is the statement being asked for.
The remaining three statements are all genuine gas properties, and they all follow from the same cause: loose packing and large empty spaces between the particles.
❌ Why the other options are incorrect
✗ The particles are very loosely packed — true; gas particles are the most loosely packed of the three states, which is why a gas has the lowest density.
✗ It is highly compressible — true; the large empty spaces can be squeezed, which is how LPG and CNG are filled into cylinders and oxygen into hospital cylinders.
✗ The rate of diffusion is very high — true; free, fast-moving particles mix quickly, which is why the smell of an agarbatti or perfume spreads across a room within seconds.
📌 SSC & Railway Exam Facts
• Solid → fixed shape and volume; Liquid → fixed volume only; Gas → neither fixed.
• Compressibility and diffusion: gas > liquid > solid; intermolecular force: solid > liquid > gas.
• Gases exert pressure on the walls of the container because of the continuous collisions of their particles.
• LPG = Liquefied Petroleum Gas (mainly butane); CNG = Compressed Natural Gas (mainly methane, CH₄).
• Liquids and gases together are called fluids, as both can flow.

Correct Answer: Option 1 — 10⁻¹⁰ m
Atoms are extremely small. The radius of a hydrogen atom — the smallest atom of all — is about 0.53 Å, that is 5.3 × 10⁻¹¹ m, which to the nearest power of ten is 10⁻¹⁰ m. This value, 0.53 Å, is the famous Bohr radius (a₀), the radius of the first orbit (n = 1) in Bohr's model of the hydrogen atom.
Because atomic radii are so small, chemists prefer the unit angstrom: 1 Å = 10⁻¹⁰ m = 10⁻⁸ cm. Atomic radii of ordinary elements lie in the range 0.3 Å to 2.5 Å, so 10⁻¹⁰ m is the correct order of magnitude for any atom, hydrogen included.
Compare this with the nucleus, which is only about 10⁻¹⁵ m across — the atom is roughly 100,000 times larger than its nucleus, which is why Rutherford concluded that an atom is mostly empty space.
❌ Why the other options are incorrect
✗ 10⁻⁸ m — this is 100 times too large; 10⁻⁸ m = 100 Å, a size that belongs to large molecules and small colloidal particles, not to a single atom.
✗ 10⁻⁶ m — this is 1 micrometre (1 μm), about 10,000 times an atom's size; it is the scale of bacteria and fine dust, visible under a microscope.
✗ 10⁻¹² m — this is 1000 times too small; it lies between the size of an atom and that of a nucleus (10⁻¹⁵ m), and no atom is that small.
📌 SSC & Railway Exam Facts
• 1 Å = 10⁻¹⁰ m; atomic radius ≈ 10⁻¹⁰ m, nuclear radius ≈ 10⁻¹⁵ m (1 fermi = 10⁻¹⁵ m).
• Bohr radius of hydrogen = 0.53 Å; hydrogen is the smallest and lightest atom.
• Atomic radius decreases across a period (left → right) and increases down a group.
• An atom is about 10⁵ times bigger than its nucleus — the atom is mostly empty space.
• 1 nm = 10⁻⁹ m = 10 Å; colloidal particles are 1–1000 nm.

Correct Answer: Option 3 — The particles of a suspension can be seen by the naked eye.
A suspension is a heterogeneous mixture in which the solute particles are larger than 1000 nm and do not dissolve — they merely remain suspended in the medium. Because these particles are so large, they are big enough to be seen with the naked eye, which makes the mixture look turbid or cloudy. That is exactly what Option 3 states.
The other characteristic tests of a suspension all follow from this large particle size: the particles scatter light (Tyndall effect), they settle down on standing under gravity, and they can be removed by ordinary filtration. A suspension is therefore an unstable mixture.
Standard examples are chalk powder in water, muddy water, paint, slaked lime used for whitewashing and a shaken medicine bottle marked "shake well before use".
❌ Why the other options are incorrect
✗ The solute particles cannot be separated from the mixture — false; suspension particles are easily separated by simple filtration or by just letting them settle and decanting.
✗ The solute particles don't settle down when a suspension is left undisturbed — false; they do settle under gravity, and this is the key difference from a colloid, whose particles stay suspended.
✗ It is a homogeneous mixture — false; a suspension is heterogeneous, with the particles forming a clearly separate phase.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Suspension particles are visible, settle on standing and can be filtered; colloidal particles do none of these.
• Tyndall effect is shown by suspensions and colloids, never by a true solution.
• Chalk is calcium carbonate (CaCO₃); marble and limestone have the same formula.
• A suspension is unstable, a colloid is stable, and a true solution is perfectly homogeneous.

Correct Answer: Option 4 — Mixture of oil in water
A homogeneous mixture has a uniform composition throughout with no visible boundary between its parts, while a heterogeneous mixture has a non-uniform composition and shows a clear boundary of separation.
Oil and water are immiscible — oil is non-polar while water is polar, and "like dissolves like". However hard the two are shaken, the oil soon separates and floats as a distinct layer on top, because oil is less dense than water. Since two separate phases with a visible boundary are present, the mixture is heterogeneous.
The other three choices are all true solutions — uniform, transparent, with particles smaller than 1 nm that never settle and cannot be filtered out.
❌ Why the other options are incorrect
✗ Tincture of iodine — a homogeneous solution of iodine dissolved in alcohol; a solid-in-liquid solution used as an antiseptic.
✗ Lemonade — a homogeneous solution of sugar, salt and lemon juice in water; the dissolved substances are uniformly spread and invisible.
✗ Mixture of ethanol in water — ethanol and water are completely miscible in all proportions (both are polar and form hydrogen bonds), giving a single uniform liquid.
📌 SSC & Railway Exam Facts
• Homogeneous: true solutions, alloys, air. Heterogeneous: suspensions, colloids, oil + water, sand + water.
• Tincture of iodine = iodine in alcohol; aerated drink = CO₂ gas in water; air = gas in gas.
• Immiscible liquids (oil–water, kerosene–water) are separated using a separating funnel.
• "Like dissolves like" — polar solutes dissolve in polar solvents, non-polar in non-polar.
• Oil floats on water because its density is lower; a soap or detergent forms an emulsion between the two.

Correct Answer: Option 1 — Orange juice with pulp
A homogeneous mixture has the same composition and properties throughout, and its components cannot be distinguished by the eye. A heterogeneous mixture does not — its components form visibly separate parts.
Orange juice with pulp contains solid pulp particles that are clearly visible, do not dissolve in the juice, and settle to the bottom if the glass is left standing. Two distinct phases are present, so it is a heterogeneous mixture (a suspension) — hence the odd one out.
The other three are all homogeneous: their components are mixed at the molecular level and no boundary can be seen.
❌ Why the other options are incorrect
✗ Sugar solution — a classic true solution; sugar dissolves completely in water, giving particles < 1 nm that never settle, so it is homogeneous.
✗ Vinegar — a homogeneous solution of about 5–8% acetic acid (ethanoic acid, CH₃COOH) in water; it is uniform and transparent throughout.
✗ Pure Air — a homogeneous mixture of gases (N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.04%); pure, dust-free air has a uniform composition. (Air carrying smoke or dust would be heterogeneous — hence the word "Pure".)
📌 SSC & Railway Exam Facts
• Homogeneous: sugar/salt solution, vinegar, air, alloys (brass, bronze, steel).
• Heterogeneous: pulpy juice, muddy water, sand + salt, oil + water, milk and other colloids.
• Vinegar = dilute ethanoic acid, CH₃COOH; glacial acetic acid freezes at about 17 °C.
• Composition of air: nitrogen ≈ 78%, oxygen ≈ 21%, argon ≈ 0.93%, carbon dioxide ≈ 0.04%.
• All solutions are homogeneous, but a mixture may be either homogeneous or heterogeneous.