RRB Technician Grade III 2024-25 23 December 2024 Shift 1 GK Questions – with answers & solutions

Correct Answer: Option 1 — Rhizopus
Rhizopus is a simple multicellular fungus that grows as a cottony mass of thread-like hyphae on moist bread. Some hyphae grow upright and their tips swell into tiny knob-like structures called sporangia. Inside each sporangium, spores are produced by spore formation, a method of asexual reproduction.
Each spore is covered by a thick protective wall that lets it survive drought and unfavourable conditions. When the sporangium bursts, the spores are carried by air, and on reaching a moist surface each spore germinates into a new Rhizopus.
❌ Why the other options are incorrect
✗ Yeast — a unicellular fungus that reproduces by budding; it has no hyphae and forms no sporangia.
✗ Amoeba — a unicellular protozoan that reproduces by binary fission, splitting into two daughter cells in any plane.
✗ Leishmania — a unicellular flagellated protozoan (causes kala-azar) that divides by binary fission in a definite longitudinal orientation.
📌 SSC & Railway Exam Facts
• Spore formation — Rhizopus, *Mucor*, ferns and mosses; Budding — Hydra and yeast.
• Binary fission — Amoeba, Paramecium, Leishmania; Multiple fission — Plasmodium.
• Fragmentation — Spirogyra; Regeneration — Planaria and Hydra.
• Rhizopus is commonly called bread mould; the black dots seen on stale bread are its sporangia.
• Asexual reproduction involves only one parent and produces genetically identical clones.

Correct Answer: Option 1 — Rhizopus
Rhizopus stolonifer is commonly called bread mould because it is the whitish, cottony fungus that grows on moist bread kept in the open for a few days. Its body is made of thread-like hyphae that together form a network called the mycelium.
Rhizopus is a saprophyte — it secretes digestive enzymes onto the bread, breaks the food down outside its body and then absorbs the dissolved nutrients. This is called saprophytic nutrition. It reproduces asexually by spore formation inside sporangia borne on upright hyphae, and the black colour of stale bread comes from these sporangia.
❌ Why the other options are incorrect
✗ Yeast — a unicellular fungus used in baking and brewing; it causes fermentation, but it is not called bread mould.
✗ Spirogyra — a green, filamentous freshwater alga (pond silk) that reproduces by fragmentation; it is not a fungus.
✗ Planaria — a flatworm (phylum Platyhelminthes) famous for regeneration; it is an animal, not a mould.
📌 SSC & Railway Exam Facts
• Fungi have chitin in their cell walls and store food as glycogen.
• Saprophytes — Rhizopus, mushroom, bread mould; Parasites — Cuscuta (amarbel), *Plasmodium*, tapeworm.
• Penicillium gave the first antibiotic Penicillin, discovered by Alexander Fleming (1928).
• Yeast (*Saccharomyces cerevisiae*) converts sugar into alcohol and CO₂, which makes bread rise.
• Lichen is a symbiotic association of a fungus and an alga; mycorrhiza is that of a fungus and plant roots.

Correct Answer: Option 3 — Neuron
Nervous tissue is made of an organised network of nerve cells called neurons, and the neuron is the unit specialised for conducting information via electrical impulses from one part of the body to another.
A neuron has three parts — a cell body (cyton) containing the nucleus, short branched dendrites that receive the stimulus, and a long axon that carries the impulse away. At the end of the axon the electrical signal is converted into a chemical signal (neurotransmitter) which crosses the synapse to the next neuron or to a muscle cell. The neuron is the longest cell in the human body.
❌ Why the other options are incorrect
✗ Alveoli — tiny balloon-like air sacs of the lungs where exchange of gases (O₂ and CO₂) between air and blood takes place.
✗ Glomerulus — a bunch of blood capillaries inside the Bowman's capsule of a nephron where filtration of blood occurs.
✗ Spinal cord — an organ made of nervous tissue that relays impulses and controls reflex actions; it is not the specialised cell asked about.
📌 SSC & Railway Exam Facts
• The four animal tissues are epithelial, connective, muscular and nervous.
• Nervous tissue forms the brain, spinal cord and nerves.
• A synapse is the junction between two neurons; a neuromuscular junction is that between a neuron and a muscle.
• Impulse travel is electrical along the axon and chemical across the synapse.
• Neurons do not normally divide; neuroglia are the supporting cells of nervous tissue.

Correct Answer: Option 4 — lactic acid
Respiration begins in the cytoplasm with glycolysis, in which one molecule of glucose (6 C) is broken down into two molecules of pyruvate (3 C). What happens next depends on the supply of oxygen.
During vigorous exercise, the demand for energy in our muscle cells rises faster than oxygen can be supplied. In this lack of oxygen, pyruvate is converted into lactic acid (3 C) by anaerobic respiration, releasing only a little energy. The accumulation of lactic acid in the muscles causes cramps and muscular pain, which are relieved by rest, massage and a hot-water bath, as the extra oxygen then oxidises the lactic acid.
❌ Why the other options are incorrect
✗ acetic acid — the acid of vinegar, produced by Acetobacter bacteria; it is not the product of anaerobic respiration in muscles.
✗ methyl alcohol — methanol, a poisonous industrial alcohol that causes blindness and death; it is not formed in the body.
✗ ethyl alcohol — formed with CO₂ during anaerobic respiration in yeast (fermentation), not in human muscle cells.
📌 SSC & Railway Exam Facts
• Glycolysis occurs in the cytoplasm; the Krebs cycle occurs in the mitochondria.
• Aerobic respiration gives CO₂ + H₂O + much energy (38 ATP); anaerobic gives little energy (2 ATP).
• Yeast → ethanol + CO₂; muscle cells → lactic acid; green plants → CO₂ + H₂O.
• ATP is the energy currency of the cell; mitochondria are the powerhouse.
• The breakdown of glucose into pyruvate is common to all three pathways of respiration.

Correct Answer: Option 4 — sclerenchyma
Sclerenchyma is a simple permanent tissue that makes the plant hard and stiff. Its cells are long, narrow and dead, with thick walls uniformly deposited with lignin — a chemical that acts like cement and glues the cells together. There are no intercellular spaces between them.
Because the cells are dead and heavily lignified, sclerenchyma gives mechanical strength and rigidity to plant parts. It is found in the stems, around vascular bundles, in the veins of leaves, and in the hard covering of nuts and seeds. The husk of coconut is made of sclerenchymatous tissue.
❌ Why the other options are incorrect
✗ collenchyma — living cells thickened at the corners with cellulose and pectin; it provides flexibility, allowing tendrils and climber stems to bend without breaking.
✗ chlorenchyma — parenchyma containing chlorophyll; its function is photosynthesis, not support.
✗ aerenchyma — parenchyma with large air cavities; found in aquatic plants, it gives buoyancy so that they float.
📌 SSC & Railway Exam Facts
• Simple permanent tissues — parenchyma, collenchyma and sclerenchyma.
• Parenchyma is the basic packing tissue, living, with thin cellulose walls and large intercellular spaces.
• Jute, coir and hemp fibres come from sclerenchyma.
• Lignin is the chemical that makes sclerenchyma walls hard; sclerenchyma cells are dead.
• Complex permanent tissues are xylem (water) and phloem (food); meristematic tissue causes growth.

Correct Answer: Option 3 — water
Plants have no muscles and no nervous system, so they move their parts by changing the amount of water in their cells. When water enters a cell it becomes turgid (swells), and when water leaves it becomes flaccid (shrinks). This change in turgor pressure changes the shape of the cell and hence the position of the plant part.
The classic example is the sensitive plant, *Mimosa pudica* (touch-me-not). On being touched, water moves out of the cells at the base of the leaflets, those cells shrink, and the leaves fold up and droop immediately. This is a nastic (non-directional) movement, independent of growth. The opening and closing of stomata by guard cells works on exactly the same principle.
❌ Why the other options are incorrect
✗ chlorophyll — the green pigment in chloroplasts that traps sunlight for photosynthesis; its amount does not swell or shrink cells.
✗ fat — a stored food (energy reserve) in seeds and in animal adipose tissue; it plays no role in changing cell shape.
✗ carbon dioxide — the raw material taken in through stomata for photosynthesis; it is a gas, not the cell's shape-controlling content.
📌 SSC & Railway Exam Facts
• Plant movements are nastic (touch-me-not, independent of growth) and tropic (growth-dependent, directional).
• Tropic movements — phototropism (light), geotropism (gravity), hydrotropism (water), chemotropism (chemicals).
• Auxin promotes cell elongation; gibberellin causes stem growth; cytokinin promotes cell division.
• Abscisic acid (ABA) is the stress/inhibitor hormone — it closes stomata and causes wilting.
• Loss of turgor pressure in herbaceous plants causes wilting; excess water loss from a cell in a strong salt solution causes plasmolysis.

Correct Answer: Option 3 — Only (i)
Non-biodegradable substances are materials that cannot be broken down by decomposers such as bacteria and fungi — for example plastic, polythene, glass, metal and DDT.
Checking each statement:
• (i) is correct — such substances may be inert and persist for a long time, or they may harm the various members of the ecosystem, choking drains, killing cattle and marine life, and entering food chains.
• (ii) is wrong — it is biodegradable substances that are broken down by biological processes; non-biodegradable ones are not.
• (iii) is wrong — these substances do persist in the environment, often for hundreds of years.
Hence only statement (i) is correct.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — statements (ii) and (iii) contradict the very definition of a non-biodegradable substance.
✗ Only (i) and (ii) — (ii) wrongly says non-biodegradable substances are broken down biologically.
✗ Only (ii) and (iii) — both statements are false, and this option also leaves out the true statement (i).
📌 SSC & Railway Exam Facts
• Biodegradable — vegetable peels, paper, cotton, cow dung, leaves; Non-biodegradable — plastic, glass, metal, DDT.
• Decomposers (bacteria and fungi) are also called saprophytes; they recycle nutrients back to the soil.
• Increase of harmful chemicals at successive trophic levels is biomagnification.
• Waste management follows the 4 R's — Refuse, Reduce, Reuse, Recycle.
• The term ecosystem was coined by A. G. Tansley (1935); ocean is the largest ecosystem.

Correct Answer: Option 4 — geotropism
Geotropism (gravitropism) is the directional growth movement of a plant part in response to the stimulus of gravity. The root grows towards gravity, i.e. downwards — this is positive geotropism; the shoot grows away from gravity, i.e. upwards — this is negative geotropism.
The response is controlled by the plant hormone auxin, which collects on the lower side of a horizontally placed seedling. In the stem, auxin promotes cell elongation, so the lower side grows faster and the shoot curves upward; in the root, a high concentration of auxin inhibits elongation, so the upper side grows faster and the root curves downward.
❌ Why the other options are incorrect
✗ chemotropism — growth towards a chemical, e.g. the pollen tube growing down the style towards the ovule.
✗ hydrotropism — growth of roots towards water (moisture) in the soil; the stimulus here is water, not gravity.
✗ phototropic movement — growth in response to light; shoots are positively phototropic and roots negatively phototropic.
📌 SSC & Railway Exam Facts
• Tropic (directional) movements — phototropism, geotropism, hydrotropism, chemotropism, thigmotropism (touch, as in tendrils).
• Nastic (non-directional) movements — the folding of Mimosa pudica (touch-me-not) leaves.
• Auxin was the first plant hormone discovered; F. W. Went isolated it.
• Gibberellin — stem elongation; Cytokinin — cell division; Abscisic acid — inhibits growth, causes wilting.
• Ethylene is the only gaseous plant hormone and is used for ripening of fruits.

Correct Answer: Option 2 — coconut husk
The husk of a coconut is made of sclerenchyma, a simple permanent tissue whose cells are long, narrow and dead. Their walls are uniformly thickened with lignin, which cements the cells together and leaves no intercellular spaces. Since the cell walls are so thick, the living contents (protoplasm) die off — hence the husk is a mass of dead tissue.
This is exactly why the husk is hard, tough and fibrous: it gives mechanical strength and protects the seed inside. The coir used for ropes, mats and brushes is obtained from this sclerenchymatous husk.
❌ Why the other options are incorrect
✗ orange peel — a living tissue made mainly of parenchyma with oil glands; it is soft and juicy, not dead.
✗ banana peel — soft, living parenchymatous tissue that protects the fruit and even continues to respire after plucking.
✗ guava peel — the living outer layer of the fruit, made of thin-walled parenchyma and epidermal cells with a few stone cells.
📌 SSC & Railway Exam Facts
• Dead plant tissues — sclerenchyma, xylem vessels and tracheids, xylem fibres and cork.
• Living plant tissues — parenchyma, collenchyma, phloem (except phloem fibres) and all meristems.
• Sclereids (stone cells) give the gritty texture of pear and the hardness of nut shells.
• Cork (phellem) cells are dead and contain suberin, making them impervious to water and gases.
• Jute, coir and hemp are commercially important sclerenchymatous fibres.

Correct Answer: Option 3 — blue colour of eyes
The blue colour of the eyes is an inherited trait decided by the genes received from the parents — it is present from birth and has nothing to do with sexual maturation. All the other choices are genuine signs of puberty.
Puberty is the age (about 10–14 years) at which the reproductive organs mature and secondary sexual characters appear, under the control of hormones — testosterone from the testes in boys and oestrogen from the ovaries in girls, both triggered by the pituitary gland.
❌ Why the other options are incorrect
✗ different patterns of hair growth — thick hair in the armpits and genital area, and a beard and moustache in boys, are classic signs of puberty.
✗ increased size and shape of breast or penis — enlargement of the breasts in girls and of the penis in boys is a direct sign of sexual maturation.
✗ girls begin to menstruate at around this time — the first menstruation (menarche) marks the start of the reproductive phase in girls.
📌 SSC & Railway Exam Facts
• Secondary sexual characters are controlled by testosterone (male) and oestrogen (female).
• The menstrual cycle is of about 28 days; menopause is its permanent stopping at about 45–50 years.
• Fertilisation in humans occurs in the fallopian tube (oviduct); the embryo implants in the uterus.
• The placenta supplies the foetus with nutrition and oxygen and removes its wastes; gestation lasts about 9 months (280 days).
• Eye colour, hair colour and blood group are hereditary traits, fixed by genes at the time of fertilisation.

Correct Answer: Option 1 — Tall plant and round seed
Gregor Johann Mendel, the Father of Genetics, worked on the garden pea (*Pisum sativum*) and studied seven pairs of contrasting characters. When he crossed a pure tall plant with a pure dwarf plant, all the plants of the F₁ generation were tall — the dwarf character had disappeared. Likewise, a cross between round and wrinkled seeds gave only round seeds in F₁.
The character that expresses itself in the F₁ hybrid is called the dominant trait, and the one that stays hidden is the recessive trait. So tallness (T) and round seed (R) are dominant, while dwarfness (t) and wrinkled seed (r) are recessive. In the F₂ generation the hidden trait reappears in the ratio 3 : 1.
❌ Why the other options are incorrect
✗ Short plant and wrinkled seed — both are recessive traits; they reappear only in F₂, in one-fourth of the plants.
✗ Tall plant and wrinkled seed — tallness is dominant, but wrinkled seed is recessive, so the pair is wrong.
✗ Short plant and round seed — round seed is dominant, but shortness (dwarfness) is recessive, so the pair is wrong.
📌 SSC & Railway Exam Facts
• Mendel's monohybrid cross gives a phenotypic ratio 3 : 1 and a genotypic ratio 1 : 2 : 1.
• The dihybrid ratio is 9 : 3 : 3 : 1, which gave the Law of Independent Assortment.
• Mendel's laws — Law of Dominance, Law of Segregation (Purity of Gametes) and Law of Independent Assortment.
• Other dominant traits in pea — green pod, axial flower, violet flower; recessive — yellow pod, terminal flower, white flower.
• The word gene was coined by Johannsen; genetics by W. Bateson.

Correct Answer: Option 2 — Gastric glands
The gastric glands are present in the wall of the stomach and secrete the gastric juice, which has three components:
• Hydrochloric acid (HCl) — makes the medium acidic, kills germs entering with food and activates the enzyme.
• Pepsin — a protein-digesting enzyme that works only in an acidic medium and breaks proteins into peptones.
• Mucus — a slimy layer that protects the inner lining of the stomach from the corrosive action of its own acid.
If mucus is not secreted properly, the acid injures the stomach wall and causes acidity and peptic ulcers.
❌ Why the other options are incorrect
✗ Gall bladder — only stores and concentrates bile made by the liver; bile is alkaline and has no enzymes.
✗ Salivary glands — secrete saliva containing salivary amylase (ptyalin), which digests starch in the mouth in an alkaline medium.
✗ Pancreas — secretes pancreatic juice with trypsin (proteins), amylase (starch) and lipase (fats); it also makes insulin.
📌 SSC & Railway Exam Facts
• Gastric juice is acidic (pH about 1.5–2); HCl is secreted by the oxyntic (parietal) cells.
• Pepsin is secreted in an inactive form called pepsinogen, activated by HCl.
• The liver is the largest gland in the body; the pancreas is a mixed gland (exocrine + endocrine).
• Bile does not digest, it only emulsifies fats and makes the medium alkaline for pancreatic enzymes.
• Absorption of digested food occurs mainly in the small intestine through the villi.

Correct Answer: Option 1 — Stomata
The epidermis is the outermost protective layer of a plant. In the epidermis of a leaf there are small pores called stomata (singular: stoma). Each stoma is enclosed by two kidney-shaped guard cells, which contain chloroplasts and swell or shrink with water to open or close the pore.
Stomata perform two vital functions — the exchange of gases (CO₂ and O₂) with the atmosphere for photosynthesis and respiration, and the loss of water vapour by transpiration, which creates the pull that draws water up the plant.
❌ Why the other options are incorrect
✗ Aerenchyma — parenchyma with large air cavities, found in aquatic plants to give them buoyancy; it lies in the ground tissue, not the epidermis.
✗ Vascular bundle — the xylem and phloem strand that conducts water and food; in a leaf it forms the veins, deep inside the mesophyll.
✗ Chlorenchyma — chlorophyll-containing parenchyma of the mesophyll that carries out photosynthesis; it lies between the two epidermal layers.
📌 SSC & Railway Exam Facts
• In desert plants the epidermis has a thick waxy cuticle to check water loss.
• Stomata are usually more numerous on the lower surface of a dorsiventral (dicot) leaf.
• Guard cells open the stoma when they become turgid and close it when they become flaccid.
• Root hairs are extensions of epidermal cells that absorb water and minerals.
• In older stems, the epidermis is replaced by cork (phellem), whose dead cells contain suberin; gas exchange then occurs through lenticels.

Correct Answer: Option 4 — Only (ii) and (iii)
Ozone (O₃) is a molecule of three oxygen atoms that forms a protective shield in the stratosphere.
Checking each statement:
• (i) is wrong — UV radiation is highly damaging to organisms; it causes skin cancer, cataract and damage to DNA.
• (ii) is correct — high up in the atmosphere, UV radiation splits an oxygen molecule (O₂) into free oxygen (O) atoms, which then combine with O₂ to form ozone (O₃).
• (iii) is correct — the amount of ozone in the atmosphere began to drop sharply in the 1980s because of synthetic chemicals, mainly CFCs.
Hence only (ii) and (iii) are correct.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — statement (i) is false; UV rays are seriously harmful to living beings.
✗ Only (i) — (i) is the only statement that is actually wrong, so this option is the exact opposite of the truth.
✗ Only (i) and (ii) — it wrongly includes (i) and wrongly leaves out the true statement (iii).
📌 SSC & Railway Exam Facts
• Ozone is formed high in the stratosphere but is a harmful pollutant in the troposphere.
• CFCs (Chlorofluorocarbons), used in refrigerators, ACs and fire extinguishers, are the main ozone-depleting substances.
• The ozone hole over Antarctica was reported in 1985.
• The Montreal Protocol (1987) was signed to freeze and phase out CFC production.
• 16 September is observed as World Ozone Day; ozone thickness is measured in Dobson Units (DU).

Correct Answer: Option 4 — protons
The atomic number (Z) of an element is defined as the number of protons present in the nucleus of one atom of that element. It is the element's identity card: change the number of protons and the element itself changes. The idea was established experimentally by Henry Moseley (1913), and the modern periodic law states that the properties of elements are a periodic function of their atomic number.
In a neutral atom the number of electrons equals the number of protons, so the atomic number also tells us the number of electrons. But the defining quantity is the proton count, because an ion such as Na⁺ or Cl⁻ has a different number of electrons and yet is still the same element.
Examples: H = 1, C = 6, N = 7, O = 8, Na = 11, P = 15, Ca = 20.
❌ Why the other options are incorrect
✗ both protons and neutrons — protons plus neutrons gives the mass number (A), also called the number of nucleons, not the atomic number.
✗ electrons — the electron count equals Z only in a neutral atom; in ions it is different, so electrons cannot define the element.
✗ neutrons — the number of neutrons can vary within the same element; atoms differing only in neutrons are isotopes (e.g. ₆C¹² and ₆C¹⁴) and still have the same atomic number.
📌 SSC & Railway Exam Facts
• Atomic number (Z) = number of protons = number of electrons in a neutral atom.
• Mass number (A) = protons + neutrons; therefore number of neutrons = A − Z.
• An element is written as ZX^A, e.g. ₁₇Cl³⁵.
• Modern periodic law (Moseley) — properties are a periodic function of atomic number; Mendeleev had used atomic mass.
• Isotopes: same Z, different A. Isobars: same A, different Z. Isotones: same number of neutrons.

Correct Answer: Option 1 — CaO
A chemical formula is written by the criss-cross method, using the valency of each ion. Calcium is a Group 2 metal with valency 2, forming Ca²⁺; oxygen is a Group 16 non-metal with valency 2, forming the oxide ion O²⁻.
Criss-crossing the valencies gives Ca₂O₂, and dividing both subscripts by their HCF (2) gives the simplest formula, CaO. Chemically it makes perfect sense: one Ca²⁺ ion exactly balances one O²⁻ ion, so the compound is electrically neutral in a 1 : 1 ratio.
Calcium oxide (CaO) is the white solid commonly called quicklime or burnt lime. It is made by the calcination of limestone, CaCO₃ → (heat) CaO + CO₂, and it reacts vigorously with water to give slaked lime, CaO + H₂O → Ca(OH)₂, releasing a large amount of heat.
❌ Why the other options are incorrect
✗ Ca₂O₃ — this would require calcium to have a valency of 3; calcium belongs to Group 2 and is always divalent.
✗ CaO₂ — this is the formula of calcium peroxide, which contains the peroxide ion (O₂²⁻) and not the ordinary oxide ion; it is a different compound altogether.
✗ Ca₂O — this would mean calcium has a valency of 1, which is wrong, and the formula would not be electrically balanced.
📌 SSC & Railway Exam Facts
• CaO = quicklime; Ca(OH)₂ = slaked lime; CaCO₃ = limestone/marble/chalk; CaSO₄·2H₂O = gypsum.
• CaCO₃ → (heat) CaO + CO₂ is a decomposition reaction; CaO + H₂O → Ca(OH)₂ is an exothermic combination reaction.
• Limewater is a solution of Ca(OH)₂; it turns milky with CO₂ because insoluble CaCO₃ is formed.
• Valency: Group 1 = 1, Group 2 = 2, Group 13 = 3, Group 16 = 2, Group 17 = 1; noble gases = 0.
• The same criss-cross rule gives Na₂O, MgO, Al₂O₃, CO₂ and SO₂.

Correct Answer: Option 3 — Diatomic and tetra-atomic
Atomicity is the number of atoms present in one molecule of an element. On this basis elements are called monoatomic (1), diatomic (2), triatomic (3), tetra-atomic (4) or polyatomic.
Oxygen normally exists as the molecule O₂, which contains two atoms, so its atomicity is 2 — diatomic. Phosphorus, in its common white/yellow form, exists as the molecule P₄, a tetrahedral cluster of four atoms, so its atomicity is 4 — tetra-atomic.
Other standard values worth memorising: He, Ne, Ar and all noble gases — monoatomic; H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ — diatomic; O₃ (ozone) — triatomic; S₈ (sulphur) — polyatomic (octa-atomic).
❌ Why the other options are incorrect
✗ Diatomic and triatomic — oxygen is right, but phosphorus exists as P₄, not P₃; triatomic describes ozone, O₃.
✗ Diatomic and diatomic — phosphorus does not exist as P₂ under ordinary conditions; its stable molecule is P₄.
✗ Diatomic and monoatomic — monoatomic elements are the noble gases (He, Ne, Ar, Kr, Xe, Rn) and metals in the vapour state, certainly not phosphorus.
📌 SSC & Railway Exam Facts
• Atomicity = number of atoms in one molecule of an element.
• Monoatomic: noble gases; diatomic: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂; triatomic: O₃; tetra-atomic: P₄; polyatomic: S₈.
• Ozone (O₃) is an allotrope of oxygen and shields the Earth from harmful ultraviolet rays.
• White phosphorus (P₄) is stored under water because it catches fire spontaneously in air.
• Allotropes of carbon: diamond, graphite and fullerene (C₆₀); of sulphur: rhombic and monoclinic.

Correct Answer: Option 1 — Solids have indistinct boundaries.
The question asks which statement is NOT true of solids. In a solid the particles are packed very close together, the intermolecular force of attraction is the strongest and the intermolecular space is the least, so the particles can only vibrate about fixed positions and cannot move from place to place.
This rigid arrangement gives solids their standard properties: a definite shape, a fixed volume, distinct (sharp) boundaries, negligible compressibility, a high density and rigidity. A solid therefore always has a distinct boundary, and the statement "Solids have indistinct boundaries" is false — which is exactly what the question is asking for.
It is gases that have no distinct boundary at all: a gas spreads out and completely fills whatever container it is placed in.
❌ Why the other options are incorrect (they ARE true of solids)
✗ Solids have a definite shape — true; the strong intermolecular forces lock the particles in fixed positions, so the shape does not change with the container.
✗ Solids have negligible compressibility — true; the intermolecular space is already at a minimum, so a solid can hardly be squeezed any further.
✗ Solids have fixed volume — true; a solid keeps both its shape and its volume unless it is heated or a large force is applied.
📌 SSC & Railway Exam Facts
• Solids: definite shape, definite volume, distinct boundary, high density, rigid, negligible compressibility.
• Liquids have a definite volume but no definite shape; gases have neither shape nor volume fixed.
• Intermolecular force: solid > liquid > gas; intermolecular space: gas > liquid > solid.
• A rubber band changes shape and a sponge can be compressed, yet both are still solids.
• Sublimation — a solid changing directly into vapour: camphor, naphthalene, iodine, ammonium chloride and dry ice.

Correct Answer: Option 2 — Solids have a definite shape, distinct boundary and fixed volume.
In a solid the particles are packed extremely close together. The intermolecular force of attraction is maximum and the intermolecular space is minimum, so each particle can only vibrate about its own fixed position and cannot travel from place to place.
Because of this rigid arrangement a solid has:
• a definite shape, which does not change with the container,
• a fixed volume,
• a distinct (sharp) boundary,
• negligible compressibility and a high density,
• rigidity — a large force is needed to change its shape.
All three properties listed in Option 2 — definite shape, distinct boundary and fixed volume — therefore belong together to the solid state, and that makes it the only true statement here.
❌ Why the other options are incorrect
✗ Solids do not have a definite shape, distinct boundary and fixed volume — this actually describes a gas, which fills its container completely and has no boundary of its own.
✗ Solids have only definite shape, but not fixed volume — a solid's volume is fixed as well; a substance with a fixed volume but no fixed shape is a liquid.
✗ Solids have only definite shape, but not distinct boundary — a sharp boundary is the most obvious feature of a solid; only gases lack a distinct boundary.
📌 SSC & Railway Exam Facts
• Solid — fixed shape + fixed volume; liquid — fixed volume only; gas — neither fixed.
• Density: solid > liquid > gas; compressibility: gas > liquid > solid.
• Diffusion is fastest in gases and slowest (almost nil) in solids.
• Solid → liquid = melting (fusion); liquid → gas = vaporisation; solid → gas = sublimation.
• Latent heat of fusion of ice = 3.34 × 10⁵ J/kg; latent heat of vaporisation of water = 22.5 × 10⁵ J/kg.

Correct Answer: Option 1 — Air
A mixture contains two or more substances mixed in any proportion, with no chemical bond between them. Each component keeps its own properties, and the components can be separated by physical methods such as filtration, distillation or evaporation.
Air is the standard example of a homogeneous mixture of gases — roughly 78% nitrogen, 21% oxygen, about 0.93% argon and 0.03–0.04% carbon dioxide, together with water vapour, dust and traces of other gases. These are not chemically combined: the proportion varies from place to place, each gas keeps its own properties, and the gases are separated industrially by the fractional distillation of liquid air.
❌ Why the other options are incorrect
✗ Water (H₂O) — a compound: hydrogen and oxygen are chemically combined in a fixed ratio (2 : 1 by atoms, 1 : 8 by mass) and can be separated only by electrolysis, a chemical method.
✗ Salt (NaCl) — also a compound, formed by an ionic bond between Na⁺ and Cl⁻ in a fixed ratio; it shows neither the properties of sodium metal nor those of chlorine gas.
✗ Iron (Fe) — a pure element, made of only one kind of atom; an element can be neither a mixture nor a compound.
📌 SSC & Railway Exam Facts
• Matter is classified into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous).
• Air is a homogeneous mixture: N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.03–0.04%.
• A mixture's components keep their own properties and are separated physically; a compound needs chemical separation.
• Brass (Cu + Zn), bronze (Cu + Sn) and steel (Fe + C) are alloys, i.e. homogeneous mixtures.
• Liquid air is separated by fractional distillation; the most abundant gas in the atmosphere is nitrogen.

Correct Answer: Option 2 — Propanone and ethanol
An IUPAC name is built as root word + suffix, where the root tells the number of carbon atoms and the suffix tells the functional group.
CH₃COCH₃ has 3 carbon atoms (root prop-) and a ketone group (>C=O) sitting in the middle of the chain, whose suffix is -one. Its IUPAC name is therefore propanone; its common name is acetone, the familiar nail-polish remover and laboratory solvent.
C₂H₅OH has 2 carbon atoms (root eth-) and an alcohol group (–OH), whose suffix is -ol. Its IUPAC name is therefore ethanol; its common name is ethyl alcohol, the alcohol present in all alcoholic drinks.
❌ Why the other options are incorrect
✗ Propanone and methanol — the ketone is right, but methanol (CH₃OH) has only one carbon atom, while C₂H₅OH has two, so it must be ethanol. Methanol is highly poisonous and can cause blindness.
✗ Ethanol and propanone — both names are correct in themselves, but their order is reversed; the question asks for the two names "respectively".
✗ Propanoic acid and ethanol — propanoic acid is CH₃CH₂COOH, which carries a carboxylic acid group (–COOH), not a ketone group, so the first name is wrong.
📌 SSC & Railway Exam Facts
• Root words: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6) carbon atoms.
• Suffixes: -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid); -ane/-ene/-yne for single/double/triple bonds.
• CH₃COCH₃ = propanone (acetone); C₂H₅OH = ethanol; CH₃COOH = ethanoic acid (acetic acid).
• Vinegar is a 5–8% solution of ethanoic acid in water; pure ethanoic acid freezes in winter and is called glacial acetic acid.
• Denatured alcohol is ethanol made unfit for drinking by adding methanol and a dye.

Correct Answer: Option 4 — substitution reaction
Alkanes (CnH₂n₊₂) are saturated hydrocarbons — every carbon atom is joined to its neighbours by single bonds only, so the molecule is already "full" and has no room to take on extra atoms. The only way a new atom can enter is by replacing an atom already present, and a reaction of that kind is called a substitution reaction.
In the presence of sunlight (ultraviolet light), chlorine replaces the hydrogen atoms of an alkane one at a time.
⚗️ The Reaction
CH₄ + Cl₂ → (sunlight) CH₃Cl + HCl
• One hydrogen atom of methane is replaced by a chlorine atom — that is substitution.
• The reaction needs sunlight/UV and goes on stepwise to CH₂Cl₂, CHCl₃ (chloroform) and finally CCl₄.
• Because alkanes are saturated, they are generally unreactive and give substitution, never addition.
❌ Why the other options are incorrect
✗ rearrangement reaction — here the atoms of a molecule simply reorganise into an isomer; no new atom enters the molecule, which is not what chlorination does.
✗ both addition and substitution chain reaction — chlorination of an alkane is a substitution chain reaction only; there is no addition step, because a saturated alkane has no double or triple bond to add across.
✗ addition reaction — addition takes place across a double or triple bond, that is, in unsaturated compounds (alkenes and alkynes), e.g. ethene + H₂ → ethane in the presence of a nickel catalyst.
📌 SSC & Railway Exam Facts
• Saturated hydrocarbons (alkanes) give substitution reactions; unsaturated ones (alkenes, alkynes) give addition reactions.
• CH₄ + Cl₂ → (sunlight) CH₃Cl + HCl is the classic substitution reaction of the syllabus.
• Hydrogenation of vegetable oil with a nickel catalyst gives vanaspati ghee — an addition reaction.
• General formulas: alkane CnH₂n₊₂, alkene CnH₂n, alkyne CnH₂n₋₂.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones burn with a yellow sooty flame.

Correct Answer: Option 1 — Potassium
The reactivity series (activity series) arranges metals in decreasing order of their reactivity, that is, of their tendency to lose electrons and form positive ions:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au
Potassium (K) stands at the very top of this series, so among the four options it is the most reactive metal. It reacts so violently with cold water that the hydrogen produced catches fire at once, and for this reason potassium is stored under kerosene.
The practical use of the series is simple: a metal higher in the list can displace any metal lower down from its salt solution — for example Zn + CuSO₄ → ZnSO₄ + Cu.
❌ Why the other options are incorrect
✗ Zinc — a moderately reactive metal placed below aluminium; it reacts with steam, not with cold water, and is far less reactive than potassium.
✗ Sodium — very reactive indeed and placed just below potassium, so it is highly reactive but not the most reactive among those given.
✗ Gold — the least reactive metal of all; it is found in the native (free) state and dissolves only in aqua regia.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au.
• Reactivity of metals increases down Group 1 and decreases across a period from left to right.
• K and Na are stored under kerosene; K, Na, Ca react with cold water, Mg with hot water, and Al, Zn, Fe with steam.
• Metals below hydrogen (Cu, Hg, Ag, Au) cannot displace hydrogen from dilute acids.
• A metal higher in the series displaces a lower one from its salt solution — the basis of all displacement reactions.

Correct Answer: Option 3 — Ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃)
This is the standard thermal decomposition reaction of the NCERT syllabus. Ferrous sulphate crystals, FeSO₄·7H₂O, are light green. On heating they first lose their water of crystallisation and turn into white anhydrous FeSO₄; on stronger heating the salt decomposes completely.
⚗️ The Reaction
2FeSO₄(s) → (heat) Fe₂O₃(s) + SO₂(g) + SO₃(g)
• The residue left behind is Fe₂O₃, reddish brown ferric oxide.
• Two gases with the choking smell of burning sulphur are released — SO₂ and SO₃.
• A coefficient of 2 is needed before FeSO₄, because one Fe₂O₃ unit contains two iron atoms.
• Before this step: FeSO₄·7H₂O → (heat) FeSO₄ + 7H₂O, the green crystals turning white.
❌ Why the other options are incorrect
✗ Ferric oxide (Fe₂O₃) only — incomplete; the sulphate radical also decomposes, and it must give off gaseous oxides of sulphur.
✗ Ferric oxide (Fe₂O₃) and sulphur dioxide (SO₂) only — the most common half-answer; it leaves out SO₃, which is produced as well.
✗ Ferric oxide (Fe₂O₃) and sulphur trioxide (SO₃) only — again incomplete, because SO₂ is also among the products.
📌 SSC & Railway Exam Facts
• Green vitriol = FeSO₄·7H₂O; blue vitriol = CuSO₄·5H₂O; white vitriol = ZnSO₄·7H₂O.
• Decomposition is an endothermic reaction; it may be thermal (heat), electrolytic (electricity) or photochemical (light).
• CaCO₃ → (heat) CaO + CO₂ and 2Pb(NO₃)₂ → (heat) 2PbO + 4NO₂ + O₂ (brown fumes) are the other standard decompositions.
• 2H₂O → (electricity) 2H₂ + O₂ is electrolytic; 2AgCl → (sunlight) 2Ag + Cl₂ is photochemical decomposition.
• SO₂ dissolves in rain water to cause acid rain; Fe₂O₃ is the iron ore haematite.

Correct Answer: Option 1 — interacts with hydrocarbons and interacts with water, respectively
Every soap molecule — for example sodium stearate, C₁₇H₃₅COO⁻Na⁺ — has two ends of opposite nature, and this dual nature is the whole secret of cleaning:
• The long hydrocarbon tail is hydrophobic (water-hating). Being non-polar, it interacts with oil, grease and hydrocarbons and dissolves in them.
• The ionic –COO⁻Na⁺ head is hydrophilic (water-loving). Being polar and charged, it interacts with water and dissolves in it.
The question names them in the order hydrophobic end first, hydrophilic end second, so the correct pairing is hydrocarbons, then water.
When soap is put into water carrying greasy dirt, the tails bury themselves in the oil droplet while the heads face outwards into the water, forming a sphere called a micelle. The dirt stays trapped inside the micelle and is carried away with the rinse water.
❌ Why the other options are incorrect
✗ interacts with water and interacts with acid, respectively — the order is reversed and "acid" is wrong; the hydrophilic end interacts with water, not with an acid.
✗ interacts with water and interacts with hydrocarbons, respectively — the two ends have simply been swapped; this is the commonest trap set in this question.
✗ interacts with hydrocarbons and interacts with base, respectively — the first half is right but the second is wrong; the hydrophilic head interacts with water, not with a base.
📌 SSC & Railway Exam Facts
• Hydrophobic = water-hating hydrocarbon tail; hydrophilic = water-loving ionic head.
• The spherical cluster of soap molecules that traps the dirt is called a micelle.
• Soap = sodium/potassium salt of a higher fatty acid, made by saponification: fat/oil + NaOH → soap + glycerol.
• Soap does not lather in hard water — Ca²⁺ and Mg²⁺ ions form an insoluble scum; detergents work even in hard water.
• A soap solution shows the Tyndall effect, because a solution of micelles is a colloid.

Correct Answer: Option 1 — 15
The atomic number (Z) of an element is the number of protons in the nucleus of its atom. Phosphorus (P) has 15 protons, so its atomic number is 15. A neutral phosphorus atom therefore also has 15 electrons, arranged as 2, 8, 5 (K = 2, L = 8, M = 5).
Since its outermost shell holds 5 valence electrons, phosphorus needs 3 more to complete its octet — hence its usual valency of 3, seen in PH₃ (phosphine) and P₂O₃, though it also shows a valency of 5 in P₂O₅. It belongs to Group 15, Period 3 of the modern periodic table and has a mass number of 31 (15 protons + 16 neutrons).
Phosphorus occurs mainly as white/yellow phosphorus (P₄), stored under water because it ignites in air, and as the safer red phosphorus used on matchbox strikers.
❌ Why the other options are incorrect
✗ 17 — this is the atomic number of chlorine (Cl), whose electronic configuration is 2, 8, 7.
✗ 14 — this is the atomic number of silicon (Si), the element standing just before phosphorus in Period 3.
✗ 16 — this is the atomic number of sulphur (S), the element just after phosphorus; 16 is also the number of neutrons in a P-31 atom, which makes it a tempting wrong choice.
📌 SSC & Railway Exam Facts
• Period 3 in order: Na (11), Mg (12), Al (13), Si (14), P (15), S (16), Cl (17), Ar (18).
• Phosphorus: Z = 15, A = 31, configuration 2, 8, 5; Group 15, Period 3; valency 3 (also 5).
• White phosphorus (P₄) is kept under water; red phosphorus is used in safety matches.
• Phosphorus is the P of NPK fertilisers; in the body it builds bones and teeth and forms part of ATP and DNA.
• Atomic number = protons; mass number = protons + neutrons; neutrons in P-31 = 31 − 15 = 16.

Correct Answer: Option 3 — Displacement reaction
In a displacement reaction a more reactive element pushes out (displaces) a less reactive element from its compound. Its general form is A + BC → AC + B.
Here iron (Fe) is more reactive than copper (Cu) — in the reactivity series Fe lies above Cu — so iron displaces copper from copper sulphate solution. The blue colour of the CuSO₄ solution slowly fades to the light green of FeSO₄, and a reddish-brown deposit of copper appears on the iron.
⚗️ The Reaction
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• Fe stands above Cu in the reactivity series, so it takes the sulphate radical away from copper.
• Blue CuSO₄ solution turns into light green FeSO₄ solution — the visible proof of the reaction.
• It is also a redox reaction: Fe → Fe²⁺ + 2e⁻ (oxidation) and Cu²⁺ + 2e⁻ → Cu (reduction).
❌ Why the other options are incorrect
✗ Combination reaction — there two or more substances join to give a single product, e.g. CaO + H₂O → Ca(OH)₂; here two separate products are formed.
✗ Combustion reaction — a substance burns in oxygen, giving out heat and light; in this reaction there is no oxygen and no burning at all.
✗ Double Displacement reaction — that needs two compounds to exchange their ions (AB + CD → AD + CB), e.g. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. Here one reactant is a free element (Fe), so only a single displacement takes place.
📌 SSC & Railway Exam Facts
• Displacement: A + BC → AC + B; double displacement: AB + CD → AD + CB.
• Fe + CuSO₄ → FeSO₄ + Cu and Zn + CuSO₄ → ZnSO₄ + Cu are the standard PYQ examples.
• Cu + FeSO₄ → no reaction, because copper is less reactive than iron.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au.
• An iron vessel must not be used to store copper sulphate solution, because the iron is eaten away by this very displacement reaction.

Correct Answer: Option 2 — 250 J
📖 Given
• Mass of the ball, m = 5 kg
• Height above the ground, h = 5 m
• Acceleration due to gravity, g = 10 m/s²
• To find: the energy possessed by the ball at that height (gravitational potential energy)
✍️ Formula
Potential energy, E(p) = mgh
🔢 Solution
• E(p) = 5 × 10 × 5
• E(p) = 50 × 5
• E(p) = 250 J
In plain language: the work done against gravity in lifting the 5 kg ball through 5 m is 250 joule, and exactly that much energy is stored in the ball as potential energy because of its raised position.
❌ Why the other options are incorrect
✗ 2 J — this looks like 10/5 or a stray division; it ignores the formula mgh altogether.
✗ 2.5 J — obtained by dividing instead of multiplying (5 × 5 ÷ 10); potential energy grows with height, it does not shrink.
✗ 10 J — this is just the numerical value of g, mistaken for an energy.
📌 SSC & Railway Exam Facts
• Potential energy E(p) = mgh; kinetic energy E(k) = ½mv²; SI unit of both is the joule (J).
• 1 J = 1 N m = 1 kg m² s⁻²; the joule is named after James Prescott Joule.
• If this ball were dropped, it would hit the ground with v = √(2gh) = √100 = 10 m/s, its 250 J of PE having become KE.
• Law of conservation of energy: energy can neither be created nor destroyed, only transformed from one form to another.
• Work W = F × s × cos θ; work done is zero when the force is perpendicular to the displacement (θ = 90°).

Correct Answer: Option 4 — Tyndall effect
When a beam of light passes through a medium containing colloidal particles — particles large enough to scatter light but too small to be seen — the light is scattered and the path of the beam becomes visible. This phenomenon is called the Tyndall effect, named after the scientist John Tyndall.
Familiar examples are a beam of sunlight entering a dark room through a small hole, which reveals the dust particles floating in the air, and the beam of a torch or a cinema projector seen in smoke or mist.
Scattering also explains many everyday sights: the sky looks blue because the tiny molecules of air scatter the shorter blue wavelengths most (Rayleigh scattering, scattering ∝ 1/λ⁴), and the Sun looks red at sunrise and sunset because the blue light has been scattered away over the long slanting path, leaving the red to reach our eyes.
❌ Why the other options are incorrect
✗ rainbow — a rainbow is formed by refraction, total internal reflection and dispersion of sunlight inside water droplets, not by scattering.
✗ twinkling of stars — this is caused by atmospheric refraction, because the starlight keeps bending through air layers of changing density.
✗ delay in sunset — the apparent advance sunrise and delayed sunset are also effects of atmospheric refraction, not of scattering.
📌 SSC & Railway Exam Facts
• Tyndall effect is used to distinguish a colloid from a true solution — a true solution shows no visible beam.
• Rayleigh's law: amount of scattering ∝ 1/λ⁴, so blue (shorter λ) scatters more than red.
• Danger signals and traffic stop lights are red because red has the longest wavelength and is scattered the least, so it is visible from far away.
• The sky appears black to an astronaut because there is no atmosphere in space to scatter light.
• Clouds appear white because their large water droplets scatter all wavelengths almost equally.

Correct Answer: Option 3 — 500 Pa
📖 Given
• Thrust (force acting normally on the surface), F = 50 N
• Area of the surface, A = 1000 cm²
• 1 cm² = 10⁻⁴ m², so A = 1000 × 10⁻⁴ = 0.1 m²
• To find: pressure, P
✍️ Formula
Pressure = Thrust / Area, i.e. P = F/A
🔢 Solution
• Convert the area: A = 1000 cm² = 1000 × 10⁻⁴ m² = 0.1 m²
• P = 50/0.1
• P = 500 Pa
In plain language: a force of 50 N spread over just one-tenth of a square metre presses on that surface with 500 pascal, i.e. 500 newton on every square metre.
❌ Why the other options are incorrect
✗ 5 Pa — obtained by dividing 50 N by 10 instead of by 0.1 m²; a unit-conversion error of a factor of 100.
✗ 20 Pa — this has no basis in the given data; it comes from mixing up 1000 and 50 by subtraction or a wrong ratio.
✗ 30 Pa — again unrelated to the data; the only correct route is F ÷ A with A converted into m².
📌 SSC & Railway Exam Facts
• P = F/A (or Thrust/Area); SI unit pascal (Pa), where 1 Pa = 1 N/m², named after Blaise Pascal.
• 1 m² = 10⁴ cm², so 1 cm² = 10⁻⁴ m² — this conversion decides the answer here.
• Pressure decreases as area increases — that is why a sharp knife cuts easily, a camel walks on sand, and school bags have broad straps.
• Atmospheric pressure at sea level = 1.013 × 10⁵ Pa = 1 atm ≈ 760 mm of Hg; measured by a barometer.
• Pressure in a liquid at depth h: P = hρg, and it acts equally in all directions (Pascal's law).

Correct Answer: Option 3 — Only (C)
Newton's universal law of gravitation states that every body in the universe attracts every other body with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them:
F = G m₁m₂/r², where G = 6.67 × 10⁻¹¹ N m² kg⁻².
(A) is related — the Moon revolves around the Earth because the Earth's gravitational pull supplies the centripetal force that keeps it in orbit.
(B) is related — the tides in the oceans are produced by the gravitational attraction of the Moon (helped by the Sun) on the water of the Earth.
(C) is NOT related — an electron revolves around the nucleus because of the electrostatic force of attraction between the negative electron and the positive nucleus, given by Coulomb's law. Gravitational force between them is about 10³⁹ times weaker and is completely negligible. Hence only (C) is unrelated.
❌ Why the other options are incorrect
✗ (A) and (B) — both of these are classic consequences of gravitation, so neither can be "not related".
✗ (B) and (C) — the tides in (B) are directly caused by the Moon's gravitational pull, so (B) cannot be included.
✗ Only (B) — (B) is related to gravitation; the statement that is unrelated is (C).
📌 SSC & Railway Exam Facts
• F = G m₁m₂/r²; G = 6.67 × 10⁻¹¹ N m² kg⁻², a universal constant measured by Henry Cavendish.
• g = GM/R²; g = 9.8 m/s² on the Earth, 1.63 m/s² on the Moon (about 1/6 of the Earth's).
• Gravitational force is the weakest of the four fundamental forces; the strong nuclear force is the strongest.
• Gravitation explains the motion of planets (Kepler's laws), the orbits of satellites and the tides.
• The electrostatic force follows Coulomb's law, F = kq₁q₂/r² — the same inverse-square form, but enormously stronger.

Correct Answer: Option 2 — The acceleration experienced by an object is independent of its mass.
Free fall is the motion of a body under the action of gravity alone, with air resistance neglected. The gravitational force on the body is F = mg, and by Newton's second law this force also equals F = ma. Equating them:
ma = mg, so a = g — the mass m cancels out completely.
Therefore every freely falling body, heavy or light, falls with the same acceleration g = 9.8 m/s² (often taken as 10 m/s²), directed vertically downwards. This is why a coin and a feather dropped together in an evacuated tube reach the bottom at the same instant — the famous experiment that confirms Galileo's conclusion.
❌ Why the other options are incorrect
✗ The velocity of the object remains constant during free fall — false; the velocity increases steadily at 9.8 m/s every second, since the motion is accelerated.
✗ The acceleration experienced by an object is opposite to the direction of motion — false for a falling body; g acts downwards, the same direction as the motion. It is opposite only for a body thrown upwards.
✗ The acceleration experienced by an object depends on its mass — false; the mass cancels in ma = mg, so a = g for every body.
📌 SSC & Railway Exam Facts
• In free fall: v = gt, h = ½gt², v² = 2gh (taking u = 0).
• g = 9.8 m/s² on the Earth; g = GM/R², so it is independent of the falling body's mass.
• g is maximum at the poles, minimum at the equator, and becomes zero at the centre of the Earth.
• In a freely falling lift a person feels weightlessness, because the apparent weight becomes zero.
• Galileo is said to have demonstrated at the Leaning Tower of Pisa that bodies of different masses fall together.

Correct Answer: Option 3 — 6 m/s
📖 Given
• Constant force, F = 4 N
• Mass of the box, m = 2 kg
• Time for which the force acts, t = 2 s
• Initial velocity, u = 2 m/s
• To find: final velocity, v
✍️ Formula
Newton's second law: F = ma, so a = F/m; then the first equation of motion v = u + at
🔢 Solution
• a = F/m = 4/2
• a = 2 m/s²
• v = u + at = 2 + (2 × 2)
• v = 2 + 4
• v = 6 m/s
In plain language: the 4 N force makes the box speed up by 2 m/s in every second, so in 2 s it gains 4 m/s, which added to its starting 2 m/s gives 6 m/s.
❌ Why the other options are incorrect
✗ 2 m/s — this is the initial velocity u; it would be the answer only if no force acted at all.
✗ 3 m/s — obtained by wrongly taking a = 0.5 m/s² (dividing 2 by 4 instead of 4 by 2).
✗ 4 m/s — this is only the gain in velocity (at = 4 m/s); the initial velocity of 2 m/s has been forgotten.
📌 SSC & Railway Exam Facts
• Newton's second law: F = ma = (mv − mu)/t — force equals the rate of change of momentum.
• SI unit of force is the newton (N): 1 N = 1 kg m/s², the force that gives a 1 kg mass an acceleration of 1 m/s².
• The three equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• Change of momentum here = m(v − u) = 2 × (6 − 2) = 8 kg m/s, which equals impulse = F × t = 4 × 2 = 8 N s.
• Newton's third law: every action has an equal and opposite reaction, acting on two different bodies.

Correct Answer: Option 2 — 80 J
📖 Given
• Mass of the object, m = 10 kg
• Initial velocity, u = 3 m/s
• Final velocity, v = 5 m/s
• To find: change in kinetic energy, ΔE(k)
✍️ Formula
Kinetic energy, E(k) = ½mv², so ΔE(k) = ½mv² − ½mu² = ½m(v² − u²)
🔢 Solution
• Initial KE = ½ × 10 × (3)² = 5 × 9 = 45 J
• Final KE = ½ × 10 × (5)² = 5 × 25 = 125 J
• ΔE(k) = 125 − 45
• ΔE(k) = 80 J
In plain language: the object's store of motion energy rises from 45 J to 125 J, so 80 J of work must have been done on it — this is the work–energy theorem in action.
❌ Why the other options are incorrect
✗ 50 J — obtained by using the difference of the velocities squared wrongly, e.g. ½ × 10 × (5 − 3)² × 2.5; the velocities must be squared separately.
✗ 160 J — this is double the correct value, from forgetting the factor ½ in ½m(v² − u²).
✗ 40 J — obtained by taking ½ × 10 × (5 − 3)² = 5 × 4 = 20, then doubling, or similar mis-handling; (v − u)² is not the same as v² − u².
📌 SSC & Railway Exam Facts
• E(k) = ½mv²; SI unit joule (J); kinetic energy is a scalar and is always positive.
• Work–energy theorem: work done by the net force = change in kinetic energy.
• KE is proportional to the square of the speed — doubling the speed makes the kinetic energy four times.
• Relation with momentum: E(k) = p²/2m and p = √(2mE(k)).
• Potential energy E(p) = mgh; the total mechanical energy E(k) + E(p) stays constant when only gravity acts.

Correct Answer: Option 1 — 3 × 10²¹
📖 Given
• Current through the filament, I = 1 A
• Time, t = 8 mins = 8 × 60 = 480 s
• Charge on one electron, e = 1.6 × 10⁻¹⁹ C
• To find: number of electrons, n
✍️ Formula
Q = I × t and Q = n × e, so n = Q/e = It/e
🔢 Solution
• t = 8 × 60 = 480 s
• Q = I × t = 1 × 480 = 480 C
• n = Q/e = 480/(1.6 × 10⁻¹⁹)
• n = 300 × 10¹⁹
• n = 3 × 10²¹ electrons
In plain language: a current of 1 A means 1 coulomb of charge passes every second, so in 480 seconds 480 coulomb flows, and since each electron carries only 1.6 × 10⁻¹⁹ C, about 3 × 10²¹ electrons must cross the filament.
❌ Why the other options are incorrect
✗ 3 × 10¹⁹ — a hundred times too small. This is the slip of forgetting to convert minutes into seconds properly, or of dividing by 1.6 × 10⁻¹⁷ instead of 1.6 × 10⁻¹⁹.
✗ 3 × 10²² — ten times too large, from a power-of-ten slip while dividing by 1.6 × 10⁻¹⁹.
✗ 3 × 10²⁰ — ten times too small, again a decimal-place error in the same division.
📌 SSC & Railway Exam Facts
• I = Q/t, so Q = It; SI unit of charge is the coulomb (C) and of current the ampere (A).
• Charge is quantised: Q = ne, where e = 1.6 × 10⁻¹⁹ C — the electron was discovered by J. J. Thomson.
• 1 coulomb ≈ 6.25 × 10¹⁸ electrons.
• Conventional current flows opposite to the direction of electron flow.
• Current is measured with an ammeter, connected in series in the circuit.

Correct Answer: Option 2 — 1V/1A
According to Ohm's law, the potential difference V across a conductor is directly proportional to the current I flowing through it, provided its physical conditions such as temperature remain constant:
V ∝ I, so V = IR, where R is the resistance of the conductor.
Rearranging gives R = V/I. Putting the SI units into this relation gives the definition of the ohm:
1 ohm = 1 volt / 1 ampere
So a conductor has a resistance of 1 Ω if a potential difference of 1 V across its ends drives a current of exactly 1 A through it. The unit is named after the German physicist Georg Simon Ohm.
❌ Why the other options are incorrect
✗ 1V * 1A — volt multiplied by ampere gives power in watts (P = VI), not resistance.
✗ 1V/2A — this works out to 0.5 Ω, not 1 Ω; the definition needs the current to be exactly 1 A.
✗ 1A/1V — this is the reciprocal, ampere per volt, which is the unit of conductance, the siemens (S) or mho.
📌 SSC & Railway Exam Facts
• Ohm's law: V = IR; R = V/I; SI unit ohm (Ω) = volt/ampere.
• Resistance R = ρl/A — it increases with length and decreases with area of cross-section; ρ is the resistivity.
• Resistivity of metals increases with temperature; that of semiconductors decreases.
• Conductance = 1/R, SI unit siemens (S); conductivity = 1/ρ.
• Power P = VI = I²R = V²/R; SI unit watt (W).

Correct Answer: Option 1 — We can use Fleming's left-hand rule to determine the direction of magnetic field.
A straight current-carrying conductor produces a magnetic field whose lines are concentric circles around the wire. The direction of this field is found by the right-hand thumb rule (also called Maxwell's corkscrew rule): if the thumb of the right hand points along the current, the curled fingers give the direction of the magnetic field lines.
Fleming's left-hand rule is not used for this purpose at all. It is used to find the direction of the force on a current-carrying conductor placed in a magnetic field — the principle of the electric motor. Its three mutually perpendicular fingers stand for: forefinger → magnetic field, centre finger → current, thumb → force/motion. Hence Option 1 is the statement that is NOT true.
❌ Why the other options are incorrect (i.e. they are true statements)
✗ The magnetic field will reverse the direction if we reverse the current — true; reversing the current reverses the sense of the circular field lines, as the right-hand thumb rule shows.
✗ The magnetic field decreases as we move away from the wire — true; for a long straight wire B ∝ 1/r, so the circles get farther apart as r grows.
✗ The strength of the magnetic field increases if we increase the current — true; B ∝ I, so a larger current gives a stronger field.
📌 SSC & Railway Exam Facts
• Field of a long straight wire: B = μ₀I/2πr — directly proportional to I and inversely proportional to r.
• Right-hand thumb rule → direction of the magnetic field of a current; Fleming's left-hand rule → force on a conductor (motor); Fleming's right-hand rule → induced current (generator).
• Oersted (1820) first showed that a current-carrying wire deflects a magnetic compass needle.
• μ₀ = 4π × 10⁻⁷ T m A⁻¹; SI unit of magnetic field is the tesla (T), and 1 T = 10⁴ gauss.
• Inside a long solenoid the field is B = μ₀nI and is uniform; magnetic field lines never intersect.

Correct Answer: Option 2 — 200 J
📖 Given
• Object A: mass = m, velocity = v, kinetic energy E(A) = 100 J
• Object B: mass = 2m, velocity = v (the same velocity)
• To find: kinetic energy of B, E(B)
✍️ Formula
Kinetic energy, E(k) = ½mv² — at a fixed velocity, E(k) ∝ m
🔢 Solution
• For A: E(A) = ½ × m × v² = 100 J
• For B: E(B) = ½ × (2m) × v²
• E(B) = 2 × (½ × m × v²)
• E(B) = 2 × 100
• E(B) = 200 J
In plain language: the velocity is unchanged, so only the mass matters — and since the mass is doubled, the kinetic energy is also simply doubled to 200 J.
❌ Why the other options are incorrect
✗ 50 J — this is half of A's energy; it would follow only if the mass were halved, not doubled.
✗ 400 J — this is four times; the KE becomes four times when the velocity is doubled, not the mass.
✗ 300 J — three times the original; this would need a mass of 3m, which is not what is given.
📌 SSC & Railway Exam Facts
• E(k) = ½mv²: E(k) ∝ m at constant v, and E(k) ∝ v² at constant m.
• Doubling the velocity makes the kinetic energy 4 times; tripling it makes the energy 9 times.
• In terms of momentum, E(k) = p²/2m; here B has twice the momentum of A but only twice the energy.
• SI unit of energy is the joule (J); 1 J = 1 N m; 1 kWh = 3.6 × 10⁶ J.
• Work–energy theorem: the work done by the net force equals the change in kinetic energy.

Correct Answer: Option 1 — 3.6 × 10⁶
📖 Given
• Energy to convert = 1 kWh (kilowatt-hour)
• 1 kilowatt = 1000 watt = 10³ W
• 1 hour = 60 × 60 = 3600 s
• 1 watt = 1 joule/second, so 1 W s = 1 J
✍️ Formula
Energy = Power × Time, i.e. 1 kWh = 1 kW × 1 h
🔢 Solution
• 1 kWh = 1000 W × 3600 s
• 1 kWh = 3 600 000 W s
• 1 kWh = 3 600 000 J
• 1 kWh = 3.6 × 10⁶ J
In plain language: an appliance of 1000 watt running for one full hour consumes 3.6 million joule of energy — and that is what the electricity meter counts as one unit.
❌ Why the other options are incorrect
✗ 3.6 — this is the value in megajoules (MJ), not in joules; the factor of 10⁶ is missing.
✗ 3.6 × 10⁻⁵ — a negative power of ten would make 1 kWh a tiny amount of energy, which is clearly wrong.
✗ 3.6 × 10⁵ — ten times too small; this comes from using 360 s or 100 W by mistake.
📌 SSC & Railway Exam Facts
• 1 kWh = 1 unit of electricity = 3.6 × 10⁶ J; this is the commercial unit of electrical energy.
• The joule (J) is the SI unit of energy; 1 J = 1 N m = 1 W s.
• Power P = W/t; SI unit watt (W), and 1 horsepower = 746 W.
• Electrical energy consumed = P × t, and electrical power P = VI = I²R = V²/R.
• A 100 W bulb burning for 10 hours consumes 1 kWh, i.e. exactly one unit.

Correct Answer: Option 3 — Unitless
The refractive index of a medium tells us how much light slows down and hence bends when it enters that medium. It is defined as
n = speed of light in vacuum / speed of light in the medium = c/v
Both the numerator and the denominator are speeds measured in m/s, so the units cancel out completely. The refractive index is therefore a pure number — dimensionless and unitless.
The same conclusion follows from Snell's law, n = sin i / sin r: the sine of an angle is itself a ratio of two lengths, so it has no unit either. A medium with a higher refractive index is optically denser and bends light more towards the normal.
❌ Why the other options are incorrect
✗ m³ — the cubic metre is the SI unit of volume, which has nothing to do with a ratio of speeds.
✗ m/s² — this is the unit of acceleration, not of an optical ratio.
✗ m/s — this is the unit of speed itself; but refractive index is the ratio of two speeds, so the unit cancels.
📌 SSC & Railway Exam Facts
• n = c/v and Snell's law: n = sin i/sin r — both give a dimensionless number.
• Speed of light in vacuum, c = 3 × 10⁸ m/s — the maximum possible speed in nature.
• Refractive indices: air ≈ 1.0003, water = 1.33, glass ≈ 1.5, diamond = 2.42 (the highest among common media).
• A higher n means a smaller speed of light in that medium and greater bending towards the normal.
• Other unitless quantities: relative density (specific gravity), strain, magnification, coefficient of friction, angle in radian.