RRB Technician Grade III 2024-25 30 December 2024 Shift 1 GK Questions – with answers & solutions

Correct Answer: Option 3 — Mitochondria
Mitochondria are called the powerhouse of the cell. They carry out aerobic respiration, breaking down glucose in the presence of oxygen and releasing energy that is stored as ATP (adenosine triphosphate) — the energy currency of the cell. This ATP is spent on every activity: muscle contraction, protein synthesis and nerve conduction.
A mitochondrion has a double membrane; the outer membrane is smooth while the inner membrane is deeply folded into cristae, which greatly increase the surface area available for ATP-producing reactions. Mitochondria possess their own DNA and ribosomes and can make some of their own proteins, so they are called semi-autonomous organelles.
❌ Why the other options are incorrect
✗ Plastids — present only in plant cells; chloroplasts (with chlorophyll) carry out photosynthesis, chromoplasts give colour and leucoplasts store food.
✗ Endoplasmic reticulum — a network of membranes; RER (with ribosomes) makes proteins while SER makes lipids and detoxifies poisons.
✗ Golgi apparatus — the packaging and dispatching centre of the cell; it modifies, stores and sends out materials and forms lysosomes.
📌 SSC & Railway Exam Facts
• Mitochondria were discovered by Altmann (1890) and named by Benda.
• The lysosome is the suicidal bag of the cell, discovered by Christian de Duve.
• The ribosome is the protein factory; the Golgi body was discovered by Camillo Golgi.
• Mitochondrial DNA is inherited maternally (from the mother).
• The chloroplast is called the kitchen of the cell and the mitochondrion the powerhouse.

Correct Answer: Option 4 — Nerve cells
Nerve cells (neurons) are the only cells that possess dendrites and an axon. A neuron has a cell body (cyton) containing a single nucleus, many short branched dendrites that receive impulses from other cells, and one long fibre — the axon — that carries the impulse away to the next neuron, muscle or gland.
Neurons build up nervous tissue, which is found in the brain, spinal cord and nerves. The impulse travels along the axon as an electrical signal and crosses the synapse in the form of a chemical neurotransmitter. Neurons are the longest cells of the human body.
❌ Why the other options are incorrect
✗ Fibres — such as muscle fibres in animals or sclerenchyma fibres in plants; they provide contraction or mechanical strength and have no dendrites or axon.
✗ Red blood cells — round, biconcave cells packed with haemoglobin to carry oxygen; mature human RBCs have no nucleus at all.
✗ White blood cells — nucleated, amoeboid defence cells that fight germs and make antibodies; they move by pseudopodia, not by dendrites.
📌 SSC & Railway Exam Facts
• Neuron = cyton + dendrites + axon; it is the structural and functional unit of the nervous system.
• Nerve impulses travel at about 100–120 m/s; the gap between two neurons is the synapse.
• RBCs live about 120 days, are made in the bone marrow and destroyed in the spleen (the graveyard of RBCs).
• The WBC : RBC ratio in human blood is roughly 1 : 600.
• The four animal tissues are epithelial, connective, muscular and nervous.

Correct Answer: Option 4 — Rhizopus
Rhizopus, the bread mould, reproduces asexually by spore formation. Its thread-like hyphae form a cottony mycelium over moist bread. Some hyphae grow upright as sporangiophores, each tipped by a blob-like sporangium in which hundreds of spores are produced.
When the sporangium ruptures, the light spores are scattered by air currents. Each spore is protected by a thick wall, so it can survive drought and heat for a long time; on reaching a warm, moist surface it germinates into a new Rhizopus. Mucor, ferns and mosses also reproduce by spores.
❌ Why the other options are incorrect
✗ Hydra — a simple multicellular animal that reproduces by budding, and can also regrow lost parts by regeneration.
✗ Earthworm — reproduces sexually; it is a hermaphrodite (bisexual) animal, but cross-fertilisation takes place between two worms.
✗ Leishmania — a unicellular protozoan that multiplies by binary fission in a definite (longitudinal) plane; it causes kala-azar.
📌 SSC & Railway Exam Facts
• Spore formation — Rhizopus, Mucor, ferns and mosses; budding — yeast and Hydra.
• Binary fission — Amoeba, Paramoecium and Leishmania; multiple fission — Plasmodium.
• Regeneration — Hydra and Planaria; fragmentation — Spirogyra.
• Spores have thick walls so that they can withstand unfavourable conditions.
• Rhizopus grows on moist bread; keeping bread dry and cold prevents moulding.

Correct Answer: Option 1 — Helps in the synthesis of starch
Starch is made only by green plants during photosynthesis, where the glucose produced in the chloroplast is stored in the form of starch. Human blood takes no part in making starch, so this is not a function of blood.
Blood is a fluid connective tissue made of a liquid matrix called plasma together with RBCs, WBCs and platelets. It transports oxygen and carbon dioxide, digested food, hormones and nitrogenous wastes, helps in clotting (platelets), defends the body against germs (WBCs) and helps to regulate body temperature.
❌ Why the other options are incorrect
✗ Transports waste materials — true; blood carries urea, uric acid and CO₂ to the kidneys and lungs for removal.
✗ Transports hormones — true; endocrine glands pour hormones straight into the blood, which delivers them to the target organs.
✗ Transports gases — true; haemoglobin in the RBCs carries oxygen, while CO₂ travels mostly dissolved in the plasma.
📌 SSC & Railway Exam Facts
• Blood is the only fluid connective tissue; plasma forms about 55% of blood.
• Haemoglobin is an iron-containing red pigment; its shortage causes anaemia.
• Platelets (thrombocytes) help in blood clotting; vitamin K is needed for clotting.
• Blood groups A, B, AB and O were discovered by Karl Landsteiner; O⁻ is the universal donor and AB⁺ the universal recipient.
• The human heart has four chambers; arteries carry blood away from the heart and veins bring it back.

Correct Answer: Option 1 — style
After pollination, a pollen grain that lands on the stigma germinates and puts out a pollen tube. This tube grows down through the style, the slender stalk joining the stigma to the ovary, and enters the ovule to deliver the male gamete. There the male gamete fuses with the egg cell — fertilisation — and a zygote is formed.
The female part of the flower, the pistil (carpel), is therefore made of three parts: stigma + style + ovary. After fertilisation the ovule becomes the seed and the ovary becomes the fruit.
❌ Why the other options are incorrect
✗ petal — the coloured, scented member of the corolla; it only attracts insects for pollination and is an accessory whorl.
✗ filament — the stalk of the stamen that simply holds up the anther; it is a male part and no pollen tube passes through it.
✗ anther — the sac-like tip of the stamen in which pollen grains are produced; the pollen tube forms only after the pollen has left it.
📌 SSC & Railway Exam Facts
• Stamen = anther + filament (male); Pistil = stigma + style + ovary (female).
• Self-pollination occurs within the same flower; cross-pollination between two flowers of the same species.
• In angiosperms double fertilisation occurs — one male gamete forms the zygote, the other the endosperm.
• Agents of pollination — wind, water, insects and birds.
• Bisexual flowers — hibiscus and mustard; unisexual flowers — papaya, watermelon and maize.

Correct Answer: Option 1 — In immediate response to some stimulus
Plants show two kinds of movement — growth-dependent (tropic) movements, which are slow, and growth-independent (nastic) movements, which are immediate responses to a stimulus. The action described here belongs to the second kind.
Since plants have no nervous system and no muscles, a plant cell changes its shape by changing the amount of water inside it. Water entering makes the cell swell (turgid) and water leaving makes it shrink (flaccid), and this change of shape produces the movement. This is exactly how the leaflets of the touch-me-not plant (Mimosa pudica) fold up the moment they are touched, and how guard cells open and close the stomata.
❌ Why the other options are incorrect
✗ To maintain balance — plants are fixed in the soil and do not need to balance their body as animals do while walking or running.
✗ To conserve energy — such water movement actually uses energy to pump ions into and out of the cells; saving energy is not its purpose.
✗ In fear — plants have no nervous system, so emotions like fear are impossible; the response is purely physical and chemical.
📌 SSC & Railway Exam Facts
• Mimosa pudica is the classic example of a growth-independent (nastic) movement.
• Tropic movements — phototropism (light), geotropism (gravity), hydrotropism (water), chemotropism (chemicals) and thigmotropism (touch).
• Auxin makes the shoot tip bend towards light; the growth of the pollen tube is chemotropism.
• Guard cells swell to open and shrink to close the stomata.
• Plant hormones — auxin, gibberellin and cytokinin (promoters) and abscisic acid (inhibitor, causes wilting).

Correct Answer: Option 3 — (i), (ii) and (iii)
All three statements are incorrect.
(i) Translocation is the transport of the soluble products of photosynthesis — chiefly sucrose — through the phloem; the statement wrongly calls the product "insoluble".
(ii) The evaporative loss of water through stomata is transpiration, not respiration. Respiration is the breakdown of food to release energy (ATP), giving out CO₂.
(iii) Photosynthesis takes place in the presence of sunlight, never in its absence. Green plants use chlorophyll to trap sunlight and convert CO₂ and water into glucose, storing the energy in the form of food.
❌ Why the other options are incorrect
✗ Only (i) — wrong; statements (ii) and (iii) are also factually incorrect, so the answer cannot be limited to (i).
✗ Only (ii) and (iii) — wrong; it treats statement (i) as correct, but the word "insoluble" makes (i) false as well.
✗ Only (i) and (ii) — wrong; it leaves out (iii), which wrongly places photosynthesis in the absence of sunlight.
📌 SSC & Railway Exam Facts
• Photosynthesis equation — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of sunlight and chlorophyll.
• Xylem carries water and minerals upward; phloem carries food in both directions (translocation).
• Translocation in the phloem needs ATP and is explained by the pressure-flow (Munch) hypothesis.
• Transpiration creates the transpiration pull that lifts water in tall trees; it is called a necessary evil.
• Stomata are opened and closed by guard cells, and most gas exchange in a leaf occurs through them.

Correct Answer: Option 4 — sporangia
In Rhizopus (bread mould) the spores are formed inside tiny blob-like structures borne at the tips of upright hyphae. These blob-on-a-stick structures are called sporangia (singular sporangium); the stalk that carries them is the sporangiophore.
Each sporangium is a sac in which hundreds of spores develop. When the sporangium bursts, the light spores are carried away by air currents. Every spore has a thick protective wall, so it can survive unfavourable conditions, and on landing on a moist surface it germinates into a new Rhizopus. This is asexual reproduction by spore formation.
❌ Why the other options are incorrect
✗ fruiting bodies — the large reproductive structures of higher fungi such as the mushroom; the Rhizopus blob is not called this in NCERT.
✗ sacs — a loose everyday word, not the scientific term for the spore-bearing structure of a mould.
✗ buds — small outgrowths formed during budding, as in yeast and Hydra; a bud contains no spores.
📌 SSC & Railway Exam Facts
• Sporangium = the spore-producing sac; sporangiophore = the stalk that holds it up.
• Rhizopus, Mucor, ferns and mosses reproduce by spore formation.
• Spores survive drought and heat because of their thick walls.
• Yeast, though a fungus, reproduces by budding under favourable conditions, not by spores.
• The study of fungi is Mycology; Rhizopus is commonly called bread mould.

Correct Answer: Option 3 — Phloem fibres
Phloem is the food-conducting complex permanent tissue of plants. It is made of four elements — sieve tubes, companion cells, phloem parenchyma and phloem fibres (bast fibres). Of these, only the phloem fibres are dead; they have thick lignified walls and give the plant mechanical strength and support.
The other three elements are living, which is unusual among conducting tissues — that is why phloem is described as a living conducting tissue, unlike xylem, in which the tracheids, vessels and xylem fibres are all dead.
❌ Why the other options are incorrect
✗ Phloem parenchyma — living, thin-walled cells that store food such as starch and fats and also store resins and latex.
✗ Sieve cells (sieve tubes) — living, tube-like cells with perforated sieve plates through which food flows; they lack a nucleus but remain alive.
✗ Companion cells — living, nucleated cells attached to the sieve tubes; they control the activity of the enucleate sieve tube.
📌 SSC & Railway Exam Facts
• Xylem = tracheids, vessels, xylem parenchyma and xylem fibres — only the xylem parenchyma is living.
• Phloem = sieve tubes, companion cells, phloem parenchyma and phloem fibres — only the phloem fibres are dead.
• Xylem carries water and minerals upward; phloem carries food in both directions (translocation).
• Commercial fibres such as jute, flax and hemp are obtained from phloem (bast) fibres.
• Xylem and phloem are complex permanent tissues and together form the vascular bundle.

Correct Answer: Option 1 — Ligament
A ligament is a connective tissue that joins bone to bone at a joint. It is very elastic and contains very little matrix, so it can stretch; it holds the two bones in position and prevents dislocation while still allowing free movement.
Connective tissue in general connects, supports, binds and packs the other tissues of the body. Its cells lie loosely spaced in an intercellular matrix. Bone, cartilage, blood, adipose tissue, areolar tissue, tendon and ligament are all connective tissues.
❌ Why the other options are incorrect
✗ Cartilage — a solid but flexible connective tissue with a matrix of proteins and sugars; it smoothens bone surfaces at joints and is found in the nose, ear, trachea and larynx.
✗ Adipose tissue — fat-storing connective tissue below the skin and between organs; it acts as an insulator and a shock absorber.
✗ Tendon — a strong but inelastic connective tissue with great strength, which joins muscle to bone, not bone to bone.
📌 SSC & Railway Exam Facts
• Ligament — bone to bone (elastic); Tendon — muscle to bone (tough and inelastic).
• The bone matrix is rich in calcium and phosphorus; blood is the fluid connective tissue.
• Areolar tissue fills the space inside organs and supports the internal organs.
• The four animal tissues are epithelial, connective, muscular and nervous.
• The adult human skeleton has 206 bones; the femur is the longest bone.

Correct Answer: Option 3 — Only (i) and (ii)
Adrenaline (epinephrine) is secreted by the adrenal glands, which sit like caps on top of the kidneys. It is called the emergency hormone or the fight-or-flight hormone, because it is poured into the blood whenever a person is angry, frightened or excited.
Its effects are: the heart beats faster (i), so that more oxygen reaches the muscles; the breathing rate increases (ii) because of faster contraction of the diaphragm and rib muscles; blood is diverted from the skin and the digestive system to the skeletal muscles; and the blood pressure rises. Hence statement (iii) "Decreases blood pressure" is wrong — adrenaline increases blood pressure.
❌ Why the other options are incorrect
✗ (i), (ii) and (iii) — wrong; it accepts statement (iii), but adrenaline raises rather than lowers blood pressure.
✗ Only (ii) and (iii) — wrong; it rejects the true statement (i) and accepts the false statement (iii).
✗ Only (i) and (iii) — wrong; it leaves out the true statement (ii) and includes the false statement (iii).
📌 SSC & Railway Exam Facts
• The adrenal gland is also called the supra-renal gland; its cortex secretes cortisol and aldosterone.
• Adrenaline raises heart rate, breathing rate, blood sugar and blood pressure.
• Pituitary — the master gland; thyroid — thyroxine (needs iodine); pancreas — insulin.
• Noradrenaline is a closely related hormone secreted by the same gland.
• Iodine deficiency causes goitre; insulin deficiency causes diabetes mellitus.

Correct Answer: Option 2 — Regulation of hearing
Hearing is a sensation, and it is received and interpreted in the temporal lobe of the cerebrum, a part of the fore-brain. The medulla is concerned only with involuntary, life-supporting actions, not with the perception of sensations.
The medulla oblongata, a part of the hind-brain, controls the involuntary actions of the body — blood pressure, salivation, vomiting, heart beat, breathing, coughing and sneezing. Because it regulates the heart and the breathing, it is often called the vital centre, and an injury to it is usually fatal.
❌ Why the other options are incorrect
✗ Blood pressure regulation — an involuntary function clearly stated in NCERT as being controlled by the medulla.
✗ Vomiting control — the vomiting centre lies in the medulla, so this action is indeed regulated by it.
✗ Salivary regulation — salivation is another involuntary action listed under the control of the medulla.
📌 SSC & Railway Exam Facts
• Hind-brain = cerebellum + pons + medulla; the medulla controls involuntary actions.
• The cerebellum maintains posture and balance and makes movements precise (e.g. riding a bicycle).
• The cerebrum is the largest part of the brain — the seat of thinking, memory, will and sensations such as hearing, sight and smell.
• The hypothalamus controls hunger, thirst, sleep and body temperature.
• The brain is protected by the cranium (skull), three meninges and the cerebrospinal fluid (CSF).

Correct Answer: Option 1 — Amoeba exhibit regeneration.
This statement is incorrect. Amoeba is a unicellular organism; it reproduces by binary fission, and by multiple fission inside a cyst when conditions turn unfavourable. Regeneration requires a multicellular body whose specialised cells can rebuild the lost parts, so Amoeba does not exhibit regeneration.
Regeneration is the ability of a fully differentiated organism to give rise to new individual organisms from its cut body parts. It is carried out by specialised cells that proliferate (multiply rapidly) to make large numbers of cells, which then change into different cell types and tissues in an organised sequence called development.
❌ Why the other options are incorrect
✗ Specialised cells are responsible for regeneration — true; NCERT states that regeneration is carried out by specialised cells.
✗ These specialist cells multiply rapidly and produce a large number of new cells — true; this rapid proliferation is what rebuilds the missing part.
✗ Hydra and Planaria show regeneration mode of reproduction — true; these are the standard NCERT examples of regeneration.
📌 SSC & Railway Exam Facts
• Regeneration — Hydra and Planaria; a cut piece can grow into a complete organism.
• Regeneration is not the same as reproduction, because most organisms do not depend on it to multiply.
• Amoeba divides by binary fission in any plane; Leishmania in a definite plane.
• Budding — yeast and Hydra; fragmentation — Spirogyra.
• Planaria is a flatworm; even a small piece of its body can regenerate a whole worm.

Correct Answer: Option 2 — Half molecules of water
Plaster of Paris is calcium sulphate hemihydrate, whose formula is CaSO₄·½H₂O (often written as 2CaSO₄·H₂O). The prefix hemi means half, so one formula unit of the salt holds only half a molecule of water of crystallisation — put the other way round, two formula units share one water molecule.
It is manufactured by heating gypsum (CaSO₄·2H₂O) to about 373 K (100 °C). At this temperature gypsum loses three-fourths of its water of crystallisation and changes into plaster of Paris, a white powder.
When plaster of Paris is mixed with water it takes the water back and sets into a hard solid mass of gypsum again. This setting property is why it is used for plaster casts of fractured bones, for statues, toys, chalk and false ceilings, and as a fire-proofing material.
⚗️ The Reaction
CaSO₄·2H₂O → (heating, 373 K) CaSO₄·½H₂O + 1½H₂O
• Gypsum gives up part of its water of crystallisation to become plaster of Paris.
• Setting is simply the reverse change: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
• Because it absorbs moisture from the air, it must be kept in a moisture-proof container.
❌ Why the other options are incorrect
✗ Three molecules of water — no hydrate of calcium sulphate carries three water molecules; gypsum, the most hydrated form, carries only two (CaSO₄·2H₂O).
✗ Zero molecules of water — water-free CaSO₄ is anhydrite, and the form made above 473 K is dead burnt plaster, which has lost its setting property.
✗ One molecule of water — that would be the monohydrate CaSO₄·H₂O. The formula 2CaSO₄·H₂O confuses aspirants: one water is shared by two units, so each unit gets half.
📌 SSC & Railway Exam Facts
• Plaster of Paris — CaSO₄·½H₂O; gypsum — CaSO₄·2H₂O.
• The name comes from the huge gypsum deposits of Montmartre, Paris (France).
• Gypsum is heated to 373 K (100 °C); above 473 K it becomes dead burnt plaster.
• Chemical name of plaster of Paris: calcium sulphate hemihydrate.
• Uses: plaster casts for fractures, statues and toys, chalk, false ceilings, fire-proofing.

Correct Answer: Option 4 — Mixture of iron chloride and iron fillings
A heterogeneous mixture is one whose composition is not uniform. Its components remain in separate phases, the boundary between them can usually be seen, and two samples taken from different parts of the mixture are not identical.
A mixture of iron chloride (a salt) and iron filings (the free metal) is exactly such a mixture. The two do not dissolve in each other, the grey metallic bits stay visibly apart from the salt, and they can be picked out by hand or lifted out with a magnet — a purely physical method, which proves it is a mixture and not a compound.
A homogeneous mixture (a true solution), by contrast, has the same composition throughout; its solute particles are smaller than 1 nm, cannot be seen even with a microscope, and never settle down.
❌ Why the other options are incorrect
✗ Sugar in water — sugar dissolves completely and spreads uniformly, giving a homogeneous solution; no separate phase is visible.
✗ Salt in water — common salt splits into Na⁺ and Cl⁻ ions that spread evenly through the water; a classic true solution.
✗ Copper sulphate solution — a uniformly blue homogeneous liquid of CuSO₄ in water; the word *solution* itself tells you it is single-phased.
📌 SSC & Railway Exam Facts
• Mixtures are of two kinds — homogeneous (solutions) and heterogeneous.
• The NCERT classic heterogeneous mixture is iron filings + sulphur powder; the iron is removed with a magnet, the sulphur with carbon disulphide.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Air is homogeneous, while muddy water, soil and oil in water are heterogeneous.
• In a mixture the components keep their own properties and are separable by physical methods; in a compound they do not.

Correct Answer: Option 4 — CuO + 2HCl → CuCl₂ + H₂O
Metal oxides are basic in nature, so when a metal oxide meets an acid the two neutralise each other and give a salt and water. The general rule is Metal oxide + Acid → Salt + Water, which is why such a reaction is also called a neutralisation and belongs to the double displacement family.
Here copper(II) oxide (CuO), a black solid, reacts with hydrochloric acid to give copper(II) chloride (CuCl₂) and water. Copper in CuO is in the +2 state, so it needs two chloride ions — hence CuCl₂ — and therefore two molecules of HCl. The black CuO dissolves and the solution turns blue-green.
⚗️ The Reaction
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• Count the atoms — Cu 1 = 1, O 1 = 1, H 2 = 2, Cl 2 = 2: the equation is balanced.
• O²⁻ of the oxide picks up the 2H⁺ of the acid to make H₂O; Cu²⁺ pairs with 2Cl⁻.
• No hydrogen gas is released — a metal *oxide* is not a metal, so this is not a displacement reaction.
❌ Why the other options are incorrect
✗ CuO + HCl → CuCl + H₂ — wrong on two counts: copper here is +2, so CuCl (Cu in +1) is the wrong salt, and H₂ gas is never produced when an oxide meets an acid. It is also unbalanced (O appears only on the left).
✗ CuO + HCl → CuOH + HCl — HCl cannot appear unchanged on both sides, and CuOH is not the product of an acid reaction; nothing is really reacting here.
✗ CuO + 2HCl → CuCl₂ + O₂ — the formula demands H₂O, not O₂: on the left there are 2 H and 1 O, so the hydrogen would vanish and the oxygen atoms would not balance.
📌 SSC & Railway Exam Facts
• Metal oxide + Acid → Salt + Water (metal oxides are basic).
• Non-metal oxide + Base → Salt + Water (non-metal oxides are acidic), e.g. CO₂ + 2NaOH → Na₂CO₃ + H₂O.
• CuO is black; CuCl₂ solution is blue-green; CuSO₄·5H₂O is blue vitriol.
• Metal + Acid → Salt + Hydrogen gas — that is where H₂ comes from, e.g. Zn + 2HCl → ZnCl₂ + H₂.
• A balanced equation obeys the law of conservation of mass (Lavoisier).

Correct Answer: Option 4 — Helium
The atomicity of an element is the number of atoms present in one molecule of that element. An element whose molecule contains only one atom is called monoatomic.
Helium (He) has the electronic configuration K = 2, i.e. its only shell is completely filled (duplet). Having no tendency to gain, lose or share electrons, a helium atom is already chemically stable and does not bond with a second atom. It therefore exists as a single free atom — He, and is monoatomic. All noble gases (He, Ne, Ar, Kr, Xe, Rn) behave this way, and so do metals such as Na, Fe and Cu when their atomicity is counted.
❌ Why the other options are incorrect
✗ Nitrogen — exists as N₂, in which the two atoms are held by a triple bond; it is diatomic, not monoatomic.
✗ Hydrogen — exists as H₂; a single H atom has only one electron in the K shell and must pair up to complete its duplet, so it is diatomic.
✗ Oxygen — exists as O₂ with a double bond, hence diatomic (its allotrope ozone, O₃, is triatomic).
📌 SSC & Railway Exam Facts
• Monoatomic: all noble gases — He, Ne, Ar, Kr, Xe, Rn — and metals such as Na, Cu, Fe.
• Diatomic: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
• Triatomic: O₃ (ozone); Tetratomic: P₄; Octatomic: S₈.
• Helium is the second most abundant element in the universe and is used in weather balloons and in divers' breathing mixtures (with oxygen).
• Noble gases are inert because their outermost shell is complete — 2 electrons in He, 8 in the rest.

Correct Answer: Option 2 — Mg + ½O₂ → MgO
A chemical equation is balanced when the number of atoms of every element is the same on both sides, as demanded by the law of conservation of mass. The skeletal equation Mg + O₂ → MgO has 2 oxygen atoms on the left but only 1 on the right, so it must be balanced.
Burning magnesium ribbon in air is a combination reaction and also an oxidation: the metal takes up oxygen, burning with a dazzling white light and leaving a white powder of magnesium oxide (MgO). This is why the ribbon is first cleaned with sandpaper — to remove the protective MgO layer already on it.
⚗️ The Reaction
Mg + ½O₂ → MgO
• Mg: 1 = 1 and O: ½ × 2 = 1 = 1 — every atom balances.
• Multiplying throughout by 2 gives the whole-number form 2Mg + O₂ → 2MgO, which is the same equation.
• MgO + H₂O → Mg(OH)₂, a base that turns red litmus blue — proof that metal oxides are basic.
❌ Why the other options are incorrect
✗ 2Mg + ½O₂ → 2MgO — magnesium balances (2 = 2) but oxygen does not: ½ × 2 = 1 atom on the left against 2 on the right.
✗ Mg + 3/2O₂ → MgO — the left side carries 3 oxygen atoms against just 1 on the right; 3/2 comes from copying the coefficient used for oxides like Al₂O₃.
✗ 2Mg + 3/2O₂ → 3MgO — here the oxygen happens to balance (3/2 × 2 = 3 = 3), which makes the option look convincing, but magnesium does not: 2 atoms on the left against 3 on the right. One unbalanced element is enough to reject an equation.
📌 SSC & Railway Exam Facts
• Whole-number form: 2Mg + O₂ → 2MgO — a standard combination (synthesis) reaction.
• Burning magnesium gives a blinding white flame; it is used in flash powder, fireworks and flares.
• Magnesium ribbon is cleaned before burning to remove the layer of magnesium oxide.
• Balancing follows the law of conservation of mass, given by Antoine Lavoisier.
• Magnesium is the metal atom at the centre of chlorophyll.

Correct Answer: Option 1 — Magnesium hydroxide
Our stomach produces hydrochloric acid (HCl) to digest food. When too much of it is secreted the result is acidity or indigestion, and the pain is relieved by an antacid — a mild, non-corrosive base that neutralises the excess acid without harming the stomach lining.
Magnesium hydroxide, Mg(OH)₂, is exactly such a base. Its suspension in water is the familiar milk of magnesia, a white liquid which is only slightly soluble, so it is weakly basic and perfectly safe to swallow. It neutralises the excess HCl to give magnesium chloride and water.
⚗️ The Reaction
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
• A neutralisation reaction: Base + Acid → Salt + Water.
• The OH⁻ of the base combines with the H⁺ of the stomach acid, so the pH rises towards neutral.
• Milk of magnesia is also used as a mild laxative.
❌ Why the other options are incorrect
✗ Calcium oxide — CaO (quicklime) is a strong, corrosive base that reacts violently with water giving out heat; it would burn the stomach and is used in whitewashing and cement, never as a medicine.
✗ Sodium chloride — common salt is a neutral salt (pH 7) of a strong acid and a strong base; having no OH⁻ to spare, it cannot neutralise anything.
✗ Calcium hydroxide — Ca(OH)₂ (slaked lime) is a fairly strong base used in whitewashing, bleaching powder and in treating acidic soil; it is too caustic to be taken internally.
📌 SSC & Railway Exam Facts
• Common antacids: Mg(OH)₂ (milk of magnesia), Al(OH)₃, NaHCO₃ (baking soda), CaCO₃.
• Milk of magnesia = suspension of magnesium hydroxide; its pH is about 10.
• Baking soda — NaHCO₃; washing soda — Na₂CO₃·10H₂O; quicklime — CaO; slaked lime — Ca(OH)₂.
• Stomach acid is HCl, with a pH of about 1.2–3.0.
• Tooth decay begins when the mouth's pH falls below 5.5; toothpastes are basic to correct it.

Correct Answer: Option 4 — Ethene
When ethanol (C₂H₅OH) is heated at 443 K (170 °C) with excess concentrated sulphuric acid, it undergoes dehydration — the removal of a water molecule — and is converted into the unsaturated hydrocarbon ethene (C₂H₄).
The concentrated H₂SO₄ does not appear in the products: it acts as a dehydrating agent, pulling out one H and one OH from adjacent carbon atoms of the alcohol. The two carbons are then left with a spare electron pair each and join through a double bond, giving an alkene. Because the acid is recovered unchanged, it is counted as a catalyst/dehydrating agent and written over the arrow.
⚗️ The Reaction
C₂H₅OH → (conc. H₂SO₄, 443 K) C₂H₄ + H₂O
• H and OH are removed from neighbouring carbons, so a C=C double bond forms.
• Ethene is an unsaturated hydrocarbon (alkene, general formula CₙH₂ₙ).
• At the lower temperature of 413 K, excess ethanol instead gives diethyl ether, not ethene.
❌ Why the other options are incorrect
✗ Ether — diethyl ether (C₂H₅OC₂H₅) is formed when excess ethanol is heated with conc. H₂SO₄ at the lower temperature of about 413 K (140 °C); at 443 K with excess acid the product is the alkene.
✗ Acetic acid — ethanoic acid (CH₃COOH) comes from the oxidation of ethanol by alkaline KMnO₄ or acidified K₂Cr₂O₇, not from dehydration.
✗ Ethane — C₂H₆ would need hydrogenation; in fact it is obtained by adding H₂ to ethene over a nickel catalyst, so it is the reverse direction.
📌 SSC & Railway Exam Facts
• Dehydration of ethanol at 443 K with conc. H₂SO₄ → ethene; conc. H₂SO₄ is the dehydrating agent.
• Oxidation of ethanol gives ethanoic acid; alkaline KMnO₄ and acidified K₂Cr₂O₇ are the oxidising agents.
• Ethanol + Na → sodium ethoxide + H₂; 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.
• Ethene is the plant hormone used for the artificial ripening of fruits.
• Ethanol is the alcohol of alcoholic drinks; methanol is poisonous and causes blindness.

Correct Answer: Option 1 — Salt and water
The reaction between an acid and a base is called a neutralisation reaction, and it always follows the pattern Acid + Base → Salt + Water.
An acid supplies H⁺ (hydrogen) ions in water, a base supplies OH⁻ (hydroxide) ions. In the reaction these two combine to form H₂O, and the remaining positive ion of the base joins the remaining negative ion of the acid to give the salt. Because both the acidic and the basic character are destroyed, the mixture moves towards pH 7 — the two have *neutralised* each other. It is also a double displacement reaction and is normally exothermic.
⚗️ The Reaction
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• The essential step is H⁺ + OH⁻ → H₂O.
• Na⁺ (from the base) pairs with Cl⁻ (from the acid) to give the salt NaCl.
• Heat is given out, so neutralisation is an exothermic reaction.
❌ Why the other options are incorrect
✗ Only water — the metal ion of the base and the acid radical do not disappear; they must combine and appear as a salt, so water alone is an incomplete answer.
✗ Salt and hydrogen ion — the H⁺ ion is used up in forming water; if free H⁺ were left over the solution would still be acidic and no neutralisation would have occurred.
✗ Salt and hydrogen gas — H₂ gas is released when an acid reacts with a metal, not with a base: Zn + 2HCl → ZnCl₂ + H₂↑. This is the most common confusion in this question.
📌 SSC & Railway Exam Facts
• Acid + Base → Salt + Water (neutralisation, a double displacement reaction).
• Acid + Metal → Salt + H₂ gas; the gas burns with a pop sound.
• Acid + Metal carbonate/bicarbonate → Salt + Water + CO₂; CO₂ turns lime water milky.
• pH 7 is neutral, below 7 acidic, above 7 basic; the pH scale was given by Sørensen.
• Everyday neutralisation: antacid for acidity, baking soda on a bee sting, lime on acidic soil.

Correct Answer: Option 1 — sodium zincate
Metals normally do not react with bases. Zinc is one of the few exceptions, because it is an amphoteric metal — it reacts both with acids and with strong alkalis. When zinc is heated with sodium hydroxide solution it dissolves, forming the salt sodium zincate (Na₂ZnO₂) and liberating hydrogen gas.
The reaction happens because zinc's oxide layer, ZnO, is amphoteric: it can behave as a base towards an acid and as an acid towards an alkali. Aluminium behaves in the same way, giving sodium aluminate. The H₂ given off burns with a pop sound, the standard laboratory test.
⚗️ The Reaction
Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g)
• The product Na₂ZnO₂ is sodium zincate; hydrogen is set free.
• Atoms balance: Zn 1 = 1, Na 2 = 2, O 2 = 2, H 2 = 2.
• With an acid the same metal gives a different salt: Zn + 2HCl → ZnCl₂ + H₂ — this double behaviour is what amphoteric means.
❌ Why the other options are incorrect
✗ zinc hydroxide — Zn(OH)₂ is formed when a zinc salt (say ZnSO₄) meets a small amount of NaOH, as a white precipitate; with excess NaOH it dissolves again to sodium zincate. Zinc metal does not stop at the hydroxide.
✗ zinc oxide — ZnO is made by burning zinc in air (2Zn + O₂ → 2ZnO) or by heating zinc carbonate; no alkali is involved.
✗ sodium hydride — NaH is an ionic hydride made by heating sodium metal in hydrogen gas; sodium here is already fixed in NaOH and is not reduced by zinc to a hydride.
📌 SSC & Railway Exam Facts
• Zn + 2NaOH → Na₂ZnO₂ + H₂↑ (sodium zincate); 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂↑ (sodium aluminate).
• Amphoteric metals/oxides: Zn, Al, Pb, Sn — ZnO, Al₂O₃, PbO, SnO.
• Galvanisation is coating iron with a layer of zinc to prevent rusting.
• Zinc's place in the reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• Hydrogen gas burns with a pop sound — the standard test in the laboratory.

Correct Answer: Option 3 — Colour of the liquid
Evaporation is the change of a liquid into vapour from its surface alone, at any temperature below the boiling point. It occurs because a few surface particles always possess more than the average kinetic energy and break free from the attraction of their neighbours.
Everything that decides how many particles escape therefore controls the rate — surface area, temperature, humidity and wind speed. The colour of the liquid is only an optical property; it changes nothing about the particles' kinetic energy or about the forces holding them together, so it does not affect the rate of evaporation. (Colour can matter only indirectly, when a dark liquid absorbs more sunlight and so gets hotter — the real factor is still the temperature.)
❌ Why the other options are incorrect
✗ Surface area of the liquid — evaporation is a surface phenomenon; a larger exposed surface lets more particles escape, which is why wet clothes are spread out to dry.
✗ Temperature of the liquid — a rise in temperature increases the kinetic energy of the particles, so more of them cross the surface; clothes dry faster on a hot day.
✗ Humidity of the surrounding air — high humidity means the air already holds a lot of water vapour, so it accepts less; evaporation slows down, which is why clothes dry slowly in the rainy season.
📌 SSC & Railway Exam Facts
• Factors that increase evaporation: greater surface area, higher temperature, lower humidity, higher wind speed.
• Evaporation causes cooling — the particles take latent heat of vaporisation from the surroundings.
• This is why we sweat, why water stays cool in an earthen pot (matka), and why acetone on the palm feels cold.
• Evaporation happens at all temperatures and only at the surface; boiling happens at a fixed temperature and throughout the liquid.
• Latent heat of vaporisation of water = 2260 kJ/kg (22.5 × 10⁵ J/kg).

Correct Answer: Option 2 — 35
The mass number (A) of an atom is the total number of nucleons, i.e. the protons plus the neutrons present in its nucleus. Electrons are not counted, because an electron's mass is only about 1/1836 of a proton's and is negligible in comparison.
Mass number (A) = number of protons (Z) + number of neutrons (n)
Here the atom has 17 protons and 18 neutrons, so A = 17 + 18 = 35. The atomic number Z = 17, which identifies the element as chlorine; with 18 neutrons this is the isotope Cl-35. The 17 electrons simply equal the protons, telling us the atom is electrically neutral.
🔢 The Calculation
A = Z + n = 17 + 18 = 35
• Protons 17 → atomic number = 17 → the element is chlorine (Cl).
• Protons = electrons = 17, so the net charge is zero — a neutral atom.
• The symbol is written ₁₇Cl³⁵, i.e. ³⁵₁₇Cl.
❌ Why the other options are incorrect
✗ 34 — obtained by adding protons + electrons (17 + 17); electrons have negligible mass and never enter the mass number.
✗ 24 — matches no combination in the data; it is the mass number of magnesium (Mg-24), a distractor from the periodic table.
✗ 18 — that is only the number of neutrons; taken alone it gives the neutron number, not the mass number.
📌 SSC & Railway Exam Facts
• Atomic number (Z) = number of protons; Mass number (A) = protons + neutrons; neutrons = A − Z.
• Chlorine has two natural isotopes, Cl-35 and Cl-37, mixed in the ratio 3 : 1, giving the average atomic mass 35.5 u.
• Isotopes — same Z, different A (₁H¹, ₁H², ₁H³); Isobars — same A, different Z (₂₀Ca⁴⁰ and ₁₈Ar⁴⁰).
• Protons and neutrons together are called nucleons; the nucleus was discovered by Rutherford.
• Proton — Goldstein, Electron — J. J. Thomson, Neutron — James Chadwick.

Correct Answer: Option 3 — Ethanol
Fermentation is the slow anaerobic (oxygen-free) breakdown of sugars by enzymes present in yeast, in which the sugar is converted into ethanol (C₂H₅OH) and carbon dioxide. It is the oldest industrial method of making alcohol, carried out at about 303 K (30 °C).
The change takes place in two enzyme-controlled steps. Invertase first splits sucrose (C₁₂H₂₂O₁₁) into glucose and fructose; then zymase, also from yeast, breaks each C₆H₁₂O₆ unit into two molecules of ethanol and two of CO₂. Molasses, grains and fruit juices are the usual raw materials.
⚗️ The Reaction
C₁₂H₂₂O₁₁ + H₂O → (invertase) C₆H₁₂O₆ + C₆H₁₂O₆
C₆H₁₂O₆ → (zymase) 2C₂H₅OH + 2CO₂
• Yeast supplies the enzymes invertase and zymase; the process needs no oxygen.
• The escaping CO₂ is what makes bread and dough rise.
• The alcohol so obtained is concentrated afterwards by fractional distillation.
❌ Why the other options are incorrect
✗ Ethanoic acid — CH₃COOH is the next stage: it forms when the ethanol is further oxidised by acetic acid bacteria in air, which is how vinegar is produced. Fermentation of sugar itself stops at ethanol.
✗ Ethene — C₂H₄ is obtained by dehydrating ethanol with conc. H₂SO₄ at 443 K, a laboratory reaction and not a biological one.
✗ Methane — CH₄ is the main gas of biogas/marsh gas, produced by the anaerobic decay of animal and plant waste by bacteria, not by the fermentation of sugar with yeast.
📌 SSC & Railway Exam Facts
• Fermentation: sugar → ethanol + CO₂, by the enzymes of yeast, at about 303 K.
• Enzymes: invertase (sucrose → glucose + fructose) and zymase (glucose → ethanol + CO₂).
• Ethanol is also called grain alcohol; spirit and wine are made this way.
• Ethanol + O₂ (bacteria) → ethanoic acid; a 5–8% solution of ethanoic acid in water is vinegar.
• Pure ethanoic acid freezes at 290 K (16.6 °C) and is therefore called glacial acetic acid.

Correct Answer: Option 2 — Aluminium oxide
An amphoteric oxide is one that shows both basic and acidic behaviour — it reacts with acids like a base, and with bases (alkalis) like an acid, giving a salt and water in each case.
Aluminium oxide (Al₂O₃) is the standard example. With hydrochloric acid it behaves as a base and gives aluminium chloride; with sodium hydroxide it behaves as an acid and gives sodium aluminate. This is why the thin, tough layer of Al₂O₃ that forms on aluminium metal dissolves in both acids and alkalis. Zinc oxide (ZnO) behaves in exactly the same way.
⚗️ The Reactions
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O *(acts as a base)*
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O *(acts as an acid)*
• With an acid it gives the salt AlCl₃; with an alkali it gives sodium aluminate (NaAlO₂).
• Reacting with both an acid and a base is precisely what makes an oxide amphoteric.
• The same duality is seen with ZnO: ZnO + 2HCl → ZnCl₂ + H₂O and ZnO + 2NaOH → Na₂ZnO₂ + H₂O.
❌ Why the other options are incorrect
✗ Copper oxide — CuO is a basic oxide of the metal copper; it dissolves in acids (CuO + 2HCl → CuCl₂ + H₂O) but does not react with alkalis such as NaOH.
✗ Sodium peroxide — Na₂O₂ is a peroxide of a highly reactive alkali metal and is strongly basic (with water it gives NaOH and O₂); a peroxide is not the same thing as an amphoteric oxide.
✗ Calcium oxide — CaO (quicklime) is a typical basic oxide; with water it gives the base Ca(OH)₂, and it reacts only with acids, never with alkalis.
📌 SSC & Railway Exam Facts
• Amphoteric oxides: Al₂O₃, ZnO, PbO, SnO, BeO — they react with both acids and bases.
• Metal oxides are basic, non-metal oxides are acidic; CO, NO and H₂O are neutral oxides.
• The ore of aluminium is bauxite (Al₂O₃·2H₂O); the metal is extracted by the Hall–Héroult electrolytic process.
• Anodising deliberately thickens the Al₂O₃ layer to protect aluminium from corrosion.
• Corundum is crystalline Al₂O₃; ruby and sapphire are its coloured forms.

Correct Answer: Option 3 — Magnetic field lines
Each iron filing is a tiny piece of soft iron. Placed near a magnet it becomes a small induced magnet and turns until it points along the direction of the magnetic field at that spot. Thousands of such filings line up end to end and trace out curved chains — these chains are the magnetic field lines (also called lines of force) of the magnet.
The pattern shows that the field is strongest at the two poles, where the filings crowd most densely, and weaker in the middle. Outside the magnet every line runs from the North pole to the South pole, and inside the magnet it continues from South to North, so the lines form closed loops. Two field lines can never intersect, because at a crossing point the compass needle would have to point in two directions at once.
❌ Why the other options are incorrect
✗ Both electric and magnetic field lines — a bar magnet carries no net charge, so it produces no electric field; only the magnetic pattern appears.
✗ Electric field lines — these are produced by electric charges and are mapped with a test charge, not with iron filings.
✗ Neither electric nor magnetic field lines — the pattern is the standard classroom demonstration of a magnetic field, so this is plainly wrong.
📌 SSC & Railway Exam Facts
• Magnetic field is a vector quantity; SI unit tesla (T); CGS unit gauss; 1 T = 10⁴ gauss.
• Field lines are closed, continuous curves; they never cross each other.
• Closer the field lines, stronger the field — hence maximum crowding at the poles.
• The direction of the field at a point is the direction in which the north pole of a compass points there.
• Magnetic field lines were introduced by Michael Faraday.

Correct Answer: Option 1 — principal axis
The principal axis of a spherical mirror is the imaginary straight line joining the pole (P) — the geometrical centre of the reflecting surface — to the centre of curvature (C), the centre of the sphere of which the mirror is a part. Because it passes through C, the principal axis is always normal (perpendicular) to the mirror at the pole.
Every important point of the mirror lies on this line: the pole P, the principal focus F, and the centre of curvature C, with PC = R and PF = f, related by R = 2f. All object and image distances in the mirror formula 1/v + 1/u = 1/f are measured along the principal axis.
A ray travelling along the principal axis strikes the mirror normally and is reflected straight back on itself, while rays parallel to the principal axis converge at (or appear to diverge from) the focus.
❌ Why the other options are incorrect
✗ reflected ray — this is the ray that bounces off the mirror after striking it; it is a real light ray, not a fixed geometrical line of the mirror.
✗ emergent ray — this term belongs to refraction: it is the ray that comes out of a glass slab or a prism.
✗ incident ray — this is the ray falling on the mirror; its direction changes with the position of the source, so it cannot define the mirror's axis.
📌 SSC & Railway Exam Facts
• Key points of a spherical mirror: Pole (P), Centre of curvature (C), Principal focus (F), Radius of curvature (R), Aperture.
• R = 2f, so f = R/2.
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u = h'/h.
• Laws of reflection: ∠i = ∠r, and the incident ray, reflected ray and normal all lie in the same plane.
• A concave mirror is converging; a convex mirror is diverging.

Correct Answer: Option 3 — P = m v
Momentum is the "quantity of motion" of a body. It is defined as the product of the mass of the body and its velocity:
p = m × v
It is a vector quantity whose direction is the same as that of the velocity, and its SI unit is kg m/s (equivalently N s). A loaded truck moving slowly and a light bullet moving very fast can both carry large momentum, because momentum depends on both mass and speed.
Momentum is the heart of Newton's second law, which says that the rate of change of momentum is proportional to the applied force: F = (mv − mu)/t = ma. It also gives the law of conservation of momentum — in the absence of an external force the total momentum of a system stays constant, which explains the recoil of a gun and the working of a rocket.
❌ Why the other options are incorrect
✗ P = m v⁻² — this is m/v², which would make momentum *decrease* as the body moves faster; the units kg s²/m² are meaningless here.
✗ P = v m⁻¹ — this is v/m, so a heavier body would have less momentum at the same speed, which contradicts everyday experience.
✗ p = (mv)⁻² — this is 1/(mv)², a completely different quantity with units of s²/(kg² m²); momentum is directly, not inversely, related to mass and velocity.
📌 SSC & Railway Exam Facts
• Momentum p = mv; SI unit kg m/s or N s; it is a vector.
• Newton's second law: F = dp/dt = ma; Impulse = F × t = change in momentum.
• Law of conservation of momentum: total momentum before = total momentum after, for an isolated system.
• Kinetic energy in terms of momentum: KE = p²/2m.
• Recoil velocity of a gun: m₁v₁ = m₂v₂, so a heavier gun recoils more slowly.

Correct Answer: Option 4 — N m⁻²
Pressure is defined as the thrust (force acting normally) per unit area:
P = F/A
So its unit must be the unit of force divided by the unit of area — that is newton per square metre, N m⁻². This unit is given the special name pascal (Pa), so 1 Pa = 1 N m⁻².
Because area sits in the denominator, the same force spread over a smaller area produces a much larger pressure. That is why a sharp knife cuts easily, why a nail is pointed, and why a camel's broad feet stop it from sinking into sand. Pressure is a scalar quantity even though force is a vector, since it acts equally in all directions inside a fluid.
❌ Why the other options are incorrect
✗ N m — newton × metre is the unit of work / energy (joule) and also of torque, not of pressure.
✗ N m⁻¹ — newton per metre is the unit of surface tension and of spring constant, not pressure.
✗ N — the newton alone is the unit of force; pressure needs force *divided by area*.
📌 SSC & Railway Exam Facts
• Pressure P = F/A; SI unit pascal (Pa) = N m⁻²; named after Blaise Pascal.
• 1 atmosphere = 1.013 × 10⁵ Pa = 760 mm of Hg = 1.013 bar.
• Pressure in a liquid at depth h: P = hρg; it depends on depth and density, not on the shape of the vessel.
• Pascal's law: pressure applied to an enclosed fluid is transmitted equally in all directions — used in hydraulic lifts and brakes.
• Atmospheric pressure is measured by a barometer; pressure decreases with altitude.

Correct Answer: Option 4 — 6.7 × 10⁻¹¹ N m² kg⁻²
By Newton's universal law of gravitation, two bodies of masses M and m separated by a distance r attract each other with a force
F = G M m / r²
The constant G in this equation is the universal gravitational constant. Its accepted value is
G = 6.67 × 10⁻¹¹ N m² kg⁻², usually rounded to 6.7 × 10⁻¹¹ N m² kg⁻².
It is called *universal* because it has the same value everywhere in the universe — it does not depend on the masses, the medium between them, or the place. Its value is extremely small, which is why gravitational attraction between ordinary objects around us is far too weak to notice; it becomes important only when at least one body is of planetary size.
❌ Why the other options are incorrect
✗ 1 × 10⁻¹¹ N m² kg⁻² — the exponent is right but the number is wrong; the accepted value is about 6.7, not 1.
✗ 6.7 N m² kg⁻² — the digits are right but the factor 10⁻¹¹ is missing, making the value about 10¹¹ times too large.
✗ 5 × 10⁻¹ N m² kg⁻² — that is 0.5, enormously bigger than G; a value like this would make everyday objects stick together by gravity.
📌 SSC & Railway Exam Facts
• F = GMm/r²; gravitational force obeys the inverse-square law and is always attractive.
• G = 6.67 × 10⁻¹¹ N m² kg⁻²; it is a scalar and a universal constant.
• g = GM/R²; on Earth g ≈ 9.8 m s⁻², and on the Moon it is about g/6.
• G was first measured experimentally by Henry Cavendish using a torsion balance.
• Do not confuse G (universal constant) with g (acceleration due to gravity, unit m s⁻²).

Correct Answer: Option 4 — 1000 J s⁻¹
Power is the rate of doing work, so
P = W/t, giving the unit joule per second, which is named the watt (W). Therefore 1 W = 1 J/s = 1 J s⁻¹.
The kilowatt is simply one thousand watts:
1 kW = 1000 W = 1000 J s⁻¹
So a 1 kW heater converts 1000 joules of electrical energy into heat every second. Note carefully that the kilowatt measures power (a rate), while the kilowatt-hour (kWh) measures energy — the energy used by a 1 kW appliance running for one hour, equal to 3.6 × 10⁶ J. This is the "unit" on an electricity bill.
❌ Why the other options are incorrect
✗ 1000 J s — joule multiplied by second is not power at all; it is the unit of "action", as in Planck's constant.
✗ 1000 J s⁻² — joule per second squared would be the *rate of change of power*, not power itself.
✗ 1000 J — the joule alone is the unit of work and energy; without dividing by time it says nothing about how fast the work is done.
📌 SSC & Railway Exam Facts
• Power P = W/t = Energy/Time; SI unit watt (W) = J s⁻¹.
• 1 kW = 1000 W; 1 MW = 10⁶ W; 1 horsepower (hp) = 746 W.
• 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J — the commercial unit of electrical energy.
• Electrical power: P = VI = I²R = V²/R.
• The watt is named after James Watt; the joule after James Prescott Joule.

Correct Answer: Option 4 — Electric motor
Fleming's Left Hand Rule gives the direction of the force on a current-carrying conductor placed in a magnetic field. Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular: the Forefinger points along the magnetic Field, the Middle finger along the Current, and then the Thumb gives the direction of Thrust (force / motion).
An electric motor works on exactly this principle. A current-carrying coil placed in a magnetic field experiences equal and opposite forces on its two arms; this pair of forces forms a couple that makes the coil rotate, converting electrical energy into mechanical energy. A split-ring commutator reverses the current every half turn so that the rotation continues in the same sense.
❌ Why the other options are incorrect
✗ Electric Iron — works on the heating effect of current (Joule's law, H = I²Rt), using a nichrome heating element.
✗ Electric Bulb — also the heating effect: a tungsten filament is heated until it glows white hot.
✗ Electric Geyser — again the heating effect, with a resistive coil warming the water.
📌 SSC & Railway Exam Facts
• Fleming's Left Hand Rule → motor (force on a current-carrying conductor).
• Fleming's Right Hand Rule → generator / dynamo (induced current, electromagnetic induction).
• Force on a conductor: F = BIL sin θ; maximum when the conductor is perpendicular to the field.
• A motor converts electrical → mechanical energy; a generator converts mechanical → electrical.
• Electromagnetic induction was discovered by Michael Faraday; the right-hand thumb rule gives the field direction around a straight current.

Correct Answer: Option 3 — increased by an amount mgH
To lift a body of mass m slowly through a height H, you must apply an upward force just equal to its weight, F = mg. The work you do against gravity is
W = Force × displacement = mg × H = mgH
This work is not lost — it is stored in the body as gravitational potential energy because of its new position. Hence the potential energy increases by mgH. If the body is released, this stored energy converts back into kinetic energy, and just before hitting the ground ½mv² = mgH, giving v = √(2gH).
Raising a body always increases its potential energy; lowering it decreases the PE. Only the change in height matters, so the actual path taken — straight up, along a ramp or a staircase — makes no difference. Gravity is therefore called a conservative force.
❌ Why the other options are incorrect
✗ decreased by an amount mgH/2 — wrong in both respects: PE rises, not falls, and there is no factor of ½ (that ½ belongs to kinetic energy, ½mv²).
✗ decreased by an amount mgH — the magnitude is right but the sign is reversed; PE falls only when a body comes down.
✗ increased by an amount mgH/2 — the direction of change is right, but the work done against gravity is the full mgH, since the force mg acts over the whole height H.
📌 SSC & Railway Exam Facts
• Gravitational PE = mgh; SI unit joule (J).
• KE = ½mv²; Mechanical energy = KE + PE, conserved when friction is absent.
• Speed on reaching the ground from height h: v = √(2gh).
• PE depends only on the change in height, not on the path — gravity is a conservative force.
• Work done is zero when a body is carried horizontally, since the height does not change.

Correct Answer: Option 1 — I²R
Electric power is the rate at which electrical energy is converted into heat in a resistor. Starting from
P = V × I
and substituting Ohm's law V = IR, we get
P = (IR) × I = I²R
Similarly, putting I = V/R gives the third standard form P = V²/R. So the three equivalent expressions are P = VI = I²R = V²/R, all measured in watts.
The form P = I²R is the one behind Joule's law of heating, H = I²Rt. Notice that the heat depends on the square of the current — doubling the current makes the heating four times greater. This is why transmission lines carry electricity at high voltage and low current, to keep I²R losses small, and why fuse wire melts when the current becomes excessive.
❌ Why the other options are incorrect
✗ IR — by Ohm's law this is simply the potential difference V across the resistor, measured in volts, not power.
✗ I/R — current divided by resistance has no physical meaning here; power must increase with resistance at a fixed current, not decrease.
✗ IR² — squaring the resistance instead of the current is a common slip; it is dimensionally wrong for power.
📌 SSC & Railway Exam Facts
• P = VI = I²R = V²/R; SI unit watt (W).
• Joule's law of heating: H = I²Rt joules.
• For a series combination the same current flows, so the largest resistance produces the most heat.
• Electrical energy is billed in kWh; 1 kWh = 3.6 × 10⁶ J = 1 "unit".
• A fuse is a short wire of low melting point and high resistance, always connected in series with the live wire.

Correct Answer: Option 2 — m g h
The gravitational potential energy of a body is the energy it possesses because of its position above the ground. To raise a body of mass m through a height h, a force equal to its weight mg must act over the distance h, so the work done is
W = mg × h = mgh
This work is stored in the body, hence
Gravitational PE = m g h
Here g is the acceleration due to gravity, about 9.8 m s⁻² on the Earth's surface. The unit works out correctly: kg × m s⁻² × m = N m = joule. Potential energy is measured from a chosen reference level (usually the ground, where PE is taken as zero), and only the difference in PE between two positions is physically meaningful.
❌ Why the other options are incorrect
✗ m g v — this mixes velocity into a position-based quantity; mass × g × velocity has the unit watt, i.e. power, not energy.
✗ m a v — again mass × acceleration × velocity gives power (J/s), not potential energy.
✗ m a — mass × acceleration is simply force (newton); multiplying by a distance is still needed to obtain energy.
📌 SSC & Railway Exam Facts
• Gravitational PE = mgh; Kinetic energy KE = ½mv²; both in joules.
• g = 9.8 m s⁻² on Earth (often taken as 10), about 1.6 m s⁻² on the Moon.
• Energy is a scalar; it can be transformed but never created or destroyed (law of conservation of energy).
• A body falling from height h hits the ground with v = √(2gh).
• Other kinds of potential energy: elastic PE in a stretched spring, chemical PE in fuel, electrical PE in a charged capacitor.

Correct Answer: Option 2 — 20 m/s
📖 Given
• Initial velocity, u = 0 (the body starts from rest)
• Acceleration, a = g = 10 ms⁻² (free fall, downwards)
• Time of fall, t = 2 s
✍️ Formula
v = u + at
🔢 Solution
• v = 0 + (10 × 2)
• v = 0 + 20
• v = 20 m/s
So the body strikes the ground moving downwards at 20 metres every second. (Its height can also be found: h = ½gt² = ½ × 10 × 4 = 20 m.)
❌ Why the other options are incorrect
✗ 5 m/s — this comes from dividing instead of multiplying, 10/2 = 5; time must be multiplied by g.
✗ 15 m/s — no combination of the given data gives 15; it is only a distractor lying between 10 and 20.
✗ 10 m/s — this is the velocity after just 1 second, or the value of g mistaken for the answer.
📌 SSC & Railway Exam Facts
• Equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• In free fall put u = 0 and a = g; then v = gt, h = ½gt², v = √(2gh).
• g ≈ 9.8 m s⁻² on Earth (10 m s⁻² in exams); it is independent of the mass of the falling body.
• In the absence of air, a feather and a stone dropped together reach the ground at the same time — shown in vacuum-tube experiments.
• g is maximum at the poles and minimum at the equator; it decreases with both altitude and depth.

Correct Answer: Option 1 — Firing of a gun
Newton's universal law of gravitation states that every body attracts every other body with a force
F = G M m / r²
It explains all phenomena caused by mutual gravitational attraction between large masses.
The firing of a gun is *not* one of them. When a gun is fired, the expanding gases push the bullet forward and, by Newton's third law, push the gun backwards — the recoil. The recoil velocity follows from the law of conservation of momentum, m₁v₁ = m₂v₂. Gravity plays no part in producing the bullet's forward motion, so this phenomenon belongs to the laws of motion, not to gravitation.
❌ Why the other options are incorrect
✗ Motion of planets around the Sun — the Sun's gravitational pull supplies the centripetal force that keeps each planet in orbit; this was Newton's greatest success and it explains Kepler's laws.
✗ Motion of the moon around the earth — the Earth's gravity continuously deflects the Moon from a straight path, holding it in orbit.
✗ Tides — the rise and fall of sea water is caused by the gravitational attraction of the Moon (and to a lesser extent the Sun) on the oceans.
📌 SSC & Railway Exam Facts
• F = GMm/r²; G = 6.67 × 10⁻¹¹ N m² kg⁻²; gravitation is always attractive.
• The universal law of gravitation explains: falling bodies, g, planetary orbits, the Moon's motion, tides and the shape of galaxies.
• Gun recoil follows from conservation of momentum and Newton's third law — action and reaction are equal and opposite.
• Gravitational force is the weakest of the four fundamental forces, yet it acts over infinite range.
• Kepler's three laws of planetary motion were later derived from Newton's law of gravitation.

Correct Answer: Option 1 — 23/60 kg
📖 Given
• Force applied in both cases, F = 2 N
• Acceleration of the first mass, a₁ = 8 m s⁻²
• Acceleration of the second mass, a₂ = 15 m s⁻²
✍️ Formula
Newton's second law: F = ma, so m = F/a
🔢 Solution
• m₁ = 2/8 = 1/4 kg
• m₂ = 2/15 kg
• Total mass = m₁ + m₂ = 1/4 + 2/15
• Taking LCM of 4 and 15 as 60: = 15/60 + 8/60
• Total mass = 23/60 kg
So the two bodies tied together weigh 23/60 kg ≈ 0.383 kg. (A check: the same 2 N force would now give them an acceleration of 2 ÷ 23/60 = 120/23 ≈ 5.2 m s⁻², less than either separate value — as expected for a bigger mass.)
❌ Why the other options are incorrect
✗ 10/40 kg — this is 1/4 kg, i.e. only m₁; the second mass has been left out.
✗ 4/10 kg — this is 0.4 kg, slightly more than the true answer; it comes from rounding 2/15 up to 0.15 and adding carelessly.
✗ 3/10 kg — 0.3 kg; obtained if you wrongly take m₂ = 2/15 as about 0.05 kg instead of 0.133 kg.
📌 SSC & Railway Exam Facts
• Newton's second law: F = ma = rate of change of momentum; SI unit of force is the newton (N).
• 1 N = 1 kg m s⁻² — the force that gives a 1 kg mass an acceleration of 1 m s⁻².
• For a fixed force, a ∝ 1/m: the heavier the body, the smaller its acceleration.
• When masses are tied together they move as one body of mass m₁ + m₂.
• Newton's first law defines inertia; the third law gives action = −reaction.