RRB Technician Grade III 2024-25 24 December 2024 Shift 2 GK Questions – with answers & solutions

Correct Answer: Option 4 — Flower
Vegetative propagation is a kind of asexual reproduction in which a new plant develops from a vegetative part of the parent — the root, stem, leaf or bud. No gametes and no fertilisation are involved, so the new plants are clones, genetically identical to the parent.
The flower is the sexual reproductive organ of an angiosperm. It carries the stamen (which makes pollen grains) and the pistil (which holds the ovules), and it works through pollination and fertilisation. Since it is a reproductive and not a vegetative organ, the flower never takes part in vegetative propagation.
❌ Why the other options are incorrect
✗ Leaves — a true vegetative part; Bryophyllum grows new plantlets from buds present in the notches of its leaf margin.
✗ Stem — the most widely used vegetative part; potato (tuber), ginger (rhizome), sugarcane, rose and mint are all propagated from stem pieces or cuttings.
✗ Root — sweet potato, dahlia, asparagus and Dalbergia develop new shoots from adventitious buds on roots.
📌 SSC & Railway Exam Facts
• Natural methods — runner (grass), tuber (potato), rhizome (ginger), bulb (onion), sucker (banana), leaf buds (Bryophyllum).
• Artificial methods — cutting, grafting, layering and tissue culture (micropropagation).
• Banana, orange, rose and jasmine have lost the ability to form viable seeds and are grown only vegetatively.
• Advantage — earlier flowering and fruiting, and all offspring keep the parental characters.
• Disadvantage — no variation, so the whole crop can be wiped out by a single disease.

Correct Answer: Option 2 — On a suitable stigma pollen tubes grow out of the pollen grain and travel through the style and reach the ovary.
In flowering plants sexual reproduction begins with pollination — the transfer of pollen grains from the anther to the stigma. When a pollen grain falls on a suitable (compatible) stigma, it germinates and puts out a pollen tube. This tube grows downward through the tissue of the style, enters the ovary and reaches an ovule through the micropyle.
The male germ-cell is then released and fuses with the egg cell (female gamete) inside the ovule. This fusion is fertilisation, and it produces a zygote. The zygote divides repeatedly to build the embryo.
❌ Why the other options are incorrect
✗ Option 1 — after fertilisation the ovary develops into the fruit, not into a seed.
✗ Option 3 — the zygote must divide again and again (mitosis) to form the embryo; an embryo can never form without division.
✗ Option 4 — the ovule develops into the seed, while the ovary becomes the fruit; this statement interchanges the two.
📌 SSC & Railway Exam Facts
• Self-pollination occurs within the same flower or plant; cross-pollination is between two flowers of different plants of the same species.
• Agents of cross-pollination — wind (anemophily), water (hydrophily), insects (entomophily), birds (ornithophily).
• After fertilisation — zygote → embryo, ovule → seed, ovary → fruit; sepals, petals and stamens usually wither.
• Angiosperms show double fertilisation: zygote (2n) and endosperm (3n) are formed together.
• A seed has seed coat, cotyledons and embryo (plumule + radicle); the plumule forms the shoot and the radicle the root.

Correct Answer: Option 2 — Regeneration is not the same as reproduction.
Regeneration is the ability of a simple organism to grow back a lost or damaged body part, or to develop a complete organism from a broken piece of its body. It is carried out by specialised cells which proliferate and then differentiate into different tissues and organs in a definite sequence.
NCERT clearly states that regeneration is not the same as reproduction, because most organisms that can regenerate do not normally multiply by breaking themselves into pieces. Regeneration is basically a repair and survival mechanism; reproduction is a deliberate process of producing new individuals to continue the species.
Hydra and Planaria are the classic examples of regeneration.
❌ Why the other options are incorrect
✗ Option 1 — regeneration and reproduction are different processes; calling them the same in all organisms is wrong.
✗ Option 3 — many organisms can regenerate — Hydra, Planaria, starfish, and even the human liver and a lizard's tail.
✗ Option 4 — regeneration usually only restores a lost part; it does not, as a rule, create a totally new independent organism.
📌 SSC & Railway Exam Facts
• Hydra and Planaria regenerate fully; Hydra also reproduces by budding.
• Starfish can regrow a lost arm; a lizard can regrow its tail (autotomy).
• The liver is the only human organ with a strong power of regeneration.
• Other modes of asexual reproduction — binary fission (Amoeba), multiple fission (Plasmodium), fragmentation (Spirogyra), spore formation (Rhizopus), budding (yeast).
• Regeneration was first studied in Hydra by Abraham Trembley.

Correct Answer: Option 4 — Only a
Plants have two conducting (vascular) tissues with completely different jobs. Xylem carries water and dissolved minerals upward from the roots to the stem and leaves, while phloem carries prepared food (sucrose) from the leaves to the rest of the plant, a process called translocation. Phloem therefore has no role in absorbing water and minerals from the soil, so statement a is incorrect.
Checking the rest:
• b is correct — the root tip bears thousands of fine root hairs that enormously increase the surface area for absorption; water enters them by osmosis and minerals largely by active transport.
• c is correct — the vascular tissue (xylem) is exactly the special tissue that moves water and dissolved nutrients taken from the soil.
• d is correct — the xylem of root, stem and leaf is joined into a continuous network of channels, so water rises in an unbroken column up to the leaves.
❌ Why the other options are incorrect
✗ c and d — both these statements are factually correct, so neither can be the incorrect one.
✗ Only d — the continuous xylem network between roots and leaves is a correct NCERT statement.
✗ b and c — root hairs do increase absorbing surface and vascular tissue does transport water; both statements are right.
📌 SSC & Railway Exam Facts
• Xylem — dead tissue, unidirectional (upward) flow; phloem — living tissue, bidirectional flow of food.
• Xylem elements — tracheids, vessels, xylem parenchyma, xylem fibres; phloem elements — sieve tubes, companion cells, phloem parenchyma, phloem fibres.
• Water rises through the stem by root pressure and mainly by the transpiration pull (cohesion–tension theory).
• Transpiration — loss of water vapour through stomata; it also cools the plant and is called a necessary evil.
• Translocation in phloem needs energy (ATP) and was explained by Münch's pressure-flow hypothesis.

Correct Answer: Option 1 — Vas deferens and scrotum
Both structures named in this option belong to the male reproductive system.
• The scrotum is a muscular pouch of skin hanging outside the abdominal cavity that holds the testes. It keeps them 2–3 °C cooler than the body, the temperature needed for sperm formation.
• The vas deferens (sperm duct) is the tube that carries sperms from the epididymis of each testis and joins the urethra, through which semen finally leaves the body.
The complete male system is testes → epididymis → vas deferens → seminal vesicle and prostate → urethra → penis.
❌ Why the other options are incorrect
✗ Fallopian tube and penis — a mixed pair; the fallopian tube (oviduct) is a female organ where fertilisation occurs, though the penis is male.
✗ Uterus and Vagina — both are purely female organs; the uterus houses the developing embryo and the vagina is the birth canal.
✗ Vas deferens and oviduct — again mixed; the oviduct is simply another name for the fallopian tube, a female structure.
📌 SSC & Railway Exam Facts
• Testes have a dual role — production of sperms and secretion of the hormone testosterone.
• Semen = sperms + secretions of the seminal vesicle and prostate gland, which nourish sperms and ease their transport.
• Female reproductive organs — ovaries, fallopian tubes, uterus, cervix and vagina.
• The human ovum is the largest cell in the body; the sperm is the smallest and is motile with a tail.
• Vasectomy in males and tubectomy in females are surgical methods of family planning.

Correct Answer: Option 1 — Two
The Endoplasmic Reticulum (ER) is a large network of membrane-bound tubules, vesicles and sheets spread through the cytoplasm. It occurs in two types:
• Rough Endoplasmic Reticulum (RER) — its surface is studded with ribosomes, which make it look rough. It is the site of protein synthesis and of the transport of those proteins to the Golgi apparatus.
• Smooth Endoplasmic Reticulum (SER) — it carries no ribosomes. It synthesises lipids and steroids and, in liver cells, detoxifies poisons and drugs.
Because the ER also serves as a channel between the cytoplasm and the nucleus and provides a huge surface for reactions, it is called the cytoplasmic framework of the cell.
❌ Why the other options are incorrect
✗ Four — no textbook recognises four kinds of ER; only the rough and smooth forms exist.
✗ Three — a third variety of ER does not exist; the Golgi apparatus, though related, is a separate organelle.
✗ Five — far too many; the ER membrane is simply either ribosome-bearing (RER) or ribosome-free (SER).
📌 SSC & Railway Exam Facts
• The endoplasmic reticulum was discovered by Keith Porter (1945).
• ER takes part in membrane biogenesis, since its membrane resembles the plasma membrane.
• Ribosomes were discovered by George Palade; they are the protein factories of the cell.
• The Golgi apparatus packages, modifies and dispatches the material produced by the ER.
• ER is absent in prokaryotes such as bacteria, which have no membrane-bound organelles.

Correct Answer: Option 1 — Both A and C
Non-biodegradable substances are materials that decomposers such as bacteria and fungi cannot break down. Since no enzyme of any microbe acts on them, they stay in the environment almost unchanged.
Checking each statement:
• A is correct — they are not broken down by biological processes; plastic, glass and metals are not attacked by decomposers.
• B is wrong — they do not persist for a short period; they remain for decades or even centuries, which is precisely the problem.
• C is correct — they harm many members of the ecosystem: plastic chokes cattle and marine animals, blocks drains and soil pores, and pesticides like DDT pass up the food chain by biomagnification.
Hence only A and C are correct.
❌ Why the other options are incorrect
✗ A, B and C — includes B, which wrongly says these substances stay for only a short period.
✗ Only B — statement B is itself false, so it can never be the sole correct answer.
✗ Both B and C — again accepts the false statement B and leaves out the correct statement A.
📌 SSC & Railway Exam Facts
• Non-biodegradable — plastic, polythene, glass, metals, DDT, synthetic fibres, aluminium foil.
• Biodegradable — food and vegetable waste, paper, cotton, wool, wood, leather, jute, cow dung.
• Biomagnification — the rise in concentration of harmful chemicals at each higher trophic level; humans, being at the top, are worst affected.
• 3R principle — Reduce, Reuse, Recycle; segregate wet waste in green bins and dry waste in blue bins.
• Burning plastic releases poisonous gases such as dioxins; India has banned many single-use plastics.

Correct Answer: Option 3 — Variation
Variations are the small differences that appear among individuals of the same species. They arise because DNA copying during reproduction is never perfectly accurate, and also because of recombination of genes during sexual reproduction.
NCERT states that variation is useful for the survival of species over time. If a population is completely uniform and the environment changes suddenly — a rise in temperature, a drought, or a new disease — the entire population could be wiped out. But if variations exist, a few individuals may already possess the character needed to survive the new condition. These survive, reproduce and rebuild the population. Variation therefore gives a species its long-term survival value and provides the raw material for natural selection and evolution.
❌ Why the other options are incorrect
✗ Growth and development — these only increase the size and maturity of an individual; they cannot protect a species against a changing environment.
✗ Fragmentation — a mode of asexual reproduction seen in *Spirogyra*; it produces identical copies and so generates no variation at all.
✗ Reproduction — it certainly continues the species, but by itself, if it made only exact copies, it would not ensure long-term survival; it is the variation arising during reproduction that does.
📌 SSC & Railway Exam Facts
• Sexual reproduction produces far more variation than asexual reproduction.
• Variations arise from errors in DNA copying, mutations and recombination of genes.
• Charles Darwin explained evolution by natural selection in his book *On the Origin of Species* (1859).
• Lamarck gave the theory of inheritance of acquired characters.
• Speciation — the formation of a new species — needs variation, geographical isolation and genetic drift.

Correct Answer: Option 3 — Vascular tissue
Vascular (conducting) tissue is the special complex permanent tissue of higher plants, made of xylem and phloem. NCERT describes it as a distinctive feature of complex plants, one that has made possible their survival in the terrestrial environment.
On land, water is available only in the soil while sunlight and CO₂ are available only above ground. A land plant must therefore move materials over long distances:
• Xylem conducts water and dissolved minerals from the roots upward to the stem and leaves and also gives mechanical strength.
• Phloem conducts the food (sucrose) prepared in the leaves to the roots, stems, fruits and storage organs — a process called translocation.
Without this plumbing system, tall land plants could never survive away from water.
❌ Why the other options are incorrect
✗ Epithelial tissue — an animal tissue that covers and lines body organs, forming a protective sheet; it is not found in plants.
✗ Connective tissue — again an animal tissue (bone, blood, cartilage, ligament) whose cells lie in an intercellular matrix.
✗ Meristematic tissue — a plant tissue indeed, but its job is cell division and growth at root and shoot tips; it does not conduct water or food.
📌 SSC & Railway Exam Facts
• Xylem — dead tissue, unidirectional (upward) flow; phloem — living tissue, bidirectional flow.
• Xylem components — tracheids, vessels, xylem parenchyma, xylem fibres; phloem components — sieve tubes, companion cells, phloem parenchyma, phloem fibres.
• Permanent plant tissues — simple (parenchyma, collenchyma, sclerenchyma) and complex (xylem, phloem).
• Apical meristem increases length, lateral meristem (cambium) increases girth, intercalary meristem lies at the base of leaves and internodes.
• Water rises in the plant mainly by the transpiration pull; root pressure helps at night.

Correct Answer: Option 2 — Biceps
The biceps is a skeletal muscle of the upper arm, attached to bone by tendons. Skeletal muscles are striated (striped) and, above all, voluntary — they contract only when we consciously decide to move the arm, and they relax when we decide to stop. Their fibres are long, cylindrical, unbranched and multinucleate, and they get tired after continuous work. So the biceps is the one body part in the list that contains no involuntary muscle.
Involuntary muscles, by contrast, work on their own under the autonomic nervous system, without our will, and never tire easily.
❌ Why the other options are incorrect
✗ Stomach — its wall is made of smooth (unstriated) involuntary muscle that churns food and pushes it forward by peristalsis.
✗ Lungs — the walls of the bronchi and bronchioles contain smooth involuntary muscle, which widens or narrows the air passages automatically.
✗ Heart — made of cardiac muscle, which is striated but branched, uninucleate and involuntary, beating rhythmically all life without fatigue.
📌 SSC & Railway Exam Facts
• Three muscle types — striated (skeletal, voluntary), unstriated (smooth, involuntary) and cardiac (involuntary).
• Smooth muscle occurs in the alimentary canal, blood vessels, iris, ureters and bronchi.
• Contractile proteins of muscle are actin and myosin; muscle cells are called muscle fibres.
• Tendon attaches muscle to bone; ligament attaches bone to bone.
• Build-up of lactic acid in skeletal muscle during heavy exercise causes cramps; cardiac muscle never fatigues.

Correct Answer: Option 2 — (i), (ii) and (iii)
Enzymes are biological catalysts — proteins that speed up chemical reactions in the body without being used up themselves. All three statements about them are true:
• (i) is correct — the food we eat is digested by a series of enzymes: salivary amylase in the mouth, pepsin in the stomach, and trypsin, lipase and amylase from the pancreas, together with the enzymes of intestinal juice.
• (ii) is correct — every enzyme is specific in its action and fits only one kind of substrate, just as a particular key fits only one lock (the lock-and-key model).
• (iii) is correct — because of this specificity, a separate enzyme is required for each substance: amylase only for starch, pepsin and trypsin for proteins, and lipase for fats.
This specificity is also why non-biodegradable substances survive — no decomposer has an enzyme that fits them.
❌ Why the other options are incorrect
✗ Only (ii) and (iii) — it leaves out (i), which correctly says that our food is digested by various enzymes.
✗ Only (i) and (ii) — it wrongly rejects (iii); specific enzymes are indeed needed for particular substances.
✗ Only (i) — it rejects both (ii) and (iii), although enzyme specificity is a basic and well-established fact.
📌 SSC & Railway Exam Facts
• Enzymes are mostly proteins; they work best at body temperature (about 37 °C) and at a specific pH.
• Pepsin needs an acidic medium (stomach), while trypsin needs an alkaline medium (small intestine).
• Salivary amylase (ptyalin) converts starch → maltose; lipase converts fat → fatty acids + glycerol.
• High temperature denatures enzymes and destroys their activity permanently.
• Bile contains no enzyme; it only emulsifies fat and makes the medium alkaline for pancreatic enzymes.

Correct Answer: Option 2 — Chromosomes
Chromosomes are the rod-shaped structures seen inside the nucleus only when the cell is about to divide. In a non-dividing cell the genetic material lies as a loose, thread-like network called chromatin. As division approaches, this chromatin condenses and coils tightly and becomes visible as distinct rod-like chromosomes.
Each chromosome is made of DNA and protein and carries the hereditary information in units called genes. The tight packing ensures that the DNA is distributed equally and without tangling to the two daughter cells.
❌ Why the other options are incorrect
✗ Lysosomes — membrane-bound sacs of digestive enzymes in the cytoplasm, called the 'suicide bags'; they are not in the nucleus and are not rod-shaped.
✗ Mitochondria — rod-shaped indeed, but they are cytoplasmic organelles, the powerhouse producing ATP; they stay in the cell at all times, not only at division.
✗ Chromatin — the same material, but in its uncondensed, thread-like (network) form seen in a non-dividing cell; it is not rod-shaped.
📌 SSC & Railway Exam Facts
• The term chromosome was coined by W. Waldeyer; chromatin by W. Flemming.
• Human body cells have 46 chromosomes (23 pairs); gametes have 23.
• Chromosome number — pea 14, onion 16, housefly 12, dog 78, rice 24.
• The point at which the two chromatids of a chromosome are joined is the centromere.
• Genes are the functional segments of DNA on the chromosome; DNA + protein (histone) = chromatin.

Correct Answer: Option 1 — Plastids
Plastids are large, double-membrane organelles found only in plant cells. The type that helps in photosynthesis is the chloroplast, which contains the green pigment chlorophyll.
Inside a chloroplast the inner membrane is folded into stacks of flat sacs called grana (thylakoids), suspended in a fluid stroma. Chlorophyll in the grana traps sunlight and converts it to chemical energy, which is then used in the stroma to build glucose from CO₂ and water, releasing oxygen. This is why the chloroplast is called the kitchen of the cell.
Other plastids are chromoplasts, which give colour to flowers and fruits, and leucoplasts, which store starch, oils and proteins.
❌ Why the other options are incorrect
✗ Lysosomes — membrane sacs containing powerful digestive enzymes; called the 'suicide bags', they clear worn-out organelles but make no food.
✗ Nucleus — the control centre of the cell; it holds the chromosomes (DNA) and directs all activities, but it does not carry out photosynthesis.
✗ Golgi body — the packaging and dispatching centre that modifies and sends out materials made by the ER; it forms lysosomes but has no pigment.
📌 SSC & Railway Exam Facts
• Photosynthesis — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of sunlight and chlorophyll.
• Chloroplast is the kitchen of the cell; mitochondria the powerhouse; ribosome the protein factory.
• Chloroplast and mitochondria both have their own DNA and ribosomes — they are semi-autonomous.
• Magnesium (Mg) is the central metal atom of the chlorophyll molecule.
• Gas exchange during photosynthesis takes place through the stomata of the leaf, guarded by guard cells.

Correct Answer: Option 4 — nucleons
Almost the whole mass of an atom is concentrated in its nucleus, which contains protons and neutrons. Protons and neutrons taken together are called nucleons, and their total number is the mass number (A) of the atom.
Mass number (A) = number of protons (Z) + number of neutrons (n)
A proton and a neutron each weigh about 1 u (1.67 × 10⁻²⁷ kg), while an electron is nearly 1/1836 times as heavy, so the electrons contribute almost nothing to the mass. Therefore the atomic mass is decided primarily by the total number of nucleons. For example, carbon-12 has 6 protons + 6 neutrons = 12 nucleons, so its mass number is 12.
❌ Why the other options are incorrect
✗ neutrons — neutrons are only one kind of nucleon; ignoring the protons, which weigh practically the same, gives a wrong mass.
✗ electrons — an electron's mass (9.1 × 10⁻³¹ kg) is negligible; electrons decide the chemical properties and valency, not the mass.
✗ protons — the number of protons alone is the atomic number (Z), which fixes the identity of the element, not its mass.
📌 SSC & Railway Exam Facts
• Mass number (A) = protons + neutrons = nucleons; atomic number (Z) = number of protons.
• Number of neutrons n = A − Z.
• Mass of proton ≈ mass of neutron ≈ 1 u; 1 u = 1/12 the mass of a carbon-12 atom.
• Isotopes — same Z, different A (Cl-35, Cl-37); isobars — same A, different Z (Ca-40, Ar-40).
• In a neutral atom, number of electrons = number of protons.

Correct Answer: Option 3 — boiling point
The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the atmospheric pressure, so that bubbles of vapour form throughout the bulk of the liquid and it changes rapidly into vapour. At 1 atmosphere (101.3 kPa) pure water boils at 100 °C (373 K).
While a liquid boils its temperature does not rise, even though heat is being supplied. This heat is used up as the latent heat of vaporisation — for water 22.5 × 10⁵ J/kg — to overcome the forces of attraction between the molecules.
The boiling point increases with pressure, which is why a pressure cooker cooks food faster, and decreases at high altitudes, which is why water boils below 100 °C on a hill.
❌ Why the other options are incorrect
✗ triple point — the single temperature and pressure at which solid, liquid and gas coexist in equilibrium (for water, 0.01 °C); it is not the boiling temperature.
✗ fusion point — another name for the melting point, the solid → liquid change, not the liquid → gas change.
✗ melting point — the temperature at which a solid turns into a liquid (ice melts at 0 °C / 273 K).
📌 SSC & Railway Exam Facts
• Boiling point of water = 100 °C = 373 K; melting point of ice = 0 °C = 273 K.
• K = °C + 273.
• Boiling is a bulk phenomenon at a fixed temperature; evaporation is a surface phenomenon at all temperatures.
• Latent heat of vaporisation of water = 22.5 × 10⁵ J/kg; latent heat of fusion of ice = 3.34 × 10⁵ J/kg.
• Adding a non-volatile solute (salt) raises the boiling point and lowers the freezing point.

Correct Answer: Option 2 — Gangue
An ore dug out of the earth is never pure — it is always mixed with earthy and rocky impurities such as sand, clay, limestone and mica. These unwanted impurities present in an ore are collectively called gangue (also called matrix).
Removing the gangue is the first step of metallurgy and is called concentration or dressing of the ore. It is done by hydraulic (gravity) washing, magnetic separation, froth flotation or chemical leaching, depending on the nature of the ore and the gangue.
Inside the furnace, a substance called flux is added to combine with the leftover gangue; the fusible product formed is called slag.
❌ Why the other options are incorrect
✗ Calcinations — this is the process of heating a carbonate ore strongly in the absence or limited supply of air to get the oxide: ZnCO₃ → (heat) ZnO + CO₂.
✗ Anode mud — the insoluble impurities (Ag, Au, Pt) that settle below the anode during electrolytic refining; they are impurities of a metal, not of an ore.
✗ Roasting — heating a sulphide ore strongly in excess of air: 2ZnS + 3O₂ → (heat) 2ZnO + 2SO₂; it is a process, not the name of an impurity.
📌 SSC & Railway Exam Facts
• Steps of metallurgy: concentration → roasting/calcination → reduction → refining.
• Gangue + flux = slag; CaCO₃ is the flux used in the blast furnace.
• Froth flotation is used for sulphide ores (e.g. ZnS, PbS, CuFeS₂).
• Bauxite (Al₂O₃·2H₂O) is concentrated by Bayer's leaching process using NaOH.
• Anode mud is an important commercial source of gold, silver and platinum.

Correct Answer: Option 3 — Centrifugation technique
In a colloidal solution the particle size lies between 1 nm and 1000 nm. Such particles are too small to be held back by filter paper and they never settle down under gravity, so ordinary filtration and sedimentation both fail.
They are separated by centrifugation. The mixture is placed in a centrifuging machine and rotated at a very high speed. The denser colloidal particles are thrown outwards and collect at the bottom of the tube, while the lighter dispersion medium stays above and is decanted off.
Everyday and laboratory uses: the milk cream separator, the spin-drier of a washing machine, separating butter from cream, and testing blood and urine samples in pathology laboratories.
❌ Why the other options are incorrect
✗ Sublimation technique — used to separate a solid that sublimes (camphor, naphthalene, NH₄Cl, iodine) from a non-subliming solid; colloidal particles do not sublime.
✗ Filtration technique — the pores of filter paper are much larger than colloidal particles, so the whole colloid simply passes through; filtration works for a suspension, not a colloid.
✗ Recrystallisation technique — purifies an impure solid by dissolving it and letting pure crystals form again; it has nothing to do with colloids.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• A colloid scatters a beam of light — the Tyndall effect; a true solution does not.
• A colloid is heterogeneous but looks homogeneous; a suspension settles on standing.
• Milk, blood, fog, smoke, ink, butter, cheese are common colloids.
• Centrifuge works on the principle that denser particles are pushed outward during fast rotation.

Correct Answer: Option 2 — Increase in humidity
Evaporation is the change of a liquid into vapour from its surface at any temperature, even below the boiling point. Its rate increases with surface area, temperature and wind speed, but it decreases when humidity increases.
Humidity is the amount of water vapour already present in the air. When humidity is high, the air is nearly saturated and can hold very little extra vapour, so molecules that escape from the liquid surface are pushed back into it. The net rate of evaporation therefore falls. This is exactly why clothes take a very long time to dry on a humid, rainy day and dry quickly in dry summer air.
Evaporation always causes cooling, because the fastest (most energetic) molecules leave the liquid and carry away latent heat.
❌ Why the other options are incorrect
✗ Increase in surface area — a bigger exposed surface lets more molecules escape, so evaporation increases; hot tea cools faster in a saucer than in a cup.
✗ Increase in wind speed — moving air sweeps the vapour away and keeps the air above the liquid unsaturated, so evaporation increases.
✗ Increase in temperature — more molecules gain enough kinetic energy to escape the surface, so evaporation increases.
📌 SSC & Railway Exam Facts
• Rate of evaporation ∝ surface area, temperature, wind speed; ∝ 1/humidity.
• Water stays cool in an earthen pot (matka) because it evaporates through the pores.
• Sweating cools the body by evaporation.
• Evaporation — surface phenomenon at all temperatures; boiling — bulk phenomenon at a fixed temperature.
• Evaporation is a cooling process; it absorbs latent heat of vaporisation.

Correct Answer: Option 4 — 3 : 1
Chlorine occurs in nature as two isotopes, Cl-35 and Cl-37. Both have the same atomic number 17 (17 protons) but different mass numbers, because one has 18 neutrons and the other 20 neutrons. In nature they are found in the ratio 3 : 1.
⚗️ The Calculation
Average atomic mass = (35 × 3 + 37 × 1) / (3 + 1) = (105 + 37)/4 = 142/4 = 35.5 u
• This is exactly the atomic mass of chlorine, 35.5 u, given in the periodic table.
• A fractional atomic mass is itself proof that the element exists as a mixture of isotopes — no single chlorine atom weighs 35.5 u.
❌ Why the other options are incorrect
✗ 1 : 3 — reversing the ratio gives (35 + 111)/4 = 36.5 u, which is not chlorine's atomic mass.
✗ 1 : 5 — gives (35 + 185)/6 = 36.67 u, far too high.
✗ 1 : 1 — gives (35 + 37)/2 = 36 u, again not 35.5 u.
📌 SSC & Railway Exam Facts
• Isotopes — same atomic number, different mass number (Cl-35/Cl-37; H-1, H-2, H-3).
• Isobars — same mass number, different atomic number (Ca-40 and Ar-40).
• Isotopes have identical chemical properties but different physical properties.
• U-235 is used in nuclear reactors; Co-60 in cancer treatment; I-131 in treating goitre.
• Hydrogen's isotopes are protium (¹H), deuterium (²H) and tritium (³H).

Correct Answer: Option 3 — Sublimation
Sublimation is the change of a substance directly from the solid state to the gaseous state without passing through the liquid state. It occurs in solids whose particles are held by weak intermolecular forces and which have a high vapour pressure even at ordinary temperature. The process absorbs the latent heat of sublimation.
The common subliming solids are camphor, naphthalene (mothballs), ammonium chloride (NH₄Cl), iodine, anthracene and dry ice (solid CO₂).
This property is used as a separation technique: a mixture of camphor and common salt, or NH₄Cl and sand, is heated in a china dish covered with an inverted funnel — the subliming solid vaporises and re-solidifies on the cool funnel.
❌ Why the other options are incorrect
✗ Fusion — this is melting, the change of a solid into a liquid at its melting point.
✗ Deposition — the reverse of sublimation: a gas changes directly into a solid (formation of frost or snow), not solid → gas.
✗ Solidification — this is freezing, the change of a liquid into a solid.
📌 SSC & Railway Exam Facts
• Solid → gas = sublimation; gas → solid = deposition.
• Liquid → gas = vaporisation; gas → liquid = condensation; solid → liquid = fusion/melting.
• Dry ice is solid CO₂; it sublimes at −78.5 °C and is used as a refrigerant.
• Classic subliming solids: camphor, naphthalene, NH₄Cl, iodine, anthracene.
• Sublimation is used to purify camphor, naphthalene and iodine in the laboratory.

Correct Answer: Option 1 — The negative and positive charges are not equal in magnitude. So, the atom as a whole is not electrically neutral.
J. J. Thomson (1898) put forward the plum-pudding model of the atom, also called the watermelon model. According to it, an atom is a sphere of uniformly spread positive charge in which the negatively charged electrons are embedded, just as seeds are studded in the red edible part of a watermelon.
A central postulate of this model is that the total negative charge of the electrons is exactly equal in magnitude to the total positive charge of the sphere, so that the atom as a whole is electrically neutral. Option 1 states the opposite of this postulate, so it is the INCORRECT statement.
❌ Why the other options are incorrect
✗ The positive charge in the atom is spread all over like the red edible part of a watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon. — this is the standard textbook description of Thomson's model, hence a correct statement.
✗ An atom consists of a positively charged sphere and the electrons are embedded in it. — this is Thomson's first postulate, hence correct.
✗ The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral. — this is Thomson's second postulate, hence correct.
📌 SSC & Railway Exam Facts
• J. J. Thomson discovered the electron (1897) and received the Nobel Prize in Physics, 1906.
• Thomson's model is called the plum-pudding, watermelon or christmas-pudding model.
• Goldstein discovered the proton (canal rays); Chadwick discovered the neutron (1932).
• Rutherford's α-particle scattering experiment disproved Thomson's model and established the nucleus.
• Thomson's model could not explain the results of the scattering experiment or the stability of the atom.

Correct Answer: Option 4 — thermal decomposition
A decomposition reaction is one in which a single compound breaks up into two or more simpler substances. When the energy required for this break-up is supplied in the form of heat, the reaction is called thermal decomposition.
⚗️ The Reaction
CaCO₃(s) → (heat) CaO(s) + CO₂(g)
• Limestone (calcium carbonate, CaCO₃), heated to about 1000 °C, gives quick lime (calcium oxide, CaO) and carbon dioxide.
• One reactant → two products, so it is a decomposition; the energy comes from heat, so it is thermal decomposition.
• The reaction absorbs heat, so it is also an endothermic reaction. It is used industrially to make cement, glass and quick lime.
❌ Why the other options are incorrect
✗ double displacement reaction — two compounds exchange their ions, e.g. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq); here there is only one reactant.
✗ combination reaction — two or more substances join to form a single product, e.g. CaO + H₂O → Ca(OH)₂; that is the exact opposite of decomposition.
✗ exothermic reaction — a reaction that releases heat. Limestone decomposition absorbs heat, so it is endothermic, not exothermic.
📌 SSC & Railway Exam Facts
• Decomposition by heat = thermal, by light = photolytic, by electricity = electrolytic.
• 2AgBr → (sunlight) 2Ag + Br₂ — photolytic decomposition, used in black-and-white photography.
• 2H₂O → (electricity) 2H₂ + O₂ — electrolysis of water; H₂ : O₂ = 2 : 1 by volume.
• CaO + H₂O → Ca(OH)₂ (slaked lime) is a vigorous exothermic reaction.
• Decomposition reactions are the opposite of combination reactions and are generally endothermic.

Correct Answer: Option 2 — The size of the particles of a colloid is very large.
In a colloidal solution the particles of the dispersed phase are of size 1 nm to 1000 nm. Such particles are too small to be seen with the naked eye and they pass straight through ordinary filter paper, which is why a colloid looks homogeneous to the eye.
So describing colloidal particles as "very large" is wrong — that description belongs to a suspension, whose particles are bigger than 1000 nm, are visible to the eye, are retained by filter paper and settle down on standing. Colloidal particles, in contrast, are stable and never settle.
❌ Why the other options are incorrect
✗ The components of a colloidal solution are only the dispersed phase and the dispersion medium. — correct; the solute-like part is the dispersed phase and the solvent-like part is the dispersion medium.
✗ A colloid is a heterogeneous mixture. — correct; a colloid is heterogeneous, though it appears homogeneous to the naked eye.
✗ A colloid is big enough to scatter a beam of light passing through it and make its path visible. — correct; this scattering is the Tyndall effect, the standard test for a colloid.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Milk, blood, fog, smoke, ink, cheese, butter, shaving cream are common colloids.
• Milk = liquid fat in water → emulsion; fog = liquid in gas → aerosol; smoke = solid in gas → aerosol.
• A colloid cannot be separated by filtration but can be by centrifugation.
• The Tyndall effect is seen when sunlight enters a dark room through a small hole, or through the canopy of a dense forest.

Correct Answer: Option 3 — CH₃OH(l)
When carbon monoxide (CO) reacts with hydrogen (H₂) under pressure in the presence of a catalyst, the product is methanol (methyl alcohol, CH₃OH). The 1 : 2 mixture of CO and H₂ used for this is called synthesis gas; the industrial mixture of CO + H₂ is also known as water gas.
⚗️ The Reaction
CO(g) + 2H₂(g) → (ZnO–Cr₂O₃, 300 °C, 200 atm) CH₃OH(l)
• The reaction is a combination (addition) of hydrogen to carbon monoxide and needs a zinc oxide–chromium oxide catalyst at high temperature and pressure.
• Check the balance: 1 C, 1 O and 4 H on each side — the equation is balanced.
• Methanol belongs to the alcohol family, whose functional group is −OH.
❌ Why the other options are incorrect
✗ CH₄(g) + O₂(g) — these are the reactants for the combustion of methane, not products of this reaction; no free oxygen is produced when CO adds hydrogen.
✗ H₂O(l) — water alone would need the carbon of CO to be left over as free carbon; hydrogen adds to CO instead of merely stripping its oxygen.
✗ C(s) + H₂O(l) — this would mean CO is completely reduced to carbon; industrially, hydrogen builds an organic molecule with CO rather than knocking the carbon out.
📌 SSC & Railway Exam Facts
• Water gas = CO + H₂; producer gas = CO + N₂; synthesis gas is the CO + H₂ feed for methanol.
• Methanol (CH₃OH) is called wood spirit; it is poisonous and can cause blindness or death.
• Ethanol (C₂H₅OH) is the alcohol of drinks; it is made unfit to drink by adding methanol — denatured alcohol.
• Carbon monoxide is a colourless, odourless, poisonous gas that binds haemoglobin far more strongly than oxygen.
• Alcohols end in −ol and contain the functional group −OH.

Correct Answer: Option 1 — Potassium and gold
The reactivity series (activity series) arranges metals in the decreasing order 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, so it is the most reactive metal, while gold (Au) stands at the very bottom, so it is the least reactive. Hence *potassium and gold* is the pair representing the most and the least reactive metals.
Potassium is so reactive that it catches fire on contact with water and is stored under kerosene. Gold, being least reactive, occurs in the free (native) state, does not tarnish or corrode, and is therefore called a noble metal.
❌ Why the other options are incorrect
✗ Calcium and gold — gold is correctly the least reactive, but calcium lies below both potassium and sodium, so it is not the most reactive.
✗ Magnesium and gold — magnesium is only moderately reactive; it burns in air but does not react with cold water.
✗ Sodium and gold — sodium is very reactive, yet it still stands below potassium in the series.
📌 SSC & Railway Exam Facts
• Memory order: K Na Ca Mg Al Zn Fe Pb H Cu Hg Ag Au.
• K and Na are stored in kerosene because they react violently with air and water.
• Metals above hydrogen displace H₂ from dilute acids; Cu, Hg, Ag, Au do not.
• Au and Pt occur in the native (free) state; reactive metals occur as oxides, carbonates and sulphides.
• A more reactive metal displaces a less reactive one: Fe + CuSO₄ → FeSO₄ + Cu.

Correct Answer: Option 3 — Formation of soap scum due to the reaction with calcium and magnesium ions.
Hard water contains dissolved salts of calcium (Ca²⁺) and magnesium (Mg²⁺). A soap is the sodium or potassium salt of a long-chain fatty acid, e.g. sodium stearate, C₁₇H₃₅COONa. In hard water the Na⁺ of the soap is replaced by Ca²⁺ or Mg²⁺, and the calcium/magnesium salt formed is insoluble. It separates as a sticky, curdy white precipitate called scum.
⚗️ The Reaction
2C₁₇H₃₅COONa(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s) + 2Na⁺(aq)
• The (s) shows the calcium salt is an insoluble precipitate — this is the scum.
• Soap keeps getting used up in removing the hardness, so very little lather forms and cleaning is poor; the scum also sticks to clothes.
❌ Why the other options are incorrect
✗ Soap dissolves too quickly in hard water. — the opposite is true; soap is precipitated out and becomes less available in solution.
✗ Soap creates too much lather. — in hard water soap gives very poor lather until all the Ca²⁺ and Mg²⁺ ions have been removed.
✗ Soap loses its colour in hard water. — colour has nothing to do with hardness; the real problem is a chemical reaction producing scum.
📌 SSC & Railway Exam Facts
• Detergents lather well in hard water because their calcium and magnesium salts are soluble.
• Soap is made by saponification — heating oil or fat with NaOH.
• A soap molecule has a hydrophilic (−COONa) head and a hydrophobic hydrocarbon tail; in water it forms micelles.
• Temporary hardness (bicarbonates) is removed by boiling; permanent hardness (chlorides, sulphates) by washing soda, Na₂CO₃·10H₂O.
• Soap = soft soap (KOH) and hard soap (NaOH); detergents are sodium salts of sulphonic acids.

Correct Answer: Option 2 — force
This is exactly the statement of Newton's second law of motion. It says that the rate of change of momentum of a body is directly proportional to the applied unbalanced force, and the change takes place in the direction of that force.
If a body of mass m changes its velocity from u to v in time t, then rate of change of momentum = (mv − mu)/t = m × (v − u)/t = ma. Taking the constant of proportionality as 1 in SI units gives the familiar form F = ma. The SI unit of force, the newton (N), is defined from this very relation: 1 N = 1 kg m s⁻².
❌ Why the other options are incorrect
✗ torque — the turning (rotating) effect of a force, τ = r × F. It equals the rate of change of angular momentum, not of linear momentum.
✗ angular momentum — L = Iω, the rotational partner of linear momentum. It is a quantity of motion, not the cause of a change in momentum.
✗ moment of inertia — I, the rotational partner of mass; it measures how strongly a body resists a change in its rotational motion.
📌 SSC & Railway Exam Facts
• Momentum p = mv; SI unit kg m/s; it is a vector quantity.
• Newton's second law: F = dp/dt = ma — it gives the measure of force.
• 1 N = 1 kg m s⁻² and 1 dyne = 10⁻⁵ N.
• Impulse = F × t = change in momentum; SI unit N s.
• The first law gives the definition of force, the third law gives action–reaction pairs.

Correct Answer: Option 1 — Power is inversely proportional to focal length.
The power (P) of a lens measures its ability to converge or diverge a beam of light. It is defined as the reciprocal of the focal length taken in metres: P = 1/f. So a lens with a short focal length bends light sharply and therefore has a large power, while a lens with a long focal length bends light only gently and has a small power.
The SI unit of power of a lens is the dioptre (D), where 1 D = 1 m⁻¹ — the power of a lens whose focal length is 1 m. The power of a convex (converging) lens is positive and that of a concave (diverging) lens is negative.
❌ Why the other options are incorrect
✗ Power is inversely proportional to square of the focal length. — wrong; power depends on 1/f, not 1/f². The inverse-square rule belongs to the intensity of light, not to lens power.
✗ Power is directly proportional to focal length. — this reverses the relation; a larger focal length actually gives a weaker lens.
✗ Power is directly proportional to square of the focal length. — no such relation exists anywhere in ray optics.
📌 SSC & Railway Exam Facts
• P = 1/f with f in metres; SI unit dioptre (D).
• A convex lens of f = 50 cm = 0.5 m has power +2 D.
• For thin lenses in contact, P = P₁ + P₂.
• Myopia is corrected by a concave lens (negative power); hypermetropia by a convex lens (positive power).

Correct Answer: Option 1 — 1 watt hour
📖 Given
• Power, P = 1 W
• Time, t = 1 hour
✍️ Formula
Energy consumed, E = Power × Time
🔢 Solution
• E = 1 W × 1 h
• E = 1 watt hour (1 Wh)
• In pure SI units, E = 1 W × 3600 s = 3600 J
In plain words, an appliance that draws 1 watt continuously for 1 hour uses up 1 watt hour of electrical energy — this is exactly how the "units" on an electricity bill are built up.
❌ Why the other options are incorrect
✗ 1 kilowatt hour — this is 1000 times bigger; it needs a 1000 W appliance running for 1 hour, not a 1 W one.
✗ 60 watt hour — comes from multiplying by 60 (minutes in an hour) even though the time was already taken in hours.
✗ 6 kilowatt hour — no step of the calculation produces this figure; it is 6000 times the correct answer.
📌 SSC & Railway Exam Facts
• 1 kWh = 1 "unit" of electricity = 3.6 × 10⁶ J.
• Electric power P = VI = I²R = V²/R; SI unit watt (W).
• 1 W = 1 J/s and 1 horsepower (hp) = 746 W.
• The joule (J) is the SI unit of energy; the kilowatt hour is its commercial unit.

Correct Answer: Option 3 — Larger the focal length, larger the power.
This statement is false, and since the question asks which one is NOT correct, it is the answer. The power of a lens is P = 1/f (f in metres), so power and focal length are inversely related. A larger focal length always means a smaller power, never a larger one.
By the New Cartesian sign convention, a convex lens has a positive focal length and hence a positive power, while a concave lens has a negative focal length and hence a negative power. The unit of power is the dioptre (D), equal to m⁻¹.
❌ Why the other options are incorrect
✗ The power of convex lens is positive. — this is true; a convex lens converges light and carries a + sign, so it cannot be the answer.
✗ Larger the focal length, smaller is the power. — true, and it is just the statement of P = 1/f in words.
✗ The power of concave lens is negative. — true; a concave lens diverges light and is given a − sign.
📌 SSC & Railway Exam Facts
• P = 1/f (metre); unit dioptre (D), given by Monoyer.
• f = +25 cm convex lens ⇒ P = +4 D; f = −50 cm concave lens ⇒ P = −2 D.
• Lens formula: 1/v − 1/u = 1/f; magnification m = v/u = h′/h.
• A thick, highly curved lens has a short focal length and therefore a high power.

Correct Answer: Option 3 — 2 N
📖 Given
• Mass, m = 2 kg
• Initial velocity, u = 8 m/s
• Final velocity, v = 12 m/s
• Time, t = 4 s
✍️ Formula
a = (v − u)/t and F = ma
🔢 Solution
• a = (12 − 8)/4
• a = 4/4 = 1 m/s²
• F = m × a = 2 × 1
• F = 2 N
So a steady push of just 2 newton is enough to speed this 2 kg body up by 4 m/s over 4 seconds.
❌ Why the other options are incorrect
✗ 1 N — this is the acceleration (1 m/s²) copied down as the force; you still have to multiply it by the mass 2 kg.
✗ 3 N — no step of the calculation gives 3; it is a distractor placed between 2 and 4.
✗ 4 N — this is the change in velocity (12 − 8 = 4 m/s) mistaken for the force; the division by t = 4 s has been skipped.
📌 SSC & Railway Exam Facts
• F = ma = m(v − u)/t; SI unit newton (N), 1 N = 1 kg m s⁻².
• Impulse = F × t = m(v − u) = change in momentum; here impulse = 8 N s.
• Equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
• 1 kgf = 9.8 N and 1 dyne = 10⁻⁵ N.

Correct Answer: Option 4 — at the principal focus
A concave mirror is a converging mirror. When a beam of rays parallel to the principal axis strikes it, every ray obeys the laws of reflection and, after reflection, they all pass through one single point on the principal axis. That point is called the principal focus (F) of the mirror.
The distance of this point from the pole (P) of the mirror is the focal length (f), and for a spherical mirror it is half the radius of curvature: f = R/2. Because the reflected rays actually meet there, the focus of a concave mirror is a real focus — a screen placed at F shows a bright point. This is why concave mirrors are used in solar cookers, torches, headlights and shaving mirrors.
❌ Why the other options are incorrect
✗ in between the pole and infinity — too vague; the rays meet at one exact point, the focus, which lies midway between the pole and the centre of curvature.
✗ at infinity — the rays are coming from infinity; a converging mirror brings them to a finite point, it does not send them back to infinity.
✗ at the pole — the pole is the centre of the mirror surface itself; a ray hitting the pole is reflected making equal angles with the principal axis.
📌 SSC & Railway Exam Facts
• f = R/2 for a spherical mirror; mirror formula 1/v + 1/u = 1/f.
• Concave mirror — real focus, converging; convex mirror — virtual focus, diverging.
• Magnification of a mirror, m = −v/u = h′/h.
• Convex mirrors are used as rear-view mirrors because they give an erect, diminished image and a wide field of view.

Correct Answer: Option 2 — It is normal to the mirror at its pole.
The principal axis of a spherical mirror is the straight line joining the pole (P) and the centre of curvature (C) of the mirror. Since the centre of curvature is the centre of the sphere from which the mirror was cut, this line is a radius of that sphere — and a radius always meets the spherical surface at right angles. Hence the principal axis is normal (perpendicular) to the mirror at its pole.
This is why a ray travelling along the principal axis hits the pole head-on, with angle of incidence 0°, and is reflected straight back along the same path. The whole sign convention of mirrors is measured along this axis, with the pole as the origin.
❌ Why the other options are incorrect
✗ It is parallel to the surface of the mirror. — the opposite is true; it is perpendicular to the mirror surface at the pole.
✗ It is located at the edge of the mirror. — the principal axis passes through the middle of the mirror, i.e. the pole, not through its rim.
✗ It intersects the surface of the mirror at an angle of 45 degrees. — the angle is 90°, not 45°, because a radius is always normal to a sphere.
📌 SSC & Railway Exam Facts
• Pole (P) — the geometric centre of the reflecting surface; Centre of curvature (C) — centre of the sphere.
• Aperture — the diameter of the reflecting surface; f = R/2.
• A ray through C retraces its path; a ray through F becomes parallel to the principal axis.
• Laws of reflection: ∠i = ∠r, and the incident ray, reflected ray and normal lie in the same plane.

Correct Answer: Option 4 — law of inertia
Newton's first law of motion states that a body at rest stays at rest, and a body in uniform motion along a straight line keeps moving that way, unless an external unbalanced force acts on it. Because the law simply describes the natural tendency of matter to resist a change in its state of motion, it is called the law of inertia.
Inertia is measured by mass — the heavier the body, the greater its inertia. It appears in three forms: inertia of rest, inertia of motion and inertia of direction. Everyday proofs are a passenger falling backwards when a bus starts suddenly (inertia of rest), falling forwards when it brakes (inertia of motion), and dust flying off a carpet that is beaten.
❌ Why the other options are incorrect
✗ law of angular momentum — this is the conservation of angular momentum (L = Iω), a rotational result, not Newton's first law.
✗ law of linear momentum — conservation of linear momentum follows from Newton's third law together with the second law.
✗ law of torque — torque τ = r × F describes the turning effect of a force; there is no "first law of torque".
📌 SSC & Railway Exam Facts
• First law — defines force and inertia; Second law — measures force, F = ma; Third law — action and reaction.
• Inertia ∝ mass; SI unit of mass kilogram (kg).
• Newton's laws were published in Philosophiæ Naturalis Principia Mathematica (1687).
• Galileo gave the idea of inertia before Newton formalised it.

Correct Answer: Option 2 — depends on the masses of the objects and the distance between them
Newton's universal law of gravitation states that every two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres:
F = G M m / r²
Here G is the universal gravitational constant, whose value is 6.67 × 10⁻¹¹ N m² kg⁻². So the force clearly depends on both the masses and the separation. Doubling either mass doubles the force, while doubling the distance cuts the force to one-fourth.
❌ Why the other options are incorrect
✗ only depends on the masses of the objects — incomplete; it ignores the inverse-square dependence on r, the reason gravity weakens with distance.
✗ does not depend on either the masses of the objects or the distance between them — completely wrong; it would make gravity the same everywhere, which contradicts orbits and free fall.
✗ only depends on the distance between the objects — incomplete; the product Mm is very much part of the formula, which is why the Sun pulls far harder than the Moon.
📌 SSC & Railway Exam Facts
• F = GMm/r²; G = 6.67 × 10⁻¹¹ N m² kg⁻², first measured by Henry Cavendish.
• G is a universal constant; g is not — it changes with place, g = GM/R².
• Average value of g on Earth = 9.8 m/s²; on the Moon it is about 1/6 of this.
• Gravitational force is the weakest of the four fundamental forces but always attractive.

Correct Answer: Option 4 — 1/3
📖 Given
• Height of object, h = 30 cm
• Height of image, h′ = 10 cm
• The lens is a concave (diverging) lens
✍️ Formula
Magnification, m = h′/h = v/u
🔢 Solution
• m = h′/h = 10/30
• m = 1/3
A concave lens always forms a virtual, erect and diminished image, so the magnification comes out positive and less than 1 — here the image is exactly one-third the size of the object.
❌ Why the other options are incorrect
✗ −20 — obtained by subtracting the heights (10 − 30 = −20); magnification is a ratio, never a difference.
✗ 20 — the same subtraction done the other way (30 − 10 = 20); again a difference, not a ratio.
✗ 3 — the ratio taken upside down, h/h′ = 30/10 = 3. That would mean the image is 3 times bigger, which a concave lens can never do.
📌 SSC & Railway Exam Facts
• For a lens, m = h′/h = v/u; for a mirror, m = h′/h = −v/u.
• |m| > 1 ⇒ enlarged image; |m| < 1 ⇒ diminished image; m = 1 ⇒ same size.
• A concave lens always gives a virtual, erect, diminished image between the optical centre and F.
• Lens formula 1/v − 1/u = 1/f; power P = 1/f (m) in dioptre.

Correct Answer: Option 3 — When it is about to hit the ground.
During a free fall the total mechanical energy is conserved: KE + PE = constant. At the top the body is at rest, so KE = 0 and PE = mgh is maximum. As it falls, the height keeps decreasing and the speed keeps increasing, so potential energy is steadily converted into kinetic energy.
Just before touching the ground the height is almost zero, so PE ≈ 0 and the whole of the initial potential energy has become kinetic energy:
KE(max) = mgh, with speed v = √(2gh).
So the kinetic energy is largest at the instant the body is about to strike the ground, which is exactly why a fall from a greater height hurts more.
❌ Why the other options are incorrect
✗ At one-fourth way through the fall. — after falling h/4 the body has gained only mgh/4 of kinetic energy.
✗ At the start of the fall. — here the velocity is zero, so KE = ½mv² = 0; this is where potential energy is maximum instead.
✗ At halfway through the fall. — at h/2 the kinetic energy is mgh/2, exactly half of its maximum value; at that point KE = PE.
📌 SSC & Railway Exam Facts
• KE = ½mv², PE = mgh; SI unit of both is the joule (J).
• Law of conservation of energy — energy is neither created nor destroyed, only transformed.
• Free-fall speed on hitting the ground, v = √(2gh), is independent of mass.
• KE = p²/2m, where p = mv is the momentum.

Correct Answer: Option 1 — If magnetic field lines are parallel and equidistant, they represent zero field strength.
This statement is false, so it is the answer to a "NOT true" question. Parallel and equally spaced field lines represent a uniform magnetic field — a field that has the same magnitude and same direction at every point, not a zero field. The field inside a long current-carrying solenoid and the field in a small region between the poles of a horseshoe magnet are of this kind.
A zero field would be shown by no field lines at all in that region, for example at the neutral point where the field of a bar magnet exactly cancels the Earth's field.
❌ Why the other options are incorrect
✗ The direction of the magnetic field ... north pole of a magnetic compass needle points. — true; this is the standard definition of field direction, and it is why field lines run N → S outside a magnet.
✗ The relative strength ... degree of closeness of the field lines. — true; crowded lines mean a stronger field, which is why lines bunch up near the poles.
✗ Magnetic field lines are closed curves. — true; a line goes from N to S outside the magnet and continues from S to N inside it, forming a closed loop.
📌 SSC & Railway Exam Facts
• Two magnetic field lines never intersect, otherwise the compass would point in two directions at once.
• SI unit of magnetic field B is the tesla (T); 1 T = 10⁴ gauss.
• Magnetic field lines are closed because isolated magnetic poles (monopoles) do not exist.
• Field due to a straight current-carrying wire is given by the right-hand thumb rule; force on a conductor by Fleming's left-hand rule.

Correct Answer: Option 1 — +10D
📖 Given
• Constant force, F = 10 N
• Displacement, s = D metre
• Displacement is in the direction of the force, so the angle θ = 0°
✍️ Formula
W = F × s × cos θ
🔢 Solution
• cos 0° = 1
• W = 10 × D × 1
• W = +10D joule
Because the force and the displacement point the same way, the force is helping the motion, so the work done on the object is positive.
❌ Why the other options are incorrect
✗ −10D — this is the answer only if the force acted opposite to the displacement (θ = 180°, cos 180° = −1), like friction or a braking force.
✗ D — here the 10 N has been dropped; work is force × displacement, not displacement alone.
✗ 0 — work is zero only when the displacement is zero or when the force is perpendicular to the displacement (θ = 90°), neither of which is the case here.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit joule (J), 1 J = 1 N m; work is a scalar.
• θ = 0° ⇒ positive work; θ = 90° ⇒ zero work; θ = 180° ⇒ negative work.
• A coolie carrying a load on his head on level ground does zero work against gravity (θ = 90°).
• Power = Work/Time; SI unit watt (W); 1 W = 1 J/s.