General Science: previous-year questions

Answer: (A) $50h$

The energy of a photon is given by Planck's relation $E = h\nu$, where $\nu$ is the frequency. Putting $\nu = 50$ Hz gives $E = 50h$ joules, option A. Option C is the trap: $E = \frac{hc}{\lambda}$ is the form of the same relation in terms of wavelength, so $\frac{hc}{50}$ would be right only if 50 were the wavelength in metres; options B and D are dimensionally wrong. Facts to remember: $h = 6.626 \times 10^{-34}$ J s, photon energy rises with frequency (gamma rays highest, radio waves lowest), and 50 Hz is also the frequency of India's AC mains supply.

Answer: (B) 15-20%

Answer as of 2024 — this may have changed since.

Ordinary commercial solar panels made of crystalline silicon convert roughly 15 to 20% of the sunlight falling on them into electricity (the best current modules reach a little over 22%), so option B is the accepted range. A single-junction silicon cell cannot exceed about 33% in theory (the Shockley-Queisser limit); efficiencies above 40% have been reached only by multi-junction cells in laboratories and on satellites, and 80 to 90% is impossible for photovoltaics. Solar cells work on the photovoltaic effect and are made mainly of silicon, a semiconductor. Related facts: the International Solar Alliance is headquartered at Gurugram, and India aims at 500 GW of non-fossil power capacity by 2030.

Answer: (A) Bats

Fruit bats of the genus Pteropus (flying foxes) are the natural reservoir of the Nipah virus, an RNA virus of the genus Henipavirus. It reaches humans through infected pigs, through fruit or raw date-palm sap contaminated by bats, or from person to person, and causes fever and encephalitis with high fatality; there is no licensed vaccine. It was first identified in Malaysia in 1998-99 (named after Sungai Nipah village). India's outbreaks include Siliguri (2001) and Nadia (2007) in West Bengal, repeated outbreaks in Kerala since 2018, and two cases in West Bengal in January 2026. Horses (D) are the intermediate host of the related Hendra virus in Australia, and migratory birds (C) are linked with avian influenza.

Answer: (D) it has nodes and inter-nodes.

Ginger is a rhizome, an underground stem, and the proof is that it bears nodes and internodes with scale leaves and buds, features that only stems have. The other options are true of ginger but are not what makes it a stem: roots such as sweet potato and carrot also grow in soil, store food and lack chlorophyll. Underground stem modifications to remember: rhizome (ginger, turmeric), tuber (potato, whose 'eyes' are nodes), corm (colocasia, gladiolus, saffron) and bulb (onion, garlic). Storage roots, by contrast, include sweet potato, carrot, radish, turnip and beet.

Answer: (B) Algae

Agar-agar is a jelly-like polysaccharide obtained from the cell walls of red algae (Rhodophyceae) such as Gelidium and Gracilaria. It is used as the solid culture medium for growing microbes in laboratories and as a vegetarian substitute for gelatin in food. Other algal products: carrageenan from red algae, algin and iodine from brown algae (kelps such as Laminaria), and protein-rich food supplements from Chlorella and Spirulina. The other groups give different products: bryophytes, peat (Sphagnum moss); fungi, antibiotics such as penicillin, and yeast; pteridophytes (ferns), ornamentals and biofertiliser (Azolla).

Answer: (A) zero

Work done is $W = Fs\cos\theta$, where $\theta$ is the angle between the force and the displacement. In a circular orbit the gravitational pull acts along the radius towards the Sun while the planet moves along the tangent, so $\theta = 90^\circ$ at every instant and the work done is zero over any part of the path, including half the circumference. That is also why the planet's speed and kinetic energy stay constant in a circular orbit. Options B, C and D wrongly multiply the force by the path length or the diameter. General rule: a centripetal force never does work. (In an elliptical orbit gravity does work over parts of the path, but the total over one full revolution is still zero.)

Answer: (B) The leaf contains green chlorophyll but it remains hidden under other pigments.

Leaves that look red, purple or yellow still contain chlorophyll; its green colour is simply masked by larger amounts of other pigments such as anthocyanins (red and purple, dissolved in the cell sap) and carotenoids (yellow and orange). Photosynthesis therefore goes on normally, as in Coleus, croton, red amaranth and red cabbage. Option A is wrong because heat cannot replace the light reactions of photosynthesis, and option C is fiction. Carotenoids act as accessory pigments that pass absorbed light energy on to chlorophyll a. Plants that truly lack chlorophyll, such as the parasite Cuscuta (dodder), cannot make food and draw it from a host through haustoria.

Answer: (B) Viruses neither have cell walls nor their own metabolic machinery that can be targeted by antibiotics.

Antibiotics act on structures and processes found only in bacteria: the peptidoglycan cell wall (penicillin), the 70S ribosome (streptomycin, tetracycline) or bacterial enzymes for copying DNA (ciprofloxacin). A virus is just nucleic acid inside a protein coat; it has no cell wall, ribosomes or metabolism of its own and multiplies by using the host cell's machinery, so an antibiotic has nothing to attack. Option C is a true statement about viruses but explains why vaccines and antiviral drugs lose effect, not why antibiotics fail; option A is false. Viral infections are handled with antivirals (acyclovir, oseltamivir) and prevented by vaccines. Needless antibiotic use promotes antimicrobial resistance (AMR). Penicillin was discovered by Alexander Fleming in 1928.

Answer: (A) Respiratory roots

Pneumatophores are 'breathing roots' that grow vertically upward out of the mud (negative geotropism) in mangrove plants such as Avicennia, Sonneratia and Heritiera (sundari). Waterlogged saline mud has almost no oxygen, so these roots take in air through pores called lenticels or pneumathodes. Mangroves of the Sundarbans also show stilt roots (Rhizophora) and vivipary, where seeds germinate while still on the tree. The other options describe different modifications: photosynthetic (assimilatory) roots occur in Tinospora and Trapa; food-storing roots in carrot, radish and sweet potato; and the spongy velamen of epiphytic orchid roots absorbs moisture from the air.

Answer: (A) Proton + Neutron

Nearly all the mass of an atom lies in its nucleus, which contains protons and neutrons; the mass number is $A = Z + N$ (protons plus neutrons). An electron is about $\frac{1}{1836}$ as heavy as a proton, so its contribution is ignored, which rules out options B, C and D. Related facts: the atomic number $Z$ (number of protons) fixes the identity of an element; isotopes have the same $Z$ but different $A$; isobars have the same $A$ but different $Z$; isotones have the same number of neutrons. Discoveries: electron, J.J. Thomson (1897); proton, Rutherford; neutron, James Chadwick (1932); nucleus, Rutherford's gold-foil experiment (1911).

Answer: (D) Rubisco

RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase) is the enzyme that fixes atmospheric carbon dioxide in the first step of the Calvin ($C_3$) cycle in the chloroplast stroma. Because it works slowly, green plants and algae make it in huge quantities, and it is regarded as the most abundant enzyme, indeed the most abundant protein, on Earth. It can also bind oxygen, which causes wasteful photorespiration; $C_4$ plants such as maize and sugarcane avoid this by first fixing carbon dioxide with PEP carboxylase. The distractors: lipase digests fats, DNase breaks down DNA, and zymase is the yeast enzyme complex that ferments sugar to alcohol. The most abundant protein in animals is collagen.

Answer: (D) Silicon

Silicon is a semiconductor: its conductivity lies between that of conductors and insulators and rises with temperature or when small amounts of impurities are added (doping). Germanium is the other elemental semiconductor; both belong to Group 14 and are metalloids, while gallium arsenide is a compound semiconductor. Doping with phosphorus or arsenic gives n-type and with boron or aluminium gives p-type material, the basis of diodes, transistors, integrated circuits and solar cells. Iron and zinc are ordinary metallic conductors, and mercury, the only metal that is liquid at room temperature, also conducts electricity.

Answer: (C) Hydrogen bonding

A hydrogen bond forms when hydrogen is attached to a small, highly electronegative atom: fluorine, oxygen or nitrogen. Water and HF therefore have hydrogen bonding between their molecules, but $\mathrm{H_2S}$ does not, because sulphur is larger and much less electronegative. This explains why water boils at $100^\circ$C while $\mathrm{H_2S}$ is a gas, why HF has a far higher boiling point than HCl, and why ice is lighter than water (an open cage-like structure). All three molecules have covalent bonds within the molecule, so A does not distinguish them, and none is ionic or metallic. Hydrogen bonds also hold together the two strands of DNA and give proteins their shape.

Answer: (C) Zygotene

Synapsis, the pairing of homologous chromosomes to form bivalents, takes place in zygotene, the second stage of prophase I of meiosis, with the help of the synaptonemal complex. The five stages in order: leptotene (chromosomes condense into thin threads), zygotene (synapsis), pachytene (crossing over between non-sister chromatids), diplotene (the synaptonemal complex dissolves and chiasmata become visible) and diakinesis (terminalisation of chiasmata, nuclear membrane disappears). Memory aid: 'Lazy Zebra Pulls Down Dates'. The commonest confusion is with pachytene (D), which is the stage of crossing over, not of pairing.

Answer: (D) Thorium

India has small uranium reserves but some of the world's largest reserves of thorium, found in the monazite sands of the Kerala, Tamil Nadu, Odisha and Andhra Pradesh coasts. Thorium-232 is 'fertile': in a reactor it absorbs a neutron and turns into fissile uranium-233. Homi Bhabha's three-stage nuclear programme is built around this: Stage 1, pressurised heavy water reactors using natural uranium; Stage 2, fast breeder reactors using plutonium (Kalpakkam); Stage 3, thorium-based reactors. Actinium and neptunium are scarce radioactive elements not available in quantity, and copper is not a nuclear fuel. Uranium is mined at Jaduguda (Jharkhand) and Tummalapalle (Andhra Pradesh).

Answer: (C) 44

Molecular weight of $\mathrm{MCl_4} = M + 4 \times 35.5 = 154$, so the atomic weight of M is $154 - 142 = 12$, i.e. carbon, and the chloride is carbon tetrachloride. The formula $\mathrm{MCl_4}$ shows that M has valency 4, so its oxide is $\mathrm{MO_2}$, with molecular weight $12 + 2 \times 16 = 44$ (carbon dioxide). The other options are the weights of familiar oxides and act as traps: 64 is $\mathrm{SO_2}$, 72 is FeO and 40 is MgO. Two-step method for such questions: first find the atomic weight from the given compound, then use the valency to write the formula of the compound asked.

Answer: (B) $2n$

Equal numbers of moles contain equal numbers of molecules. 14 g of $\mathrm{N_2}$ (molar mass 28) is $\frac{14}{28} = 0.5$ mole and contains $n$ molecules, so 1 mole contains $2n$ molecules. 46 g of $\mathrm{NO_2}$ (molar mass $14 + 32 = 46$) is exactly 1 mole and therefore contains $2n$ molecules. Option A assumes equal masses mean equal molecules, the usual mistake. One mole of any substance contains Avogadro's number of particles, $6.022 \times 10^{23}$, and one mole of any gas occupies 22.4 litres at STP.

Answer: (B) Pulmonary veins

Arteries normally carry oxygenated blood and veins deoxygenated blood, and the pulmonary vessels are the exception. The four pulmonary veins bring oxygenated blood from the lungs to the left atrium, while the pulmonary artery (D) carries deoxygenated blood from the right ventricle to the lungs. The hepatic portal vein (A) carries nutrient-rich but deoxygenated blood from the intestine to the liver, and the renal veins (C) carry deoxygenated blood, low in urea, from the kidneys. In the foetus the umbilical vein also carries oxygenated blood. Humans have double circulation (pulmonary and systemic); the circulation of blood was discovered by William Harvey.

Answer: (C) low pressure and high temperature.

An ideal gas is assumed to have molecules of negligible volume with no forces between them. A real gas comes closest to this at low pressure, when the molecules are far apart, and at high temperature, when they move too fast for intermolecular attraction to matter, so option C is correct. The opposite conditions, high pressure and low temperature (B), are exactly those used to liquefy gases, where deviation from ideal behaviour is greatest. The ideal gas equation is $PV = nRT$; real gases are described by the van der Waals equation $\left(P + \frac{an^2}{V^2}\right)(V - nb) = nRT$, in which $a$ corrects for intermolecular attraction and $b$ for molecular volume.

Answer: (D) Abscisic acid

Abscisic acid (ABA) is the main growth-inhibiting plant hormone. It induces dormancy of seeds and buds, closes stomata during water shortage (hence 'stress hormone') and promotes ageing and shedding of leaves, acting against gibberellins. The other three are growth promoters: auxin (IAA) causes cell elongation, apical dominance and phototropism, and is used for rooting cuttings; gibberellin causes stem elongation and bolting and breaks dormancy (first found in the fungus Gibberella, which causes the 'foolish seedling' disease of rice); cytokinin promotes cell division and delays ageing. The fifth major hormone, ethylene, is a gas that ripens fruits.