Class 10 Science Competency-Based Questions: 50 Practice Questions with Answers

Answer: ✓ Hydrochloric acid (HCl). It creates the acidic medium needed for the enzyme pepsin to digest proteins, and it also kills many harmful bacteria entering with food.

Answer: ✓ Milk of magnesia is magnesium hydroxide, a mild base. It neutralises the excess hydrochloric acid in the stomach, raising the pH back toward a comfortable level and relieving the burning sensation.

Answer: ✓ A strong base would react violently with the stomach acid and would itself be corrosive to the delicate lining of the stomach, causing serious damage. A mild base neutralises the excess acid gently and safely, without harming the stomach wall or removing all the acid needed for digestion.

The "why this and not that" sub-question is extremely common in competency sets — the marks are in the reasoning, not in naming the chemical.

Answer: ✓ A pH of 4.5 is well below 7, indicating the soil is distinctly acidic. Most crops grow poorly in strongly acidic soil because nutrient availability and root function are impaired.

Answer: ✓ The farmer should add a basic substance such as quicklime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate). These bases neutralise the excess acid in the soil, raising the pH toward the neutral range where crops grow well.

Answer: ✓ No — the opposite treatment is needed. A pH of 8.5 means the soil is basic, so adding more base would worsen the problem. Such soil is treated by adding organic matter such as compost or manure, or gypsum, which help lower the pH toward neutral.

Answer: ✓ Because the correct treatment depends entirely on whether the soil is acidic or basic, and these require opposite remedies. Adding lime to already-basic soil, or organic acid to already-acidic soil, would push the pH further from neutral and damage the crop further. Testing first prevents an intervention that actively makes the problem worse.

Answer: ✓ The process is rusting (corrosion of iron). It requires both air (specifically oxygen) and moisture (water). Neither alone is sufficient — iron does not rust in dry air or in water free of dissolved oxygen.

Answer: ✓ The process is galvanisation — coating iron with a layer of zinc. It works in two ways: the zinc layer physically blocks air and moisture from reaching the iron, and since zinc is more reactive than iron, even if the coating is scratched, the zinc corrodes in preference to the iron, protecting it.

The second half of that answer — sacrificial protection through greater reactivity — is what distinguishes a full-mark answer from a partial one.

Answer: ✓ Rainwater collects and remains at the base for longer, keeping that region in prolonged contact with both moisture and air. Since both conditions for rusting are sustained there far longer than on the drier upper portions, corrosion proceeds faster at the base.

Answer: ✓ Their differing reactivity. The extraction method depends on the metal's position in the reactivity series — the more reactive a metal, the more strongly it is bound to its ore and the more energy is required to separate it.

Answer: ✓ Carbon is more reactive than iron, so it can displace iron from its oxide. Sodium, however, is more reactive than carbon, so carbon cannot displace it — only electrolysis, which supplies energy directly through electricity, is powerful enough to separate such a highly reactive metal from its compound.

Answer: ✓ Gold is very low in the reactivity series, meaning it has little tendency to react with oxygen, water, or acids in the environment. Because it does not readily form compounds, it remains as the pure metal — which is also why gold jewellery does not tarnish over time.

Answer: ✓ Pure-breeding tall parent: TT. Pure-breeding dwarf parent: tt. All F₁ plants: Tt (each receives one T from one parent and one t from the other).

Answer: ✓ Because T is dominant over t. In a Tt individual, the presence of a single dominant allele is sufficient to produce the tall phenotype, so the recessive dwarf allele is present in the genotype but not expressed in the appearance.

Crossing Tt × Tt gives genotypes in the ratio 1 TT : 2 Tt : 1 tt.

TT and Tt both appear tall (3 plants); only tt appears dwarf (1 plant).

Answer: ✓ Expected phenotypic ratio = 3 tall : 1 dwarf

Answer: ✓ Human females have two X chromosomes (XX). Human males have one X and one Y chromosome (XY).

Answer: ✓ The father determines the sex of the child. Since the mother is XX, every egg she produces carries an X chromosome — she has no other option to give. The father, being XY, produces two kinds of sperm: half carrying X and half carrying Y. If an X-bearing sperm fertilises the egg the child is XX (female); if a Y-bearing sperm does, the child is XY (male).

Answer: ✓ It is entirely unjustified. The mother contributes an X chromosome in every case and has no biological influence whatsoever over the child's sex. The outcome depends solely on which type of sperm from the father fertilises the egg, and that is itself a matter of chance — with roughly equal probability for each. Blaming the mother contradicts the basic biology of sex determination.

Answer: ✓ Aquatic plants → small fish → large fish. The aquatic plants are the producers, small fish are primary consumers, and large fish are secondary consumers.

Answer: ✓ This is biological magnification (biomagnification). Pesticides are not biodegradable and are not excreted efficiently, so they accumulate in the bodies of organisms. Since each consumer eats many organisms from the level below, the pesticide accumulated in all of them concentrates in the consumer's body, increasing at every successive trophic level.

Answer: ✓ Humans consuming the large fish would face the highest risk, because the large fish occupy the highest trophic level shown and therefore carry the greatest accumulated concentration of pesticide. Eating organisms from higher trophic levels exposes consumers to correspondingly higher doses.

Answer: ✓ Energy decreases because most of it is used by each organism for its own life processes and lost as heat, leaving only a small fraction available to the next level. Pesticide behaves oppositely because it is neither used up nor broken down — it simply passes on and accumulates. So the same feeding relationship that dilutes energy concentrates the persistent chemical.

This contrast — energy down, toxins up, through the same food chain — is one of the most conceptually satisfying ideas in the chapter, and a favourite for higher-order sub-questions.

Answer: ✓ Biodegradable: vegetable peels, paper, used tea leaves. Non-biodegradable: plastic bottles, glass.

Answer: ✓ A substance is biodegradable if decomposer organisms such as bacteria and fungi possess enzymes capable of breaking it down into simpler substances. Non-biodegradable materials persist because no natural decomposer has enzymes that can act on them.

Answer: ✓ Once mixed, biodegradable waste contaminates recyclable materials with moisture and organic residue, making them harder and costlier to recycle, while the biodegradable portion can no longer be cleanly composted. Separating at source keeps both streams usable, whereas separating after mixing recovers far less of either.

Answer: ✓ The defect is myopia (short-sightedness). In such an eye, light from a distant object converges too strongly and forms an image in front of the retina rather than on it, so distant objects appear blurred.

A concave (diverging) lens is needed. It must form an image of a distant object at the eye's far point, 80 cm away, so f = −80 cm.

Answer: ✓ Concave lens of focal length −80 cm (that is, −0.8 m)

Formula: P = 1/f (with f in metres) = 1/(−0.8)

Answer: ✓ P = −1.25 D

The negative sign isn't optional decoration — it identifies the lens as concave. Reporting the magnitude alone loses the mark.

Answer: ✓ The defect is hypermetropia (long-sightedness). Light from a nearby object is not converged strongly enough, so the image forms behind the retina, making near objects appear blurred.

The lens must take an object at 25 cm and form a virtual image at the reader's near point, 50 cm. So u = −25 cm and v = −50 cm.

Formula: 1/f = 1/v − 1/u = 1/(−50) − 1/(−25) = −1/50 + 2/50 = 1/50

Answer: ✓ f = +50 cm (a convex lens)

Formula: P = 1/f = 1/0.50

Answer: ✓ P = +2 D

Answer: ✓ Air molecules scatter sunlight, and shorter wavelengths are scattered much more strongly than longer ones. Blue light, having a shorter wavelength, is scattered far more than red across the sky, so scattered blue light reaches our eyes from all directions and the sky appears blue.

Answer: ✓ Near the horizon, sunlight travels through a much greater thickness of atmosphere to reach the observer. Over this longer path, most of the blue and shorter-wavelength light is scattered away, leaving mainly the longer-wavelength red light to travel through directly — so the sun appears reddish.

Answer: ✓ Because red light has the longest wavelength among visible colours, it is scattered the least by atmospheric particles such as fog, mist, or dust. This means red light travels furthest without being scattered away and remains clearly visible from a greater distance in poor conditions — exactly what a danger or stop signal needs.

All three parts of this scenario rest on the same single principle — scattering depends on wavelength. Competency questions often reward recognising that one idea explains several apparently unrelated observations.

Answer: ✓ The device is an electromagnet. A current-carrying coil produces a magnetic field, which magnetises the soft iron core and allows it to attract the pins. When the current stops, the magnetic field disappears and the soft iron loses its magnetism almost immediately, so the pins fall.

Answer: ✓ Soft iron is easily magnetised and just as easily demagnetised, which is exactly what an electromagnet requires — magnetism that can be switched on and off with the current. Steel retains its magnetism after the current stops, which would make the device a permanent magnet rather than a controllable one.

Answer: ✓ Increase the current flowing through the coil, or increase the number of turns in the coil. Both increase the strength of the magnetic field produced, and therefore the lifting power of the electromagnet.

Formula: I = P/V. Heater: 1000/220 ≈ 4.55 A. Geyser: 2000/220 ≈ 9.09 A

Answer: ✓ Heater ≈ 4.55 A, Geyser ≈ 9.09 A

Total current with both running ≈ 4.55 + 9.09 ≈ 13.64 A

Answer: ✓ The combined current of about 13.64 A far exceeds the fuse's 5 A rating. A fuse is deliberately made of a wire with a low melting point, so when current exceeds its rating the heat produced melts it, breaking the circuit. This is overloading — drawing more current than the circuit is rated for.

Answer: ✓ Extremely dangerous. A thick copper wire has a high melting point and would not break the circuit during an overload, allowing dangerously high current to continue flowing. The wiring itself would then overheat, risking insulation damage, electric shock, and fire. The fuse is designed to be the deliberate weak point that fails safely — removing that protection defeats the entire purpose.

Safety-reasoning sub-questions like this appear often, and full marks require explaining the consequence, not just saying "it is unsafe."

Answer: ✓ The correct solution is to use a circuit with an appropriately higher-rated fuse and adequately thick wiring for the intended load, or to avoid running both high-power appliances on the same circuit simultaneously. The earth wire serves a separate purpose: it provides a low-resistance path to the ground, so that if a fault causes the metal body of an appliance to become live, the current flows safely to earth instead of through a person touching it.

Formula: Energy (kWh) = (Power in kW) × hours × days

Incandescent: 0.060 × 6 × 30 = 10.8 kWh

LED: 0.009 × 6 × 30 = 1.62 kWh

Answer: ✓ 10.8 kWh and 1.62 kWh respectively

Incandescent: 10.8 × 8 = ₹86.40. LED: 1.62 × 8 = ₹12.96

Answer: ✓ Monthly saving = ₹86.40 − ₹12.96 = ₹73.44

Answer: ✓ The incandescent bulb consumes far more electrical energy for the same amount of light because most of its energy is converted into heat rather than light — which is exactly why it feels hot. The LED converts a much larger proportion of its input energy into light, wasting far less as heat, which is why it stays cool and consumes only a fraction of the energy.

Answer: ✓ This is a reflex action — an involuntary, automatic and very rapid response to a stimulus.

Answer: ✓ The pathway is the reflex arc: receptors in the skin detect the heat → a sensory neuron carries the impulse to the spinal cord → a relay neuron within the spinal cord passes it on → a motor neuron carries the impulse to the effector → the arm muscles contract and withdraw the hand.

Answer: ✓ The reflex is processed in the spinal cord, which sends the motor command directly without waiting for the signal to travel to the brain and be processed consciously. The signal does also travel onward to the brain, which is why pain is felt a moment later — but the protective movement has already happened by then.

Answer: ✓ It saves time in situations where delay causes harm. Routing the response through the spinal cord rather than the brain shortens the pathway considerably, allowing the body to withdraw from danger in a fraction of the time conscious processing would require — which limits the extent of injury.