CBSE Class 10 Science 2025 — Paper 31/1/2

Chemicals wash into soil and water, magnify along food chains to harm humans, and kill soil micro-organisms, so the statement is justified.

Power is minimum (14.4 W) in circuit (ii) and maximum (57.6 W) in circuit (iii), because $P=\frac{V^2}{R}$ at the same voltage.

The two lamps draw $I=\frac{8}{11}\text{A}\approx 0.73\text{A}$ from the supply.

$v=-60$ cm: the image is formed 60 cm in front of the mirror, beyond $C$, real and inverted.

Plugging a leak keeps the pressure of the circulation; platelets stick at the cut and start clotting, and fibrin threads seal it.

Plants have no pump and low energy needs, so transport is slow; phloem translocates sugars from leaves to other parts, upwards and downwards.

Budding in Hydra is shown in four stages: a bud forms, grows, develops mouth and tentacles, then detaches as a new Hydra.

Lead nitrate decomposes on heating into lead oxide, nitrogen dioxide and oxygen; natural gas burns to give carbon dioxide, water and heat.

One ampere is one coulomb of charge passing a cross-section per second; the wire is $1$ m long.

Total resistance is $30\Omega$, the current is $0.5$ A, and the $10\Omega$ and $15\Omega$ parallel pair has $3$ V across it.

A convex mirror gives a virtual, erect, diminished image: at F for an object at infinity, and between P and F otherwise.

Iron rusts only when both air and water are present: only the nail in the tube with air and water rusts.

Potassium and sodium react violently with cold water, giving hydrogen, which burns with a pop sound near a burning splinter.

Aluminium displaces middle-of-series metals such as iron and manganese from their oxides; carbon cannot reduce the oxides of highly reactive metals.

Round : wrinkled and yellow : green are each about $3:1$ and the four classes about $9:3:3:1$, so the two traits are inherited independently.

Adrenaline is secreted by the adrenal glands and thyroxin by the thyroid; feedback regulates hormone release, as blood sugar level controls insulin secretion.

(i) Concave lens, $f=-40$ cm, for myopia; (ii) real, inverted, magnified image at $v=+40$ cm; (iii) lens A, the larger power.

Emergent ray is parallel to the incident ray; a convex lens gives a virtual image only inside $F$ ($m>1$), a concave lens always ($m<1$).

A is ethanol, B ethanoic acid, C ethene (28 u); sodium carbonate gives brisk effervescence of carbon dioxide and sodium ethanoate.

Methanoic, ethanoic and propanoic acids form a homologous series; cyclohexane (${\text{C}}_6{\text{H}}_{12}$) and benzene (${\text{C}}_6{\text{H}}_6$) are ring compounds.

Ovary makes the ovum and hormones, fallopian tube is the site of fertilisation, uterus nourishes the foetus; condom and vasectomy are male methods.

Self-pollination is within one plant, cross-pollination between plants; A is the stigma, B the pollen tube, C the female germ cell.

Live wire is red, neutral black; the iron needs a 5 A circuit ($I\approx 4.55$ A); the earth wire carries leakage current to the earth.

Photosynthesis makes carbohydrate from carbon dioxide and water; desert plants fix carbon dioxide at night; cloudy weather and blocked stomata lower the rate.

Brine electrolysis gives $\text{NaOH}$, ${\text{Cl}}_2$ and ${\text{H}}_2$; A is baking soda, B is sodium carbonate; water of crystallisation is fixed water in a salt.