CBSE Class 10 Science 2025 — Paper 31(B)

A white precipitate of barium sulphate forms in a double displacement (precipitation) reaction.

Green crystals turn white and then reddish brown; $2{\text{FeSO}}_4\to {\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$, a thermal decomposition reaction.

Zinc gives brisk effervescence of hydrogen gas and the test tube becomes warm, as the reaction is exothermic.

Gases are exchanged through the stomata of leaves, the lenticels of woody stems and the surface of the roots, by simple diffusion.

Salivary amylase starts starch digestion, HCl provides an acidic medium for pepsin, trypsin digests proteins and villi increase absorption.

Blood from the organs flows through capillaries, venules and veins, whose valves prevent backflow, into the venae cavae and the right atrium.

The image is virtual, erect, the same size, as far behind the mirror as the object is in front, and laterally inverted.

A convex mirror always forms a virtual, erect, diminished image behind the mirror, between the pole and the focus.

Biodegradable waste is broken down by micro-organisms into simpler harmless substances; here, agricultural waste and sewage.

Bacteria and fungi break dead remains and wastes into simple substances; they clean the environment and return nutrients to the soil.

Hydrogen is collected at the cathode and oxygen at the anode, in a $2:1$ volume ratio; the acid makes water conduct.

Sodium and water give $\text{NaOH}$ and ${\text{H}}_2$; lead nitrate gives $\text{PbO}$, ${\text{NO}}_2$ and ${\text{O}}_2$; nitrogen and hydrogen combine as ${\text{N}}_2+3{\text{H}}_2\to 2{\text{NH}}_3$.

Hydrogen at the cathode (fuel), chlorine at the anode (water disinfectant) and sodium hydroxide near the cathode (soaps and detergents).

A synapse is the gap between a nerve ending and the next neuron's dendrite; it passes the message on chemically, in one direction.

Thyroxin comes from the thyroid and regulates metabolism, insulin from the pancreas lowers blood sugar, and growth hormone from the pituitary controls growth.

Plant hormones are chemical compounds made in the plant that coordinate its growth; auxin made at the shoot tip bends the shoot towards light.

The image is 60 cm from the optical centre, on the opposite side, and 8.0 cm tall (real, inverted, magnified).

Fuse and earthing; the appliance needs $9.09$ A against the 5 A rating, so the fuse melts and the circuit breaks.

(i) Right-hand thumb rule gives the field around a straight conductor; (ii) Fleming's left-hand rule gives the force on a conductor in a field.

Gas G is ozone, which absorbs harmful UV; CFCs deplete it; UNEP's 1987 agreement froze CFC production at 1986 levels.

Carbon shares electrons; methanal ${\text{CH}}_2\text{O}$ and ethanal ${\text{C}}_2{\text{H}}_4\text{O}$ are consecutive aldehydes; benzene has 9 single and 3 double bonds.

Ethanol gives ethene with excess conc. sulphuric acid, ethyl ethanoate with ethanoic acid, sodium ethoxide with sodium and ethanoic acid on oxidation.

An unfertilised egg leads to menstruation; fertilisation leads to an embryo nourished by the placenta; pills, Copper-T and tubectomy each have side effects.

Power is the rate of consuming electric energy, $P=\frac{V^2}{R}$; the air-conditioner is 2200 W, draws 10 A and has resistance $22\Omega$.

Impure copper is the anode and pure copper the cathode in copper sulphate solution; impurities settle below the anode as anode mud.

$F_1$ plants are all violet ($Vw$); $VV:Vw=1:2$; 200 of 800 $F_2$ plants are white; the father's sex chromosome decides the child's sex.

Myopia: nearby objects clear, distant ones not; causes are a too curved lens or a long eyeball; presbyopia needs bifocals; OR hypermetropia, $f=+4$ m.