CBSE Class 10 Science 2025 — Paper 31/3/2

Heat, electricity or light can supply the energy; for example, electricity splits water into hydrogen and oxygen.

Green crystals turn white, then reddish brown, giving off gases with the smell of burning sulphur.

Baking powder gives soft, spongy bread without the bitter taste, because its acid reacts with ${\text{NaHCO}}_3$ and leaves no sodium carbonate.

Arteries carry blood away from the heart to the organs; the blood is under high pressure, so their walls are thick and elastic.

(a) An altered gene gives a less efficient enzyme, so less growth hormone is made and the pea plant stays short; (b) DNA.

Hypermetropia, caused by a long eye-lens focal length or a short eyeball, corrected with a convex lens.

The ray bends towards the normal at the first face and away at the second, turning towards the base; $\delta =i+e-A$.

The voltage source supplies $4.8\times 10^4$ J (48 kJ) of electrical energy.

Displacement: a more reactive element replaces another in a compound; double displacement: two compounds exchange ions, often forming a precipitate.

Sodium hydroxide: electrolysis of brine (chlor-alkali process); sodium hydrogencarbonate: ammonia and carbon dioxide passed into brine; both with balanced equations.

The bulb glows with dilute hydrochloric acid but not with glucose or alcohol, as they give no ${\text{H}}^{+}$ ions in water.

Urea and uric acid; the glomerulus; and urine flows from the kidney through the ureter, urinary bladder and urethra to the outside.

${\text{F}}_1$ seeds are all round and yellow; ${\text{F}}_2$ gives four combinations in the ratio $9:3:3:1$ because the two traits are inherited independently.

Hypermetropia; caused by a long eye-lens focal length or a small eyeball; corrected by convergent convex glasses, $f=+50$ cm.

Concentric circles with clockwise arrows seen from above for downward current; the right-hand thumb rule confirms the directions.

Energy enters only as sunlight and about 90% is lost as heat at each level, so flow is one-way; secondary consumers get 100 J.

Parallel faces give $r_2=i_1$, so the emergent ray is parallel but shifted; a normal ray goes straight; $v=-12$ cm.

Real, inverted image that shrinks towards F as the flame moves away; the bus's image is 2.0 m behind the mirror.

Ag, Al, K and Cu match I to IV; amphoteric oxides react with both acids and bases, as ${\text{Al}}_2{\text{O}}_3$ does; alkalis are water-soluble bases.

Mercury comes from roasting then heating cinnabar, copper from self-reduction of ${\text{Cu}}_2\text{O}$ with ${\text{Cu}}_2\text{S}$; coatings are black silver sulphide and green basic copper carbonate.

X is the stigma, Y the anther; pollen from Y reaches X by pollination, and fertilisation follows, turning the ovule into the seed.

Leishmania divides by binary fission; asexual reproduction needs one parent and gives identical offspring quickly; Hydra reproduces by budding.

Test tube III gives least foam; A is a soap and B a detergent; an ester with alkali gives a sodium salt and alcohol.

Adrenaline, from the adrenal medulla; it speeds the heartbeat and breathing; hormones act slower but longer and wider than nerve impulses.

Graph A is series and D is parallel; two $10\Omega$ in parallel plus one in series give $15\Omega$; or the current is 4 A.