CBSE Class 10 Science 2025 — Paper 31/5/2

Gas X is ozone (${\text{O}}_3$), depleted mainly by CFCs; UV splits oxygen molecules into atoms that combine with oxygen to form ozone.

The potential difference across the $6\Omega$ resistor is 3 V.

A 100% increase doubles the current, so the power becomes four times; the increase in power is 300%.

Medium X is optically denser than air, and light travels more slowly in X than in air.

Plasma and proteins leak from capillaries into tissue spaces as lymph, which carries absorbed fat and drains excess fluid back into the blood.

X is the Bowman's capsule, which collects the initial filtrate; useful substances are reabsorbed selectively because the body still needs them.

The testes hang outside the body because sperm formation needs a lower temperature; the glands' secretions carry and nourish the sperms.

The reaction is a double displacement reaction and a precipitation reaction, forming white barium sulphate.

Short-circuiting is live–neutral contact; a very large current flows, overheating wires, so a fuse of proper rating melts to break the circuit.

The length of the wire is $5\times 10^{-3}$ m, that is 0.005 m.

Concave lens, $f=-4$ m; used for myopia; the image is virtual, erect and diminished, between $F$ and $O$.

M is iron; it rusts in tube A but not in B (oil) or C (dry air), so rusting needs both air and moisture.

Al gives three electrons to N to form ${\text{Al}}^{3+}$ and ${\text{N}}^{3-}$, giving AlN; ionic solids are brittle because like charges repel when layers slip.

Washing soda is ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, made by heating baking soda and recrystallising the sodium carbonate formed.

A gamete has half the chromosomes; the zygote, formed by fusion of two gametes, has the full number and starts the new individual.

Water holds little dissolved oxygen, cartilage rings keep the windpipe open, and oxygen-starved muscle cells turn pyruvate into 3-carbon lactic acid.

Light is fastest in C ($v_A=v_B$ less than $v_C$); A and B are equally dense and C is the rarest.

(a) $f=+20$ cm; (b) $m=-2$; (c) observation 1 gives a virtual, erect image, observation 2 a real, inverted one.

Aldehydes (methanal, ethanal, propanal) form the $-\text{CHO}$ series; sodium hydrogencarbonate gives effervescence with a carboxylic acid but not with an alcohol.

Ethene (X) shows addition because it has a double bond; propane (Y) is saturated. Hydrogenation with nickel is used in the vegetable oil industry.

Iodine deficiency causes goitre, too little growth hormone causes dwarfism, testosterone brings facial hair; hormones reach every cell and give a steady signal.

Voluntary actions are controlled by the cerebrum, involuntary ones by the mid-brain and medulla; a reflex action is processed by the spinal cord.

Lines emerge from N and merge at S; closer lines mean a stronger field; lines never cross; the field inside a solenoid is uniform.

Chromosomes carry genes; the male XY pair is mismatched; boys and girls are $50:50$; turtles and snails show non-genetic sex.

Element A gains two electrons (${\text{A}}^{2-}$); butene has 10 single and 1 double bond; ammonia has three shared pairs and a lone pair.