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

An alloy is a homogeneous mixture of metals; solder is lead and tin, and its low melting point suits welding wires.

Anode is impure silver and cathode is pure silver; anode mud is the insoluble noble-metal impurity that settles below the anode.

The balanced equations are $2\text{Al}+3{\text{CuCl}}_2\to 2{\text{AlCl}}_3+3\text{Cu}$ and $\text{Zn}+2\text{NaOH}\to {\text{Na}}_2{\text{ZnO}}_2+{\text{H}}_2$.

It is a long, coiled tube whose villi increase the absorbing surface, and their blood vessels carry the food to every cell.

The ray bends towards the normal at $AB$ and away at $AC$, giving angle of deviation $\angle D=37.4^{\circ }$ for $\angle i=45^{\circ }$.

$v=-37.5$ cm: the image is formed 37.5 cm in front of the mirror, real, inverted and enlarged.

The part of a tendril touching a support grows more slowly, so the tendril coils round it and anchors the weak stem.

Impulses reach only cells joined by nervous tissue, and a cell needs time to reset before it can send another impulse.

Biological magnification is the progressive rise of harmful chemicals along a food chain; it is highest in human beings, the last level.

X has the higher hydrogen ion concentration (low pH); the green mixture is neutral (pH 7).

X is the medulla, controlling involuntary actions; Y is the cerebellum, giving posture, balance and precision to voluntary actions.

Sodium hydrogencarbonate, ${\text{NaHCO}}_3$, is made from brine, ammonia and carbon dioxide; being a mild base, it neutralises excess stomach acid.

Below the wire the field points south; above the wire it points north.

All three in parallel give $1\Omega$; the $3\Omega$ and $6\Omega$ in parallel, in series with the $2\Omega$, give $4\Omega$.

Sucrose is loaded into the phloem using ATP, water follows by osmosis, and the resulting pressure moves food from source to sink.

Xylem moves water and minerals upwards passively; phloem moves sucrose and other food both ways using ATP energy.

Carbon dioxide is released; it turns lime water milky, and excess carbon dioxide clears it by forming soluble calcium hydrogencarbonate.

Copper oxide gives blue-green ${\text{CuCl}}_2$ solution; X is gypsum, Y is Plaster of Paris, and adding water to Y gives X back.

Raindrops refract, internally reflect and refract sunlight to form a rainbow opposite the Sun; the Moon has no atmosphere, so none forms.

Hypermetropia is corrected by a convex lens; the concave $-0.25$ D lens is for distant vision, with $f=-4$ m.

Inheritance information is carried as DNA; Amoeba divides in any plane, Leishmania lengthwise; complex bodies cannot fragment because a piece lacks all tissues.

The placenta exchanges glucose, oxygen and wastes between mother and embryo through villi; contraceptives are barrier, chemical and surgical.

With ${\text{K}}_1$ only the fan and L work; ${\text{K}}_2$ adds A to E; $I_L\approx 0.09$ A, $R_A=193.6\Omega$, total $968\Omega$.

It is propyne, an alkyne; saturated compounds burn with a clean blue flame, unsaturated with a sooty yellow flame; alkenes and alkynes undergo addition.

Sex chromosomes (XX, XY) decide sex; turtle sex depends on egg temperature; the father's sperm decides a child's sex; each parent gives 23 chromosomes.