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

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

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.

Residual air keeps gas exchange going between breaths; carbon dioxide is far more soluble than oxygen, so it travels dissolved in plasma.

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.

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

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

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

The thermite reaction between aluminium and ferric oxide gives molten iron that welds rail ends; aluminium displaces iron because it is more reactive.

Yeast gives ethanol and carbon dioxide; muscle cells give lactic acid; both processes take place in the cytoplasm.

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.

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

Rod moves left for current B to A, right for A to B; parallel to the field there is no force.

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

Chlor-alkali process; A is chlorine (anode), B is hydrogen (cathode); C is bleaching powder, ${\text{CaOCl}}_2$, made from chlorine and ${\text{Ca(OH)}}_2$.

The droplets are water of crystallisation; green crystals turn dirty white, then reddish brown; ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}$ has 7 water molecules.

Image distance stays constant as the ciliary muscles change the lens focal length; the person has myopia, corrected by a concave lens.

Tyndall effect makes the beam visible, with fine particles scattering blue; the concave $-0.5$ D lens is for distant vision, with $f=-2$ 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.

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.

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 $968\Omega$.