CBSE Class 10 Science 2024 — Paper 31/2/3

Zinc with dilute sulphuric acid gives hydrogen gas; an iron nail in copper sulphate solution turns the blue solution pale green.

The balanced equations are $2{\text{H}}_2\text{S}+3{\text{O}}_2\to 2{\text{H}}_2\text{O}+2{\text{SO}}_2$ and $2\text{AgBr}\to 2\text{Ag}+{\text{Br}}_2$ in sunlight.

The renal artery brings blood to the kidneys; blood is filtered in each nephron's glomerulus, and the tubule selectively reabsorbs useful substances.

All ${\text{F}}_1$ seeds are round and yellow; ${\text{F}}_2$ shows four combinations in the ratio 9 : 3 : 3 : 1.

The potential difference across the $3\Omega$ resistor is 1 V.

Myopia; caused by an over-curved eye lens or an elongated eyeball; corrected by a concave (diverging) lens.

An age-related loss of accommodation; corrected by bifocal lenses whose upper part is concave (diverging).

They are called non-biodegradable substances; they persist and pollute soil and water, and some, like pesticides, magnify along food chains.

Sodium is kept in kerosene to cut it off from air; soluble oxides give alkalis; the surface layer is oxide, which protects the metal.

Hydrogen bubbles stick to calcium and make it float; carbon dioxide turns the lime water milky by forming insoluble calcium carbonate.

Oral pills stop release of eggs and blocked fallopian tubes stop the egg reaching the uterus; the testes make sperms and secrete testosterone.

Capillaries give alveoli a large exchange surface; haemoglobin binds oxygen strongly; without oxygen, pyruvate becomes lactic acid, not carbon dioxide.

Peristalsis pushes food along the gut; bile, stored in the gall bladder, makes food alkaline and emulsifies fats.

Power of accommodation is the eye lens's ability to change focal length; the ciliary muscles contract for near objects and relax for distant ones.

The ray is refracted and dispersed at A, internally reflected at B and refracted at C; the Sun must be behind the observer.

A solenoid is a closely wound coil; its field is like a bar magnet's, and soft iron inside it becomes an electromagnet.

Biological magnification: non-degradable pesticides enter the food chain and build up at each trophic level, reaching the maximum in human beings.

Plant hormones lack glands while animal hormones come from endocrine glands; cerebrum, cerebellum, medulla and hypothalamus control the four activities; cranium and backbone protect.

Tropic movements are directional growth responses to a stimulus; abscisic acid inhibits growth, cytokinin promotes cell division, and auxin makes the pea tendril coil.

(a) Image at $2F_2$, real, inverted, same size but dimmer; brighter when uncovered. (b) $v=-10$ cm.

Carbon shares its four valence electrons; ${\text{CH}}_2\text{O}$ and ${\text{C}}_2{\text{H}}_4\text{O}$ are consecutive aldehydes; cyclohexane is a ring of six carbon atoms with single bonds.

Ethanol (${\text{C}}_2{\text{H}}_6\text{O}$) gives sodium ethoxide, ethene, ethyl ethanoate and ethanoic acid with sodium, conc. sulphuric acid, ethanoic acid and dichromate.

All pieces (L, M, N, O, P) regenerate; Hydra is another example; development is the organised sequence of changes forming the lost parts.

Heating elements need high resistivity and melting point; a fuse uses the heating effect to break the circuit; or the heater draws $5$ A.

$\text{HCl}$ and $\text{NaOH}$ form sodium chloride; calcium sulphate has ${\text{Ca}}^{2+}$ and ${\text{SO}}_4^{2-}$; sodium chloride and washing soda share ${\text{Na}}^{+}$.