CBSE Class 10 Science 2024 — Paper 31/2/1

A precipitate forms only when the two reactants exchange ions, as ${\text{Ba}}^{2+}$ and ${\text{SO}}_4^{2-}$ combine to give insoluble barium sulphate.

In displacement a more reactive element replaces a less reactive one, but in double displacement two compounds exchange ions.

Translocation: sucrose made in leaves is loaded into the phloem using ATP and carried by pressure to roots, fruits, seeds and buds.

Germ cells form with half the chromosomes and DNA, and fertilisation restores the full number, so the DNA content stays constant.

The incident ray, refracted ray and normal lie in one plane, and $\frac{\sin i}{\sin r}$ is constant (Snell's law).

The absolute refractive index is $n=\frac{c}{v}$ (vacuum speed over medium speed); light travels at $2\times 10^8$ m/s in glass.

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

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

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

The impure copper anode dissolves and pure copper deposits on the cathode; insoluble impurities settle as anode mud.

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.

Fertilisation in the oviduct gives a zygote that becomes an implanted embryo, then a foetus; the placenta supplies nutrients and removes wastes.

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.

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

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

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.

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.

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

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.