Science — solved board-pattern questions

(b) 100 J.

By Lindeman's 10% law only about 10% of the energy at one trophic level passes to the next: Grass 10000 J → Deer 1000 J → Lion 100 J.

(a) Both A and R are true and R is the correct explanation of A.

Dry HCl gas exists as covalent molecules with no free H⁺ ions, so it shows no acidic action on dry litmus. Acidic character appears only after HCl dissolves in water and releases H⁺ (H₃O⁺) ions — exactly the reason stated in R.

(a) Lead oxide is reduced and carbon is oxidised.

PbO loses oxygen → it is reduced; carbon gains oxygen → it is oxidised (carbon acts as the reducing agent). It is a redox reaction.

(i) 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

Type: displacement (redox) reaction — iron displaces hydrogen from steam.

(ii) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)↓

Type: double displacement (precipitation) reaction — insoluble white BaSO₄ is precipitated.

Check (ii): Ba 3=3, Cl 6=6, Al 2=2, SO₄ 3=3.

1. Nature of oxide: Metals form basic oxides (e.g. MgO, Na₂O turn red litmus blue), whereas non-metals form acidic or neutral oxides (e.g. SO₂, CO₂ turn blue litmus red).

2. Physical property: Metals are good conductors of heat and electricity and are malleable and ductile (e.g. copper, aluminium), whereas non-metals are poor conductors and brittle (e.g. sulphur, carbon — except graphite, which conducts).

(i) Micelle: In water, soap molecules cluster into a tiny sphere called a micelle, with the hydrophobic hydrocarbon tails pointing inward (towards oil/dirt) and the hydrophilic –COO⁻Na⁺ heads pointing outward into the water.

(ii) Cleansing action: The oily tail dissolves into the greasy dirt while the ionic head stays in water; the dirt is trapped at the centre of the micelle, forming an emulsion that is rinsed away with water.

(iii) Hard water contains Ca²⁺ and Mg²⁺ ions that react with soap to form an insoluble precipitate (scum), so soap is wasted and gives poor lather.

Given (New Cartesian sign convention): f = −15 cm, u = −30 cm, object height h = +4 cm.

Mirror formula: 1/v + 1/u = 1/f

1/v = 1/f − 1/u = (1/−15) − (1/−30) = −1/15 + 1/30 = −1/30

So v = −30 cm → image is 30 cm in front of the mirror (real).

Magnification m = −v/u = −(−30)/(−30) = −1.

Image height = m × h = −1 × 4 = −4 cm.

The image is real, inverted, same size (4 cm), formed at the centre of curvature (object is at C).

1. Direction of flow: Arteries carry blood away from the heart; veins carry blood towards the heart.

2. Wall and valves: Arteries have thick, elastic, muscular walls (blood flows under high pressure) and no valves; veins have thin walls with internal valves that prevent the backflow of blood.

(Also: arteries, except the pulmonary artery, carry oxygenated blood, while veins, except the pulmonary vein, carry deoxygenated blood.)

(i) After fertilisation the ovary develops into the fruit and the ovules develop into seeds.

(ii) Advantages of sexual reproduction: (a) it produces variations because DNA from two different parents combines, which helps the species adapt and survive; (b) these variations are the raw material for evolution and increase the diversity of the species.

(i) Series: Rs = R1 + R2 = 6 + 3 = 9 Ω. Current I = V/Rs = 6/9 = 0.67 A (approximately 0.67 A).

(ii) Parallel: 1/Rp = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2 ⇒ Rp = 2 Ω. Current I = V/Rp = 6/2 = 3 A.

(iii) The total resistance is greater in the series arrangement (9 Ω > 2 Ω).

(iv) Power in parallel: P = V²/Rp = (6)²/2 = 36/2 = 18 W (or P = VI = 6 × 3 = 18 W).

(i) It is a reflex action (a spinal reflex).

(ii) A reflex action is a rapid, automatic, involuntary response of the body to a stimulus, which occurs without conscious thinking.

(iii) Reflex arc: Receptor in the skin (hot stimulus) → sensory (afferent) neuron → spinal cord (relay neuron) → motor (efferent) neuron → effector muscle of the hand → hand is withdrawn.

(iv) The withdrawal is controlled quickly by the spinal cord, but the sensation of pain is felt only when the impulse later reaches the brain — so the hand is pulled back first and the pain is registered a moment later.

(i) Power of accommodation: the ability of the eye lens to change (adjust) its focal length, by the action of the ciliary muscles altering its curvature, so that objects at different distances are focused sharply on the retina.

(ii) Near point = 25 cm (least distance of distinct vision); far point = infinity for a normal eye.

(iii) Defect: Myopia (short-sightedness / near-sightedness). Cause: the eyeball is too long or the eye lens is too converging, so the image of a distant object forms in FRONT of the retina. Correction: a concave (diverging) lens of suitable power.

(iv) Opposite defect: Hypermetropia (long-sightedness) — near objects appear blurred and the image forms BEHIND the retina; it is corrected using a convex (converging) lens.

(i) Electromagnetic induction is the production of an induced current (and EMF) in a conductor or coil whenever the magnetic flux linked with it changes — for example, by moving a magnet into or out of a coil.

(ii) Fleming's right-hand rule: Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular. If the forefinger points along the magnetic field and the thumb along the direction of motion of the conductor, the middle finger points along the direction of the induced current.

(iii) Principle/working of an AC generator: It works on electromagnetic induction. When a rectangular coil is rotated in a magnetic field, the magnetic flux linked with it changes continuously, inducing a current that reverses its direction every half rotation, giving alternating current.

(iv) Difference: an AC generator uses two slip rings, whereas a DC generator uses a split-ring commutator (which reverses the connection every half turn to give a unidirectional current).

(v) Two ways: (a) move the magnet/coil faster (increase the rate of change of flux); (b) use a stronger magnet or increase the number of turns in the coil.

(i) Homologous organs have the same basic structure and origin but perform different functions — e.g. the forelimbs of a human (arm), a bird (wing) and a frog. Analogous organs have different basic structure and origin but perform the same function — e.g. the wings of a bird and the wings of an insect.

(ii) Homologous organs indicate a common ancestry, i.e. divergent evolution; analogous organs indicate convergent evolution (different ancestors evolving a similar function).

(iii) An acquired trait develops during an organism's lifetime (e.g. the strong muscles of a wrestler, or low body weight from starvation) and is NOT inherited, because it does not change the DNA of the germ (reproductive) cells. An inherited trait (e.g. eye colour) is present in the DNA of the germ cells and so IS passed on to the next generation.