Q&A: practice questions with answers

Concept: $W=mgh$ when lifting a load.

Solution:

1. $W=10\times 10\times 2.5$.
2. $W=250$ J.

Yaad rakhein: Work against gravity = increase in potential energy (mgh).

Answer: (D) 250 J

Concept: Saturated hydrocarbons burn completely with a clean (blue) flame.

Solution:

1. Alkanes burn in air giving a clean, non-sooty blue flame.
2. Unsaturated hydrocarbons burn with a yellow, sooty flame (incomplete combustion).

Yaad rakhein: Saturated = clean blue flame; unsaturated = sooty yellow flame.

Answer: (A) Clean flame

Concept: Reactions that release heat are exothermic.

Solution:

1. C + O₂ → CO₂ + heat.
2. All combustion reactions release heat → exothermic.

Yaad rakhein: Burning, respiration and neutralisation are exothermic; photosynthesis is endothermic.

Answer: (C) Exothermic reaction

Concept: Plant cells have a cellulose cell wall; animal cells do not.

Solution:

1. Cell wall is outside the plasma membrane in plants.
2. Mitochondria, ER and cytoplasm are present in both.

Yaad rakhein: Plant-only: cell wall, plastids (chloroplasts), large central vacuole.

Answer: (A) Cell wall

Concept: Each trophic level depends on the level below for food.

Solution:

1. Herbivores are food for carnivores.
2. Fewer herbivores → less food → carnivore population falls.
3. Producers would not decrease from overgrazing because herbivores are fewer.

Yaad rakhein: Change at one level affects the next level up first.

Answer: (A) Carnivore population will decrease due to less food.

Concept: A concave lens always forms a virtual, erect, diminished image.

Solution:

1. For any real object position, rays diverge after refraction.
2. Image is always virtual, erect and between optical centre and F (same side).

Yaad rakhein: Object at infinity → image at F (highly diminished); otherwise between F and O.

Answer: (C) At any position in front of the lens.

Concept: $a=\frac{v-u}{t}$ in SI units.

Solution:

1. $v=216\times \frac{5}{18}=60$ m/s; $u=0$.
2. $t=10\text{min}=600\text{s}$.
3. $a=\frac{60}{600}=0.1$ m/s².

Trick: km/h → m/s: multiply by $\frac{5}{18}$ (216 → 60).

Answer: (C) 0.1 m/s2

Concept: Distance = area under velocity–time graph.

Solution:

1. Area = 10 m/s × 5 s (rectangle).
2. Distance = 50 m.

Yaad rakhein: Slope of v–t = acceleration; area under v–t = displacement.

Answer: (B) 50 m

Concept: $v=f\times \lambda$.

Solution:

1. f = 33 kHz = 33,000 Hz.
2. $v=33,000\times 0.01=330$ m/s.

Yaad rakhein: Ultrasound has frequency above 20 kHz but in air it still travels at sound speed (~330–343 m/s).

Answer: (C) 330 m/s

Concept: Criss-cross the valencies of the ions.

Solution:

1. Ca²⁺ and PO₄³⁻.
2. Cross valencies: Ca₃(PO₄)₂.
3. Check charge: $3\times (+2)=+6,2\times (-3)=-6$ ✓.

Trick: Swap the charge numbers: 2 goes to PO₄, 3 goes to Ca.

Answer: (B) Ca3(PO4)2

Concept: W = F × s; work is zero if displacement is zero.

Solution:

1. Standing still → load does not move → s = 0.
2. W = F × 0 = 0, even though muscles use energy (we feel tired).

Yaad rakhein: Work is also zero when force ⟂ displacement (e.g. carrying a load horizontally).

Answer: (C) There is no displacement of the load.

Concept: Cilia beat rhythmically to move substances.

Solution:

1. In the respiratory tract, cilia push mucus (with trapped dust and germs) upward to be cleared.
2. Secretion is done by goblet/gland cells, not cilia.

Yaad rakhein: Ciliated columnar epithelium is also found in fallopian tubes (moves the ovum).

Answer: (A) To move and push mucus forward to clear it

Concept: Balanced use of manures and fertilisers keeps soil healthy and raises yield.

Solution:

1. Supplies macro- and micronutrients in the right amount.
2. Maintains soil fertility and gives optimum crop yield; other options are harmful or irrelevant.

Yaad rakhein: Manures add organic matter; fertilisers give quick nutrients — integrated use is best.

Answer: (B) To optimize soil fertility and crop yield

Concept: Golgi = cell's packaging and dispatch centre.

Solution:

1. Modifies, packages and secretes materials made in the ER.
2. Also forms lysosomes.
3. Protein synthesis → ribosomes; ATP → mitochondria; photosynthesis → chloroplasts.

Yaad rakhein: Golgi apparatus was first described by Camillo Golgi.

Answer: (D) Packaging and secretion of materials

Concept: Food moves through sieve tubes of phloem.

Solution:

1. Sieve tubes have sieve plates through which food (sucrose) flows.
2. Companion cells support sieve tubes; fibres give strength.

Yaad rakhein: Phloem elements: sieve tubes, companion cells, phloem parenchyma, phloem fibres (only fibres are dead).

Answer: (C) Sieve tubes

Concept: Speed of sound depends on the medium's nature and temperature.

Solution:

1. Speed increases with temperature (e.g. ~331 m/s at 0 °C, ~344 m/s at 22 °C in air).
2. At constant temperature, pressure has no effect; amplitude does not change speed.

Yaad rakhein: Speed: solids > liquids > gases.

Answer: (C) Temperature

Concept: Rhizopus reproduces by spore formation.

Solution:

1. Thread-like hyphae bear erect stalks.
2. The tiny blobs on top are sporangia, which contain spores.

Yaad rakhein: Sporangium is the case; spores are inside it — don't confuse the two.

Answer: (D) Sporangia

Concept: Soap molecules have a hydrophilic ionic head and a hydrophobic hydrocarbon tail.

Solution:

1. In water, tails turn inward (away from water) and heads face outward.
2. This forms a micelle that traps oil/dirt in its centre.

Yaad rakhein: Soap does not work well in hard water — it forms scum with Ca²⁺/Mg²⁺.

Answer: (A) hydrophobic and hydrophilic interactions in water

Concept: Lens formula: $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$.

Solution:

1. Applies to both convex and concave lenses with sign convention.
2. Mirror formula differs: $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$.

Trick: Lens has minus, mirror has plus — 'L for Less (minus)'.

Answer: (A) (1/v) - (1/u) = 1/f

Concept: Solid sol = solid dispersed in a solid medium.

Solution:

1. Coloured gemstone, milky glass: solid in solid ✓.
2. Jelly, cheese = gel (liquid in solid); smoke = aerosol (solid in gas).

Yaad rakhein: Sol = solid in liquid (paint); aerosol = in gas (fog, smoke); emulsion = liquid in liquid (milk).

Answer: (A) Coloured gem