Q&A: practice questions with answers

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

1. $u=-20$ cm, $f=+15$ cm: $\frac{1}{v}=\frac{1}{15}-\frac{1}{20}=\frac{1}{60}$, so $v=+60$ cm
2. $m=\frac{60}{-20}=-3$ (real, inverted, 3 times enlarged)

Trick: The object is between f and 2f (20 cm lies between 15 and 30 cm), so the image is real, inverted and magnified: m must be negative with $|m|>1$, which is −3.

Answer: (B) -3

Concept: The pistil (carpel) is the female reproductive part.

1. Pistil = stigma + style + ovary (ovules inside)
2. Stamen (anther + filament) is the male part; sepals and petals are non-reproductive

Yaad rakhein: After fertilisation the ovary becomes the fruit and the ovules become seeds.

Answer: (D) pistil

Concept: Thumb, first finger and middle finger are held mutually perpendicular.

1. First (index) finger = magnetic Field
2. Middle finger = Current
3. Thumb = Motion (force on the conductor)

Trick: FBI order: F (thumb) = Force, B (index) = magnetic field, I (middle) = current. Or 'Father, Mother, Child' = Motion (thumb), Field (first), Current (middle).

Answer: (C) First (index) finger

Concept: $R=\frac{V}{I}$.

1. $R=1\text{V}÷1\text{A}$
2. $=1\Omega$

Yaad rakhein: This is the definition of 1 ohm: the resistance of a conductor through which 1 A flows when 1 V is applied across it.

Answer: (D) 1 Ω

Concept: Add the atomic masses: N = 14, H = 1, S = 32, O = 16 u. The formula is $({\text{NH}}_4{)}_2{\text{SO}}_4$.

1. $2\times {\text{NH}}_4=2\times (14+4)=36$
2. ${\text{SO}}_4=32+4\times 16=96$
3. Total = $36+96=132$ u

Trick: Remember ${\text{NH}}_4=18$ and ${\text{SO}}_4=96$ (${\text{H}}_2{\text{SO}}_4=98$, take away 2 H). So $2\times 18+96=132$.

Answer: (C) 132 u

Concept: Acid + base → salt + water; H⁺ and OH⁻ cancel each other.

1. When neutralisation is complete, the solution is neither acidic nor basic, so pH ≈ 7
2. The reaction is exothermic, so the temperature rises (it does not fall)
3. Blue → red litmus shows an acid, not neutralisation; a pungent gas is not a sign of it

Trick: 'Neutral' means pH 7. The option with pH 7 is the right one.

Answer: (C) The pH of the solution becomes close to 7.

Concept: Each observation of α-scattering leads to one conclusion.

1. Most α-particles passed straight through, so the atom is mostly empty space
2. A few were deflected, so the positive charge is concentrated in a small region
3. Very few (about 1 in 12,000) bounced back, so the nucleus is very small and dense

Yaad rakhein: The nucleus is about 10⁵ times smaller than the atom (radius ~10⁻¹⁵ m vs ~10⁻¹⁰ m).

Answer: (B) the atom is mostly empty space

Concept: A complex tissue is made of more than one type of cell, all working together for one function.

1. Xylem: tracheids, vessels, xylem parenchyma, xylem fibres; carries water and minerals
2. Phloem: sieve tubes, companion cells, phloem parenchyma, phloem fibres; carries food
3. They are 'permanent' because their cells have lost the ability to divide

Yaad rakhein: Simple permanent tissues (parenchyma, collenchyma, sclerenchyma) have only one type of cell.

Answer: (C) They contain different types of cells working together for transport.

Concept: A molecule of an element contains only one kind of atom.

1. Examples: O₂, N₂, O₃, P₄, S₈ (same atoms bonded together)
2. Noble gases (He, Ne) are monoatomic, but not all elements are, so B is wrong
3. Different types of atoms make a compound, not an element (A is wrong)

Yaad rakhein: Atomicity: Ar = 1, O₂ = 2, O₃ = 3, P₄ = 4, S₈ = 8.

Answer: (C) A molecule of an element consists of only one type of atom bonded together.

Concept: 3R = Reduce, Reuse, Recycle.

1. Steel plates and bottles are used again and again, which is reuse
2. Fewer disposable cups thrown away means waste is reduced
3. Nothing is reprocessed, so this is not recycling

Yaad rakhein: Reuse is better than recycle because it needs no energy for reprocessing.

Answer: (D) Reduction and reuse of waste

Concept: Osmosis is the movement of solvent (water) through a semipermeable membrane from a dilute to a concentrated solution.

1. The membrane lets water pass but not sugar
2. Water moves into the concentrated side, so its level rises and the other side falls
3. Diffusion is wrong because sugar cannot cross a semipermeable membrane

Trick: Semipermeable membrane + change in levels = osmosis. Water moves toward more solute.

Answer: (D) Osmosis – because solvent molecules move from the dilute solution to the concentrated solution through the semipermeable membrane

Concept: In a hypotonic medium water enters the cell by osmosis and the cell swells.

1. The rigid cellulose cell wall resists the swelling, so the cell becomes turgid but does not burst
2. Animal cells have no cell wall, so they can burst (e.g. RBCs in water)

Yaad rakhein: The pressure of the cell contents against the wall is called turgor pressure; it keeps soft plant parts firm.

Answer: (C) Cell wall made of cellulose

Concept: At maximum height all the initial KE has changed into PE.

1. $KE=\frac{1}{2}m{v}^2=\frac{1}{2}\times 10\times {10}^2=500$ J
2. PE at the top = 500 J (check: $h=\frac{v^2}{2g}=5$ m, so $mgh=10\times 10\times 5=500$ J)

Trick: PE at the top = KE at launch, so you do not need to find the height.

Answer: (A) 500 J

Concept: To convert km/h to m/s, multiply by $\frac{5}{18}$.

1. $40\times \frac{5}{18}=\frac{200}{18}$
2. $=11.11$ m/s (the 10 minutes is extra data)

Trick: Divide by 3.6: $40÷3.6\approx 11.1$.

Answer: (D) 11.11 m/s

Concept: The atomic number (number of protons) decides which element an atom is.

1. Same atomic number but different mass numbers means isotopes of the same element (e.g. ${}^{12}\text{C}$, ${}^{14}\text{C}$)
2. Different atomic numbers always mean different elements (isobars, isotones)

Yaad rakhein: Isotopes: same Z. Isobars: same A (e.g. ${}^{40}\text{Ca}$, ${}^{40}\text{Ar}$). Isotones: same number of neutrons.

Answer: (D) Same atomic number, different mass numbers

Concept: Ions are held by strong electrostatic forces in a rigid crystal lattice.

1. Strong attraction gives a high melting point (A ✓); the rigid lattice makes them brittle (D ✓)
2. They dissolve in water but not in non-polar solvents like kerosene (B ✗)
3. They conduct only when molten or in solution, not as solids, since the ions cannot move (C ✗)

Trick: Rule out B and C (both are false), which leaves only A and D.

Answer: (D) Only (A) and (D)

Concept: Dendrite → cell body → axon → nerve ending.

1. Dendrites receive the signal; the axon carries the electrical impulse away from the cell body
2. At the axon ending, chemicals are released across the synapse to the next neuron or to a muscle/gland

Yaad rakhein: The longest cell in the human body is the nerve cell (neuron).

Answer: (C) Transmit electrical impulses to other neurons or target organs.

Concept: Acid + carbonate/hydrogen carbonate → salt + CO₂ + water.

1. ${\text{NaHCO}}_3+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$
2. CO₂ turns lime water milky

Trick: 'Carbonate' means CO₂ will be formed. Acid + metal gives H₂; acid + carbonate gives CO₂.

Answer: (B) Salt, carbon dioxide and water

Concept: Smooth muscle is involuntary and found in the walls of internal organs.

1. Peristalsis (movement of food in the alimentary canal) is done by smooth muscle
2. Walking and waving use striated (voluntary) muscle; the heart has cardiac muscle (striated + involuntary)

Trick: Heart = involuntary but striated, so it is the trap option. Only gut movement is both involuntary and unstriated.

Answer: (C) Movement of food in the alimentary canal

Concept: Mature plant cells have one large central vacuole filled with cell sap.

1. It stores water, sugars, amino acids, salts and wastes
2. It keeps the cell turgid (turgor pressure), giving rigidity to the plant
3. Heredity = nucleus, respiration = mitochondria, protein synthesis = ribosomes

Yaad rakhein: A vacuole can take up 50–90% of a mature plant cell's volume; animal cells have small vacuoles.

Answer: (D) Store cell sap and help maintain turgor pressure.