Q&A: practice questions with answers

Concept: $\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$; real image of concave mirror is in front (v negative).

1. $u=-120$ cm, $v=-40$ cm
2. $\frac{1}{f}=-\frac{1}{40}-\frac{1}{120}=-\frac{3+1}{120}=-\frac{4}{120}$
3. $f=-30$ cm

Trick: Concave mirror f is negative → only −60 or −30; $f=\frac{uv}{u+v}=\frac{4800}{-160}=-30$.

Answer: (C) - 30 cm

Concept: Kharif crops grow in the rainy (monsoon) season; rabi crops in winter.

1. Kharif: sown June–July, e.g. paddy, maize, cotton
2. Rabi: sown Oct–Nov, harvested March–April, e.g. wheat, gram, mustard

Yaad rakhein: Rabi = winter: wheat, gram, peas, mustard.

Answer: (B) Winter season

Concept: Position is always stated relative to a fixed reference point called the origin.

1. Example: 'school is 2 km north of the station' — station is the origin
2. Displacement and final position are measured from this origin

Yaad rakhein: Motion is relative — the same object can be at rest for one observer and moving for another.

Answer: (A) Origin

Concept: Rough ER has ribosomes on its surface, which make proteins.

1. Ribosomes give RER its rough look → site of protein synthesis
2. Smooth ER (no ribosomes) makes lipids and detoxifies
3. DNA replication occurs in the nucleus

Trick: Rough = Ribosomes = pRotein; Smooth = Lipids.

Answer: (C) Protein synthesis

Concept: A food web is a network of interconnected food chains.

1. Each organism has several food options and predators
2. If one species declines, others can substitute → ecosystem stays balanced
3. Energy flows (not recycled); nutrients are recycled

Yaad rakhein: Energy flow in an ecosystem is unidirectional; only ~10% passes to the next trophic level.

Answer: (D) They allow multiple feeding relationships, ensuring balance.

Concept: Structural isomers have the same molecular formula but different structures.

1. Pentane and 2-methylbutane: both C₅H₁₂ → isomers
2. Butane and 2-methylpropane: both C₄H₁₀ → isomers
3. Benzene C₆H₆ vs cyclohexene C₆H₁₀ → different formulas, not isomers

Trick: First count C and H — different formula means not isomers.

Answer: (A) A and B only

Concept: Slope of a v–t graph = acceleration.

1. Straight line → constant slope → uniform acceleration
2. Curved line → slope keeps changing → non-uniform acceleration
3. Horizontal line → constant velocity

Trick: Straight = uniform, curved = non-uniform.

Answer: (D) Non-uniform acceleration

Concept: Areolar tissue fills spaces, supports organs and helps repair; fat storage is done by adipose tissue.

1. Areolar: between skin and muscle, around blood vessels, nerves, bone marrow
2. Functions: fills space inside organs, supports internal organs, helps tissue repair
3. Storing fat globules for insulation → adipose tissue

Yaad rakhein: Adipose tissue is found below the skin and between organs; acts as an insulator.

Answer: (B) Storing fat globules for insulation

Concept: Isobars are atoms of different elements with the same mass number.

1. Same mass number, different atomic numbers
2. Example: ${}^{40}\text{Ca}$ ($Z=20$) and ${}^{40}\text{Ar}$ ($Z=18$)
3. Same atomic number/protons = isotopes, not isobars

Trick: iso-BAR: Greek 'barys' = heavy → same mass number; iso-TOPE: 'topos' = place → same atomic number.

Answer: (B) Same mass number

Concept: Work done = change in kinetic energy = $\frac{1}{2}m(v^2-u^2)$.

1. 72 km/h = 20 m/s; 108 km/h = 30 m/s
2. $W=\frac{1}{2}\times 120\times ({30}^2-{20}^2)=60\times (900-400)$
3. $W=60\times 500=30000\text{J}=3.0\times {10}^4\text{J}$

Trick: km/h → m/s: multiply by $\frac{5}{18}$ (72 → 20, 108 → 30).

Answer: (A) 3.0 × 10⁴ J

Concept: Average atomic mass = Σ (isotope mass × fractional abundance).

1. $35\times \frac{25}{100}=8.75$
2. $37\times \frac{75}{100}=27.75$
3. Average = $8.75+27.75=36.5$ u

Trick: Answer lies closer to the more abundant isotope (37): $35+\frac{3}{4}\times 2=36.5$.

Answer: (D) 36.5 u

Concept: Impure metal is the anode, pure metal strip is the cathode, salt solution of the metal is the electrolyte.

1. Anode: impure copper — dissolves into solution
2. Cathode: thin pure copper strip — pure Cu deposits
3. Electrolyte: acidified copper sulphate; impurities settle as anode mud

Trick: Mnemonic 'AI–CP': Anode = Impure, Cathode = Pure.

Answer: (D) Impure copper block

Concept: Mass can neither be created nor destroyed in a chemical reaction: mass of reactants = mass of products.

1. Reactants: 5.6 g Na + 8.0 g Cl₂
2. Product NaCl = 5.6 + 8.0 = 13.6 g

Yaad rakhein: Law given by Lavoisier; just add masses of reactants.

Answer: (C) 13.6 g

Concept: A negative ion (anion) forms by gaining electrons; charge −2 means 2 electrons gained, valency 2.

1. Electrons are negative, so gaining 2 e⁻ gives charge −2
2. Example: $\text{O}+2e^{-}\to {\text{O}}^{2-}$ (valency 2)
3. Protons never change in ions; electrons are not in the nucleus

Yaad rakhein: Gain of e⁻ → negative ion (non-metals); loss of e⁻ → positive ion (metals).

Answer: (B) The element has gained 2 electrons.

Concept: A ray through (or directed towards) the principal focus emerges parallel to the principal axis after refraction.

1. Convex lens: ray through F → parallel to axis
2. Concave lens: ray heading towards F on the other side → parallel to axis
3. Reverse of: parallel ray → passes through / appears from F

Yaad rakhein: Ray through optical centre goes undeviated; parallel ray ↔ focus ray are reverses of each other.

Answer: (C) It emerges parallel to the principal axis after refraction.

Concept: Non-biodegradable chemicals (e.g. DDT) accumulate more at each higher trophic level.

1. They cannot be broken down or excreted, so they pass up the food chain
2. Top consumers (e.g. humans, eagles) get the highest concentration
3. Biodegradation = breakdown by microbes (opposite idea)

Yaad rakhein: Maximum concentration of such chemicals is found in top carnivores.

Answer: (A) Biological magnification

Concept: Prokaryotic cells are small and simple; eukaryotic cells are larger and complex.

1. Prokaryotes: no nuclear membrane, no membrane-bound organelles (e.g. bacteria)
2. Eukaryotes: true nucleus and membrane-bound organelles (e.g. plant, animal cells), generally larger

Yaad rakhein: Pro = 'before' nucleus (no true nucleus); Eu = 'true' nucleus.

Answer: (D) Larger and complex

Concept: Male gamete is small and motile; female gamete is large, non-motile and stores food.

1. Sperm: tiny, has a tail → moves to reach the egg
2. Egg (ovum): larger, stores food for the zygote, does not move

Yaad rakhein: In humans the ovum is the largest cell and the sperm one of the smallest.

Answer: (A) Male gamete is small and motile; and female gamete is larger and non-motile.

Concept: $P=VI$; with $V=IR$, $P=I^{2}R=\frac{V^2}{R}$.

1. $P=VI$ ✓
2. $P=I^{2}R$ ✓ (put $V=IR$)
3. $P=\frac{V^2}{R}$ ✓ (put $I=\frac{V}{R}$)
4. $P=\frac{R}{V^2}$ is the reciprocal of $\frac{V^2}{R}$ → incorrect

Trick: Check units: $\frac{R}{V^2}$ is not watt — it is 1/W.

Answer: (B) P=R/V2

Concept: For mirrors, $m=-\frac{v}{u}$.

1. $m=+\frac{1}{3}$, $u=-30$ cm
2. $v=-m\times u=-\frac{1}{3}\times (-30)=+10$ cm

Trick: Convex mirror image is always behind the mirror → v positive; only +10 or +90, and $v=\frac{u}{3}$ in size → 10.

Answer: (B) + 10 cm