Q&A: practice questions with answers

Concept: Non-degradable chemicals accumulate and increase in concentration at each trophic level.

Solution:

1. Pesticides like DDT are not broken down and get stored in fat
2. Their concentration rises step by step up the food chain
3. Humans are at the top trophic level → highest concentration

Yaad rakhein: This phenomenon is called biomagnification; it is why DDT levels are highest in top carnivores.

Answer: (D) They occupy the top trophic level in food chains

Concept: Cytoplasm is the fluid between nucleus and membrane that contains organelles.

Solution:

1. Stains like safranin/methylene blue colour the nucleus darkly
2. Cytoplasm takes a light stain
3. Cytoplasm holds organelles like mitochondria, ER, Golgi

Yaad rakhein: Onion peel has a cell wall; cheek cells do not — both have nucleus and cytoplasm.

Answer: (B) Cytoplasm

Concept: Pair each muscle type with its control: voluntary or involuntary.

Solution:

1. Skeletal muscle → voluntary (under our will)
2. Cardiac muscle → involuntary (beats on its own)
3. 'Striated' is a structure feature, not a control type

Yaad rakhein: Cardiac muscle is striated, branched and involuntary — it never tires.

Answer: (B) Involuntary

Concept: Chemical fertilisers mainly supply N, P and K.

Solution:

1. NPK = Nitrogen, Phosphorus, Potassium
2. These are major macronutrients needed in large amounts
3. Zn, Ca, Mg are needed but are not the main supply of fertilisers

Yaad rakhein: Urea is a common nitrogen fertiliser (about 46% N).

Answer: (C) Nitrogen

Concept: Combination reaction: two or more reactants → a single product.

Solution:

1. 2Cu + O₂ → 2CuO, NH₃ + HCl → NH₄Cl, CaO + H₂O → Ca(OH)₂ — one product each
2. CuO + H₂ → Cu + H₂O gives two products
3. It is a redox (displacement) reaction: CuO reduced, H₂ oxidised

Trick: Count products: more than one product → not combination.

Answer: (B) $\mathrm{C}\mathrm{u}\mathrm{O}+{\mathrm{H}}_2\stackrel{\text{Heat}}{\to }\mathrm{C}\mathrm{u}+{\mathrm{H}}_2\mathrm{O}$

Concept: $P=P_1+P_2$ and $f=\frac{1}{P}$ (in metres).

Solution:

1. $P=+4+(-6)=-2$ D
2. $f=\frac{1}{-2}=-0.5$ m
3. = −50 cm (diverging)

Trick: $f(\text{cm})=\frac{100}{P}$: $\frac{100}{-2}=-50$ cm.

Answer: (B) - 50 cm

Concept: Weeds are unwanted plants that compete with crops.

Solution:

1. They take water, nutrients, light and space meant for crops
2. This lowers crop growth and yield
3. So weeding (manual, weedicides like 2,4-D) is needed

Yaad rakhein: Weedicides are sprayed before weeds flower and set seeds.

Answer: (A) Weeds compete with crops for resources.

Concept: $\text{KE}=\frac{1}{2}m{v}^2$, so $\text{KE}\propto v^2$.

Solution:

1. Velocity doubles (5 → 10 m/s)
2. KE becomes $2^2=4$ times
3. $4\times 25=100$ J

Trick: Ratio method: ${\left(\frac{10}{5}\right)}^2\times 25=100$ J — no need to find m (which is 2 kg).

Answer: (B) 100 J

Concept: Boiling point rises with external pressure and falls with lower pressure.

Solution:

1. Hill stations: lower air pressure → water boils below 100°C
2. Pressure cooker: higher pressure → boils above 100°C, cooking faster
3. Boiling point clearly depends on external pressure

Yaad rakhein: A liquid boils when its vapour pressure equals the external pressure.

Answer: (A) Boiling point of a liquid increases with an increase in atmospheric pressure and decreases with a decrease in atmospheric pressure.

Concept: Mass number A = protons + neutrons (nucleons).

Solution:

1. A is always a whole number and differs for isotopes
2. A = 2Z only for some light elements (e.g. C-12, O-16)
3. Not for all: e.g. Na has Z = 11, A = 23 → statement C is false

Yaad rakhein: Isotopes: same Z, different A; isobars: same A, different Z.

Answer: (C) It is equal to twice the atomic number for all elements.

Concept: Non-uniform motion: unequal distances in equal time intervals.

Solution:

1. Speed keeps changing
2. So distance covered per second is not the same
3. Constant speed / zero acceleration describe uniform motion

Yaad rakhein: Distance–time graph of non-uniform motion is a curve; of uniform motion a straight line.

Answer: (A) Covering unequal distances in equal intervals of time

Concept: Isobars: same mass number, different atomic numbers.

Solution:

1. Same A but different Z → different number of protons
2. So neutrons differ too, and chemical properties differ
3. Example: ⁴⁰Ca (Z = 20) and ⁴⁰Ar (Z = 18)

Trick: isoBARS = same 'Bhaar' (mass); isoTOPES = same 'Place' (Z) in periodic table.

Answer: (C) They have the same number of protons.

Concept: Repeated reflection of sound can guide it to listeners far away.

Solution:

1. Megaphones, shehnai, trumpets send sound in one direction by multiple reflection
2. Curved ceilings and sound boards in halls spread sound evenly
3. Reflection does not change frequency or pitch

Yaad rakhein: Stethoscope also works on multiple reflection of sound.

Answer: (A) Hear sounds clearly over long distances

Concept: Evaporation is a surface phenomenon occurring below boiling point.

Solution:

1. Statement I is the correct definition of evaporation
2. More surface area → more molecules escape → faster evaporation
3. Both statements are true

Yaad rakhein: Evaporation rate rises with surface area, temperature and wind speed; falls with humidity.

Answer: (C) Both statements I and II are true.

Concept: A filament must glow white-hot without melting.

Solution:

1. Tungsten melting point ≈ 3380°C (very high)
2. It also has high resistivity, so it heats up well
3. Bulbs are filled with inert gas (N₂/Ar) to stop its oxidation

Yaad rakhein: Heating elements of heaters use nichrome (alloy) instead.

Answer: (D) It has a very high melting point.

Concept: Add atomic masses of all atoms in the formula.

Solution:

1. Na₂ = $2\times 23=46$
2. C = 12; O₃ = $3\times 16=48$
3. Total = $46+12+48=106$ u

Yaad rakhein: Na₂CO₃·10H₂O is washing soda; anhydrous Na₂CO₃ is soda ash.

Answer: (D) 106 u

Concept: PE at height h = work done against gravity = mgh.

Solution:

1. Lifting needs force equal to weight mg, upward
2. Work = mg × h, done against gravity
3. This work is stored as potential energy

Yaad rakhein: PE depends only on height, not on the path taken.

Answer: (C) Against gravity

Concept: NaHCO₃ is a mild, non-corrosive basic salt.

Solution:

1. Used in baking powder (releases CO₂)
2. Used in antacids (neutralises acid)
3. Used in soda-acid fire extinguishers; it is NOT an acid at all

Yaad rakhein: On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.

Answer: (C) It is used as a strong acid.

Concept: Mitochondria carry out aerobic respiration and make ATP.

Solution:

1. ATP is the energy currency of the cell
2. Hence mitochondria = powerhouse of the cell
3. Lysosome = suicide bag; ribosome = protein synthesis

Yaad rakhein: Mitochondria have their own DNA and ribosomes.

Answer: (C) Mitochondria

Concept: Xylem and phloem are complex permanent tissues (made of more than one cell type).

Solution:

1. Xylem carries water and minerals
2. Phloem carries food
3. Storage is by parenchyma; protection by epidermis/cork (simple tissues)

Yaad rakhein: Phloem's only dead component is phloem fibres; xylem's only living one is xylem parenchyma.

Answer: (B) Transport of water and food