Q&A: practice questions with answers

Concept: Ozone (${\mathrm{O}}_3$) has three oxygen atoms. It is poisonous at ground level, but in the stratosphere it shields us from UV rays.

1. (A) wrong – ozone is ${\mathrm{O}}_3$, three atoms, not two.
2. (B) wrong – UV radiation reaching Earth due to ozone loss causes skin cancer, not blood cancer from ozone.
3. (D) wrong – ozone is a deadly poison.
4. (C) correct – chlorofluorocarbons (CFCs) used in refrigerants and aerosols release chlorine that destroys ozone.

Yaad rakhein: CFC → ozone depletion (Montreal Protocol, 1987).

Answer: (C) Decrease in the amount of ozone in the atmosphere is due to chlorofluorocarbons (CFC).

Concept: Nitric acid is a strong oxidising agent: it oxidises the ${\mathrm{H}}_2$ formed to water and itself is reduced to oxides of nitrogen, so usually no ${\mathrm{H}}_2$ is released.

1. Exception: magnesium (Mg) and manganese (Mn) react with very dilute $\mathrm{H}\mathrm{N}{\mathrm{O}}_3$ to give ${\mathrm{H}}_2$ gas.
2. $\mathrm{M}\mathrm{n}+2\mathrm{H}\mathrm{N}{\mathrm{O}}_3\to \mathrm{M}\mathrm{n}(\mathrm{N}{\mathrm{O}}_3{)}_2+{\mathrm{H}}_2$.
3. Zn, Fe and Al give nitrogen oxides instead.

Yaad rakhein: Only "M-M" – Mg and Mn – give ${\mathrm{H}}_2$ with very dilute nitric acid.

Answer: (B) Mn

Concept: In Hydra, regenerative cells divide repeatedly at one site to form a bud, which grows into a new individual and detaches.

1. Spirogyra – fragmentation.
2. Amoeba – binary fission.
3. Planaria – regeneration from body pieces.
4. Hydra – budding using regenerative cells.

Yaad rakhein: Hydra and yeast → budding.

Answer: (C) Hydra

Concept: Work done by a constant force along the direction of displacement = force × displacement.

1. Force = F, displacement in the direction of force = s.
2. $W=F\times s$ (SI unit joule, $1J=1\mathrm{N}\mathrm{m}$).

Yaad rakhein: Work needs both force and displacement – multiply them.

Answer: (D) $F\times s$

Concept: Nutrition is the process of taking in food (a source of energy and raw materials) from outside into the body of an organism.

1. Metabolism = all chemical reactions inside the body.
2. Transportation = moving substances within the body.
3. Translocation = transport of food in plants through phloem.
4. Taking food from outside to inside = nutrition.

Yaad rakhein: Outside → inside (food) = nutrition; inside → outside (waste) = excretion.

Answer: (A) Nutrition

Concept: Carbon is a reducing agent: it removes oxygen from zinc oxide (smelting).

1. $\mathrm{Z}\mathrm{n}\mathrm{O}+\mathrm{C}\to \mathrm{Z}\mathrm{n}+\mathrm{C}\mathrm{O}$.
2. ZnO is reduced to zinc metal; carbon is oxidised to carbon monoxide.

Yaad rakhein: Metal oxide + C → metal + CO (standard NCERT reaction for zinc).

Answer: (A) Both Zn and CO

Concept: Energy possessed by a body due to its motion is kinetic energy: $KE=\frac{1}{2}m{v}^2$.

1. Potential energy is due to position or configuration.
2. Nuclear energy comes from the nucleus; electromagnetic energy is carried by radiation.
3. Energy due to motion = kinetic energy.

Yaad rakhein: "Kinetic" comes from the Greek word for motion.

Answer: (C) kinetic energy

Concept: A convex mirror always forms a virtual, erect and diminished image, and has a wide field of view.

1. Because the image is erect and small, a large area of traffic behind fits in the mirror.
2. Concave mirrors are used in shaving mirrors, torches and headlights.
3. Lenses are not used as rear-view devices.

Yaad rakhein: Rear-view / street-corner mirrors → convex; headlights/dentist → concave.

Answer: (D) Convex mirror

Concept: Weight $W=mg$. Mass stays the same, but g on the Moon is about one-sixth of g on Earth.

1. $g_{\text{moon}}\approx \frac{1}{6}{g}_{\text{earth}}$.
2. So $W_{\text{moon}}\approx \frac{1}{6}{W}_{\text{earth}}$ – less than on Earth.
3. This does not depend on the object's density.

Yaad rakhein: Mass is constant everywhere; weight changes with g.

Answer: (B) less than the weight on the earth

Concept: A soda-acid extinguisher contains sodium hydrogencarbonate (baking soda) solution and sulphuric acid, which react to give $\mathrm{C}{\mathrm{O}}_2$.

1. $2\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4+2{\mathrm{H}}_2\mathrm{O}+2\mathrm{C}{\mathrm{O}}_2$.
2. $\mathrm{C}{\mathrm{O}}_2$ is heavier than air and cuts off oxygen supply to the fire.

Yaad rakhein: Soda-acid = $\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}{\mathrm{O}}_3$ (soda) + ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ (acid).

Answer: (D) $\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}{\mathrm{O}}_3$

Concept: During sudden heavy exercise, oxygen falls short in muscle cells, so pyruvate is converted into lactic acid (anaerobic respiration).

1. Glucose → pyruvate (in cytoplasm).
2. Lack of oxygen: pyruvate → lactic acid + energy.
3. Build-up of lactic acid in muscles causes cramps.

Yaad rakhein: Muscles (no ${\mathrm{O}}_2$) → lactic acid; yeast (no ${\mathrm{O}}_2$) → ethanol + $\mathrm{C}{\mathrm{O}}_2$.

Answer: (D) Lactic acid

Concept: Parasites obtain food from a living host and harm it.

1. Ticks – blood-sucking parasites on animals.
2. Lice – parasites living on human hair/skin.
3. Cuscuta (amarbel) – parasitic plant that takes food from the host plant.
4. Amoeba – free-living; it engulfs food particles (holozoic nutrition), so it is not a parasite.

Yaad rakhein: Amoeba = holozoic; Plasmodium/Entamoeba = parasites – do not confuse them.

Answer: (C) Amoeba

Concept: Magnesium ribbon burns in air with a dazzling white flame, forming white powder of magnesium oxide.

1. $2\mathrm{M}\mathrm{g}+{\mathrm{O}}_2\to 2\mathrm{M}\mathrm{g}\mathrm{O}$.
2. The light is so bright that it should not be looked at directly.
3. MgO dissolves in water to give the base $\mathrm{M}\mathrm{g}(\mathrm{O}\mathrm{H}{)}_2$.

Yaad rakhein: Mg burns with a dazzling white light – used in fireworks and flares.

Answer: (A) White

Concept: Vegetative propagation produces new plants from vegetative parts – root, stem or leaf – without seeds.

1. Leaves – Bryophyllum (buds on leaf margins).
2. Stem – potato, sugarcane, rose cuttings.
3. Root – sweet potato, dahlia.
4. Seed is formed by sexual reproduction, so it is not a vegetative part.

Yaad rakhein: Vegetative = root, stem, leaf; seed/flower = sexual.

Answer: (C) Seed

Concept: Force between two masses is directly proportional to the product of masses and inversely proportional to the square of the distance between them.

1. $F\propto M\times m$ and $F\propto \frac{1}{d^2}$.
2. So $F=G\frac{M\times m}{d^2}$, where $G=6.67\times {10}^{-11}\mathrm{N}{\mathrm{m}}^2\mathrm{k}{\mathrm{g}}^{-2}$.

Yaad rakhein: Inverse-square law – distance is always squared, and both masses appear.

Answer: (A) $F=G\frac{M\times m}{d^2}$

Concept: $KE=\frac{1}{2}m{v}^2$.

1. $m=0.5\mathrm{k}\mathrm{g}$, $v=6\mathrm{m}/\mathrm{s}$.
2. $KE=\frac{1}{2}\times 0.5\times 6^2=0.25\times 36=9J$.

Trick: Half of 0.5 is 0.25 = $\frac{1}{4}$; so KE = $\frac{36}{4}=9$.

Answer: (D) 9 J

Concept: For a concave mirror, the image position depends on where the object is placed with respect to C and F.

1. A ray parallel to the axis reflects through F.
2. A ray through F reflects parallel to the axis.
3. For an object beyond C, these rays meet between C and F – the image is real, inverted and diminished.

Yaad rakhein: Object and image swap: beyond C ↔ between C and F; at C ↔ at C; at F ↔ at infinity.

Answer: (C) Between the centre of curvature and the focus point

Concept: Momentum $p=m\times v$.

1. Unit of mass = kg, unit of velocity = $\mathrm{m}{\mathrm{s}}^{-1}$.
2. Unit of momentum = $\mathrm{k}\mathrm{g}\mathrm{m}{\mathrm{s}}^{-1}$ (same as $\mathrm{N}\mathrm{s}$).

Yaad rakhein: Momentum unit = force unit × time: $\mathrm{N}\mathrm{s}=\mathrm{k}\mathrm{g}\mathrm{m}{\mathrm{s}}^{-1}$.

Answer: (C) kg m $s^{-1}$

Concept: One atomic mass unit (u) is defined as exactly $\frac{1}{12}$ of the mass of one carbon-12 atom.

1. So the mass of a C-12 atom is exactly 12 u by definition.
2. C-12 has 6 protons + 6 neutrons; mass number = 12.

Yaad rakhein: C-12 is the reference standard: 12 u exactly.

Answer: (A) 12 u

Concept: The heart is a muscular organ that pumps blood to all parts of the body through arteries.

1. Human heart has four chambers – two atria and two ventricles.
2. Pancreas secretes enzymes and insulin; liver makes bile; stomach digests food.

Yaad rakhein: Heart – pump; blood vessels – pipes; blood – fluid carrier.

Answer: (B) Heart