Q&A: practice questions with answers

Concept: For a concave mirror, the image position depends on where the object is placed relative to F and C (radius $R=2f$).

1. Object beyond C → image between F and C, diminished.
2. Object at C → image also at C, real, inverted and of the same size.
3. Object between F and C → image beyond C, enlarged; object at F → image at infinity.

Yaad rakhein: Same size real image only when object is at C ($u=-2f\Rightarrow v=-2f$, $m=-1$).

Answer: (B) at the centre of curvature

Concept: Inertia is the tendency of a body to resist a change in its state of rest or uniform motion. Mass is the measure of inertia.

1. A heavier body (more mass) is harder to start, stop or turn.
2. Pushing a loaded truck vs. an empty cart shows greater mass → greater inertia.
3. So inertia increases directly with mass: inertia $\propto$ mass.

Yaad rakhein: Mass = measure of inertia; more mass, more inertia.

Answer: (C) Inertia is proportional to mass.

Concept: Evaporation takes latent heat from the surface, producing cooling.

1. In summer we sweat more.
2. Cotton absorbs sweat and exposes it to the air, so it evaporates quickly.
3. The evaporating sweat takes heat from the body → body feels cool.

Yaad rakhein: Sweat + cotton + evaporation = cooling (same reason earthen pots keep water cool).

Answer: (C) Evaporation

Concept: Ionic compounds are formed by transfer of electrons between a metal and a non-metal.

1. Table salt $\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}$ = $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{C}{\mathrm{l}}^{-}$ ions → ionic.
2. Glucose (${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$) and sugar (sucrose) are covalent compounds of C, H, O.
3. Turmeric is a natural mixture (mainly the organic compound curcumin), not ionic.

Trick: Metal + non-metal → usually ionic; only non-metals (C, H, O) → covalent.

Answer: (C) Table salt

Concept: From $F=\frac{G{m}_1{m}_2}{r^2}$, $G=\frac{F{r}^2}{m_1{m}_2}$, so its unit is ${\text{N m}}^2{\text{kg}}^{-2}$.

1. $G=6.7\times {10}^{-11}{\text{N m}}^2{\text{kg}}^{-2}$ – option (C).
2. $9.1\times {10}^{-31}\text{kg}$ = mass of an electron; $1.6\times {10}^{-19}\text{C}$ = charge of an electron.
3. $9.8{\text{m s}}^{-2}$ = $g$, acceleration due to gravity (not $G$).

Trick: Match by unit: only $G$ has ${\text{N m}}^2{\text{kg}}^{-2}$.

Answer: (C) $6.7\times {10}^{-11}N{m}^{2}k{g}^{-2}$

Concept: pH scale (0 to 14) measures the hydrogen-ion concentration, i.e. how acidic or basic a solution is.

1. pH $<7$ → acidic; pH $=7$ → neutral; pH $>7$ → basic.
2. Lower pH = stronger acid; higher pH = stronger base.
3. It does not measure pollution, density or volume.

Yaad rakhein: 'p' = potenz (power) of ${\mathrm{H}}^{+}$; 7 is neutral.

Answer: (B) acidity or basicity

Concept: In a mixture components keep their own properties; in a compound elements combine chemically in a fixed ratio to form a new substance.

1. Mixture (e.g. iron filings + sulphur powder): no new substance → physical change.
2. Compound (heating Fe + S → $\mathrm{F}\mathrm{e}\mathrm{S}$): new substance with new properties → chemical change.
3. So the answer is physical, chemical.

Yaad rakhein: Mixture = separable by physical methods; compound = new substance, needs chemical methods.

Answer: (B) physical, chemical

Concept: The pituitary gland (at the base of the brain) secretes growth hormone, which regulates growth and development of the body.

1. Deficiency in childhood → dwarfism; excess → gigantism.
2. Adrenal → adrenaline; pancreas → insulin; thyroid → thyroxin.
3. So growth hormone comes from the pituitary.

Yaad rakhein: Pituitary = 'master gland' and source of growth hormone.

Answer: (B) Pituitary

Concept: For a freely falling body, total mechanical energy (PE + KE) remains constant; PE changes into KE as it falls.

1. At the top: all potential energy $mgh$, KE $=0$.
2. At the midpoint: height $\frac{h}{2}$, so PE $=\frac{mgh}{2}$ and KE $=\frac{mgh}{2}$.
3. Just before the ground: almost all KE. So at the midpoint it has both PE and KE.

Yaad rakhein: Midway in free fall: PE = KE = half of total energy.

Answer: (D) Both Potential and Kinetic

Concept: Colloidal particles are large enough to scatter a beam of light, making its path visible. This is the Tyndall effect.

1. Examples: sunlight beam in a dusty room or through a forest canopy; headlight beam in fog.
2. Rainbow – dispersion + refraction + total internal reflection in droplets.
3. Twinkling of stars and delayed sunset – atmospheric refraction.

Yaad rakhein: Colloid + visible light path = Tyndall effect (true solutions show no Tyndall effect).

Answer: (D) Tyndall effect

Concept: Average velocity $=\frac{\text{total displacement}}{\text{total time}}$; average speed uses total distance.

1. Uma goes A → B (60 m) and returns B → A (60 m): distance $=120\text{m}$.
2. Start and end points are the same, so displacement $=0$.
3. Average velocity $=\frac{0}{60\text{s}}=0{\text{m s}}^{-1}$ (average speed would be $2{\text{m s}}^{-1}$).

Trick: Back to the starting point → displacement 0 → average velocity 0.

Answer: (D) 0 m/s

Concept: Biodegradable substances are broken down by microorganisms (bacteria, fungi); non-biodegradable ones are not.

1. Plastic glass – not decomposed by microbes → non-biodegradable (correct pair).
2. Kulhad is made of baked clay and returns to soil → biodegradable, so 'Kulhad – non-biodegradable' is wrong.
3. Paper cup and vegetables – decomposed by microbes → biodegradable (correct).

Yaad rakhein: Kulhad, paper, food waste → biodegradable; plastic, polythene → non-biodegradable.

Answer: (B) Kulhad – non-biodegradable

Concept: Splitting of white light into its seven component colours (VIBGYOR) by a prism is called dispersion.

1. Different colours have different speeds in glass, so they bend through different angles.
2. Red bends the least and violet bends the most.
3. Refraction is the bending itself; dispersion is the splitting into colours due to it.

Yaad rakhein: Prism + white light → spectrum VIBGYOR = dispersion.

Answer: (D) Dispersion

Concept: Reproduction produces new individuals similar to the parent; the basic event is making a copy of DNA.

1. DNA in the nucleus carries the information for making proteins and body design.
2. During reproduction the cell copies its DNA (DNA replication) and the two copies separate into two cells.
3. Slight copying variations lead to variation – the basis of evolution.

Yaad rakhein: Reproduction = DNA copying + cell division.

Answer: (B) Reproduction

Concept: A colloid has a dispersed phase (scattered particles) and a dispersion medium. Liquid dispersed in a solid is called a gel.

1. Butter: tiny droplets of water (liquid) are dispersed in solid fat.
2. Dispersed phase = liquid; dispersion medium = solid → gel.
3. Other gels: cheese, jelly.

Yaad rakhein: Gel = liquid in solid; emulsion = liquid in liquid (milk); foam = gas in liquid.

Answer: (A) Only liquid

Concept: Plant cells have a rigid cell wall (mainly cellulose) outside the plasma membrane.

1. It gives structural strength and shape to the cell.
2. It protects the cell against external effects, e.g. bursting in dilute (hypotonic) solutions and mechanical injury.
3. Cell walls are not needed for photosynthesis (chloroplasts do that); plant cells do have genetic material and cytoplasm.

Yaad rakhein: Cell wall = cellulose 'armour' for protection and rigidity.

Answer: (A) they defend plant cells from a variety of external effects

Concept: Air passes from the nostrils through the throat into the trachea (windpipe) and then to the lungs.

1. The trachea is supported by C-shaped rings of cartilage.
2. These rings keep the air passage open and stop it from collapsing when air pressure inside drops.
3. Hairs and mucus in the nostrils only filter the air; they do not support the passage.

Yaad rakhein: Cartilage rings = 'support hoops' of the windpipe.

Answer: (A) Rings of cartilage

Concept: In some animals sex is not decided by genes but by other factors, and individuals can even change sex.

1. In snails, individuals can change sex – sex is not genetically determined.
2. In some reptiles (e.g. many turtles, crocodiles) the incubation temperature of eggs decides the sex.
3. In humans, sex is genetically determined by sex chromosomes (XX female, XY male).

Yaad rakhein: Snail = sex change; reptile eggs = temperature; humans = X/Y chromosomes.

Answer: (B) Snails

Concept: After pollination, the pollen grain on the stigma grows a pollen tube that carries the male germ-cell to the ovule.

1. Pollen grain → produces the male germ-cell.
2. Pollen tube grows through the style and reaches the ovule in the ovary.
3. The male germ-cell fuses with the female gamete (egg) present in the ovule → zygote.

Yaad rakhein: Male gamete from pollen, female gamete in ovule; the ovule later becomes the seed.

Answer: (A) The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule.

Concept: In parallel, $\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}+\dots$

1. Each extra path lets more current flow, so total resistance decreases.
2. Example: $2\Omega$ and $3\Omega$ → $R_p=\frac{2\times 3}{2+3}=1.2\Omega$, less than $2\Omega$.
3. Hence $R_p$ is always less than the smallest resistance.

Trick: Series → more than the largest; parallel → less than the smallest.

Answer: (A) The total resistance is always less than the smallest resistance.