Q&A: practice questions with answers

Concept: $G$ in $F=\frac{GMm}{r^2}$ is a universal constant: its value is the same everywhere, independent of masses, distance or medium.

1. $G=6.67\times {10}^{-11}\mathrm{N}{\mathrm{m}}^2\mathrm{k}{\mathrm{g}}^{-2}$ everywhere in the universe.
2. Changing the planet mass to $4M$ changes $g$, not $G$.
3. So on the second planet the constant is still $G$.

Yaad rakhein: $G$ never changes; $g=\frac{GM}{R^2}$ changes with planet mass and radius.

Answer: (C) G

Concept: Laws of reflection: (1) angle of incidence = angle of reflection; (2) incident ray, normal and reflected ray lie in one plane.

1. For a plane mirror, the normal is perpendicular to the surface.
2. For a spherical mirror, the normal at any point is the radius through that point.
3. With this normal, both laws hold at every point – so they apply to all reflecting surfaces.

Yaad rakhein: Laws of reflection work for every surface – plane, concave, convex – even rough ones, point by point.

Answer: (A) both plane and spherical surfaces

Concept: A girl is born with ovaries already containing thousands of immature eggs; at puberty some start maturing.

1. Two oviducts unite to form the uterus, not one – (A) wrong.
2. Usually only one egg is released each month by one ovary – (B) wrong.
3. The womb is the uterus, not the fallopian tube – (C) wrong.
4. (D) is the correct statement.

Yaad rakhein: Ovary → egg; oviduct/fallopian tube → fertilisation site; uterus (womb) → embryo grows.

Answer: (D) When a girl is born, the ovaries already contain thousands of immature eggs.

Concept: Calcium sulphate hemihydrate is Plaster of Paris. It is made by heating gypsum at 373 K, which removes part of its water of crystallisation.

1. Gypsum: $\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4·2{\mathrm{H}}_2\mathrm{O}$
2. $\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4·2{\mathrm{H}}_2\mathrm{O}\stackrel{373\text{K}}{\to }\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$
3. Hemi = half molecule of water per formula unit.

Yaad rakhein: PoP + water → sets back into gypsum; used for plasters on fractured bones and toys.

Answer: (B) Gypsum

Concept: Write the formula by criss-crossing valencies: ammonium ion $\mathrm{N}{\mathrm{H}}_4^{+}$ (valency 1) and sulphate ion $\mathrm{S}{\mathrm{O}}_4^{2-}$ (valency 2).

1. Valencies: $\mathrm{N}{\mathrm{H}}_4$ = 1, $\mathrm{S}{\mathrm{O}}_4$ = 2.
2. Cross over: $\mathrm{N}{\mathrm{H}}_4$ gets subscript 2, $\mathrm{S}{\mathrm{O}}_4$ gets 1.
3. Formula: $(\mathrm{N}{\mathrm{H}}_4{)}_2\mathrm{S}{\mathrm{O}}_4$ – charges $2\times (+1)+(-2)=0$.

Trick: Total positive charge must equal total negative charge; a polyatomic ion taken more than once goes in brackets.

Answer: (A) $(N{H}_4{)}_{2}S{O}_4$

Concept: Resistivity $\rho$ is a property of the material (and temperature) only. Resistance $R=\rho \frac{l}{A}$ depends on size, but $\rho$ does not.

1. Cutting into 3 equal parts: length becomes $\frac{l}{3}$, so $R$ becomes $\frac{R}{3}$.
2. Material is unchanged, so resistivity stays the same.
3. Each part has resistivity $\rho$.

Yaad rakhein: Cutting, stretching or bending changes resistance $R$; it never changes resistivity $\rho$ of the same material.

Answer: (C) $\rho$

Concept: Metals above hydrogen in the reactivity series displace hydrogen from dilute acids; metals below hydrogen do not react.

1. Order: Al > Zn > Fe > (H) > Cu.
2. Al, Zn and Fe react with dilute HCl giving ${\mathrm{H}}_2$.
3. Copper is below hydrogen, so it does not react with dilute acids – least reactive.

Yaad rakhein: Below H: Cu, Hg, Ag, Au – no ${\mathrm{H}}_2$ with dilute acids.

Answer: (D) Copper

Concept: Work $W=Fs\cos \theta$, where $\theta$ is the angle between force and displacement.

1. Same direction: $\theta =0^{\circ }$, $\cos 0^{\circ }=1$ → $W=Fs$, positive.
2. Opposite: $\theta ={180}^{\circ }$ → $W=-Fs$, negative.
3. Perpendicular: $\theta ={90}^{\circ }$ → $W=0$; no displacement → $W=0$.

Trick: Displacement along force → +ve, against force → −ve, at right angle or zero → no work.

Answer: (B) is displaced in the direction of the force

Concept: Epithelial tissue forms the covering or lining of the body and organs (skin, mouth lining, blood vessels), so its main function is protection.

1. Cells are tightly packed with almost no intercellular space – a continuous sheet.
2. It protects underlying cells from drying, injury and germs, and controls exchange of materials.
3. Involuntary movement → smooth muscle; response to stimulus → nervous tissue; fat storage → adipose tissue.

Yaad rakhein: Epithelium = body's protective cover; muscle = movement; nerve = response; connective = support/storage.

Answer: (C) Protection

Concept: Newton's law: $F=\frac{G{m}_1{m}_2}{r^2}$ – every two bodies attract each other with a force proportional to the product of their masses.

1. (i) Gravity is always attractive, never repulsive – false.
2. (ii) Its magnitude depends on masses and distance, so it is not the same everywhere (only $G$ is) – false.
3. (iii) $F\propto m_1{m}_2$ – true.
4. (iv) The Sun's gravity keeps the Earth in orbit – true.

Yaad rakhein: Gravitational force: always attractive, $F\propto m_1{m}_2$, $F\propto \frac{1}{r^2}$; constant $G$ is universal, the force is not.

Answer: (D) Both (iii) and (iv)

Concept: Meristems are growing tissues found only in plants. Apical meristem lies at the tips of stems and roots and increases their length.

1. Plants grow throughout life only in regions having meristems.
2. Apical (tips), lateral/cambium (girth) and intercalary (base of leaves, internodes) are its types.
3. Animals, bacteria and viruses have no meristematic tissue.

Yaad rakhein: Apical → length; Lateral (cambium) → girth; Intercalary → near nodes.

Answer: (A) Plants

Concept: Meiosis is reduction division: it occurs in reproductive cells and forms gametes with half the number of chromosomes.

1. Parent cell: $2n$ chromosomes (human $2n=46$).
2. After meiosis, each gamete has $n$ (human $n=23$).
3. At fertilisation $n+n=2n$, so the number stays constant over generations. Mitosis/binary fission keep the number the same.

Yaad rakhein: Meiosis → half (gametes); Mitosis → same (growth and repair).

Answer: (D) meiosis

Concept: Most non-metals are solids or gases at room temperature; bromine is the only non-metal that is a liquid.

1. Fluorine and oxygen are gases.
2. Mercury is a liquid but it is a metal, not a non-metal.
3. Bromine (reddish-brown liquid) is the liquid non-metal.

Yaad rakhein: Liquid metal → mercury; liquid non-metal → bromine.

Answer: (D) Bromine

Concept: Nerve cells (neurons) have long extensions called axons to carry messages over long distances, making them the longest cells in the body.

1. An axon can be up to about 1 m long (spinal cord to toe).
2. The ovum is the largest cell (by size/volume), not the longest.
3. Blood and fat cells are small.

Yaad rakhein: Longest cell → neuron; largest cell → ovum; smallest cell → sperm.

Answer: (A) Nerve cell

Concept: Cambium is the lateral meristem; its dividing cells add new tissue on the sides, so the stem and root become thicker.

1. Apical meristem at tips → increases length.
2. Intercalary meristem near nodes → growth of internodes/leaves.
3. Cambium (lateral) → increases girth (secondary growth).

Yaad rakhein: Girth = cambium (lateral meristem).

Answer: (C) Cambium

Concept: Basic power formula: $P=VI$. Using Ohm's law $V=IR$, it can be rewritten in two more forms.

1. Put $V=IR$: $P=(IR)I=I^{2}R$.
2. Put $I=\frac{V}{R}$: $P=V\times \frac{V}{R}=\frac{V^2}{R}$.
3. $P=IR$ is actually Ohm's law for $V$, not power – wrong relation.

Trick: Check units: $IR$ gives volts, not watts – so $P=IR$ must be wrong.

Answer: (D) $P=IR$

Concept: Characteristics of particles of matter: very small, have spaces between them, are continuously moving, and attract each other.

1. Sugar dissolving in water without raising the level shows spaces between particles.
2. Diffusion and Brownian motion show constant motion.
3. A solid piece is hard to break → particles attract each other.
4. So 'no spaces between them' is NOT true.

Yaad rakhein: 4 properties: tiny, spaces, motion, attraction.

Answer: (B) They do not have spaces in between them.

Concept: For a spherical mirror $f=\frac{R}{2}$. By the New Cartesian sign convention, positive $R$ and $f$ (behind the mirror) belong to a convex mirror.

1. $f=\frac{R}{2}=\frac{+20}{2}=+10\text{cm}$
2. Positive focal length → focus lies behind the mirror.
3. Hence the mirror is convex.

Yaad rakhein: Sign of $f$: concave → negative, convex → positive.

Answer: (B) +10 cm, convex

Concept: Saturated hydrocarbons generally burn with a clean blue flame; unsaturated hydrocarbons give a yellow, sooty flame.

1. Ethane (${\mathrm{C}}_2{\mathrm{H}}_6$) – saturated (single bonds) → clean flame.
2. Ethylene/ethene (${\mathrm{C}}_2{\mathrm{H}}_4$) – double bond, unsaturated → sooty.
3. Ethyne (${\mathrm{C}}_2{\mathrm{H}}_2$) – triple bond, unsaturated → sooty.

Yaad rakhein: Ethylene and ethene are the same compound; '-ane' = saturated = clean flame.

Answer: (B) Ethane

Concept: In electrolytic refining, impure metal is the anode, a thin strip of pure metal is the cathode, and a soluble salt of the same metal is the electrolyte.

1. Anode: impure Cu; cathode: pure Cu.
2. Electrolyte: acidified copper sulphate, $\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4$.
3. Cu dissolves from the anode and deposits on the cathode: $\mathrm{C}{\mathrm{u}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{C}\mathrm{u}$; impurities fall as anode mud.

Yaad rakhein: Electrolyte always contains ions of the metal being refined → copper needs a soluble copper salt.

Answer: (D) $CuS{O}_4$