Q&A: practice questions with answers

Concept: Different scientists gave different models of the atom.

1. Thomson – plum-pudding model.
2. Rutherford – nucleus at the centre, electrons revolving around it (but could not explain stability).
3. Niels Bohr (1913) – electrons revolve in fixed orbits (discrete energy levels/shells K, L, M…) without radiating energy.

Yaad rakhein: Discrete energy levels / fixed shells = Bohr; nucleus discovery = Rutherford.

Answer: (C) Niels Bohr

Concept: Hydra reproduces asexually by budding using its regenerative cells.

1. A bud develops as an outgrowth due to repeated cell division at one specific site.
2. The bud grows into a tiny individual and detaches when mature.
3. Yeast also buds, but the use of regenerative cells is described for Hydra; Amoeba and Leishmania use fission.

Yaad rakhein: Hydra – budding and regeneration; Planaria – regeneration; Amoeba – binary fission; Plasmodium – multiple fission.

Answer: (D) Hydra

Concept: For uniformly accelerated motion, velocity changes at a steady rate, so the average velocity is the mean of initial and final velocities.

1. $\text{Average velocity}=\frac{\text{initial velocity}+\text{final velocity}}{2}$
2. $=\frac{u+v}{2}$
3. Total displacement then $=racu+v2\times t$.

Trick: If $u=v$ (uniform motion), the formula should give $u$ – only $\frac{u+v}{2}$ does.

Answer: (C) $\frac{u+v}{2}$

Concept: Oxidation = gain of oxygen (or loss of hydrogen) by a substance.

1. $\mathrm{M}\mathrm{g}$, $C$ and $\mathrm{C}\mathrm{u}$ combine with ${\mathrm{O}}_2$ to form oxides → oxidation (A, C, D).
2. $2\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}\to 2\mathrm{A}\mathrm{g}+\mathrm{C}{\mathrm{l}}_2$ – silver chloride breaks down in sunlight.
3. No oxygen is added; it is a (photo)decomposition reaction in which silver is reduced.

Trick: Look for ${\mathrm{O}}_2$ on the reactant side – the only reaction without it is the answer.

Answer: (B) $2AgCl\to 2Ag+C{l}_2$

Concept: Uniform circular motion = motion along a circular path with constant speed (direction keeps changing).

1. A bird flies along an irregular path – not circular.
2. A walking person or a car on a straight highway moves in a straight line.
3. A satellite in a circular orbit moves round the earth at constant speed → uniform circular motion.

Yaad rakhein: In uniform circular motion speed is constant but velocity changes, so it is accelerated motion.

Answer: (C) A satellite in a circular orbit around the earth.

Concept: A reaction in which an insoluble solid (precipitate) is formed is a precipitation (double displacement) reaction.

1. $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{a}\mathrm{q})+\mathrm{B}\mathrm{a}\mathrm{C}{\mathrm{l}}_2(\mathrm{a}\mathrm{q})\to \mathrm{B}\mathrm{a}\mathrm{S}{\mathrm{O}}_4(\mathrm{s})+2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$
2. Sodium sulphate, barium chloride and sodium chloride stay dissolved (aqueous).
3. Barium sulphate $\mathrm{B}\mathrm{a}\mathrm{S}{\mathrm{O}}_4$ is insoluble and appears as a white precipitate (solid).

Yaad rakhein: Double displacement + white ppt = $\mathrm{B}\mathrm{a}\mathrm{S}{\mathrm{O}}_4$.

Answer: (B) Barium sulphate

Concept: Metals at the bottom of the reactivity series are least reactive and occur in the free (native) state.

1. Sodium, aluminium and zinc are reactive – found only as compounds (oxides, sulphides, carbonates).
2. Gold is very unreactive – it does not combine with oxygen or water.
3. So gold is found in the free state in the earth's crust.

Yaad rakhein: Native metals: Au, Pt, Ag, Cu (Cu and Ag also as sulphides/oxides).

Answer: (D) Gold

Concept: Least reactive metals at the bottom of the reactivity series occur in the native (free) state.

1. Na, K, Ca, Mg, Al are at the top – highly reactive, never found free.
2. Gold and platinum are at the bottom – they are not attacked by air, water or most acids.
3. So gold and platinum are found in native state.

Yaad rakhein: Top of series (K, Na, Ca, Mg, Al) → electrolysis; bottom (Au, Pt) → native.

Answer: (C) Gold and platinum

Concept: For lenses in contact, net power $P=P_1+P_2$; a converging (convex) lens has positive power and a diverging (concave) lens has negative power.

1. $P_1=+4.50\text{D}$, $P_2=-3.00\text{D}$
2. $P=4.50+(-3.00)$
3. $P=+1.5\text{D}$ (net converging)

Trick: Convex = plus, concave = minus; then just add.

Answer: (B) $+1.5D$

Concept: Power is the rate of doing work: $P=\frac{W}{t}$ (unit watt = joule per second).

1. $W=500\text{J}$, $t=4\text{s}$
2. $P=\frac{500}{4}$
3. $P=125\text{W}$

Trick: Work ÷ time; multiplying (2000 W) is the common trap.

Answer: (C) 125 watt

Concept: Substances not broken down by microbes (bacteria, fungi) are non-biodegradable.

1. Paper, clay and plant leaves are decomposed or merged back into soil over time.
2. Plastic is not broken down by microorganisms.
3. It persists for a long time and may harm members of the ecosystem (animals swallowing it, soil and water pollution).

Yaad rakhein: Non-biodegradable: plastic, polythene, DDT, glass, metal cans.

Answer: (A) Plastic made material

Concept: For a body moving with uniform speed (no acceleration): $\text{speed}=\frac{\text{distance}}{\text{time}}$.

1. So $\text{distance}=\text{speed}\times \text{time}$.
2. Rearranging: $\text{time}=\frac{\text{distance}}{\text{speed}}$.
3. Only option B matches this rearranged form.

Trick: Triangle D over S×T – cover the quantity you want.

Answer: (B) $time=\frac{distance}{speed}$

Concept: The principal focus F is the point on the principal axis where rays parallel to the principal axis meet (concave) or appear to diverge from (convex) after reflection.

1. (i) F always lies on the principal axis – true.
2. (ii) A ray through F becomes parallel to the axis after reflection – true.
3. (iii) Not all incident rays meet at F – only rays parallel to the principal axis do. So (iii) is NOT true.

Trick: Watch the word "all" – it usually makes a definition wrong.

Answer: (B) Only (iii)

Concept: Atomic mass is expressed in atomic mass units (amu, now written u), where $1\text{u}=\frac{1}{12}$ of the mass of one carbon-12 atom.

1. One atom is extremely light ($1\text{u}\approx 1.66\times {10}^{-27}\text{kg}$), so grams, kilograms or pounds are impractical.
2. Grams per mole is the unit of molar mass, not atomic mass.
3. So atomic mass is expressed in amu (u).

Yaad rakhein: Atomic mass → u (amu); molar mass → g/mol.

Answer: (D) Atomic mass units (amu)

Concept: Kulhads are biodegradable, but they are made by baking clay dug from the ground.

1. Large-scale making of kulhads needs huge amounts of clay.
2. This clay comes from the fertile top layer of soil.
3. So making them on a large scale would cause loss of fertile top-soil (NCERT example).

Yaad rakhein: Disposable ≠ eco-friendly: kulhads waste top-soil, plastic cups persist in the environment.

Answer: (B) losing the fertile top-soil

Concept: Receptors are specialised tips of nerve cells that detect information from the environment.

1. They are located in sense organs.
2. Inner ear – hearing and balance; nose – smell (olfactory receptors); tongue – taste (gustatory receptors).
3. The most complete option lists all three: inner ear, nose and tongue.

Trick: When options are subsets of one another, choose the most complete correct list.

Answer: (B) the inner ear, the nose and the tongue

Concept: In the New Cartesian sign convention, distances are measured from the optical centre; those in the direction of incident light are positive.

1. A convex lens forms its real focus on the other side (in the direction of light) → $f$ positive.
2. A concave lens has a virtual focus on the same side as the object → $f$ negative.
3. So convex: positive; concave: negative.

Yaad rakhein: Lens: convex $+$, concave $-$. Mirror: concave $-$, convex $+$ (just opposite).

Answer: (C) positive; negative

Concept: In eukaryotic cells the nucleus is enclosed by a double-layered nuclear membrane.

1. The nuclear membrane separates the nucleus from the cytoplasm and has pores.
2. The nucleus contains chromosomes (DNA) which carry information for all activities.
3. Hence it directs the life processes of the cell – the "control centre".

Yaad rakhein: Double membrane organelles: nucleus, mitochondria, plastids; ribosome has no membrane.

Answer: (A) Nucleus

Concept: When an object is placed between the principal focus F and pole P of a concave mirror, the reflected rays diverge.

1. A ray parallel to the axis reflects through F; a ray to P reflects at an equal angle.
2. These reflected rays do not meet in front; extended backwards they meet behind the mirror.
3. So the image is behind the mirror – virtual, erect and enlarged (used in shaving/makeup mirrors).

Yaad rakhein: Concave mirror gives a virtual image only when the object is between P and F.

Answer: (D) virtual, erect and enlarged

Concept: Isotopes are atoms of the same element with the same atomic number but different mass numbers.

1. Protium ${\mathrm{}}_1^1\mathrm{H}$ – no neutron (most abundant).
2. Deuterium ${\mathrm{}}_1^2\mathrm{H}$ (D) – one neutron.
3. Tritium ${\mathrm{}}_1^3\mathrm{H}$ (T) – two neutrons, radioactive, in traces. So there are 3 isotopes.

Yaad rakhein: P-D-T: Protium, Deuterium, Tritium – neutrons 0, 1, 2.

Answer: (D) 3