Q&A: practice questions with answers

Concept: After fertilisation in a flower, the zygote, ovule and ovary develop into new structures.

1. Zygote divides repeatedly → embryo.
2. Ovule → seed (embryo inside it).
3. Ovary → fruit. Stamen, pistil and pollen are formed before fertilisation.

Yaad rakhein: Zygote → embryo, ovule → seed, ovary → fruit.

Answer: (C) Embryo

Concept: EMF is the work done per unit charge by a source (cell) in moving charge round a circuit, so its unit is the same as potential difference.

1. $\epsilon =\frac{W}{q}$ → $\frac{J}{C}=V$ (volt).
2. Newton → force; ampere → current; second → time.
3. Hence EMF is measured in volts.

Yaad rakhein: EMF, potential difference, voltage – all in volt.

Answer: (A) Volt

Concept: Bleaching powder, $\mathrm{C}\mathrm{a}\mathrm{O}\mathrm{C}{\mathrm{l}}_2$, releases chlorine which acts as a strong oxidising agent.

1. Made by: $\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H}{)}_2+\mathrm{C}{\mathrm{l}}_2\to \mathrm{C}\mathrm{a}\mathrm{O}\mathrm{C}{\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}$.
2. Uses: bleaching cotton/linen and wood pulp, disinfecting water.
3. In chemical industries it is used as an oxidising agent.

Yaad rakhein: NCERT list: bleaching, oxidising agent in industry, disinfecting drinking water.

Answer: (C) an oxidising agent

Concept: A reflex action is a sudden, automatic, involuntary response to a stimulus, controlled by the spinal cord via a reflex arc.

1. Touching a flame → receptor → sensory neuron → spinal cord → motor neuron → muscle.
2. Hand is pulled back before the brain even 'thinks'.
3. Walking, talking and dancing are voluntary actions controlled by the brain.

Yaad rakhein: Reflex = quick + without thinking (spinal cord).

Answer: (B) Pulling hand from the flame

Concept: Gas particles are far apart; to liquefy a gas they must be brought close together.

1. Applying pressure squeezes the particles closer.
2. Reducing temperature lowers their kinetic energy so attraction can hold them.
3. Example: LPG and CNG are stored under high pressure in cylinders.

Yaad rakhein: Liquefy gas: P up, T down.

Answer: (B) Applying pressure and reducing temperature

Concept: Appliances like ovens and irons use the heating effect of electric current, $H=I^{2}Rt$.

1. Electric oven – runs on electricity ✔.
2. Electric laundry iron – runs on electricity ✔.
3. Solar water heater – uses heat from sunlight (solar energy), not electricity ✘.

Trick: The word 'solar' tells you the energy source is the Sun, not the mains.

Answer: (B) Only (A) and (B)

Concept: Regeneration is the ability of a fully differentiated organism to grow a complete new organism from its cut body pieces, using specialised cells.

1. Planaria and Hydra show regeneration.
2. Each piece of Planaria grows into a complete animal.
3. Fragmentation (Spirogyra) is breaking up naturally as a mode of reproduction; regeneration involves cut pieces regrowing lost parts.

Yaad rakhein: Planaria / Hydra → regeneration; Spirogyra → fragmentation.

Answer: (C) Regeneration

Concept: Force $F=ma$; its SI unit is the newton (N).

1. $1N=1\mathrm{k}\mathrm{g}\times 1\mathrm{m}{\mathrm{s}}^{-2}$.
2. Dyne is the CGS unit: $1N={10}^5$ dyne.
3. rad/s is angular velocity; m/s is velocity.

Yaad rakhein: SI → newton, CGS → dyne.

Answer: (C) Newton

Concept: Plants have no special excretory organs; they remove wastes in several simple ways.

1. A – excess water lost by transpiration ✔.
2. B – wastes stored in leaves, bark which later fall off ✔.
3. C – wastes stored as resins and gums, especially in old xylem ✔. Some are also stored in vacuoles or excreted into soil.

Yaad rakhein: All three lines are straight from the NCERT 'Excretion in plants' paragraph.

Answer: (D) A, B and C

Concept: Law of conservation of mass: mass is neither created nor destroyed in a chemical reaction, so total mass of reactants = total mass of products.

1. Mass of reactants $=10+15=25$ g.
2. Mass of products = mass of reactants.
3. So products weigh 25 g.

Trick: Just add the reactant masses.

Answer: (A) 25 g

Concept: Prokaryotic cells (bacteria) lack a well-defined nucleus; eukaryotic cells have one bounded by a nuclear membrane.

1. In prokaryotes the genetic material lies in the nucleoid, with no nuclear membrane.
2. Membrane-bound organelles (mitochondria, plastids) are also absent in prokaryotes, so A is false.
3. Eukaryotes have the nuclear membrane, so C and D are false.

Yaad rakhein: Pro = 'before' + karyon = 'nucleus' → no true nucleus.

Answer: (B) The nuclear membrane is absent in prokaryotic cells.

Concept: Resistance of a conductor: $R=\rho \frac{l}{A}$, where $\rho$ = resistivity, $l$ = length, $A$ = area of cross-section.

1. $R\propto l$ (longer wire → more resistance).
2. $R\propto \frac{1}{A}$ (thicker wire → less resistance).
3. So resistance is inversely proportional to $A$.

Trick: Thick wire = wide road for current → less resistance.

Answer: (D) Inversely proportional to A

Concept: On the pH scale, a higher pH (above 7) means a more basic solution.

1. Sodium hydroxide (strong base) → pH about 14.
2. Blood → about 7.4; tap water → about 7.
3. Saliva → about 6.5–7.5. So NaOH is by far the most basic.

Yaad rakhein: Strong alkali (NaOH, KOH) sits at the top end of the scale, pH $\approx 14$.

Answer: (A) Sodium hydroxide

Concept: Unicellular organisms commonly reproduce by fission – the single cell divides into two (binary) or many (multiple) daughter cells.

1. Amoeba, Paramecium → binary fission; Plasmodium → multiple fission.
2. Regeneration needs many specialised cells, so it is not possible for unicellular organisms – option A is ruled out.
3. Among the given choices, fission is the mode used by unicellular organisms.

Yaad rakhein: Unicellular → fission; multicellular (Planaria, Hydra) → regeneration.

Answer: (D) Only fission

Concept: Match each organism with its asexual mode of reproduction.

1. Bryophyllum – vegetative propagation by buds on leaf margins (not spore formation) ✘.
2. Amoeba – binary fission ✔; Spirogyra – fragmentation ✔.
3. Hydra – budding ✔. Spore formation is seen in Rhizopus.

Yaad rakhein: Bryophyllum = 'leaf buds' (vegetative propagation); Rhizopus = spores.

Answer: (A) Bryophyllum – Spore formation

Concept: In multiple fission a single cell divides into many daughter cells at the same time.

1. Plasmodium (malarial parasite) shows multiple fission.
2. Amoeba and Leishmania – binary fission (Leishmania along a definite plane).
3. Yeast – budding.

Yaad rakhein: Malaria parasite multiplies fast → many cells at once → multiple fission.

Answer: (C) Plasmodium

Concept: An object is in uniform motion when it covers equal distances in equal intervals of time, i.e. it moves with constant speed.

1. Equal distance in equal time → speed $=\frac{\text{distance}}{\text{time}}$ is constant.
2. So acceleration in uniform linear motion is zero.
3. Options B, C, D describe non-uniform motion.

Yaad rakhein: Uniform = 'same speed all the time'.

Answer: (A) The object moves with a constant speed.

Concept: An object floats if the buoyant force on it can balance its weight before it is fully immersed – this happens when it is less dense than the liquid.

1. ${\rho }_{\text{object}}<{\rho }_{\text{liquid}}$ → floats.
2. ${\rho }_{\text{object}}>{\rho }_{\text{liquid}}$ → sinks.
3. Equal densities → stays suspended wherever placed.

Yaad rakhein: Ice ($0.92\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$) floats on water ($1\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$).

Answer: (B) less than the density of the liquid

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

1. A moving bullet has mass $m$ and large speed $v$.
2. So it has kinetic energy (very large because $KE\propto v^2$).
3. Potential energy is due to position/shape, not motion.

Yaad rakhein: Moving → kinetic; raised or stretched → potential.

Answer: (B) Kinetic energy

Concept: A suspension is a heterogeneous mixture in which solid particles are dispersed in a liquid but do not dissolve.

1. Particles are large (visible to the naked eye).
2. They settle down when left undisturbed and can be separated by filtration.
3. Different composition in different parts → heterogeneous. Example: chalk in water, muddy water.

Yaad rakhein: Solution – homogeneous; colloid & suspension – heterogeneous.

Answer: (B) Heterogeneous mixture