Q&A: practice questions with answers

Concept: An emulsion is a colloid in which a liquid is dispersed in another liquid (dispersed phase liquid, medium liquid).

1. Face cream = oil and water mixed as fine droplets.
2. Liquid in liquid → emulsion (like milk).
3. Gel = liquid in solid; foam = gas in liquid.

Yaad rakhein: Emulsion examples: milk, face cream, mayonnaise.

Answer: (D) Emulsion

Concept: A more reactive metal displaces a less reactive metal from its salt solution.

1. Iron is more reactive than copper.
2. $\mathrm{F}\mathrm{e}+\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4\to \mathrm{F}\mathrm{e}\mathrm{S}{\mathrm{O}}_4+\mathrm{C}\mathrm{u}$
3. Brown copper deposits on the nail; blue solution turns light green.

Yaad rakhein: Reactivity: Zn > Fe > Pb > Cu — iron can displace only Pb and Cu here, and brown coating means copper.

Answer: (B) Copper sulphate

Concept: Energy stored in a body due to its position or change in shape (configuration) is potential energy.

1. Stretching changes the shape of the rubber band.
2. Work done in stretching gets stored in it.
3. On release, it converts to kinetic energy.

Yaad rakhein: Stretched band, compressed spring, wound clock spring → potential (elastic) energy.

Answer: (C) Potential energy

Concept: Glucose has the molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$; add atoms of each element.

1. Carbon = 6
2. Hydrogen = 12
3. Oxygen = 6
4. Total $=6+12+6=24$

Trick: Glucose 6-12-6: just add the subscripts.

Answer: (B) 24

Concept: Blood platelets circulate in the body and plug leaks by helping blood to clot at the point of injury, preventing loss of pressure.

1. RBCs carry oxygen.
2. WBCs fight infection.
3. Lymph carries digested fat and drains excess fluid.
4. Platelets → clotting.

Yaad rakhein: Platelets = 'plugs'; vitamin K is also needed for clotting.

Answer: (A) Blood platelets

Concept: Thrust is the net force acting perpendicular to a surface; pressure is thrust per unit area.

1. $\text{Pressure}=\frac{\text{Thrust}}{\text{Area}}$
2. Thrust is a force → SI unit newton (N).
3. Pressure → SI unit pascal (${\text{N/m}}^2$).

Yaad rakhein: Thrust = force (N); pressure = force ÷ area (Pa).

Answer: (B) thrust

Concept: Plastids and mitochondria are semi-autonomous organelles: they have their own DNA and ribosomes.

1. Only chloroplasts (a type of plastid) contain chlorophyll — not mitochondria.
2. Mitochondria are found in both plant and animal cells.
3. Common feature: own DNA and ribosomes, so they can make some of their own proteins.

Yaad rakhein: Organelles with own DNA = mitochondria and plastids (double-membrane bound).

Answer: (B) Both contain their own DNA and ribosomes.

Concept: The radius of the Earth used in NCERT gravitation problems is $R=6.4\times {10}^6\text{m}$.

1. $6.4\times {10}^6\text{m}=6400\text{km}$
2. Mass of Earth $M=6\times {10}^{24}\text{kg}$

Yaad rakhein: Earth: $R\approx 6400\text{km}$, $g=9.8{\text{m/s}}^2$, $M=6\times {10}^{24}\text{kg}$.

Answer: (B) 6400 km

Concept: Squamous epithelium is made of thin, flat cells forming a delicate lining.

1. Lining of mouth and oesophagus → squamous epithelium.
2. Cuboidal → kidney tubules, salivary gland ducts.
3. Ciliated columnar → respiratory tract.

Yaad rakhein: Squamous = flat 'tiles'; also seen in skin and alveoli (stratified squamous in skin).

Answer: (B) squamous epithelium

Concept: Solid $\mathrm{C}{\mathrm{O}}_2$ (dry ice) is stored under high pressure. When pressure decreases to 1 atmosphere, it changes directly into gas without becoming liquid (sublimation).

1. High pressure keeps $\mathrm{C}{\mathrm{O}}_2$ solid.
2. Lowering pressure to 1 atm → direct solid → gas.
3. Hence called 'dry ice'.

Yaad rakhein: Dry ice → 1 atmosphere → sublimation.

Answer: (A) 1 atmosphere

Concept: Human excretory system: a pair of kidneys, a pair of ureters, one urinary bladder and one urethra.

1. Kidney – a pair ✔
2. Ureter – a pair ✔
3. Urinary bladder – unpaired ✔
4. Urethra – single, not a pair ✘

Trick: Count '2-2-1-1': kidneys 2, ureters 2, bladder 1, urethra 1.

Answer: (A) Urethra – a pair

Concept: Electric power $P=V\times I$.

1. $V=200\text{V},I=0.1\text{A}$
2. $P=200\times 0.1=20\text{W}$

Trick: Multiplying by $0.1$ just shifts the decimal one place: $200\to 20$.

Answer: (B) 20 W

Concept: Puberty brings specific changes caused by sex hormones, such as new hair growth in armpits, thinner hair on arms and legs, and breast development in girls.

1. Thick hair in armpits, thinner hair on legs/arms, increase in breast size → puberty-specific.
2. Increase in height happens throughout childhood as well.
3. So height increase is not specific to teenage.

Yaad rakhein: Puberty-specific = hormone-driven secondary sexual characters; general growth is not.

Answer: (B) Increase in height

Concept: Metals react with dilute acids to form a salt and hydrogen gas.

1. $\text{Metal}+\text{dil. HCl}\to \text{Metal chloride}+{\mathrm{H}}_2$
2. e.g. $\mathrm{Z}\mathrm{n}+2\mathrm{H}\mathrm{C}\mathrm{l}\to \mathrm{Z}\mathrm{n}\mathrm{C}{\mathrm{l}}_2+{\mathrm{H}}_2$
3. Test: gas burns with a 'pop' sound.

Yaad rakhein: Pop sound with burning splinter = hydrogen.

Answer: (D) Hydrogen

Concept: Force $F=ma$, where acceleration $a=\frac{v-u}{t}$.

1. $a=\frac{10-6}{2}=2{\text{m/s}}^2$
2. $F=10\times 2=20\text{N}$

Trick: $F=\frac{m(v-u)}{t}=\frac{10\times 4}{2}=20$ — change in momentum per second.

Answer: (C) 20 N

Concept: Unsaturated hydrocarbons contain at least one carbon–carbon double or triple bond.

1. Alkenes have $\mathrm{C}=\mathrm{C}$; alkynes have $\mathrm{C}\equiv \mathrm{C}$ → unsaturated.
2. Alkanes (methane, ethane, butane) have only single bonds → saturated.
3. Cyclopentane and cyclohexane also have only single bonds → saturated.

Yaad rakhein: '-ane' = saturated; '-ene' and '-yne' = unsaturated.

Answer: (C) Alkenes and alkynes are unsaturated hydrocarbons.

Concept: Oestrogen, secreted by the ovaries, controls development of female sex organs, secondary sexual characters and regulates the menstrual cycle.

1. Adrenaline – emergency (fight or flight) hormone.
2. Thyroxin – regulates metabolism.
3. Testosterone – male hormone from testes.

Yaad rakhein: Ovary → oestrogen (female); testis → testosterone (male).

Answer: (C) Oestrogen

Concept: Nitric acid is a strong oxidising agent: it oxidises the ${\mathrm{H}}_2$ formed to water, so usually no hydrogen is evolved. Only Mg and Mn give ${\mathrm{H}}_2$ with very dilute $\mathrm{H}\mathrm{N}{\mathrm{O}}_3$.

1. $\mathrm{M}\mathrm{g}+2\mathrm{H}\mathrm{N}{\mathrm{O}}_3\to \mathrm{M}\mathrm{g}(\mathrm{N}{\mathrm{O}}_3{)}_2+{\mathrm{H}}_2$
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. Copper is below hydrogen in reactivity — gives no ${\mathrm{H}}_2$ with dilute acids.

Trick: 'M-M' rule: Magnesium and Manganese with very dilute nitric acid.

Answer: (A) Magnesium (Mg) and manganese (Mn)

Concept: The colour of scattered light depends on the size of the scattering particles.

1. Very fine particles scatter mainly blue light (shorter wavelength).
2. Larger particles scatter longer wavelengths too.
3. Very large particles scatter all colours → light looks white (e.g. clouds).

Yaad rakhein: Blue sky = tiny air molecules; white clouds = big water droplets.

Answer: (C) The size of the scattering particles.

Concept: Sun rays (parallel) converge at the principal focus, so $f=30\text{cm}$. For a concave mirror, an object at the centre of curvature $C$ ($2f$) gives a real, inverted image of the same size at $C$.

1. $f=30\text{cm}$
2. $R=2f=60\text{cm}$
3. Object at $C$ → image at $C$, same size.

Yaad rakhein: Same size image in a concave mirror only when object is at $C$ ($u=2f$).

Answer: (A) 60 cm in front of the mirror