CBSE Class 9 Chemistry Is Matter Around Us Pure MCQs Set 13

Answer: (c) sol

Explanation:
1. Colloidal particles are big enough to scatter light, which makes the beam visible.
2. A sol is a colloid, so it shows the effect.

Teacher's Note:
1. Light beam path becomes visible in a colloid.
2. True solutions do not scatter light.

Answer: (c) 10 to 1000 angstrom

Explanation:
1. Colloidal particles are between about 1 nm and 100 nm in diameter.
2. That is 10 to 1000 angstrom.

Teacher's Note:
1. 1 nm = 10 angstrom.
2. Smaller than that is a true solution.

Answer: (c) solid dispersed in gas

Explanation:
1. Smoke has tiny solid particles dispersed in air.
2. So it is a solid in gas colloid (aerosol).

Teacher's Note:
1. Fog is liquid in gas.
2. Foam is gas in liquid.

Answer: (d) two immiscible liquids

Explanation:
1. An emulsion is a liquid dispersed in another liquid that does not mix with it.
2. Milk and mayonnaise are examples.

Teacher's Note:
1. An emulsifier keeps the two liquids from separating.
2. Oil and water do not mix by themselves.

Answer: (b) the charge carried by clay particles is neutralised

Explanation:
1. Clay particles carry a negative charge.
2. The positive ions from alum neutralise it, and the particles settle together.

Teacher's Note:
1. Alum is used to purify muddy water.
2. Coagulation removes colloidal impurities.

Answer: (c) impact of molecules of dispersion medium on colloidal particles

Explanation:
1. Molecules of the dispersion medium keep hitting the colloidal particles from all sides.
2. The uneven hits make the particles move in a zig-zag path.

Teacher's Note:
1. Brownian motion keeps colloids from settling.
2. It shows that matter is made of moving particles.

Answer: (b) two

Explanation:
1. A colloid has a dispersed phase and a dispersion medium, so it has two phases.

Teacher's Note:
1. Dispersed phase is the particles, medium is what they are spread in.
2. A true solution has one phase.

Answer: (c) water is dispersed in fat

Explanation:
1. Butter is a water in fat emulsion, with tiny water droplets spread in fat.

Teacher's Note:
1. Milk is fat in water, butter is water in fat.
2. Both are emulsions.

Answer: (b) Sulphur

Explanation:
1. Carbon disulphide dissolves sulphur but not common salt or iron.
2. The filtrate has sulphur in solution, and sulphur remains after evaporation.

Teacher's Note:
1. Choose a solvent that dissolves only one component.
2. Iron and salt stay on the filter paper.

Answer: (b) stainless steel

Explanation:
1. Stainless steel is strong, does not rust and is not harmful to the body, so it is used for surgical pins.

Teacher's Note:
1. Stainless steel is an alloy of iron, chromium and nickel.
2. Copper and aluminium are too soft.

Answer: (a) crystallisation

Explanation:
1. Crystallisation separates a pure solid from its saturated solution by cooling or evaporation.
2. Simple distillation gives the liquid, not the pure solid crystals.

Teacher's Note:
1. Crystallisation is used to purify solids like alum and copper sulphate.
2. Evaporation to dryness can leave impurities.

Answer: (a) Oxygen

Explanation:
1. Boiling points are about -183 degrees C for oxygen, -186 degrees C for argon and -196 degrees C for nitrogen.
2. Hydrogen is not a significant gas in air.
3. Oxygen has the highest.

Teacher's Note:
1. In fractional distillation of liquid air, nitrogen boils off first.
2. Oxygen remains last.

Answer: (c) both (a) and (b)

Explanation:
1. Water, alcohol and other solvents are used to carry the components up the paper in chromatography.

Teacher's Note:
1. The solvent should dissolve the components.
2. Different dyes travel different distances.

Answer: (a) chemically in fixed ratio by weight

Explanation:
1. In a compound the elements are combined chemically in a fixed ratio by mass.
2. Sodium, carbon and oxygen in washing soda are in a fixed ratio.

Teacher's Note:
1. Compound: chemical combination, fixed ratio.
2. Mixture: physical combination, any ratio.

Answer: (d) bcad

Explanation:
1. First take the mixture (b).
2. Then fit the moistened filter paper in the funnel (c).
3. Pour the mixture to collect the filtrate (a).
4. Finally the solid left on the paper is the residue (d).

Teacher's Note:
1. Prepare the apparatus before pouring.
2. The order is b, c, a, d.

Answer: (c) chromatography

Explanation:
1. Chromatography separates substances that differ in how far they move with a solvent on paper, which is used to separate dyes in ink.

Teacher's Note:
1. Chromatography is good for coloured components.
2. It needs only a tiny sample.

Answer: (b) liquids

Explanation:
1. Distillation separates liquids by differences in boiling point.

Teacher's Note:
1. Simple distillation is for a liquid and a dissolved solid or liquids with a large boiling point difference.
2. Fractional distillation is for closer boiling points.

Answer: (a) adhesive forces between water and glass

Explanation:
1. Adhesive forces are attractions between unlike substances.
2. Water and glass attract each other, so water sticks to glass.

Teacher's Note:
1. Cohesion is within the same substance.
2. Adhesion is between different substances.

Answer: (b) No

Explanation:
1. Mixing the two just forms a mixture and the components keep their properties.
2. So it is only a physical change.

Teacher's Note:
1. Iron can still be separated by a magnet.
2. Chemical change comes only on heating.

Answer: (d) A to b, B to c, C to a

Explanation:
1. Iodine sublimes, so it is separated from sand by sublimation.
2. Insoluble sawdust is separated from water by filtration.
3. Salt is recovered from its solution by evaporation.

Teacher's Note:
1. Choose the method from the property that differs.
2. Sublimation: one component sublimes.

Answer: (b) Boiling of water

Explanation:
1. Boiling only changes water from liquid to vapour and no new substance forms.
2. The other three form new substances.

Teacher's Note:
1. A change of state is physical.
2. Burning is chemical.