CBSE Class 9 Chemistry Is Matter Around Us Pure MCQs Set 16

Answer: (b) 1 : 8

Explanation:
1. Water, H₂O, has 2 g of hydrogen for every 16 g of oxygen.
2. 2 : 16 = 1 : 8 by mass.

Teacher's Note:
1. The ratio 2 : 1 is the ratio of volumes of hydrogen and oxygen, not of masses.
2. Water always has this ratio of masses (law of constant proportions).

Answer: (c) temperature and pressure

Explanation:
1. Changing the temperature and pressure changes the energy and spacing of particles.
2. Shape, volume and mass do not decide the state.

Teacher's Note:
1. Heating a solid gives a liquid and then a gas.
2. Cooling and compressing a gas can liquefy it.

Answer: (a) an element

Explanation:
1. Oxygen contains only one kind of atom, so it cannot be split into simpler substances.

Teacher's Note:
1. Oxygen gas is made of O₂ molecules.
2. Ozone, O₃, is another form of the element.

Answer: (b) solids

Explanation:
1. In solids the particles are packed closely, so the forces between them are strongest.
2. Forces are weaker in liquids and weakest in gases.

Teacher's Note:
1. This is why solids have a fixed shape.
2. Order of attraction: solid > liquid > gas.

Answer: (b) sublimation

Explanation:
1. Sublimation is the change from solid to vapour without passing through the liquid state.
2. Examples: iodine, camphor, naphthalene.

Teacher's Note:
1. Deposition is the reverse change.
2. Dry ice (solid CO₂) also sublimes.

Answer: (c) free molecules

Explanation:
1. Sugar is a covalent compound, so it dissolves as separate sugar molecules and does not give ions.

Teacher's Note:
1. Salt solution has ions, sugar solution has molecules.
2. Sugar solution does not conduct electricity.

Answer: (b) Molality

Explanation:
1. Molality is moles of solute per kg of solvent, and masses do not change with temperature.
2. Molarity, formality and normality involve the volume of solution, which changes with temperature.

Teacher's Note:
1. Volume changes with temperature but mass does not.
2. Mass percent is also independent of temperature.

Answer: (b) the boiling point of the solution increases

Explanation:
1. A non-volatile solute raises the boiling point of the solvent (elevation of boiling point).
2. It also lowers the freezing point.

Teacher's Note:
1. That is why salt is spread on icy roads.
2. Option (d) is wrong because the boiling point rises.

Answer: (a) 10⁻⁷ to 10⁻⁵ cm

Explanation:
1. Colloidal particles are larger than the particles of a true solution but smaller than those of a suspension.
2. The range is 10⁻⁷ cm to 10⁻⁵ cm (1 nm to 100 nm).

Teacher's Note:
1. Suspension particles are bigger than 10⁻⁴ cm.
2. Solution particles are smaller than 10⁻⁷ cm.

Answer: (a) scattering of light

Explanation:
1. Colloidal particles are big enough to scatter a beam of light, making its path visible.
2. True solutions do not show the effect.

Teacher's Note:
1. A beam of sunlight in a dusty room shows the effect.
2. Scattering is by the particles, not by charges.

Answer: (c) silica

Explanation:
1. Ozone (O₃), graphite and diamond (carbon) are made of one kind of atom.
2. Silica is SiO₂, a compound of silicon and oxygen.

Teacher's Note:
1. Allotropes are different forms of the same element.
2. Sand is mainly silica.

Answer: (a) oxygen

Explanation:
1. Oxygen makes up about 46 per cent of the mass of the earth's crust.
2. Silicon is second (about 28 per cent).

Teacher's Note:
1. Oxygen is found in oxides and silicates.
2. Nitrogen is abundant in air, not in the crust.