Class 9 Chemistry Notes – Chapter 14: States of Matter and Phase Changes

A. Multiple-Choice Questions (MCQs)

14.1 According to kinetic theory, the basic difference between solid, liquid and gas is due to:

(a) The difference in the movements of the particles
(b) The chemical properties of the particles
(c) The size of the particles
(d) The shape of the particles

Correct Answer: (a) The difference in the movements of the particles

14.2 Upon heating, the rate of evaporation:

(a) Decreases
(b) Increases
(c) Remains the same
(d) Initially decreases and then increases upon further heating

Correct Answer: (b) Increases

14.3 The inter-particle attractions are the strongest in:

(a) Solids
(b) Liquids
(c) Plasma
(d) Gas

Correct Answer: (a) Solids

14.4 Cooling the vapours of some gases changes them directly into the solid state. This phenomenon is called:

(a) Evaporation
(b) Condensation
(c) Sublimation
(d) Deposition

Correct Answer: (d) Deposition

14.5 Physical state in which the particles possess the maximum energy:

(a) Solid
(b) Liquid
(c) Gaseous
(d) Vapour

Correct Answer: (c) Gaseous

14.6 How does the process of evaporation depend on the force of attraction present among the molecules of a liquid?

(a) It decreases with the increasing strength of attraction.
(b) It increases with the increasing strength of attraction.
(c) It is independent of the strength of the force of attraction.
(d) It first increases and then decreases with the increasing strength of the force of attraction.

Correct Answer: (a) It decreases with the increasing strength of attraction.

14.7 Which gas will diffuse at the fastest rate?

(a) HCl
(b) SO₂
(c) H₂S
(d) CO₂

Correct Answer: (c) H₂S

14.8 The phase changes A and B in gas ⇌ liquid ⇌ solid are:

(a) Melting, evaporating
(b) Condensation, melting
(c) Condensation, freezing
(d) Boiling, freezing

Correct Answer: (c) Condensation, freezing

14.9 A student noticed frost falling on the ground at 6:30 am and disappearing by 8:30 am. Which phase change was responsible for changing the frost into water vapours?

(a) Melting
(b) Evaporation
(c) Sublimation
(d) Deposition

Correct Answer: (c) Sublimation

B. Short Answer Questions

14.1 From where does the energy come when a liquid evaporates?

Energy comes from the surrounding environment and the kinetic energy of the liquid molecules themselves. Molecules with higher kinetic energy overcome intermolecular forces and escape.

14.2 Is condensation an endothermic process?

No, condensation is an exothermic process because heat is released when gas molecules lose kinetic energy and convert into liquid.

14.3 Why do naphthalene balls (used to repel insects) disappear after some time?

Naphthalene undergoes sublimation, changing directly from the solid to the vapour phase at room temperature without melting into a liquid.

14.4 Why does the temperature remain constant during a phase change?

The heat supplied (latent heat) is entirely utilized to weaken or break intermolecular forces between particles rather than increasing their kinetic energy (temperature).

14.5 Is it possible to compress a liquid like a gas?

No, liquids are nearly incompressible because their molecules are closely packed, with very small intermolecular empty spaces compared to gases.

14.6 Can you change the temperature at which water boils in an experiment in your lab?

Yes, by changing the external atmospheric pressure, for example, using a vacuum pump or pressure cooker. Lower pressure lowers the boiling point, while higher pressure raises it.

14.7 In which season do wet clothes take relatively longer to dry?

In winter (or during the humid/monsoon season), due to lower temperatures, lower kinetic energy, and higher atmospheric humidity, which slows down evaporation.

14.8 When a solid is heated, what happens to its vibrating particles?

Their vibrational kinetic energy increases, causing them to vibrate more vigorously and move slightly further apart, leading to thermal expansion.

14.9 Do solids and liquids also diffuse just like gases?

Yes, but at a much slower rate because particles in solids and liquids are tightly bound and have restricted movement compared to gases.

14.10 Why is dew formed in the early hours of the morning?

Due to cooling at night, the temperature drops to the dew point, causing water vapours in the air to condense on cold surfaces like grass and leaves.

C. Constructed Response Questions

14.1 Differentiate between evaporation and boiling.

Evaporation is a surface phenomenon occurring at all temperatures, whereas boiling is a bulk phenomenon occurring at a specific temperature (boiling point), where vapour pressure equals external pressure.

14.2 Describe how boiling is related to external pressure.

The boiling point is directly proportional to external pressure. When external pressure increases, a higher temperature is required for vapour pressure to match it, raising the boiling point.

14.3 Why does the process of evaporation not need the input of energy?

It draws thermal energy spontaneously from its surroundings and the internal kinetic energy pool of the liquid.

14.4 Why do the heating and cooling curves for phase changes follow the same path?

Because phase-change temperatures (melting and boiling points) are reversible thermodynamic constants. Heat absorbed during melting or vaporization equals heat released during freezing or condensation.

14.5 Why does ice melt when pressure is exerted on it?

Ice expands upon freezing. Applying pressure favours the denser state (liquid water), lowering the melting point and causing ice to melt.

14.6 Why do phase changes occur?

They occur when thermal energy overcomes, or is overcome by, intermolecular attractive forces, changing the degree of freedom and spacing of particles.

D. Descriptive Questions (Summary Outlines)

14.1 Rates of diffusion of two gases

According to Graham’s law, the rate of diffusion is inversely proportional to the square root of molar mass. Lighter gases diffuse faster.

[
\text{Rate} \propto \frac{1}{\sqrt{M}}
]

Here, (M) represents the molar mass of the gas.

14.2 Sublimation in printing and air fresheners

Solid sublimating inks transfer directly to the gas phase in heat-press printing. Solid air-freshener blocks sublime into the surrounding air to release fragrance continuously.

14.3 Heating curve of water

The heating curve of water shows the following stages:

Ice heats up as a solid.
Melting occurs at 0°C, shown by a plateau where temperature remains constant.
Liquid water heats up to 100°C.
Boiling occurs at 100°C, shown by another plateau.
Steam heats up after all the water has changed into vapour.

Notes by Sir Dilawar

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