Class 9 > Unit # 07:Properties of Matter > Behaviour of Gases

Talha's Physics Academy

Behavior of Gases: Pressure, Volume, and Temperature

Explain behavior of Gases by their Pressure, Volume and Temperature.

1. Pressure of Gases

Pressure is defined as the force per unit area ($P = \frac{F}{A}$). All gases exert pressure on the walls of their container due to the total force exerted per unit area by gas molecules during collisions.

The number of collisions is directly proportional to the number of molecules. For instance, if the number of molecules is doubled, the number of collisions doubles, and consequently, the pressure is also doubled.

Gas pressure can be modified via two main methods:

• Increasing the number of molecules: Pumping more air into a balloon increases the number of molecules colliding with the inner rubber walls, inflating the balloon further.
• Compressing the gas: Reducing the container size packs molecules into a smaller volume, causing them to collide more frequently with the walls and raising the pressure. For example, compressing a gas to half its original volume doubles its pressure.

2. Volume of Gases

The space occupied by a substance is known as its volume. Because gas molecules are far apart and move freely at high speeds, a gas has no definite volume and takes up the shape and volume of its container.

The volume of a gas can be increased by decreasing its pressure (e.g., by reducing the load on a container's piston). Due to random motion, gas molecules quickly expand to cover the available space.

Figure: Change of Volume due to Pressure

3. Temperature of Gases

The temperature of a gas is determined by the average translational kinetic energy of its molecules:

• Heating a gas increases the average translational kinetic energy of its molecules, raising the temperature.
• Cooling a gas decreases the average translational kinetic energy of its molecules, lowering the temperature.

Pressure-Volume Relationship (Boyle's Law Context)

$P \propto \frac{1}{V} \quad \text{or} \quad PV = \text{constant}$

If the pressure of a gas is doubled, its volume becomes half. If pressure increases by three times, its volume becomes one-third, and so on (at constant temperature).

© 2026 Talha's Physics Academy. All rights reserved.