Effects of Temperature and Pressure on the Volume of Gas

In Cambridge IGCSE science, the behavior of gases is explained using kinetic particle theory: gas particles are in rapid, continuous, random motion with large spaces between them and negligible intermolecular forces ¹.

Low Temp (T1)       High Temp (T2)      Low Pres (P1)       High Pres (P2)
 +---------+         +---------+         +---------+         +---------+
 | [Piston]|         |==[Piston|         | [Piston]|         |         |
 |---------|         |---------|         |---------|         | [Piston]|
 |  o   o  |         |  o   ^  |         | o     o |         |---------|
 |    o    |         |   / \   |         |    o    |         | o o o o |
 |  o   o  |         |<-o   o->|         | o     o |         | o o o o |
 +---------+         +---------+         +---------+         +---------+
 (V1: Small)         (V2: Large)         (V1: Large)         (V2: Small)

1. Effect of Temperature (Constant Pressure)

• Mechanism: Heating a gas increases the average kinetic energy ($E_k$) and speed of its particles ².
• Outcome: Particles collide with the container walls more frequently and with greater force. To maintain constant internal pressure against an external load (e.g., a moveable piston), the gas expands, increasing its volume ($V \propto T$, Charles’s Law) ¹.
• Cooling: Decreasing temperature reduces kinetic energy; particles collide less forcefully, and the volume contracts.

2. Effect of Pressure (Constant Temperature)

• Mechanism: Applying external pressure compresses the gas by forcing particles closer together into the empty spaces between them ¹.
• Outcome: Increasing pressure decreases volume ($V \propto \frac{1}{P}$, Boyle’s Law: $P_1 V_1 = P_2 V_2$) ².
• Decompression: Reducing external pressure allows particles to spread out, increasing volume.

Summary

Related

• Kinetic Particle Theory of Matter - Core physical model explaining the structure and motion of particles.
• State of Matter - Kinetic particle model across solids, liquids, and gases.
• Chemistry - Domain governing physical properties of matter.
• Diffusion - Random molecular motion in gases.