The Gas Phase (College Board AP® Chemistry): Revision Note

The Gas Phase

• Gases have several characteristics that differentiate them from solids and liquids:

  • Unlike solids and liquids, the particles of gases have very little intermolecular force and are in constant linear random motion

  • Given the little intermolecular force and constant random motion of the particles, gases have neither a fixed volume nor a fixed shape

  • Gases are also highly compressible because of the large distances between their particles

  • Gases have much lower densities than liquids or solids

• Also for gases, unlike for liquids and solids, pressure, volume, temperature and amount (in moles) of a gas are related, to a good approximation, by one equation: the ideal gas law

  • The pressure of a gas is a measure of the collisions of its particles with the walls of its container

    • The magnitude of the pressure is determined by how often (frequency) and how forcefully the particles strike the walls

  • The temperature of a gas is a measure of the average kinetic energy of its particles

    • Hence, an increase in temperature is an increase in average kinetic energy and an increase in the molecular motion of the particles

    • Heating a gas makes its particles move faster; the particles themselves do not get bigger

  • How often the particles collide, and how far apart they are on average, depend on the temperature, pressure and volume of the gas

  • The molecular interpretations of the temperature and pressure of a gas are explained by the kinetic molecular theory

Gas Particles

Diagram showing the arrangement and movement of gas particles