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Ideal Gases and Non-Ideal Gases

Ideal Gases and Non-Ideal Gas Behaviour

When we look at the gas laws in the next set of lessons, we will only deal with ideal gases.

Definition: Ideal gas

An ideal gas has identical particles of zero volume, with no intermolecular forces between them. The atoms or molecules in an ideal gas move at the same speed.

Almost all gases obey the gas laws within a limited range of pressures and temperatures. So we can use the gas laws to predict how real gases will behave.

Definition: Real gas

Real gases behave more or less like ideal gases except at high pressures and low temperatures.

Before we go on to look at the gas laws we will first see what happens to gases at high pressures and low temperatures.

When we defined an ideal gas, we said that an ideal gas has identical particles of zero volume and that there are no intermolecular forces between the particles in the gas. We need to look more closely at these statements because they affect how gases behave at high pressures or at low temperatures.

  1. Molecules do occupy volume

    When pressures are very high and the molecules are compressed, the volume of the molecules becomes significant. This means that the total volume available for the gas molecules to move is reduced and collisions become more frequent. This causes the pressure of the gas to be higher than what would be expected for an ideal gas (see figure below).

    Gases deviate from ideal gas behaviour at high pressure.

  2. Forces of attraction do exist between molecules

    At low temperatures, when the speed of the molecules decreases and they move closer together, the intermolecular forces become more apparent. As the attraction between molecules increases, their movement decreases and there are fewer collisions between them. The pressure of the gas at low temperatures is therefore lower than what would have been expected for an ideal gas (see figure below). If the temperature is low enough or the pressure high enough, a real gas will liquefy.

     

    Gases deviate from ideal gas behaviour at low temperatures.

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