Question

Which of the following statements is/are correct about real gases?

• A. The molecules do cause attractive forces on each another.
• B. They obey gas laws at low temperature and high pressure.
• C. They show deviations from ideal behaviour.
• D. The molecules have negligible mass.

Answer 1

Answer: (A), (C)

The molecules do cause attractive forces on each another.
They show deviations from ideal behaviour.

A gas which obeys the gas laws and the gas equation $PV=nRT$ strictly at all temperatures and pressures is said to be an ideal gas.The molecules of ideal gases are assumed to be volume less points with no attractive forces between one another. But no real gas strictly obeys the gas equation at all temperatures and pressures. Deviations from ideal behaviour are observed particularly at high pressures or low temperatures.

The causes of deviations from ideal behaviour may be due to the following two assumptions of kinetic theory of gases.

The volume occupied by gas molecules is negligibly small as compared to the volume occupied by the gas.

The forces of attraction between gas molecules are negligible.

The first assumption is valid only at low pressures and high temperature, when the volume occupied by the gas molecules is negligible as compared to the total volume of the gas. But at low temperature or at high pressure, the molecules being incompressible the volumes of molecules are no more negligible as compared to the total volume of the gas.

The second assumption is not valid when the pressure is high and temperature is low. But at high pressure or low temperature when the total volume of gas is small, the forces of attraction become appreciable and cannot be ignored.