what is an ideal gas? what are the causes of deviation of the real gases from ideal behaviour
Answers

The deviation from ideal behaviour is expressed by introducing a factor Z known as compressibility factor in the ideal gas equation. Z may be expressed as Z = PV / nRT
In case of ideal gas, PV = nRT ∴ Z = 1
In case of real gas, PV ≠ nRt ∴ Z ≠ 1
Thus in case of real gases Z can be < 1 or > 1
When Z < 1, it is a negative deviation. It shows that the gas is more compressible than expected from ideal behaviour.
When Z > 1, it is a positive deviation. It shows that the gas is less compressible than expected from ideal behaviour.

Causes of Deviation from Ideal Behaviour
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 in compressible 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.
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Answer:
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 deviation from ideal behaviour is expressed by introducing a factor Z known as compressibility factor in the ideal gas equation. Z may be expressed as Z = PV / nRT
In case of ideal gas, PV = nRT ∴ Z = 1
In case of real gas, PV ≠ nRt ∴ Z ≠ 1
Thus in case of real gases Z can be < 1 or > 1
When Z < 1, it is a negative deviation. It shows that the gas is more compressible than expected from ideal behaviour.
When Z > 1, it is a positive deviation. It shows that the gas is less compressible than expected from ideal behaviour.

Causes of Deviation from Ideal Behaviour
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 in compressible 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.
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