Explanation
At high pressure gas deviates from its ideal behaviour.
The negative slope is because of change of phase. This happens to liquids which contract on melting.
This is because a gas can be heated under different conditions of pressure and volume. The amount of heat required to raise the temperature of unit mass through unit degree is different under different conditions of heating.
A vapour above the critical temperature is a gas and gas below the critical temperature for the substance is a vapour. As gas cannot be liquified by the application of pressure alone, how so ever large the pressure may be while vapour can be liquified under pressure alone. To liquify a gas it must be cooled upto or below its critical temperature.
Helium is a monoatomic gas, while oxygen is diatomic. Therefore, the heat given to helium will be totally used up in increasing the translational kinetic energy of its molecules; whereas the heat given to oxygen will be used up in increasing the translational kinetic energy of the molecule and also in increasing the kinetic energy of rotation and vibration. Hence there will be a greater rise in the temperature of helium.
Only the energy of translatory motion of molecules is represented by temperature. Others forms of energy such as intermolecular potential energy, energy of molecular relation, etc. are not. represented by temperature. Hence at absolute zero, the translatory motion of molecules ceases but other forms of molecular energy do not become zero. Therefore absolute zero temperature is not the temperature of zero-energy. At absolute zero molecular motion ceases.
In real gas, intermolecular force exist. Work has to be done in changing the distance between the molecules. Therefore, internal energy of real gas is sum of internal kinetic and internal potential energy which are function of temperature and volume respectively. Also change in internal energy of a system depends only on initial and final states of the system.
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