State and explain zeroth law of thermodynamics.

Statement: If two bodies A and B are in thermal equilibrium and also A and C are in thermal equilibrium then B and C are also in thermal equilibrium.

Explanation:

1. Consider two sections of a container separated by an adiabatic wall containing two different gases as system A and system B.

2. Systems A and B are independently brought in thermal equilibrium with a system C.

3. When the adiabatic wall separating systems A and B is removed, there will be no transfer of heat from system A to system B or vice versa.

4. This indicates that systems A and B are also in thermal equilibrium.

5. This means, if systems A and B are separately in thermal equilibrium with a system C, then A and B are also mutually in thermal equilibrium.