'If a system A is in thermal equilibrium separately with B and C, then B and C are also in thermal equilibrium with each other.' This is 

Explanation:

There are 4 laws to thermodynamics:

Zeroth Law of thermodynamics: It states that, When a body ‘A’ is in thermal equilibrium with a body B, and also separately with a body C, then B and C will be in thermal equilibrium with each other. This is the basis of temperature measurement

• In other words, If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

Additional Information

First law of thermodynamics – Energy can neither be created nor destroyed. It can only change forms. In any process, the total energy of the universe remains the same.

For a thermodynamic cycle, the net heat supplied to the system equals the net work done by the system.

δQ = ΔU + δW

Second law of thermodynamics – The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

ΔS = ΔQ/T

⇒ ΔS⋅T = ΔQ

In differential form, dQ = Tds

ΔSTotal = ΔSSystem + ΔSsurrounding

The second law of thermodynamics introduces the concept of entropy.

Second Law of Thermodynamics:

1. Clausius statement: It is impossible for a self-acting machine to transfer heat from a colder body to a hotter one without the aid of an external agency
2. Kelvin-Planck’s statement: It is impossible to design an engine that extracts heat and fully utilizes into work without producing any other effect.

Third law of thermodynamics – As the temperature approaches absolute zero, the entropy of a system approaches a constant minimum.