1. SECOND LAW OF THERMODYNAMICS

2. Clausius statement
Heat can flow spontaneously from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.
A refrigerator requires work

3. Kelvin-Planck statement
No device is possible whose sole effect is to transform a given amount of heat completely into work.
No engine can be 100% efficient

4. general statement
The total entropy of any system plus that of its environment increases as a result of any natural process.
The total entropy of an isolated system that undergoes a change cannot decrease.