Plasma: The Fourth State of Matter

The plasma state is frequently referred to as the fourth state of matter in the sequence: solid, liquid, gas, and plasma. The statement implies that plasma is another phase. Each state is achieved by adding heat to the previous state. The first three states are the three common phases achieved via phase transitions. The statement that plasmas are the fourth state of matter is examined considering phase transitions. It is shown that the transition from gas to plasma is not a phase transition similar to the other phase transitions at which transitions the differential of the Gibbs free energy equals zero. Therefore, strictly speaking, plasmas are better not called the fourth state of matter.