Equation of State

For any thermodynamic system, the state of matter is described by the physical variables pressure, volume, temperature, and total mass of the matter or number of moles.

These physical variables are called state variables, and an equation that relates these physical variables with each other is termed the equation of state.

For example, consider a kettle filled with water. The state of the water inside the kettle is defined by the initial volume, pressure, and temperature. If the water is heated, the volume occupied by the water increases with temperature.

The heated water, converted to steam, occupies a larger volume at the top of the kettle and exerts high pressure on the water. This excess pressure is released when steam leaks out of the spout.

The steam, the gaseous water, is described by new values of these state variables. 

The state of matter can be changed by changing one of the state variables, which causes a change in the other state variables.