• The physical state of a material is described by three variables, including temperature (T), pressure (P) and density ()....
• These are called state variables....

• The variables are related by an equation of state....

For an Ideal Gas the equation of state is

• k is a constant (just a number), called the Boltzmann Constant....
• As the temperature, pressure and density of a substance changes, its physical changes as well....
• A phase change (e.g., gas to a liquid, liquid to a solid) may also occur...