The third law of thermodynamics states that there is an abslute entropy scale. The zero of entropy is at absolute zero of temperature on the absolute scale. However, a substance can only have zero entropy if it is a perfrect crystal. Any imperfections lead to a residual entropy. In this section we discuss how to define a perfect crystal and how to use the absolute entropy. The experimental test for these ideas depends on our ability to conduct experiments at low temperatures approaching T = 0 K. However, as the temperature approaches zero the heat capacity also approaches zero.
