ABSTRACT
There are many physical systems in which the quantity of matter is not fixed; this chapter shows how such systems are treated in thermodynamics by giving a very brief introduction to the concept of the chemical potential. As an example of such a system, we could imagine a block of ice floating in water; as the ice melts, its mass decreases because there is a transfer of H2O molecules across the boundary dividing the ice from the liquid water. Alternatively we may consider a chamber containing a small hole through which we allow gas to enter from the surroundings; the gas in the chamber is then a system of variable mass.
