ABSTRACT
Ice, water and steam are the three phases of the substance we generally term water. We are also familiar with the fact that two of the phases can coexist in equilibrium with each other: a beaker at 0°C and at atmospheric pressure can contain ice floating in water with the mass of the ice remaining constant. In fact, at one particular temperature and pressure, all three phases may exist together. Strictly, a phase consists of a homogeneous region of the substance having definite boundaries; this is certainly so for our ice-in-water example. It is known that, because of the indistinguishability of atomic particles, some particles can be either bosons or fermions. There is no restriction on the number of bosons that can occupy a single quantum state; however, only one fermion can occupy each quantum state. Atomic particles have the property of spin which is a measure of their intrinsic angular momentum.
