14.2 Superheating and Supercooling

At any temperature and pressure, the lower value of ? at the corresponding point in Figure 14.1 gives the chemical potential of the stable phase. However, physical meaning can still be attached to the higher ? (that is, ? ₁ when T< T _t, ? ₂ when T> T _t), even though this corresponds to the phase that is not stable at that temperature. Because there is usually an energy barrier that prevents a transition occurring from the higher ? to the lower ? phase, the higher ? phase may be metastable (see Figure 5.5). For example, if one takes a gas at a given pressure and cools it below T _t, it does not immediately condense, since surface energy makes the formation of very small drops of liquid energetically unfavorable (this is made more quantitative in Example 14.2). Under such conditions, the gas phase is metastable and may survive for a long period if not disturbed; a phenomenon known as supercooling. Similarly, if a liquid is heated above T _t it does not immediately vaporize, and the metastable liquid is said to be superheated. Once bubbles of gas form in a superheated liquid, they may expand explosively, as every cook knows.

A familiar example of supercooling is the sodium acetate hand warmer. When a hot concentrated solution of sodium acetate in water is cooled to room temperature, the solution is...