Chapter 3 Phase Changes

Figure 3.1 - A Real Gas (with lines of isotherms)

In the liquid-vapour equilibrium region volume reduces (increases) sharply at constant temperature and pressure as the vapour condenses into a liquid (or the liquid evaporates). Although the temperature is constant there is a release (absorption) of heat

D Q=Lm (3.1)

However we notice that

the temperature of the boiling point depends on the pressure
at the point where the pressure is greater than the critical pressure, cooling at constant pressure shows no clear phase change. This implies there is no clear distinction between gas and liquid. Thus at the critical point L=0 . When the pressure is less than the critical pressure, L depends on the pressure

Explaining figure 3.1 to include the solid phase, an isotherm again shows a sharp drop (rise) in volume as the liquid solidifies (as solid melts).

Here the latent heat of fusion is released (absorbed).

Figure 3.2 - p-T Diagram of Phase Regions

At the triple point the three phases co-exist, however there is no triple point value for V_m so in figure 3.1 the triple point appears as a line at constant pressure (where p=p_tr ).

At p<p_tr no liquid phase exists

	T_C(K)	P_C(atm)	T_tr	P_tr(atm)
CO₂	304	74	217	5
H₂O	647	221	273	0.006

At room temperature and one atmosphere, CO₂ is a gas. If it is cooled at constant pressure it sublimes into a solid (dry ice). H₂O expands on freezing (hence the backwards slope in figure 3.2). At room temperature and one atmosphere, H₂O is a liquid. Heating at constant pressure will cause it to evaporate, cooling at constant pressure will cause it to freeze.