Figure 25. Partial Phase Diagram for Water (distorted somewhat to distinguish
the triple point from the freezing point)TIP
The triple point is the only temperature and pressure at which all three phases of a substance
can exist.At 0°C the freezing point of water is found along the line BC at point E for
pressure at 1 atmosphere or 760 millimeters. Again, this point is affected by
pressure along the line BC so that, if pressure is decreased, the freezing point is
slightly higher up to point B or 0.01°C.
Point B represents the point at which the solid, liquid, and vapor phases may
all exist at equilibrium. This point is known as the triple point. It is the only
temperature and pressure at which three phases of a pure substance can exist in
equilibrium with one another in a system containing only the pure substance.
WATER
Water is so often involved in chemistry that it is important to have a rather
complete understanding of this compound and its properties. Pure water has
become a matter of national concern. Although commercial methods of
purification will not be discussed here, the usual laboratory method of obtaining
pure water, distillation, will be covered.