Page 232 - Physical chemistry understanding our chemical world

QUANTITATIVE EFFECTS OF PRESSURE AND TEMPERATURE CHANGE 199

Aside

If water behaved in a similar fashion to most other materials and possessed a positive
value of V m , then water would spontaneously freeze when pressure was applied,
rather than solid ice melting under pressure. Furthermore, a positive value of V m
would instantly remove the problems discussed above, caused by vehicles travelling
over ‘black’ ice, because the ice would remain solid under pressure; and remember that
the slipperiness occurs because liquid water forms on top of solid ice.
Unfortunately, a different problem would present itself if V m was positive! If V m
was positive, then Equation (5.1) shows that applying a pressure to liquid water would
◦
convert it to ice, even at temperatures slightly higher than 0 C, which provides a
different source of black ice.

Why does a pressure cooker work?

The Clausius–Clapeyron equation

A pressure cooker is a sealed saucepan in which food cooks faster than it does in a
simple saucepan – where ‘simple’, in this context means a saucepan that is open to
the air. A pressure cooker is heated on top of a cooker or hob in the conventional way
but, as the water inside it boils, the formation of steam rapidly causes the internal
pressure to increase within its sealed cavity; see Figure 5.11. The internal pressure
inside a good-quality pressure cooker can be as high 6 atm.
The phase diagram in Figure 5.12 highlights the pressure–temp-
Remember that all
erature behaviour of the boiling (gas–liquid) equilibrium. The equilibria are dynamic.
◦
normal boiling temperature T (boil) of water is 100 C, but T (boil)
increases at higher pressures and decreases if the pressure de-
creases. As a simple example, a glass of water would boil instantly at the cold
temperature of 3 K in the hard vacuum of deep space. The inset to Figure 5.12

Condensation

Vaporization

Heat
Figure 5.11 A pressure cooker enables food to cook fast because its internal pressure is high,
which elevates the temperature at which food cooks