Vapour Pressure

How vapour pressure builds up

If an evacuated container is partly filled with a liquid, some of the liquid evaporates to fill the remaining space with vapour. At first, the vapour pressure (the pressure the escaping molecules exert on the container walls) keeps rising; eventually it levels off once an equilibrium is reached between the liquid and vapour phases. The vapour pressure at this point is the equilibrium (or saturated) vapour pressure. Since evaporation is temperature-dependent, a liquid's vapour pressure must always be quoted together with the temperature.

Boiling

When a liquid is heated in an open vessel, evaporation happens only at the surface — until the liquid's vapour pressure reaches the external (atmospheric) pressure, at which point vapourisation can occur throughout the bulk of the liquid, with vapour bubbles expanding freely into the surroundings. This condition — free vapourisation throughout the liquid — is called boiling, and the temperature at which it happens is the boiling temperature at that particular external pressure.

• At $1\,\text{atm}$, this is the normal boiling point.
• At $1\,\text{bar}$ (very slightly lower pressure), it is the standard boiling point, which is therefore slightly lower than the normal boiling point.

For water: normal boiling point $= 100\,^{\circ}\text{C}$ (373 K); standard boiling point $= 99.6\,^{\circ}\text{C}$ (372.6 K).

Fig. 5.14 shows vapour pressure vs temperature curves for several common liquids — diethyl ether, ethyl alcohol, carbon tetrachloride and water — each reaching the $760\,\text{mm Hg}$ line (normal boiling point) at its own characteristic temperature (diethyl ether at 308 K; ethyl alcohol at 349.7 K; carbon tetrachloride at 351.3 K; water at 373 K).

Altitude and pressure cookers

At high altitudes, atmospheric pressure is lower, so liquids there boil at a lower temperature than at sea level. Because water boils "too easily" (at too low a temperature) on hills, food cooks slowly — hence the pressure cooker, which raises the internal pressure and pushes the boiling point back up. The same idea, taken further, is used in hospital autoclaves, where surgical instruments are sterilised at a boiling point raised well above 100 $^{\circ}$C by weighting down the vent.

Heating in a closed vessel …