Solubility, Vaporisation and Vaporisers

Abstract

In discussing humidity in the preceding chapter, the concept of equilibrium between water and its vapour has been introduced as a thermodynamic concept. The concept of vaporisation of other liquids such as volatile anaesthetic agents follows on naturally from that, but first of all it will be worth taking a detour through a discussion on solubility of gases and vapours in their own and other liquids [Davis 2003]. To maintain simplicity in the discussion on humidity, no mention was made of the presence of air or other gas above the surface of the water, only the water vapour. Depending on the solubility of the gas in the liquid, a variable amount of the gas dissolves in the liquid, whether that be air in water or carbon dioxide in blood. As will be discussed in the section on vaporisation, some molecules of gas enter the liquid and some leave it, depending on their individual kinetic energies, until equilibrium is reached. If the pressure inside the container with the gas or vapour and liquid is increased, then the partial pressure of the gas above the liquid surface increases; this increases the population density of gas molecules, resulting in more of the gas molecules dissolving in the liquid. Henry’s Law states that for a fixed temperature the solubility of a gas in a liquid is proportional to its partial pressure in equilibrium with the liquid. Note the condition of constant temperature because, in addition, solubility decreases with increased temperature. This occurs because an increase in the thermal energy of the dissolved gas molecules increases the partial pressure of the gas and encourages it to come out of solution (see below on vaporisation). Thus gas bubbles are more apparent in liquids that are heated. A historical clinical example of the relevance of ambient pressure and nitrogen solubility in body tissues is in decompression sickness associated with tunnel workers. Modern examples include underwater diving and, to a lesser extent, aviators and space walking astronauts. Nitrogen is a compressible gas and goes into solution in body tissue spaces under compression if the miner, tunnel worker, or diver is breathing ambient air.