To make gasses more soluble in water, we could think about trying to decrease the magnitude of their negative entropies of solution. One way to accomplish that is to make the gas above the solvent more ordered by increasing the pressure of the gas. In fact, William Henry discovered this property of gaseous solutes around the turn of the nineteenth century. Because he discovered that behavior first, the law that describes the increase in the solubility of a gas with increasing pressure is called Henry's law. Henry's law is given below in , C stands for the concentration of the dissolved gas and P represents the partial pressure of the gas above the solution. The units on the constant, k, are adjusted to suit the pressure and concentration units used.
Normally, solutes become more soluble in a given solvent at higher temperatures. One way to predict that trend is to use Le Chatelier's principle. Because ΔHsoln is positive for most solutions, the solution formation reaction is usually endothermic. Therefore, when the temperature is increased, the solubility of the solute should also increase. However, there are solutes that do not follow the normal trend of increasing solubility with increasing temperature. One class of solutes that becomes less soluble with increasing temperature is the gasses. Nearly every gas becomes less soluble with increasing temperature.
Another property of gaseous solutes in summarized by Henry's law (see ) which predicts that gasses become more soluble when their pressures above a liquid solution are increased. That property of gaseous solutes can be rationalized by using Le Chatelier's principle. Imagine that you have a glass of water inside of a sealed container filler with nitrogen gas. If the size of that container were suddenly halved, the pressure of nitrogen would suddenly double. To decrease the pressure of nitrogen above the solution (as is required by Le Chatelier's principle), more nitrogen gas becomes dissolved in the glass of water.