Solubility of substances

Solubility of substances

The solubility of most substances depends strongly on the temperature and, in the case of gases, on the pressure. The solubility of most solid or liquid solutes increases with increasing temperature. The components of a mixture can often be separated using fractional crystallization, which separates compounds according to their solubilities. The solubility of a gas decreases with increasing temperature. Henry’s law describes the relationship between the pressure and the solubility of a gas.

The solubility of gases in liquids decreases with increasing temperature. Attractive inter-molecular interactions in the gas phase are essentially zero for most substances. When a gas dissolves, it does so because its molecules interact with solvent molecules. Because heat is released when these new attractive interactions form, dissolving most gases in liquids is an exothermic process ( ΔH soln<0 ). Conversely, adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas.

The relationship between pressure and the solubility of a gas is described quantitatively by Henry’s law, which is named for its discoverer, the English physician and chemist, William Henry (1775–1836):                C=kP

where,
C  is the concentration of dissolved gas at equilibrium,
P  is the partial pressure of the gas, and
k  is the Henry’s law constant, which must be determined experimentally for each combination of gas, solvent, and temperature.

Gases that react chemically with water, such as  HCl  and the other hydrogen halides,  H2S , and  NH3 , do not obey Henry’s law.

Human red blood cells contain a protein called hemoglobin that specifically binds  O2  and facilitates its transport from the lungs to the tissues, where it is used to oxidize food molecules to provide energy. The concentration of hemoglobin in normal blood is about 2.2 mM, and each hemoglobin molecule can bind four  O2  molecules. Although the concentration of dissolved  O2  in blood serum at 37°C (normal body temperature) is only 0.010 mM, the total dissolved  O2  concentration is 8.8 mM, almost a thousand times greater than would be possible without hemoglobin.