Solutions, Acids, Bases and Gas Laws

The Chemistry of Solutions and Factors Affecting Solubility

A solution is a homogeneous mixture composed of two or more substances thoroughly blended at a molecular level. Within every solution, we classify components into solutes and solvents. The solute is the substance that dissolves, whereas the solvent is the substance in which the solute dissolves. The solvent is typically the component present in the greatest amount. When water serves as the solvent, the resulting mixture is specifically termed an aqueous solution. Water is widely considered the universal solvent because of its high polarity and its exceptional ability to form hydrogen bonds, enabling it to dissolve an extensive variety of substances.

The ability of a solute to dissolve in a solvent is governed by molecular compatibility, described by the principle that 'like dissolves like.' Under this rule, polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. Polar substances are generally more likely to dissolve in water than nonpolar substances, but polarity alone does not guarantee solubility. A solution’s saturation state determines how much additional solute can dissolve, but it does not determine the solute’s equilibrium solubility under the given conditions.

A solution can exist in an unsaturated, saturated, or supersaturated state. An unsaturated solution contains less dissolved solute than the maximum amount capable of dissolving at that specific temperature. Additional solute will dissolve until the solution becomes saturated under the given conditions. A saturated solution contains the maximum amount of dissolved solute possible at a specified temperature and pressure. Finally, an unstable condition can be produced to create a supersaturated solution, which contains more dissolved solute than its equilibrium solubility at a specified temperature.

Temperature and pressure directly control solubility behavior, but they affect solid and gas solutes in liquid solvents very differently. The solubility of most solid solutes in liquid solvents increases as temperature increases. For instance, more sugar or solid salt can typically dissolve in heated water than in cold water. In stark contrast, the solubility of gases in liquid solvents decreases as temperature increases. Furthermore, pressure plays a vital role for gases: the solubility of gases in liquids increases as the partial pressure of the gas above the liquid increases.