Solubility

For most solutes, the higher the temperature of the solvent, the faster its rate of dissolving and the greater its solubility.

When making iced tea in summertime, it is best to dissolve the sugar in the hot tea before adding the ice cubes and refrigerating. Trying to dissolve sugar in a mixture of tea and ice is a much slower process and will often result in a build up of sugar at the bottom of your glass.

Figure 1 shows that the solubility of sugar and the three other compounds listed increases with rising temperature. Most solid compounds show the same behavior. One theory to explain this observation suggests that hot solvent particles, which move faster than cold ones, are on average more spread out. This creates larger spaces and increases the amount of solute that can fit into the solvent.

Bases, however, are less soluble in hot water than in cold. The solubility of carbon dioxide gas in soda pop actually decreases as temperature is increased. An open bottle of pop taken from a refrigerator soon loses its fizz if stored in a warm environment. As the pop warms up the carbon dioxide gas dissolved in it becomes less soluble.

You may have noticed the same thing happening when heating a pot of water on a kitchen stove. Tap water contains dissolved air and when heated, small bubbles form, rise to the surface and leave. This reduced solubility of air is one cause of thermal pollution. Industries often use lake water as a coolant for their machinery. Before the hot water can be returned to the lake it must be allowed to cool down; otherwise it can be harmful to some fish because warm water holds less dissolved air and therefore less oxygen.