Solubility and its curves

Keep adding sugar to a glass of water and stirring. At some point, no more dissolves; it just sits at the bottom. The water has reached its limit.

Solubility is that limit: the maximum amount of solute that can dissolve in a fixed amount of solvent (100 g or 100 mL) at a given temperature. A solution that has reached the limit is saturated: it cannot dissolve any more at that temperature.

"Why must we say the temperature?" Because the limit changes with temperature:

• A solid in a liquid: usually more dissolves when it is hotter.
• A gas in a liquid: usually less dissolves when it is hotter. (That is why a warm soda goes flat faster.)

A graph of solubility against temperature is a solubility curve. In Fig. 5.6, compound B dissolves much more than A at every temperature, and its solubility climbs much faster: about 241 g per 100 g of water at 40 °C, and 287 g at 60 °C.

Table 5.4 shows how different substances behave:

• Potassium nitrate rises from 21 g at 10 °C to 167 g at 80 °C.
• Sodium chloride (common salt) hardly changes: 36 g to 37 g.

Why learn this? Solubility is one of the main properties we use to separate substances, as you will see in the next lesson.