Solubility

Solubility

The solubility of a substance is the amount of that substance that will dissolve in a given amount of solvent. Solubility is a quantitative term. Solubilities very enormously (recall Table 8.2). The terms soluble and insoluble are relative. A substance is said to be soluble if more than 0.1 g of that substance dissolves in 100 mL solvent. If less than 0.1 g dissolves in 100 mL solvent, the substance is said to be insoluble or, more exactly, sparingly soluble. The terms miscible and immiscible may be encountered when considering the solubility of one liquid in another. Miscible means soluble without limits; for example, alcohol is miscible with water Immiscible and insoluble mean the same; oil is immiscible with water, as in oil and vinegar salad dressing (see Figure 11.1).

FIGURE 11.1 Soluble and insoluble. Alcohol is soluble in water; when added to water, it forms a clear solution. Oil is insoluble in water; when added to water, the two liquids form separate layers. |

A. Determining Solubility

How is the solubility of a substance determined? A known amount of the solvent--for example, 100 mL--is put in a container. Then the substance whose solubility is to be determined is added until, even after vigorous and prolonged stirring, some of that substance does not dissolve. Such a solution is said to be saturated because it contains as much solute as possible at that temperature. In this saturated solution, the amount of solute is the solubility of that substance at that temperature in that solvent. Doing this experiment with water as the solvent and sodium chloride as the solute, we find that, at 20°C, 35.7 g of the salt dissolve in 100 mL water. The solubility of sodium chloride is, then, 35.7 g/100 mL water at 20°C. Sodium chloride is a moderately soluble salt. The solubility of sodium nitrate is 92.1 g/100 mL water at 20°C; sodium nitrate is a very soluble salt. At the opposite end of the scale is barium sulfate, which has a solubility of...