6.3: The Effect of Temperature on Solubility of Solids, Liquids, and Gases
- Describe how temperature affects solubility of different types of solute.
Table salt (NaCl) readily dissolves in water. In most cases, only a certain maximum amount of solute can be dissolved in a given amount of solvent. This maximum amount is called the solubility of the solute. It is usually expressed in terms of the amount of solute that can dissolve in 100 g of the solvent at a given temperature. The solubilities vary widely. NaCl can dissolve up to 31.6 g per 100 g of H 2 O, while AgCl can dissolve only 0.00019 g per 100 g of H 2 O.
The solubility of the majority of solid substances increases as the temperature increases. However, the effect is difficult to predict and varies widely from one solute to another. Shown below Figure \(\PageIndex{1}\) is the solubility curve , a graph of the solubility vs. temperature.
Notice how the temperature dependence of \(\ce{NaCl}\) is fairly flat, meaning that an increase in temperature has relatively little effect on the solubility of \(\ce{NaCl}\). The curve for \(\ce{KNO_3}\), on the other hand, is very steep, and so an increase in temperature dramatically increases the solubility of \(\ce{KNO_3}\). The solubility of liquids also increases as temperature increases.
Several substances such as \(\ce{HCl}\), \(\ce{NH_3}\), and \(\ce{SO_2}\) have solubility that decreases as temperature increases. They are all gases at standard pressure. When a solvent with a gas dissolved in it is heated, the kinetic energy of both the solvent and solute increase. As the kinetic energy of the gaseous solute increases, its molecules have a greater tendency to escape the attraction of the solvent molecules and return to the gas phase. Therefore, the solubility of a gas decreases as the temperature increases.When the temperature of a river, lake, or stream is raised abnormally high, usually due to the discharge of hot water from some industrial process, the solubility of oxygen in the water is decreased. Decreased levels of dissolved oxygen may have serious consequences for the health of the water’s ecosystems and, in severe cases, can result in large-scale fish kills.
Solubility curves can be used to determine if a given solution is saturated or unsaturated . Suppose that \(80 \: \text{g}\) of \(\ce{KNO_3}\) is added to \(100 \: \text{g}\) of water at \(30° \text{C}\). According to the solubility curve in Figure \(\PageIndex{1}\), approximately \(48 \: \text{g}\) of \(\ce{KNO_3}\) will dissolve at \(30° \text{C}\). This means that the solution will be saturated since \(48 \: \text{g}\) is less than \(80 \: \text{g}\). We can also determine that there will be \(80 - 48 = 32 \: \text{g}\) of undissolved \(\ce{KNO_3}\) remaining at the bottom of the container. In a second scenario, suppose that this saturated solution is heated to \(60° \text{C}\). According to the curve, the solubility of \(\ce{KNO_3}\) at \(60° \text{C}\) is about \(107 \: \text{g}\). The solution, in this case, is unsaturated since it contains only the original \(80 \: \text{g}\) of dissolved solute. Suppose in a third case, that the solution is cooled all the way down to \(0° \text{C}\). The solubility at \(0° \text{C}\) is about \(14 \: \text{g}\), meaning that \(80 - 14 = 66 \: \text{g}\) of the \(\ce{KNO_3}\) will recrystallize.
Supersaturated Solutions
Some solutes, such as sodium acetate, do not recrystallize easily. Suppose an exactly saturated solution of sodium acetate is prepared at \(50° \text{C}\). As it cools back to room temperature, no crystals appear in the solution, even though the solubility of sodium acetate is lower at room temperature. A supersaturated solution is a solution that contains more than the maximum amount of solute that is capable of being dissolved at a given temperature. The recrystallization of the excess dissolved solute in a supersaturated solution can be initiated by the addition of a tiny crystal of solute, called a seed crystal. The seed crystal provides a nucleation site on which the excess dissolved crystals can begin to grow. Recrystallization from a supersaturated solution is typically very fast.
Recrystallization of excess solute from a supersaturated solution usually gives off energy as heat. Commercial heat packs containing supersaturated sodium acetate (NaC 2 H 3 O 2 ) take advantage of this phenomenon. You can probably find them at your local drugstore.
Video \(\PageIndex{1}\): Watered-down sodium acetate trihydrate. Needle crystal is truly wonderful structures