Factors Affecting Solubility

The solubility of a solid or a liquid solute in a solvent is affected by the temperature, while the solubility of a gaseous solute is affected by both the temperature and the pressure of the gas. We will examine the effects of temperature and pressure separately.

Notice how the temperature dependence of NaCl is fairly flat, meaning that an increase in temperature has relatively little effect on the solubility of NaCl. The curve for KNO3, on the other hand, is very steep; an increase in temperature dramatically increases the solubility of KNO3.

Several substances listed on the graph - HCl, NH3, and SO2 - have solubilities that decrease as the temperature increases. These substances are all gases over the indicated temperature range when at standard pressure. When a solvent with a gas dissolved in it is heated, the kinetic energy of both the solvent and solute increases. 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 back to the gas phase. As a result, the solubility of a gas decreases as the temperature increases. This has some profound environmental consequences. Industrial plants situated near bodies of water often use that water as a coolant, returning the warmer water back to the lake or river. This increases the overall temperature of the water, which lowers the quantity of dissolved oxygen, affecting the survival of fish and other organisms.

Solubility curves can be used to determine if a given solution is saturated or unsaturated. Suppose that 80g of KNO3 is added to 100g of water at 30 ^oC. According to the solubility curve, approximately 48g of KNO3 will dissolve at 30oC. This means that the solution will be saturated, since 48g is less than 80g. We can also determine that there will be 80−48=32g of undissolved KNO3 remaining at the bottom of the container. Now, suppose that this saturated solution is heated to 60oC. According to the curve, the solubility of KNO3 at 60 ^oC is about 107 g the solution is now unsaturated, since it still contains only the original 80g of solute, all of which is now dissolved. Then, suppose the solution is cooled all the way down to 0 ^oC. The solubility at 0 ^oC is about 14g, meaning that 80−14=66g of the KNO3 will recrystallize.

Some solutes, such as sodium acetate, do not recrystallize easily. Suppose an exactly saturated solution of sodium acetate is prepared at 50 ^oC. As it cools back to room temperature, crystals do not immediately 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 is 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.

molecules come into contact with the surface of the water and dissolve into the liquid. Now suppose that more CO2 is added to the space above the container, causing a pressure increase. More CO2 molecules are now in contact with the water, so more of them dissolve. Thus the solubility increases as the pressure increases. As with a solid, the CO2 that is undissolved reaches an equilibrium with the dissolved CO2 , represented by the following equation.

At equilibrium, the rate of gaseous CO2 dissolving is equal to the rate of dissolved CO2 coming out of the solution.

When carbonated beverages are packaged, they are done so under high CO2 pressure so that a large amount of carbon dioxide dissolves in the liquid. When the bottle is opened, equilibrium is disrupted because the CO2 pressure above the liquid decreases. Immediately, bubbles of CO2 rapidly exit the solution and escape out of the top of the open bottle. The amount of dissolved CO2 decreases. If the bottle is left open for an extended period of time, the beverage becomes "flat" as more and more CO2 comes out of the liquid.

The relationship of gas solubility to pressure is described by Henry's law, named after English chemist William Henry (1774 - 1836). Henry's law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.