Experiment 4 Solubility of a Salt OUTCOMES After completing this experiment, the student should be able to: . measure the temperature at which a solution becomes saturated. · construct a solubility curve for a solute. . determine the concentration of an unknown solution from its saturation temperature. DISCUSSION Solubility is defined as the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature. Typically, solubilities are recorded as grams of solute per 100 g of solvent. The solubility of a substance can be affected by a variety of factors including the identity of the solute and solvent, the temperature of the solution, and by the pressure exerted on the solution when the solute is a gas. Substances tend to be more soluble when the intermolecular forces holding them together are similar to the intermolecular forces present in the solvent. This trend leads to the axiom "like dissolves like." In general, ionic compounds are soluble in water due to the attraction between the charges of the ions and the dipole moment of the water but are insoluble in nonpolar solvents which do not have the permanent dipole moment. For many liquid and solid solutes, increasing the temperature of the solution increases the solubility of the solute, that is, more of the solute dissolves in the same amount of solvent. This is because the increased temperature leads to greater motion within the solute which makes it easier for the intermolecular forces holding it together to be overcome. With gaseous solutes, the opposite is true. Increasing the temperature and motion of the solute increases its ability to escape from the solvent, lowering its solubility. Instead, the solubility of a gas can be increased by raising the pressure exerted on the solution, essentially "pushing" more of the gaseous solute into the solution. Regardless of the identity of the solute and the temperature of the solution, it is not possible to dissolve an unlimited amount of solute into a given solution. When the maximum amount of solute has been dissolved at a particular temperature, the solution is said to be saturated and the addition of any more solute will simply precipitate out of the solution. The behavior of a particular solute can be represented graphically with a solubility curve which plots the solubility of the solute (y-axis) vs. temperature (x-axis). When a solution is at a temperature and concentration that fall on the curve, the solution is saturated. If the solution falls below 4.1
the curve, it has less than the maximum amount of solute required for saturation, so the solution is considered unsaturated. If the solution is above the curve, it is classified as supersaturated because it has more solute than required for saturation in the solution. In this experiment, you will construct a solubility curve for KNO3. This will be done by combining different amounts of the salt in similar amounts of water and heating the solutions until all of the solute completely dissolves. Once the