Solubility

Solubility is the measure of how well a solute can dissolve in a solvent to form a solution. It dictates how substances dissolve and interact, impacting fields like pharmaceuticals, environmental science, cooking, and chemical reactions. Understanding solubility allows for better control and optimisation of processes in these areas.

Solutes, solvents and solutions

A solute is a substance that is dissolved to form a solution. A solvent is the substance that the solute dissolves in. Solutions are therefore the resulting mixture. The formation of a mixture involves attractive intermolecular forces between solutes and solvent molecules, which allow them to mix together.

In a solution, the solvent can be liquid water or something else such as alcohol-based solvents, acetone and ethyl ether. When liquid water is the solvent, the solution is an aqueous solution. If the solvent is not liquid water, we call it a non-aqueous solution.

To prepare a solution, a known mass of solute (e.g. \(5.85\textrm{ g}\) of \(\ce{NaCl}\)) is dissolved in a solvent, like water, to give a known volume of solution (e.g. \(250.0\textrm{ mL}\) aqueous solution). If the solid solute readily dissolves in the solvent, it is soluble. If it does not readily dissolve and remains as a solid, it is considered insoluble.

Saturation

The level of saturation describes the extent to which a solution contains dissolved solute relative to its maximum solubility at a given temperature and pressure. There are three ways to classify the level of saturation:

• Unsaturated solutions contain less than the maximum amount of solute that can be dissolved under given conditions.
• Saturated solutions contain the maximum amount of solute that can be dissolved under the given conditions.
• Supersaturated solutions contain more dissolved solute molecules than a saturated solution. They exist in an unstable, temporary condition. A supersaturated solution always tries to revert to a saturated state with time, through the formation of crystals.

The solubility rule

The solubility of a solute depends on whether it can form attractive forces with the solvent molecules, which in turn, depends on the types of forces it can form.

• Polar solutes dissolve in polar solvents. Polar substances can interact via electrostatic forces, due to the presence of partial or complete charges.
• Non-polar solutes dissolve in non-polar solvents. Non-polar substances are unlikely to form electrostatic forces. Instead, they interact with each other via dispersion forces.

Thus, when you are predicting the solubility of a solute in a solvent, you should check whether the solute is an ionic or molecular compound, and whether the solvent liquid contains polar or non-polar molecules. This concept is summarised by the solubility rule:

"Like dissolves like."

Factors affecting solubility

As well as the polarity of the solute and solvent, there are other external factors that affect solubility. One key example is temperature. Many solids show increased solubility in water at higher temperatures. In contrast, gases generally have a lower solubility in water with increased temperature.

Worked example – predicting solubility

Compare the solubility of hexane and glucose in water.

Step 1: Determine whether the solutes are polar or non-polar.

Hexane, \(\ce{C6H14}\), is a non-polar solute. Glucose, \(\ce{C6H12O6}\), is a polar solute due to the presence of hydroxyl (\(\ce{OH}\)) groups.

Step 2: Determine whether the solvent is polar or non-polar.

Water, \(\ce{H2O}\), is polar.

Step 3: Recall the solubility rule and apply it to the solutes and solvent in the question.

"Like dissolves like", therefore the polar water solvent will only dissolve polar solutes. This means that hexane is not soluble in water, but glucose is.