All solutions are made up of two elements – a solute and a solvent. The solute is solid matter, liquid or gas, and the solvent is liquid into which the solute is mixed. Solubility is, to put it very simply, the amount of a particular solute that will dissolve in a particular solvent. As a general rule of thumb there are three main factors that affect solubility - temperature, pressure, and the nature of the solute and solvent. Not all three factors affect all solutes in all circumstances.
Changes in pressure have a significant impact on the solubility of a gaseous solute; an increase in pressure means that the gas molecules are forced into the solution to relieve the pressure that has been applied. This can be demonstrated easily by using a carbonated drink as an example. Carbonated drinks are bottled under pressure which increases the amount of carbon dioxide which is dissolved in the liquid. When the bottle is opened the pressure in the bottle decreases to that of the atmosphere and the solution ‘effervesces.’ The fizzing and bubbling that occurs is the number of gas molecules – in this case carbon dioxide – in the solution reducing.
Although temperature does not determine whether a solute is soluble or not, the effect of differing temperatures will directly affect solubility. If a solution absorbs energy, then increasing the temperature will increase the solubility. For example, sugar will dissolve more rapidly in warm water than it will in cold. If, on the other hand, a solution dispenses energy, then increasing the temperature will decrease solubility; the solubility of oxygen in water will decrease if the temperature is raised. Fish do not thrive in very warm water because it does not contain enough oxygen to sustain them.
Substances that have a similar makeup are soluble in one another. This makeup, or ‘polarity’ describes how attracted particles of a solvent are to particles in a solute. Particles of a solvent are able to separate the particles of a solute that is similar and take up residence in the resulting spaces between those particles, resulting in the solute becoming soluble. Particles are either 'polar' or 'non polar.' Polar particles are attracted to other polar particles and vice versa. To break it down into the simplest terms, “like dissolves like.”
On a more general level larger molecules tend to be less soluble than smaller ones because they weigh more, and they have a greater surface area for the solvent to break down. While stirring does not have a direct effect on solubility it will increase the speed at which a solute will break down.
The above rules apply to all solutions, with few exceptions.