A solution is a homogenous mixture of two components. The major component is called the solvent, while the minor component is called the solute.

Depending on the physical state of the solvent, a solution can be a solid, such as an alloy like brass, gaseous, such as air, and liquid, such as a saline solution.

Liquid solutions can also have gases, or other liquids, mixed in a liquid solvent. If the solvent is water, the solution is called aqueous. 

A solute dissolved in a solvent other than water, such as iodine in carbon tetrachloride, makes a non-aqueous solution.

One solvent cannot dissolve all solutes. Salt will dissolve in water, but oil will separate. Thus, salt is said to be soluble, while oil is insoluble in water.

Solubility is the maximum amount of solute that will dissolve in a given amount of solvent at a certain temperature.

Solubility depends both on the intermolecular forces between the solute and solvent molecules and on the tendency to mix, which is driven by an increase in entropy of the system.

Entropy is a thermodynamic measurement of energy dispersal or disorder. A process occurs spontaneously when there is an increase in total entropy.

Consider two gases separated by a barrier. If the barrier is removed, the two gases mix spontaneously into a single, homogeneous solution.

At low pressure and moderate temperature, there are no significant intermolecular forces between their constituent molecules. So, the gases behave as ideal gases. 

Here, forming a solution does not lower the potential energy of the atoms, but their kinetic energy can now be distributed over a larger volume. 

This dispersal of energy increases the entropy of each of the gases making solution formation a spontaneous process.