Understanding differences between solutions, emulsions, colloids and dispersions

Homogeneous solution of a solid in liquid

Analysis of samples in laboratories more than often requires pre-treatment steps for extraction, isolation, concentration or dilution to measurable concentration ranges. This is generally achieved by dissolving the sample in a suitable solvent. However, the solubility of a compound in a solvent depends on the affinity between the compound and the solvent. Depending on the degree of affinity the resulting mixtures are classified as solutions, suspensions, emulsions or colloids. The differences between these terms are often not clear to students and researchers so the objective of the article is to offer some clarity on these terms.

Solution

A solution results when there is complete miscibility between the solute and the solvent at a given temperature. The solubility of a solute is temperature dependent and increases with increasing temperature. At a given temperature if excess solute is added to a solvent then the excess amount fails to dissolve and settles down even after mechanical homogenization.

A solution is a clear homogeneous mixture and the solute particles are no longer visible nor do they settle down .The particles of solute are of molecular size dimensions and do not scatter a light beam passing through it. As the composition is homogeneous it is not possible to separate the solute from the solvent by filtration. The only option available is to resort to distillation which removes the solvent and leaves behind a solid residue of the solute.

Suspensions

Suspensions are obtained when insoluble solid particles are dispersed in a liquid medium. On shaking the solute particles can be evenly dispersed in the medium but if left undisturbed the solute particles which range from 0.5 to 5 μm tend to settle down and can be separated from the solution by filtration or centrifugation.

Colloids

A colloid results when particles ranging between  to  m are dispersed in the liquid solvent. A colloid is a homogeneous mixture and the solute does not settle out on standing. Colloids can be distinguished from solutions as they exhibit light scattering. Milk is an example of a colloid solution which consists of fat particles evenly distributed in water.

Emulsions

An emulsion like a colloid comprises of a dispersion of two or more immiscible liquids. Oil and water are immiscible and form emulsions when mixed and shaken. Minute oil droplets evenly disperse throughout the water. Due to the phenomenon of distortion of light emulsions generally appear cloudy. Dilute emulsions often appears bluish due to greater scatter of lower wavelengths of light. It is possible to disperse any of the liquids in the other such as oil and water or water in oil. The minor component does not separate from the major component even if left to stand undisturbed indefinitely. Some emulsions tend to form a film over another phase in the solution to form globules that repel one another allowing them to remain suspended or dispersed indefinitely.

The terms colloid and emulsion are often used synonymously but it should be kept in mind that emulsions result when immiscible liquids are mixed whereas in a colloid solution it can be a liquid or solid dispersion in another liquid. In other words, an emulsion can be termed as a colloid but all colloids are not emulsions.