Which mixture involves two substances that do not normally mix and must be shaken before use?

An emulsion involves two immiscible substances, typically oil and water, that do not naturally blend together. In order to create a uniform mixture, emulsifiers are often added, which help stabilize the mixture, but shaking or agitation is usually required to temporarily disperse one substance into another before it is used. This is due to the nature of emulsions, where droplets of one liquid are dispersed in another, leading to the necessity of mixing to prevent separation.

Suspensions also consist of particles that are not fully dissolved in a liquid, but they differ in that they typically involve solid particles dispersed throughout a liquid, requiring shaking to ensure an even distribution. Solutions, on the other hand, are homogeneous mixtures where the solute is fully dissolved in the solvent, and no separation occurs, making shaking unnecessary. Ointments are semi-solid preparations that incorporate oils and fats, but they generally do not require shaking to achieve a consistent application compared to emulsions.