Liquid liquid mixing and solid liquid mixing

Two non-solids phases can come into contact or “mix” in one of two ways: by the gradual dissolution of one phase into the other (liquid–liquid mixing), or by the combination of both phases at once to give a mixed, heterogeneous phase (solid–liquid mixing). The latter process is called “gelation”.

Liquid–liquid mixtures usually involve two components that are miscible. That is, they mix homogeneously on standing. Examples include water and ethanol/water mixtures, as well as blood/air mixtures. When one component has limited solubility in the other then partial pre-mixing may occur.

A classic example occurs when oil is added to water. As the amount of oil is increased, a point is reached when the mixture will not mix (“emulsify”). At this point, tiny droplets of oil are dispersed throughout the water in a stable manner by forming what are known as Pickering emulsions.

Solid–liquid mixtures or heterogeneous mixtures consist of two components that do not mix in any proportion, but form a uniform phase whose properties differ from those of either component separately. A familiar example is adding granulated sugar to water; each component forms its own separate phase without mixing with the other. The resulting solution has different properties from pure water or pure sugar solutions. Another example is making porridge: it can be done by heating an appropriate quantity of water with the cereal. The solid particles remain separate when stirred, but gradually form a viscous mixture as heating continues. Under certain conditions, porridge can be made without stirring at all by slowly adding hot water to the dry cereal then letting it stand for a few minutes, repeating until the porridge is ready.

Another example is mixing powdered laundry detergent with water before use. As soon as enough powder is added to create a thin paste, bubbles of carbon dioxide appear immediately in large numbers throughout the solution and can be heard popping and crackling. These bubbles are caused by carbon dioxide gas that forms inside microscopic cavities on the surface of each solid particle and escapes into solution after sufficient pressure has built up, leading to sudden collapse of the bubbles.