What is the primary characteristic of a product that states to shake well before use?

A product that instructs users to shake well before use is primarily a suspension. Suspensions are mixtures in which solid particles are dispersed throughout a liquid but are not dissolved. These particles can settle at the bottom of the container over time, so shaking the product helps evenly distribute the solid particles throughout the liquid before application. This ensures that the user receives the correct formulation and benefits from the active ingredients effectively.

In contrast, a solution involves solutes that are fully dissolved in the solvent and do not require shaking, as they remain homogeneous. An emulsion is a mix of two immiscible liquids where one is dispersed in the other, typically requiring emulsifying agents to maintain stability, and often does not necessitate shaking before use. Gel products are typically stable and do not separate or require agitation, making shaking unnecessary.