Mixtures of two or more kinds of molecules, having a tendency to separate when left standing, are known as:

• A. Solutions
• B. Ointments
• C. Emulsions
• D. Suspensions

Answer: (D)

Suspensions are mixtures in which undissolved particles are dispersed in a liquid and will settle to the bottom if left standing. Because the particles aren’t dissolved, gravity pulls them down over time, so you often see a sediment forming after a while and you need to shake or stir to redisperse them. This is different from solutions, where the solute fully dissolves in the solvent and there’s no visible separation, and from emulsions, where two immiscible liquids are mixed together with an emulsifier to form droplets that can still separate into layers if not stabilized. Ointments are a semisolid form that can be based on either emulsions or suspensions, but the defining trait of a suspension is that the dispersed particles will settle out when the mixture stands still.