What type of mixture has particles that will settle over time, such as mud in water?

Mixtures can be classified by the size of their dispersed particles and their behaviour over time. A suspension is a heterogeneous mixture in which the dispersed particles are large enough (typically greater than 1 micrometre) to be seen with the naked eye and to settle out under the influence of gravity when left undisturbed. Mud in water is a classic example: the soil particles are temporarily dispersed when stirred but gradually sink to the bottom.

A solution is a homogeneous mixture in which the solute particles are at the molecular or ionic level (less than 1 nanometre). They do not scatter light and never settle - for example, salt dissolved in water. A colloid has particles between 1 nanometre and 1 micrometre that remain permanently dispersed due to Brownian motion and do not settle - for example, milk. An emulsion is a specific type of colloid consisting of two immiscible liquids, such as oil in water, stabilised by an emulsifier.

Since the question describes particles that settle over time, the correct classification is a suspension.