In chemistry, a suspension is a heterogenous flui containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometre. The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. Unlike colloids, suspensions will eventually settle. An example of a suspension would be sand in water. The suspended particles are visible under a microscope and will settle over time if left undisturbed. This distinguishes a suspension from a colloid in which the suspended particles are smaller and do not settle. In a solution, the dissolved substance does not exist as a solid and the two are homogeneously mixed.