Whether a heterogeneous mixture is a colloid or a suspension depends on the size of the particles and their ability to settle out of the mixture under the influence of gravity.

Colloids

* Particles are between 1 and 1000 nanometers in size.

* Particles are small enough to remain suspended in the mixture due to Brownian motion.

* Mixtures appear homogeneous to the naked eye.

Suspensions

* Particles are greater than 1000 nanometers in size.

* Particles are too large to remain suspended in the mixture due to Brownian motion and will eventually settle out under the influence of gravity.

* Mixtures appear heterogeneous to the naked eye.

In addition to particle size, the concentration of the particles in the mixture can also affect whether it is a colloid or a suspension. At higher concentrations, colloids can become suspensions and suspensions can become gels.