Aerosols are colloidal dispersions of liquids or solids in gases. In general, mists and fogs possess liquid disperse phases whereas smoke is a dispersion of solid particles in gases. However, no sharp distinction can be made between the two kinds because liquid is often associated with the solid particles. A mist consists of fine droplets of liquid that may or may not contain dissolved or suspended material. If the concentration of droplets becomes high it may be called a fog.
Although all the disperse systems mentioned above are less stable than colloids that have a liquid as dispersion medium, they have many properties in common with the latter and can be investigated in the same way. Particle size is usually within the colloidal range, but if it is larger than 1 µm the life of an aerosol is short because the particles settle out too quickly.