In winter, the fog is a common experience in both urban and rural areas. Fog forms when tiny water droplets are suspended in air. So, fog is nothing but a type of a suspension. There are large numbers of substances which do not mix with each other.
There are some solids that do not dissolve in water or other liquid solvents and there are liquids that do not mix with each other. The mixing of such substances results into heterogeneous mixtures. Depending on the size of the particles suspended, or dispersed in the surrounding medium, heterogeneous mixtures can be divided into colloids and suspension.
Materials of smaller particle size, insoluble in a solvent but visible to naked eyes, form suspension. Unlike a colloid, which contains smaller particles ranging in size from 1 to 1000 nanometres, a suspension contains relatively larger particles. The size of particles in suspension is over 1000 nanometres.
When flour is added to water it does not dissolve but forms a slurry, which is a suspension. However, if less amount of water is added in the flour (200 g of flour and 100 mL of water) you get dough to make chapatti. Muddy water is an example of suspension. When a suspension is allowed to stand undisturbed, the dispersed particles settle down.
Suspensions are very useful in medical sciences. For example barium sulphate (whose solubility is very low when dispersed in water) is an opaque medium. It is used for diagnostic X-rays (barium meal test). Many medicines, which are insoluble in water, are given in the form of suspension, for example, pencillin and amoxycilin.