(a) Impure water contains soil particles, bacteria etc. These particles are negatively charged. Alum is added in water to remove them. Al3+ ions present in alum precipitates impurities present in water. Pure water is separated by decantation.
(b) Colloids:
These are not diffused from animal membrane.
Crystalloids:
These are diffused from animal membrane. But the classification of substance given by Grahm was not satisfactory completely, because a specific compound behaves as crystalloids in one solvent and also as colloid in other solvent.
Example:
The aqueous solution of NaCl is crystalloid while it behaves as colloid in benzene.
(c) The wavelength of blue colour is very less hence its scattering is very high. Due to this reason, sky appears as blue.