Colloid is a heterogeneous mixture in which particle size is intermediate of true solution and suspension. Smoke from a fire is example of colloidal system in which tiny particles of solid float in air. Some common examples of colloids are gem stones, smoke, cheese, milk, soap lather and foam.
Brief History of Colloids
In 1861, T. Graham, while studying diffusion classified substances into two types: Crystalloids and Colloids. According to him, the substances which diffuse readily through a parchment membrane are called Crystalloids. Example salts, sugars, acids and bases. Other substances which diffuse through parchment paper very slowly are called colloids. Gelatin and glue are examples of Colloid. But this distinction was not valid, as under different conditions of temperature and pressure the same substance may act as crystalloid or colloid. Example: Soap shows colloidal character in water but crystalloid character in alcohol. It has been realized that absolute distinction between Colloids and Crystalloids does not exist.
Colloidal system is made up of Dispersed phase and Dispersion medium. In colloidal system one substance is dispersed as very fine particles in another substance called dispersion medium. In above example, particles of smoke are dispersed in air.
Size of colloidal particles
Size of colloidal particles are in range of 1– 1000nm while size of true solution suspension is >1000nm and that of true solution is < 1nm. Thus size of colloids lies between that of true solution and suspension. The colloidal particles can’t be seen with naked eye. This is why, colloidal system appears as homogeneous mixtures, but in reality are heterogeneous mixtures.