Colloids - Practice Questions & MCQ

Between the two extremes of suspensions and solutions we come across a large group of systems called colloidal dispersions or simply colloids. A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium.
The essential difference between a solution and a colloid is that of particle size. While in a solution, the constituent particles are ions or small molecules, in a colloid, the dispersed phase may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but small enough to remain suspended. Their range of diameters is between 1 and 1000 nm(10^–9 to 10^–6 m). Colloidal particles have an enormous surface area per unit mass as a result of their small size.

Depending upon the nature of interaction between the dispersed phase and the dispersion medium, colloidal sols are divided into two categories, namely, lyophilic (solvent attracting) and lyophobic (solvent repelling).

• Lyophilic colloids: The colloidal systems in which the particle of dispersed phase have great affinity for the dispersion medium, are called Lyophilic (solvent-loving) Colloids. Some common examples of lyophilic colloids are gum, gelatin, starch, rubber, protein, etc.
• Lyophobic colloids: The colloidal systems in which the particles of the dispersed phase have no affinity for the dispersion medium are called Lyophobic (solvent hating) Colloids. Some examples of lyophobic colloids include sols of metals and their insoluble compounds like sulphides and oxides.

Depending upon the type of the particles of the dispersed phase, colloids are classified as: multimolecular, macromolecular and associated colloids.

• Multimolecular Colloids: The multimolecular colloidal particles consist of an aggregate of atoms or small molecules with diameters less than 10^-9 or 1 nm.
  For example: gold sol, sulphur sol
• Macromolecular Colloids: The macromolecular colloidal particles themselves are large molecules. They have high molecular weights varying from thousands to millions. Naturally occurring macromolecules are such as starch, cellulose and proteins. Artificial macromolecules are such as polyethylene, nylon, polystyrene, dacron, synthetic rubber, plastics etc.

There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (T_k) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10^–4 to 10^–3 mol L^–1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.

Cleaning action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. Since the polar group can interact with water, the stearate ions is now pulled in water and remove from first surface.