Earlier the term 'Colloids' was used for a category of substances. However, later, the term colloidal state of matter was preferred. Colloidal dispersions have been classified into different types depending upon the physical state of the dispersed phase and the dispersion medium or the nature of the colloidal particles. They are prepared in the industry or in the laboratory by a number of methods and then purified. Their property has also been studied in detail. Hardy and Schulze made a substantial contribution in studying the coagulation of the colloidds. The protective action of lyophilic colloids was studied by Zsigmondy and he introduced a term, called 'gold number'.

What is the range of critical micelle concentration (CMC) of soap solutions?

10^-4 to 10^-3 M

Micelles are associated colloids. These are substances which behave as a strong electrolyte at low concentration but behave as colloids at higher concentration. These particles get associated with each other and get into colloidal range called 'micelle'. The micelle formation takes place only above a particular temperature called Kraft temperature and above a particular concentration called critical micelle concentration (CMC).

For e.g. CMC for soaps is 10⁻⁴ − 10⁻³M.