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'.

In the experiment on electro-osmosis, in which of the following the level of the dispersion medium will fall on the cathode side?

Fe(OH)₃ sol

Level on cathode side will fall when dispersion medium moves towards anode. i.e., dispersion medium carries negative charge or colloidal particles carry positive charge, viz, Fe(OH)₃.