Question

Q1. Explain why filter paper cannot be used to separate colloids.​

Answer 1

The pores of an ordinary filter paper are large enough to allow the passage of both impurity particles as well as colloidal particles. Therefore an ordinary filter paper cannot be used for removing the impurities of electrolytes from an impure sol. However, if the pore size of ordinary filter paper is reduced, it can be used for separating the impurities from impure sols. This is achieved by treating an ordinary filter paper with collodion or gelatin followed by its hardening by dipping it in formaldehyde solution. This treatment reduces the pore size and enables it to check the passage of colloidal particles through it. Filter papers thus obtained are called ultrafilters. Filtration through ultrafilters is called ultrafiltration