Q.A peptising agent is added to convert a precipitate into a colloidal solution. Explain with an example.

The study of colloids began with Thomas Graham, who found that sugar, urea and NaCl solutions diffuse readily through a parchment membrane while glue, gelatine and gum diffuse only slowly; he named the fast-diffusing substances crystalloids and the slow ones colloids (Greek: kola — glue, eidos — like). It is now understood that particle SIZE, not chemical identity, is what matters — reducing almost any substance's particle size into the 1–200 nm range converts it into a colloid. A colloid is a mixture in which one substance (smaller proportion, the dispersed phase) is finely distributed throughout another (larger proportion, the dispersion medium).

Colloids are classified three ways. By dispersion-medium identity: hydrosol/aquasol (water), alcosol (alcohol), benzosol (benzene). By interaction strength: lyophilic colloids (e.g. protein, starch sols) have real dispersed-phase/medium affinity, are stable, and can be reconstituted after precipitation just by adding medium back; lyophobic colloids (e.g. gold, silver, platinum, copper sols) have no such affinity, are much less stable, precipitate readily, cannot be reconstituted (hence 'irreversible sols'), and eventually coagulate spontaneously over their characteristic lifetime. By the physical states of BOTH phases: gas+liquid → liquid aerosol (fog); gas+solid → solid aerosol (smoke); liquid+gas → foam (whipped cream); liquid+liquid → emulsion (milk); liquid+solid → sol (paints); solid+gas → solid foam (pumice); solid+liquid → gel (butter); solid+solid → solid sol (coloured glass, alloys). …