1. An agar-type gel used in the preparation of solid culture media for micro-organisms.
2. A type of impression material which changes from a liquid to a gel-like consistency; the change may be reversible or irreversible. A reversible hydrocolloid is a material composed principally of colloidal agar agar gel with water as the dispersing medium; it becomes fluid when heated to 40°C (which is compatible with the mouth) and reverts to a stiff gel-like consistency when cooled or chilled. As an impression material it has a high degree of accuracy but requires specialized equipment; models need to be poured with minimum delay because of the long-term poor dimensional stability. The physical state of an irreversible hydrocolloid is changed by a chemical reaction to an irreversible elastic state. See also alginate.