A hydrocolloid is a state of matter
where individual particles of one substance, are uniformly distributed in a dispersion medium
of another substance; and that medium is water
The material, a colloid, is relatively fluid when the solute particles present are dispersed throughout the liquid. This is called a sol.
Alternatively, the particles can become attached to each other, forming a loose network which restricts movement of the solute molecules. The colloid becomes viscous and jelly like, and is called a gel.
Some colloids have the ability to change reversibly from the sol state to the gel state.
SOL (---) GEL
A sol can be converted into a gel in one of two ways :
1. Reduction in temperature, reversible because sol is formed again on heating (eg agar).
2. Chemical reaction which is irreversible (eg alginates). A gel can lose (syneresis which results in shrinkage) or take up (imbibition which results in expansion) water or other fluids.
When hydrocolloids are placed in the mouth in the sol state when it can record sufficient detail, then removed when it has reached the gel state. This is called taking an impression of your teeth.
To summarise, there are 2 types of hydrocolloid impression materials : the reversible hydrocolloids e.g. agar, and the irreversible hydrocolloids e.g. the alginates.
With some help from lecture notes from Birmingham Uni.