Definition: Hydrocolloids are elastic impression materials in which a colloidal dispersion of fine polymer particles in waterforms a hydrosol (liquid) that sets into a hydrogel (solid), with water trapped within an entangled network of fibrils/chain-like molecules.
Summary
Hydrocolloid impression materials are water-based elastic materials that can engage undercuts and are supplied as either reversible hydrocolloids (agar) or irreversible hydrocolloids (alginate).
They set by a sol–gel transition: agar sets reversibly via heating and cooling (gel ⇄ sol ⇄ gel), whereas alginate sets irreversibly by a chemical cross-linking reaction that converts sol → gel.
Since they contain a large water phase, hydrocolloids can be dimensionally unstable on storage (notably via water loss or uptake), so impressions generally need to be poured promptly and stored appropriately if delayed.