An irreversible hydrocolloid impression material widely used in impressions?

• A. Impression tray
• B. Alginate
• C. Polyvinyl siloxane
• D. Desensitizer

Answer: (B)

Irreversible hydrocolloids set through a chemical reaction and cannot revert to a liquid once gelled. Alginate is the material most widely used in this category. It’s popular because it’s inexpensive, easy to mix, sets quickly, and tolerates some moisture on the impression surface, making it ideal for routine preliminary impressions, study models, and many denture-related cases. Alginate comes from seaweed and forms a gel when calcium ions cross-link the alginate chains; once set, it cannot go back to liquid. Its main limitations are greater dimensional change over time and sensitivity to improper storage or delays before pouring, compared with more stable elastomeric materials used for final impressions. The other options aren’t impression materials in this sense: an impression tray is simply the carrier, polyvinyl siloxane is an elastomeric material used for final impressions, and a desensitizer is a treatment for dentin hypersensitivity, not an impression material.