National Board Dental Hygiene Examination (NBDHE) Dental Hygienist Practice Test

What type of dental material is alginate categorized as?

1. Reversible hydrocolloid

2. Irreversible hydrocolloid

3. Polyvinyl siloxane

4. Calcium sulfate

The correct answer is: Irreversible hydrocolloid

Alginate is categorized as an irreversible hydrocolloid. This classification is based on its physical properties and the chemical processes that occur when alginate is mixed with water and allowed to set. Once alginate has set into a gel-like form, it cannot be returned to its original viscous state, which is a key characteristic of irreversible hydrocolloids. Alginate is commonly used in dentistry for making impressions due to its ease of use, affordability, and adequate flow characteristics. When mixed with water, the alginate powder hydrates and forms a gel through a chemical reaction that is not reversible, hence the term 'irreversible.' In contrast, reversible hydrocolloids can transition between sol and gel states through temperature changes, which is not applicable to alginate. Polyvinyl siloxane, on the other hand, is a type of elastomeric impression material that has different setting and handling properties. Calcium sulfate is not a hydrocolloid but rather a mineral often used as a dental material in gypsum products. Understanding these differences helps clarify why alginate exclusively fits the category of irreversible hydrocolloid.