What type of impression material is known to be irreversible?

The correct choice is alginate because it is classified as an irreversible hydrocolloid. When mixed with water, alginate undergoes a chemical reaction that transforms it into a gel-like state, which cannot be reversed back to its original liquid form. This irreversible nature makes alginate suitable for creating dental impressions, especially for study models and preliminary impressions, as it can capture fine details of oral structures while setting quickly.

Silicone materials, contrary to alginate, can be either reversible or irreversible depending on their formulation. Reversible hydrocolloids, like agar, allow for the impression material to revert to a liquid state upon heating, which is not the case with alginate. Latex, while used in various applications, is not typically considered for dental impressions due to issues like potential allergic reactions and its lack of suitability in capturing detailed impressions of dental anatomy.

Thus, alginate is clearly recognized for its permanent transformation during the setting process, establishing its status as an irreversible impression material in dental practices.