What characteristic distinguishes irreversible hydrocolloid from reversible hydrocolloid?

The distinguishing characteristic of irreversible hydrocolloid from reversible hydrocolloid is its irreversible nature. Irreversible hydrocolloid, commonly known as alginate, sets through a chemical reaction that cannot be reversed once it has taken place. This means that once the material has set, it cannot return to its original state, which is a critical aspect in its usage, particularly in obtaining impressions for dental applications.

In contrast, reversible hydrocolloids can be transformed back to their soluble state by reheating, allowing for them to be reused. This property makes reversible hydrocolloids flexible in applications where multiple impressions may be needed, as they can be melted and reformed.

The other factors such as elasticity, reaction to temperature, and ability to mix with water are important properties of hydrocolloids but do not fundamentally capture the key distinction of irreversibility. While irreversible hydrocolloids may have limited elasticity compared to reversible forms, their setting mechanism and inability to revert to their original state are what fundamentally define them as irreversible.