The principal ingredient of an irreversible hydrocolloid impression is:

 #  The principal ingredient of an irreversible hydrocolloid impression is:

A. Agar

B. Tri sodium phosphate

C. Sodium alginate

D. Colloidal silica

The correct answer is C. Sodium alginate.

The chief active ingredient in the alginate impression materials is one of the soluble alginates, such as sodium, potassium, or triethanolamine alginate.

The diatomaceous earth acts as a filler to increase the strength and stiffness of the alginate gel. It also produces a smooth texture and ensures the formation of a firm gel surface that is not tacky.

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