Which is the most common agent employed in external bleaching?
**Core Concept**
Hydrogen peroxide is a widely used agent in dental bleaching, particularly for external bleaching, due to its oxidizing properties that break down chromogenic molecules in tooth enamel, thereby reducing tooth discoloration.
**Why the Correct Answer is Right**
The most common concentration of hydrogen peroxide used for external bleaching is 35% (or 10 volume). This concentration is effective in removing surface stains and chromogens, while minimizing the risk of enamel damage or pulp irritation. The oxidizing action of hydrogen peroxide leads to the formation of free radicals, which react with and break down chromogenic molecules, resulting in a lighter tooth color.
**Why Each Wrong Option is Incorrect**
**Option A:** Carbamide peroxide is also used in bleaching, but it is more commonly used for internal bleaching or in lower concentrations for external bleaching. It breaks down into hydrogen peroxide and urea, making it less potent than hydrogen peroxide itself.
**Option B:** Sodium bicarbonate is a mild abrasive and can be used as a polishing agent, but it is not effective in removing intrinsic stains or chromogens.
**Option C:** Hydrochloric acid is a strong acid and can be used in some dental procedures, but it is not suitable for external bleaching due to its harsh action on tooth enamel and potential for pulp irritation.
**Clinical Pearl / High-Yield Fact**
When using hydrogen peroxide for external bleaching, it is essential to use a rubber dam or a protective barrier to prevent the peroxide from coming into contact with the gingiva or other oral tissues, which can lead to irritation or damage.
**Correct Answer:** C. Hydrogen peroxide (35% or 10 volume)