What is the mechanism by which carbamide peroxide produces tooth whitening?

Correct answer: D. It decomposes to hydrogen peroxide, which releases free radicals that break down pigmented organic molecules

• A. No mineral layer is deposited; the existing chromogens are chemically altered in place.
• B. Bleaching gels are not primarily acidic etchants. Enamel is not removed — the discolored molecules within it are oxidized.
• C. Mechanical abrasion is the mechanism of whitening TOOTHPASTES, which remove extrinsic surface stain only. It is not how peroxide bleaching works, and it cannot alter intrinsic color.
• D. CORRECT. CARBAMIDE PEROXIDE breaks down into HYDROGEN PEROXIDE and urea — roughly a 10% carbamide peroxide gel yields about 3.5% hydrogen peroxide. The hydrogen peroxide then dissociates into reactive FREE RADICALS that diffuse through enamel and dentin and OXIDIZE the large, conjugated, pigmented organic molecules within the tooth, cleaving them into smaller, less pigmented fragments. The change is chemical, and it acts on INTRINSIC discoloration.

Note: Practical notes: carbamide peroxide releases slowly and suits overnight tray wear; hydrogen peroxide acts faster and suits in-office use. Expect transient tooth SENSITIVITY (from peroxide diffusion through dentinal tubules) and gingival irritation. Bleaching does NOT change the shade of existing restorations — plan to replace anterior composites and crowns AFTER whitening and after a two-week color-stabilization period.