Professional teeth whitening works by using hydrogen peroxide or carbamide peroxide as the active bleaching agent. These compounds penetrate the porous structure of the enamel and dentin layers of the tooth.
Once inside, the peroxide molecules break down into oxygen molecules. These oxygen molecules react with the stained molecules within the tooth structure, breaking the chemical bonds that cause discoloration. Essentially, they oxidise the stain molecules, making them colourless and thus lightening the overall appearance of the tooth.
The process is safe when supervised by a dental professional. Unlike abrasive whitening toothpastes that physically scrub surface stains (potentially causing enamel wear over time), professional whitening chemically changes the colour of the teeth.