Final answer:
Amalgam restorations can tarnish due to the chemical reaction between silver in the amalgam and hydrogen sulfide gas found in saliva, forming silver sulfide which appears as dark spots on the surface.
Step-by-step explanation:
Causes of Tarnish on Amalgam Restorations
Dental amalgams, commonly used by dentists to fill cavities, consist of a mixture of metals, including silver, tin, copper, and approximately 50% by weight of mercury. While dental amalgams are popular due to their low cost, ease of manipulation, and longevity, they can tarnish over time when exposed to certain substances. The tarnishing of amalgam restorations, specifically those containing silver, occurs due to a chemical reaction with sulfur compounds present in the mouth such as hydrogen sulfide gas, which can be found in saliva.
The chemical reaction responsible for the tarnishing process is: 2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g). Silver reacts with hydrogen sulfide, forming silver sulfide, which is a black compound that appears as tarnish on the restoration's surface. While this tarnishing does not necessarily affect the functional properties of the amalgam, it can affect the aesthetic appearance of the filling.
Due to its contact with saliva and the consequent exposure to hydrogen sulfide, amalgam restorations may gradually form dark spots and lose their initial luster. It is essential for dentists to consider this potential for tarnish when selecting materials for restorations, and for patients to be aware of the aesthetic changes that can occur over time.