Which of the following compounds is NOT a fluoride compound commonly added to water supplies?

Calcium sulfate is not a fluoride compound and is not commonly used in water fluoridation processes. It is primarily known as a mineral (gypsum), often utilized in construction materials, and while it can affect water chemistry, it does not contribute to fluoride levels in drinking water.

In contrast, sodium fluoride, sodium silicofluoride, and fluorosilic acid are all compounds specifically associated with the fluoridation of public water supplies, aimed at preventing dental caries (tooth decay). Sodium fluoride is frequently added directly for its fluoride content, while sodium silicofluoride and fluorosilic acid also release fluoride ions when added to water. Each of these three compounds serves a common purpose in enhancing dental health through fluoridation, setting them apart from calcium sulfate, which does not participate in this process.