Final answer:
Fluoride is the compound that prevents cavities when added to drinking water at a concentration of 1 ppm, by forming fluorapatite and enhancing tooth enamel resistance to decay.
Step-by-step explanation:
The compound that can help prevent cavities when added to drinking water at concentrations of 1 part per million is fluoride. Fluoride ions react with the hydroxylapatite in teeth to form fluorapatite, making the teeth more resistant to acid attacks and less soluble, and therefore better protected against tooth decay. Community water fluoridation is the process of adjusting the amount of fluoride found in water to achieve optimal prevention of tooth decay by strengthening tooth enamel and inhibiting harmful bacteria.