What is the chemical form of fluoride delivered to the treatment plant in liquid form?

The correct answer, which is fluorosilicic acid (H2SiF6), is commonly used in water treatment processes for fluoridation. Fluorosilicic acid is a byproduct of the phosphate fertilizer industry and is often delivered to water treatment plants in liquid form. Its use is preferred due to its effectiveness and cost-efficiency in providing fluoride for preventing dental cavities.

Fluorosilicic acid dissociates in water, releasing fluoride ions which are then available for the intended public health benefits. This form is advantageous because it is concentrated, making it easier to transport and store compared to other fluoride compounds. While sodium fluoride is also used for fluoridation, it is typically delivered in solid form and requires dissolving before use, making fluorosilicic acid a more efficient choice for direct use in treatment processes.

Hydrofluoric acid is a highly corrosive compound and is not suitable for use in water treatment due to its safety hazards. Therefore, while it is a form of fluoride, it does not align with the application in question.