In what two forms can silica be present in water?

Silica in water primarily occurs in two forms: silicon dioxide and silicate. Silicon dioxide, often referred to simply as silica, is a compound made up of silicon and oxygen and is often found in its crystalline form as quartz. In contrast, silicates are salts that contain silicon and oxygen combined with metals, often resulting from the dissolution of minerals that contain silica.

These two forms of silica are important in various contexts, including water quality assessment and treatment processes. For instance, understanding the forms of silica in natural water bodies is crucial for evaluating potential scaling in water treatment systems and understanding the geological impacts on water systems.

The other options do not accurately reflect the chemical forms of silica relevant to water. Silicon alone is not typically referenced in this context, and while sand is a common form of silicon dioxide, it is not a dissolved form as silica is in water. Salt does not correspond to any form of silica. Therefore, the option stating "silicon dioxide and silicate" accurately captures the relevant forms of silica that can be found in water.