Which of the following is not classified as a hydrate?

Sodium nitrate is not classified as a hydrate because it does not contain water molecules coordinated with its ions in a fixed ratio. Hydrates are crystalline solids that include water molecules in their structure, which are integral to their chemical formula and contribute to their properties. For example, substances like ferrous sulfate can exist as a hydrate (ferrous sulfate heptahydrate), meaning they contain specific amounts of water.

In contrast, sodium nitrate exists primarily as an anhydrous compound, meaning it does not include water within its structure. Therefore, it does not meet the criteria to be classified as a hydrate. This distinction is essential when evaluating how substances interact with water and influences their solubility and stability in various conditions.