What statement about anhydrous compounds is true?

Anhydrous compounds are defined as substances that do not contain water molecules within their structure. The statement indicating that they are formed by heating hydrated compounds is accurate, as heating removes the water of crystallization from hydrated salts, resulting in the formation of anhydrous compounds.

Hydrated compounds, which include water in their structure, can lose this water when subjected to heat, leading to the corresponding anhydrous form. This process is commonly used in chemistry to obtain anhydrous salts for various applications.

The other statements misrepresent the nature of anhydrous compounds. Anhydrous compounds specifically lack water, meaning the first statement is incorrect. While some anhydrous compounds can dissolve in water, it is not a universal property and varies depending on the specific compound, making the statement about dissolving in water overly general. Reactivity with water can vary, but many anhydrous salts do not react violently; therefore, the last statement does not accurately apply to all anhydrous compounds. Thus, B stands out as the only true statement regarding anhydrous compounds.