Tetrahydrate refers to a chemical compound that contains four molecules of water (H₂O) for each molecule of the substance. This term is often used in the context of hydrates, which are compounds that include water in their crystalline structure. The presence of water can significantly affect the properties and behavior of the compound, such as its solubility and stability. In many cases, tetrahydrates are formed when certain salts or minerals crystallize from aqueous solutions. For example, copper(II) sulfate can exist as a tetrahydrate, known as blue vitriol, which is commonly used in various applications, including agriculture and chemistry. Understanding tetrahydrates is important in fields like chemistry and materials science.