HYDRATE

any compound containing water in the form of H2O molecules. The best known hydrates are crystalline solids that lose their fundamental structures upon removal of the bound water. Exceptions to this are the zeolites, aluminum silicate minerals or their synthetic analogues that contain water in indefinite amounts; zeolites lose and regain water reversibly with little or no change in structure. Examples of hydrates are: Glauber's salt (sodium sulfate decahydrate; Na2SO410H2O), washing soda (sodium carbonate decahydrate; Na2CO310H2O), borax (sodium tetraborate decahydrate; Na2B4O710H2O), and also blue vitriol (copper sulfate pentahydrate; CuSO45H2O). A number of gases, notably the noble gases and simple hydrocarbon gases, form crystalline hydrates, called clathrate compounds, at relatively low temperatures and pressures. Clathrate crystals have a structure in which the water molecules form a loosely held framework surrounding the gas molecule.