A hydrate is a substance that contains water. In inorganic chemistry, it refers to salts or ionic compounds that have molecules of water incorporated into their crystal structure. Some hydrates change color when they are heated.
The chemical formula of a hydrate lists the water molecules after the other elements that form the compound. Copper (II) sulfate pentahydrate, for instance, is CuSO4 * 5H2O. Epsom salt, gypsum and borax are everyday examples of hydrates.
When the hydrate is heated, the water molecules break free of the complexes they have formed with the ions in the crystal lattice. The loss of the water molecules changes the structure of these complexes and hence their properties.
Substances appear to have color when they absorb or reflect specific wavelengths of light. When the hydrate loses the water molecules and the structure of the ion complexes changes, the orbitals available to electrons in the ions also change, so the compound will absorb and reflect different wavelengths or "colors" of light than it did before.
- "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins and Loretta Jones; 2008.
About the Author
Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.
solution and powder image by Radu Razvan from Fotolia.com