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In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understood.
| Cobalt(II) chloride hexahydrate ' (pink) |
The notation " hydrated compound ⋅n", where n is the number of water molecules per formula unit of the salt, is commonly used to show that a salt is hydrated. The n is usually a low integer, though it is possible for fractional values to occur. For example, in a monohydrate n = 1, and in a hexahydrate n = 6. Numerical prefixes mostly of Greek origin are: Nomenclature of Inorganic Chemistry . IUPAC Recommendations 2005. Table IV Multiplicative Prefixes, p. 258.
A hydrate that has lost water is referred to as an anhydride; the remaining water, if any exists, can only be removed with very strong heating. A substance that does not contain any water is referred to as anhydrous. Some anhydrous compounds are hydrated so easily that they are said to be hygroscopic and are used as drying agents or .
Many organic molecules, as well as inorganic molecules, form crystals that incorporate water into the crystalline structure without chemical alteration of the organic molecule (water of crystallization). The sugar trehalose, for example, exists in both an anhydrous form (melting point 203 °C) and as a dihydrate (melting point 97 °C). Protein crystals commonly have as much as 50% water content.
Molecules are also labeled as hydrates for historical reasons not covered above. Glucose, , was originally thought of as and described as a carbohydrate.
Hydrate formation is common for active ingredients. Many manufacturing processes provide an opportunity for hydrates to form and the state of hydration can be changed with environmental humidity and time. The state of hydration of an active pharmaceutical ingredient can significantly affect the solubility and dissolution rate and therefore its bioavailability.Surov, Artem O., Nikita A. Vasilev, Andrei V. Churakov, Julia Stroh, Franziska Emmerling, and German L. Perlovich. "Solid Forms of Ciprofloxacin Salicylate: Polymorphism, Formation Pathways and Thermodynamic Stability". Crystal Growth & Design (2019). .
Nonpolar molecules, such as methane, can form clathrate hydrates with water, especially under high pressure. Although there is no between water and guest molecules when methane is the guest molecule of the clathrate, guest–host hydrogen bonding often forms when the guest is a larger organic molecule such as tetrahydrofuran. In such cases, the guest–host hydrogen bonds result in the formation of L-type in the clathrate lattice.
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