## Naming Hydrates

#### Learning Objective

• Generate the chemical formula and systematic name of a given inorganic hydrate

#### Key Points

• Hydrates are named by the ionic compound followed by a numerical prefix and the suffix “-hydrate. ” The “· nH2O” notation indicates that “n” (described by a Greek prefix) number of loosely bonded water molecules are associated per formula unit of the salt.
• An anhydride is a hydrate that has lost water. A substance that does not contain any water is referred to as anhydrous.
• In organic chemistry, a hydrate is a compound of water, or its elements, with another molecule. Glucose, C6H12O6, was originally thought of as a carbohydrate (carbon and water), but this classification does not properly describe its structure and properties.

#### Terms

• anhydrideAny compound formally derived from another (or from others) by the loss of a water molecule; a molecule with no water.
• hydrateA solid compound containing or linked to water molecules.
• carbohydrateA sugar, starch, or cellulose that is a food source of energy for an animal or plant; a saccharide

## Inorganic Hydrates

“Hydrate” is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains loosely bonded water. The name of a hydrate follows a set pattern: the name of the ionic compound followed by a numerical prefix and the suffix “-hydrate.” For example, CuSO4 · 5 H2O is “copper(II) sulfate pentahydrate.” The notation of hydrous compound · nH2O, 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 “$\cdot$” indicates that the water is loosely bonded to the ionic compound. The “n” is usually a low integer though it is possible for fractional values to exist. The prefixes are the same Greek prefixes used in naming molecular compounds. Therefore, in a monohydrate “n” is one; in a hexahydrate “n” is 6, and so on.

The Greek prefixes used in naming hydrates for numbers 1/2 through 10 are as follows:

• 1/2: hemi-
• 1: mono-
• 2: di-
• 3: tri-
• 4: tetra-
• 5: penta-
• 6: hexa-
• 7: hepta-
• 8: octa-
• 9: nona-
• 10: deca-

A hydrate that has lost water is referred to as an anhydride. An anhydride can normally lose water only with significant heating. A substance that no longer contains any water is referred to as anhydrous.

## Organic Hydrates

In organic chemistry, hydrates tend to be rarer. An organic hydrate is a compound formed by the addition of water or its elements to another molecule. For example, ethanol, CH3–CH2–OH, can be considered a hydrate of ethene, CH2=CH2, formed by the addition of H to one C and OH to the other C. Another example is chloral hydrate, CCl3–CH(OH)2, which can be formed by the reaction of water with chloral, CCl3–CH=O.

Molecules have been labeled as hydrates for historical reasons. Glucose, C6H12O6, was originally thought of as C6(H2O)6 and was described as a carbohydrate, but this is a very poor description of its structure given what is known about it today. Methanol is often sold as “methyl hydrate,” implying the incorrect formula CH3OH2. The correct formula is CH3–OH.