Hydrous and anhydrous are terms used in chemistry to describe substances that contain water and that are devoid of water, respectively. Chemical compounds that are hydrous are called hydrates. Compounds that are anhydrous are similarly called anhydrates. The water referred to is water of crystallization, an outdated term that is still used because of its self-explanatory nature. Hydrates are used in skin-care products like shampoo or lotion to infuse moisture, while anhydrates remove water to maintain intended dryness in paper products, among a plethora of others.
The composition of hydrates falls into two chemical categories, organic and inorganic. In organic chemistry, a hydrate results from the incorporation of water or its elements in another chemical compound. This usually alters the chemical composition of the initial compound and creates a new one with different properties.
Inorganic hydrates are salts that have a crystal-lattice shape, with a fixed ratio of water, which is essential to the crystal structure. These salts either attach to a metal center or form a crystal lattice with the metal complex. The water within crystallized salts is sometimes called water of crystallization because these compounds cannot crystallize in the absence of water.
An example of an organic hydrate is ethanol, a compound with the formula CH3—CH2—OH. Ethanol occurs when ethylene, CH2=CH2, is combined with H2O, one H bonding to the left side and the OH bonding to the right. Accordingly, an inorganic salt, Magnesium sulfate, MgSO4·7H4O, is a hydrate created with the addition of seven water molecules. Magnesium sulfate is actually Epsom salt.
An anhydrous compound does not have water in its composition. It is usually created by the removal of water of crystallization, which can be done by applying heat or boiling. Other techniques for water removal include using molecular sieves, which do not allow absorption of molecules that exceed a certain size (e.g., water). Alkali bases like barium oxide are also used to attract molecules of water from a hydrous substance to render it anhydrous.
Sodium hydroxide, NaOH, is a hydrate that goes by the moniker “lye.” When water is removed, sodium hydroxide becomes sodium oxide, or Na2O, which is used to make glass or ceramics. Epsom salt, MgSO4 ·7H2O, becomes an anhydrate when heated. In many cases anhydrates are created to perform reactions in which water would be damaging or produce unintended results.
Some anhydrates are used as drying agents because they absorb water so efficiently, earning them the nickname hygroscopic. Anhydrates can be used to remove water from other substances, but they can also be used to maintain moisture when they are a part of the chemical. In this form, they are also called humectants. Humectants are added for the sole purpose of retaining moisture in products, like food and cigarettes. Hygroscopic substances can also exist in reptiles and dry-climate animals whose survival depends on their ability to store water for long periods of time.