chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. T ...

, a hydrate is a substance that contains

water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

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.

Chemical nature

Inorganic chemistry

Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the

crystal A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformatio ...

crystal

" that are either bound to a metal center or that have crystallized with the metal complex. Such hydrates are also said to contain ''

water of crystallizationIn chemistry, water(s) of crystallization or water(s) of hydration are water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's ...

'' or ''water of hydration''. If the water is

heavy water Heavy water (deuterium oxide, , ) is a form of water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). I ...

in which the constituent hydrogen is the

isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of ...

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific elemen ...

, then the term ''deuterate'' may be used in place of ''hydrate''. A colorful example is

cobalt(II) chloride Cobalt(II) chloride is an inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, ...

, which turns from blue to red upon hydration, and can therefore be used as a water indicator. The notation "''hydrated compound''⋅''n''H₂O", where ''n'' is the number of water molecules per

formula unitA formula unit in chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds compose ...

of the salt, is commonly used to show that a salt is hydrated. The ''n'' is usually a low

integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...

, 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: A hydrate that has lost water is referred to as an

anhydride An organic acid anhydride is an acid anhydride that is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. ...

; 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 A substance is anhydrous if it contains no water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the flui ...

. Some anhydrous compounds are hydrated so easily that they are said to be

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water m ...

and are used as drying agents or

desiccant A desiccant is a hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal ...

Organic chemistry

In organic chemistry, a hydrate is a compound formed by the hydration, i.e. "Addition of water or of the elements of water (i.e. H and OH) to a molecular entity". For example:

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol

, CH₃−CH₂−OH, is the product of the

hydration reactionHydration may refer to: * Hydrate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, b ...

ethene Ethylene (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative ...

, CH₂=CH₂, formed by the addition of H to one C and OH to the other C, and so can be considered as the hydrate of ethene. A molecule of water may be eliminated, for example, by the action of

sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

. Another example is

chloral hydrate Chloral hydrate is a geminal diol with the formula . It is a colorless solid. It has limited use as a sedative and hypnotic pharmaceutical drug. It is also a useful laboratory chemical reagent and precursor. It is derived from chloral (tri ...

, CCl₃−CH(OH)₂, which can be formed by reaction of water with

chloral Chloral, also known as trichloroacetaldehyde or trichloroethanal, is the organic compound with the formula Cl3CCHO. This aldehyde is a colourless oily liquid that is soluble in a wide range of solvents. It reacts with water to form chloral hydrat ...

chloral

, CCl₃−CH=O. Many organic molecules, as well as inorganic molecules, form crystals that incorporate water into the crystalline structure without chemical alteration of the organic molecule (

). The sugar

trehalose Trehalose (from Turkish 'trehala' – a sugar derived from insect cocoons + -ose) is a sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regu ...

, for example, exists in both an

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 Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

Glucose

, C₆H₁₂O₆, was originally thought of as C₆(H₂O)₆ and described as a

carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...

. Hydrate formation is common for

active ingredient An active ingredient is the ingredient An ingredient is a substance that forms part of a mixture (in a general sense). For example, in cooking Cooking or cookery is the art, science, and craft of using heat to Outline of food preparation, pr ...

s. 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 In pharmacology, bioavailability (''BA or F'') is a subcategory of absorption and is the fraction (%) of an administered medication, drug that reaches the systemic circulation. By definition, when a medication is administered intravenously, its ...

Clathrate hydrates

Clathrate hydrates Clathrate hydrates, or gas hydrates, clathrates, hydrates, etc., are crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, f ...

(also known as gas hydrates, gas clathrates, etc.) are water ice with gas molecules trapped within; they are a form of

clathrate A clathrate is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched ar ...

. An important example is

methane hydrate . Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound A clathrate is a chemical substance A chemical substance i ...

(also known as gas hydrate, methane clathrate, etc.). Nonpolar molecules such as methane can form

clathrate hydrate Clathrate hydrates, or gas hydrates, clathrates, hydrates, etc., are crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, f ...

s with water, especially under high pressure. Although there is no

hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

ing 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 Tetrahydrofuran (THF), or oxolane, is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds compose ...

. In such cases the guest–host hydrogen bonds result in the formation of L-type

Bjerrum defect A Bjerrum defect is a crystallographic defect Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek words ''crysta ...

s in the clathrate lattice.

Stability

The stability of hydrates is generally determined by the nature of the compounds, their temperature, and the relative humidity (if they are exposed to air).

References