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Anhydrous
 
 
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As a general term, a substance is said to be anhydrous if it contains no water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
. The way of achieving the anhydrous form differs from one substance to another.

any cases, the presence of water can prevent a reaction from happening, or form undesirable products. To prevent this, anhydrous solvents must be used when performing certain reactions. Examples of reactions requiring the use of anhydrous solvents are the Grignard reaction
Grignard reaction

The Grignard reaction, named after the French chemist Fran?ois Auguste Victor Grignard, is an organometallic chemistry chemical reaction in which alkyl- or aryl-magnesium halides , act as nucleophiles, attack electrophilic carbon atoms that are present within polar bonds to yield a carbon-carbon bond , thus altering hybridization about the r...
 and the Wurtz reaction
Wurtz reaction

The Wurtz reaction, named after Charles-Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium to form a new carbon-carbon bond:...
.

Solvents are commonly rendered anhydrous by boiling them in the presence of a hygroscopic substance; metallic sodium
Sodium

Sodium is an element which has the symbol Na , atomic number 11, atomic mass 23 amu , and a common oxidation number +1. Sodium is a soft, silvery white, highly reactive element and is a member of the alkali metals within "group 1" ....
 is one of the most common metals used.






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As a general term, a substance is said to be anhydrous if it contains no water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
. The way of achieving the anhydrous form differs from one substance to another.

Solvents

In many cases, the presence of water can prevent a reaction from happening, or form undesirable products. To prevent this, anhydrous solvents must be used when performing certain reactions. Examples of reactions requiring the use of anhydrous solvents are the Grignard reaction
Grignard reaction

The Grignard reaction, named after the French chemist Fran?ois Auguste Victor Grignard, is an organometallic chemistry chemical reaction in which alkyl- or aryl-magnesium halides , act as nucleophiles, attack electrophilic carbon atoms that are present within polar bonds to yield a carbon-carbon bond , thus altering hybridization about the r...
 and the Wurtz reaction
Wurtz reaction

The Wurtz reaction, named after Charles-Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium to form a new carbon-carbon bond:...
.

Solvents are commonly rendered anhydrous by boiling them in the presence of a hygroscopic substance; metallic sodium
Sodium

Sodium is an element which has the symbol Na , atomic number 11, atomic mass 23 amu , and a common oxidation number +1. Sodium is a soft, silvery white, highly reactive element and is a member of the alkali metals within "group 1" ....
 is one of the most common metals used. Other methods include the addition of molecular sieve
Molecular sieve

A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorption for gases and liquids.Molecules small enough to pass through the pores are adsorption while larger molecules are not....
s or alkali bases such as potassium hydroxide
Potassium hydroxide

Potassium hydroxide is the inorganic compound with the formula potassiumhydroxide. Along with sodium hydroxide, this colourless solid is a prototypical "strong base"....
 or barium oxide
Barium oxide

Barium oxide, BaO, is a white hygroscopic chemical compound formed by the burning of barium in oxygen, although it is often formed through the decomposition of other barium compounds....
. Column solvent purification devices (generally referred to as Grubb's columns) recently became available, reducing the hazards (water reactive substances, heat) from the classical dehydrating methods.

Ionic crystals

An example of anhydration can be seen in copper(II) sulfate
Copper(II) sulfate

Copper sulfate is the chemical compound with the chemical formula CopperSulfurOxygen4. This salt exists as a series of compounds that differ in their degree of water of crystallization....
. If the water of crystallization
Water of crystallization

Water of crystallization is water that occurs in crystals but is not covalent bond to a host molecule or ion. The term is archaic and predates modern structural inorganic chemistry, coming from an era when the relationships between stoichiometry and structure were poorly understood....
 is removed from blue crystals of copper (II) sulfate, a white powder (anhydrous copper(II) sulfate) is formed.

The formula for anhydration of pentahydrate copper (II) sulfate (CuSO4·5H2O) is as follows:

CuSO4·5H2O ? CuSO4 + 5H2O

Another example is in the heating of magnesium sulfate
Magnesium sulfate

Magnesium sulfate is a chemical compound containing magnesium and sulfate, with the formula MgSO4. In its hydrated form the pH is 6.0 ....
 heptahydrate, MgSO4·7H2O. On heating, it undergoes the following reaction:

MgSO4·7H2O ? MgSO4 + 7H2O

These equations are said to be anhydrous equations

Gases

Several substances that exist as gases at standard conditions of temperature and pressure are commonly used as concentrated aqueous solutions. To clarify that it is the gaseous form that is being referred to, the term anhydrous is prefixed to the name of the substance:

  • gaseous ammonia
    Ammonia

    Ammonia is a chemical compound with the chemical formula nitrogenhydrogen. It is normally encountered as a gas with a characteristic pungent odor....
     is generally referred to as anhydrous ammonia to distinguish it from household ammonia, which is an ammonium hydroxide
    Ammonium hydroxide

    Ammonium hydroxide, also known as ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, or aqueous ammonia, is a solution of ammonia in water....
     aqueous solution.
  • gaseous hydrogen chloride
    Hydrogen chloride

    The Chemical compound hydrogen chloride has the chemical formula HydrogenChlorine. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity....
     is generally referred to as anhydrous to distinguish it from the more commonly used 37% w/w solution in water.


See also

  • Air-free technique
    Air-free technique

    Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of Compound that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less commonly carbon dioxide and nitrogen....