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As a general term, a substance is said to be anhydrous if it contains no water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

. The way of achieving the anhydrous form differs from one substance to another. That is to say, anhydrous matter is prepared or used as a form with no water, and specifically the form of a chemical without water of crystallization
Water of crystallization
In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat...



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 is an organometallic chemical reaction in which alkyl- or aryl-magnesium halides add to a carbonyl group in an aldehyde or ketone. This reaction is an important tool for the formation of carbon–carbon bonds...

 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 is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 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 adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...

s or alkali bases such as potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

 or barium oxide
Barium oxide
Barium oxide, BaO, is a white hygroscopic compound formed by the burning of barium in oxygen, although it is often formed through the decomposition of other barium salts.It reacts with water to form barium hydroxide.-Uses:...

. 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.


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 is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

     is generally referred to as anhydrous ammonia to distinguish it from household ammonia, which is an ammonium hydroxide
    Ammonium hydroxide
    Ammonia solution, also known as ammonium hydroxide, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or simply ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3...

     aqueous solution
    Aqueous solution
    An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

  • gaseous hydrogen chloride
    Hydrogen chloride
    The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

    is generally referred to as anhydrous to distinguish it from the more commonly used 37% w/w solution in water.