As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another.


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 and the Wurtz reaction.

Solvents are commonly rendered anhydrous by boiling them in the presence of a hygroscopic substance; metallic sodium is one of the most common metals used. Other methods include the addition of molecular sieves or alkali bases such as potassium hydroxide or barium oxide. 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. [ Guidelines for solvent purification at UC Davis] ]

Ionic crystals

An example of anhydration can be seen in copper(II) sulfate. If the water of crystallizationis 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 heptahydrate, MgSO4·7H2O. On heating, it undergoes the following reaction:

MgSO4·7H2O → MgSO4 + 7H2O

These equations are said to be anhydrous equations


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 is generally referred to as "anhydrous" ammonia to distinguish it from household ammonia, which is an ammonium hydroxide aqueous solution.
* gaseous hydrogen chloride is generally referred to as "anhydrous" to distinguish it from the more commonly used 37% w/w solution in water.

ee also

* Air-free technique


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Look at other dictionaries:

  • Anhydrous — An*hy drous, a. [Gr. ? wanting water; an priv. + y dwr water.] Destitute of water; as, anhydrous salts or acids. [1913 Webster] || …   The Collaborative International Dictionary of English

  • anhydrous — (adj.) containing no water, 1819, a modern coinage from Gk. an , privative prefix (see AN (Cf. an ) (1)), + hydor water (see WATER (Cf. water) (n.1)). Greek did have anhydros waterless, used of arid lands or corpses that had not been given proper …   Etymology dictionary

  • anhydrous — ► ADJECTIVE Chemistry ▪ containing no water. ORIGIN from Greek an without + hud r water …   English terms dictionary

  • anhydrous — [an hī′drəs] adj. [Gr anydros < an , without + hydōr, WATER] 1. without water 2. Chem. having no water of crystallization; not hydrated …   English World dictionary

  • anhydrous — Containing no water, especially water of crystallization. * * * an·hy·drous ( )an hī drəs adj free from water and esp. water that is chemically combined in a crystalline substance <anhydrous ammonia> * * * an·hy·drous (an hiґdrəs) [Gr …   Medical dictionary

  • anhydrous — adjective Etymology: Greek anydros, from a + hydōr water more at water Date: 1819 free from water and especially water of crystallization …   New Collegiate Dictionary

  • anhydrous — adj. [Gr. an, without; hydor, water] Being without water; completely lacking in water …   Dictionary of invertebrate zoology

  • anhydrous — /an huy dreuhs/, adj. Chem. with all water removed, esp. water of crystallization. [1810 20; < Gk ánydros waterless (with etymological h restored). See AN 1, HYDRO 1, OUS] * * * …   Universalium

  • anhydrous — adjective a) having little or no water b) having no water of crystallization …   Wiktionary

  • Anhydrous — Безводный …   Краткий толковый словарь по полиграфии