Anhydrous 268397 209264672 2008-04-30T17:11:54Z 74.233.187.22 /* Gases */ 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. ==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]] 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 sieve]]s 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.<ref name="grubb"> [http://fire.ucdavis.edu/main/ucdfdinfobull22.htm Guidelines for solvent purification at UC Davis] </ref> ==Ionic crystals== An example of anhydration can be seen in [[copper(II) sulfate]]. If the [[water of crystallization]] 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 (CuSO<sub>4</sub>·5H<sub>2</sub>O) is as follows: CuSO<sub>4</sub>·5H<sub>2</sub>O + heat → CuSO<sub>4</sub> + 5H<sub>2</sub>O Another example is in the heating of [[magnesium sulfate]] heptahydrate, MgSO<sub>4</sub>·7H<sub>2</sub>O. On heating, it undergoes the following reaction: MgSO<sub>4</sub>·7H<sub>2</sub>O + heat → MgSO<sub>4</sub> + 7H<sub>2</sub>O ==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]] 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. ==See also== * [[Air-free technique]] ==References== <references/> [[Category:Chemical properties]] [[fr:Anhydre]] [[pt:Anidro]] [[vi:Khan (hóa học)]]