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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. 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.
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.
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