A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water, therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achieve perfect dryness; anhydrous compounds gradually absorb water from the atmosphere so they must be stored carefully.
1 Solids 2 Liquids or solvents 3 Gases 4 See also 5 References
Many salts and solids can be dried using heat, or under vacuum.
Dessicators can also be used to store reagents in dry conditions.
Common dessicants include phosphorus pentoxide or silica gel.
Chemists may also require dry glassware for sensitive reactions. This
can be achieved by drying glassware in an oven, by flame, or under
Dry solids can be produced by freeze-drying/lyophilisation.
Liquids or solvents
In many cases, the presence of water can prevent a reaction from
happening, or cause undesirable products to form. To prevent this,
anhydrous solvents must be used when performing certain reactions.
Examples of reactions requiring the use of anhydrous solvents are the
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
Reactions which produce water can be kept dry using a Dean-Stark apparatus. See also
^ Williams, D. Bradley G.; Lawton, Michelle (2010-12-17). "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants". The Journal of Organic Chemistry. 75 (24): 8351–8354. doi:10.1021/jo101589h. ISSN 0022-3263. ^ Guidelines for solvent purification at UC Davis Archived September 4, 2006, at the Wayback Machine. ^ "Drying Solvents". UCDavis Chemwiki. Retrieved 8 April 2014. ^ "New Aldrich Sure/Seal® packaging for Anhydrous Solvents and Air-Sensitive Reagents". Sigma-Aldrich. Retrieved 2018-03-26. ^ "Drying agents". www.chem.ucla.edu. Retrieved