Sodium hydroxide , also known as   lye   and   caustic soda ,   is an   inorganic compound   with the formula   NaOH . It is a white solid   ionic compound   consisting of   sodium   cations   Na ⁺   and   hydroxide   anions   OH ⁻ .

Sodium hydroxide is a highly   corrosive   base   and   alkali   that decomposes   lipids   and   proteins   at ambient   temperatures   and may cause severe   chemical burns . It is highly soluble in   water , and readily absorbs   moisture   and   carbon dioxide   from the   air . It forms a series of   hydrates   NaOH· n H ₂ O . The monohydrate   NaOH·H ₂ O   crystallizes from water solutions between 12.3 and 61.8 °C. The commercially available "sodium hydroxide" is often this monohydrate, and published data may refer to it instead of the   anhydrous   compound.

As one of the simplest hydroxides, sodium hydroxide is frequently used alongside neutral   water   and acidic   hydrochloric acid   to demonstrate the pH scale to chemistry students.

Sodium hydroxide is used in many industries: in the making of   wood pulp   and   paper ,   textiles ,   drinking water ,   soaps   and   detergents , and as a   drain opener .  

Sodium hydroxide is a popular strong   base   used in industry. Sodium hydroxide is used in the manufacture of sodium salts and detergents, pH regulation, and organic synthesis. In bulk, it is most often handled as an   aqueous solution ,   since solutions are cheaper and easier to handle.

Sodium hydroxide is used in many scenarios where it is desirable to increase the   alkalinity   of a mixture, or to neutralize acids.

For example, in the petroleum industry, sodium hydroxide is used as an additive in   drilling mud   to increase   alkalinity   in   bentonite   mud systems, to increase the mud   viscosity , and to neutralize any   acid gas   (such as   hydrogen sulfide   and   carbon dioxide ) which may be encountered in the   geological formation   as drilling progresses.

Another use is in   Salt spray testing   where pH needs to be regulated. Sodium hydroxide is used with hydrochloric acid to balance pH. The resultant salt, NaCl, is the corrosive agent used in the standard neutral pH salt spray test.

Poor quality   crude oil   can be treated with sodium hydroxide to remove   sulfurous   impurities in a process known as   caustic washing . As above, sodium hydroxide reacts with weak acids such as   hydrogen sulfide   and   mercaptans   to yield non-volatile sodium salts, which can be removed. The waste which is formed is toxic and difficult to deal with, and the process is banned in many countries because of this. In 2006,   Trafigura   used the process and then   dumped the waste in Ivory Coast .

Other common uses of sodium hydroxide include:

• for making soaps and detergents. Sodium hydroxide is used for hard bar soap, while   potassium hydroxide   is used for liquid soaps.   Sodium hydroxide is used more often than   potassium hydroxide   because it is cheaper and a smaller quantity is needed.
• as drain cleaners that contain sodium hydroxide convert fats and grease that can clog pipes into soap, which dissolves in water   (see   cleaning agent ) .
• for making artificial textile fibres (such as   rayon ).
• in the manufacture of   paper . Around 56% of sodium hydroxide produced is used by industry, 25% of which is used in the paper industry   (see   chemical pulping ) .
• in purifying   bauxite ore   from which   aluminium   metal is extracted. This is known as   Bayer process   (see   dissolving amphoteric metals and compounds ) .
• in de-greasing metals,   oil refining , and making   dyes   and   bleaches .
• in water treatment plants for pH regulation.
• to treat bagels and pretzel dough, giving the distinctive shiny finish.