Sodium hydroxide (NaOH) is extremely versatile. Used in the production of paper, soap, dyes, rayon, metallurgy, petroleum refining, cotton finishing, purification of coal tar products, as well as food processing, wood processing and machinery industries.
Can be used as a chemical experiment. In addition to being used as a reagent, it can also be used as an alkaline desiccant because of its strong water absorption and deliquescence.  It is also possible to absorb acid gases (eg in experiments where sulfur is burned in oxygen, sodium hydroxide solution can be packed into bottles to absorb toxic sulfur dioxide).
Neutral and alkaline gases are mixed with CO2, and NaOH can be used to remove impurities to form Na2CO3 (sodium carbonate) and H2O (the formed Na2CO3 is dissolved in H2O):
CO2+2NaOH = Na2CO3+H2O; H2O+CO2+Na2CO3=2NaHCO3
Sodium hydroxide is widely used in the national economy, and many industrial sectors require sodium hydroxide. The sector that uses the most sodium hydroxide is the manufacture of chemicals, followed by papermaking, aluminum smelting, tungsten smelting, rayon, rayon and soap manufacturing. In addition, in the production of dyes, plastics, pharmaceuticals and organic intermediates, the regeneration of old rubber, the production of sodium metal, water electrolysis and inorganic salt production, the preparation of borax, chromium salts, manganates, phosphates, etc., also use A large amount of caustic soda. At the same time, sodium hydroxide is one of the important raw materials for the production of polycarbonate, superabsorbent polymers, zeolites, epoxy resins, sodium phosphate, sodium sulfite and a large amount of sodium salt.
Soap: Making soap is the oldest and most extensive use of caustic soda.
Sodium hydroxide has been used in traditional living applications. To date, the demand for caustic soda in soaps, soaps and other types of detergents still accounts for about 15% of caustic soda.
The main component of fats and vegetable oils is triglyceride (triacylglycerol), whose basic hydrolysis equation is:
(RCOO)3C3H5 (fat) +3NaOH = 3 (RCOONa) (higher fatty acid sodium) + C3H8O3 (glycerol)
This reaction is the principle of producing soap, hence the name saponification reaction.
The R base may be different, but the resulting R-COONa can be made into soap. Common R- have:
C17H33-: 8-heptadecenyl group. R-COOH is oleic acid.
C15H31-: n-pentadecyl. R-COOH is palmitic acid.
C17H35-: n-heptadecyl. R-COOH is stearic acid.
Detergent: Sodium hydroxide is used to produce various detergents. Even today's washing powder (sodium dodecyl benzene sulfonate, etc.) is also made from a large amount of caustic soda. After caustic soda is used in the sulfonation reaction, Excess fuming sulfuric acid is neutralized.
Sodium hydroxide plays an important role in the paper industry. Due to its alkaline nature, it is used in the process of boiling and bleaching paper sheets.
The raw materials for papermaking are wood or grasses, which contain a considerable amount of non-cellulose (lignin, gum, etc.) in addition to cellulose. The non-cellulosic component can be dissolved and separated by adding a dilute sodium hydroxide solution to prepare a pulp having cellulose as a main component. 
Man-made fibers and textiles
Man-made fibers such as rayon, artificial wool, rayon, etc., are mostly viscose fibers, which are made of cellulose, sodium hydroxide, carbon disulfide (CS2) as a raw material, which is obtained by spinning and coagulating.
In the textile industry, sodium hydroxide is used for the treatment and dyeing of fibers and for the mercerizing treatment of cotton fibers. After the cotton fabric is treated with the caustic soda solution, the wax, grease, starch and the like covered on the cotton fabric can be removed, and the mercerized color of the fabric can be increased to make the dyeing more uniform. 
After the petroleum product is washed with sulfuric acid, it also contains some acidic substances, which must be washed with sodium hydroxide solution and then washed with water to obtain a refined product. 
China's "Sanitary Standards for the Use of Food Additives" (GB 2920-1996) stipulates that it can be used as a processing aid and used in an appropriate amount according to production needs.
Sodium hydroxide can be widely used in the following production processes: the cleaning process of the container; the processing of starch; the preparation process of carboxymethyl cellulose; the manufacturing process of sodium glutamate. 
Sodium hydroxide is widely used in water treatment. In sewage treatment plants, sodium hydroxide can reduce the hardness of water by neutralization. In the industrial field, it is a regenerant for ion exchange resin regeneration. Sodium hydroxide is strongly alkaline and has a relatively high solubility in water. Since sodium hydroxide has a relatively high solubility in water, it is easy to measure the amount and can be conveniently used in various fields of water treatment.
Sodium hydroxide is used in water treatment as follows: elimination of water hardness; adjustment of water pH; neutralization of wastewater; elimination of heavy metal ions in water by precipitation; regeneration of ion exchange resin.
Sodium hydroxide is used to treat bauxite, which contains alumina in the bauxite, which is the raw material for the production of aluminum. The alumina can be purified from the concentrate with sodium hydroxide.
Reaction equation: Al2O3+2NaOH+H2O=2Na[Al(OH)4] or Al2O3+2NaOH=2NaAlO2+H2O 
Sodium hydroxide is also used to extract tungsten raw material tungstate from wolframite:
Sodium hydroxide is also used to produce zinc alloys and zinc ingots.