Sodium hydroxide

Sodium hydroxide is an inorganic compound with the chemical formula NaOH. It is also commonly known by the names lye or caustic soda. This substance is a white, solid ionic compound made of sodium cations and hydroxide anions. It is categorized as a highly corrosive base and an alkali. Because it is so reactive, it can decompose lipids, which are fats, and proteins at room temperature. This property makes it useful in many industries, but it also means it can cause severe chemical burns at high concentrations.

The way sodium hydroxide behaves in water is quite intense. When you dissolve solid sodium hydroxide in water, it undergoes a highly exothermic reaction. This means the process releases a large amount of heat. This heat release can be a safety threat because it might cause the liquid to splash. The resulting solution is usually colorless and has no smell. A 50% aqueous solution has a specific viscosity, or thickness, of 78 mPa·s. This is much thicker than pure water, which has a viscosity of only 1.0 mPa·s. In fact, it is nearly as thick as olive oil, which has a viscosity of 85 mPa·s.

Sodium hydroxide can exist in several different forms called hydrates. A hydrate is a compound that contains water molecules within its crystal structure. Spencer Umfreville Pickering described these complex solubility patterns in 1893. There are many known hydrates, such as the heptahydrate, pentahydrate, and tetrahydrate. The monohydrate, which is NaOH·H2O, is very common. In fact, much of the sodium hydroxide sold commercially is actually this monohydrate form. The monohydrate crystallizes from water solutions at temperatures between 12.3 and 61.8 °C.

Chemical reactions allow sodium hydroxide to perform many different tasks. When it reacts with a protic acid, it performs a neutralization reaction. This process produces water and a corresponding salt. For example, reacting sodium hydroxide with hydrochloric acid produces sodium chloride, which is common table salt. This reaction is also exothermic, meaning it releases heat. Sodium hydroxide can also react with acidic oxides like sulfur dioxide. This is used to "scrub" or clean harmful gases from the air produced by burning coal.

In industrial settings, sodium hydroxide is used for a variety of specialized purposes. It is used in the production of wood pulp, paper, and textiles. It is also used to treat drinking water and to make soaps and detergents. In one water treatment process, sodium hydroxide reacts with aluminum sulfate to create aluminum hydroxide. This substance acts as a gelatinous flocculant, which helps filter out tiny particles from the water. Additionally, it is a common ingredient in many drain cleaners.

One interesting chemical process involving sodium hydroxide is saponification. This is the base-driven hydrolysis of esters. If sodium hydroxide touches bare skin, it creates a slippery feeling. This happens because the chemical converts the natural oils on your skin, such as sebum, into soap. While it is useful for making soap, the chemical is also quite aggressive toward other materials. It can react with glass to form soluble silicates, which can cause glass joints to "freeze" or stick together. It can even react with aluminum to produce hydrogen gas, which can cause pressure to build up in containers.

The production of sodium hydroxide has changed significantly over time. In the early 19th century, people used the causticizing process. This involved treating sodium carbonate with calcium hydroxide. This worked because sodium hydroxide is soluble in water while calcium carbonate is not. Today, most sodium hydroxide is made through the electrolytic chloralkali process. This modern method produces sodium hydroxide in a single step and creates chlorine gas as a byproduct. Global production has grown massively over the years. In 1998, world production was about 45 million tonnes. By 2022, worldwide production reached approximately 83 million tons.