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Chemical Reactions - Sodium Hydroxide

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Chemical Reactions- Sodium Hydroxide

A chemical reaction is a change where two or more substances are changed into a new substance. You can identify a chemical reaction by colour change, effervescence (bubbles), when light or heat given off, and the change is usually irreversible. There are 6 main types of chemical reactions- combustion (often called burning), synthesis, decomposition, neutralization, single replacement and double replacement. A combustion reaction is a reaction with oxygen, and heat is evolved (given off). A common example of combustion is

Hydrogen +Oxygen= Water

Synthesis is a reaction where two or more substances combine to make a compound. An example of a synthesis reaction is

Iron + Oxygen = Iron Oxide

Decomposition is the opposite of synthesis. It is the breakdown of a compound, usually through electrolysis. Electrolysis is a method of separating bonded elements or compounds by passing an electric current through them . An example of a decomposition reaction is

Calcium carbonate calcium oxide+ carbon dioxide

Single displacement is the replacement of one element with another in a compound. One of the elements is one of the reactants, and a metal replaces a metal or a non-metal replaces a non-metal. Eg.

Magnesium + zinc sulphate = magnesium sulphate + zinc

Double displacement is the swapping of elements, usually forming a solid. Eg.

Lead nitrate + sodium chloride = sodium nitrate + lead chloride

Neutralization is the reaction between an acid and alkali/base/carbonate . One example of neutralization is

Sulphuric acid + magnesium oxide = magnesium sulphate + water

Sodium hydroxide is a corrosive metallic base, and it is also called caustic soda. Nowadays, it is made along with hydrogen and chlorine, using the chloralkali process (any process that produces chlorine or a related oxidizer ) Electrolysis of a solution of sodium chloride (common salt) makes chlorine and sodium hydroxide.

2NaCl + 2H2O --electric current = 2NaOH + H2 + Cl2

This is a double displacement reaction, because both substances 'swap'. To stop the sodium hydroxide from reacting with the chlorine, three processes can be used. The first one is called the Mercury cell process, where sodium metal forms an amalgam at the bottom of a mercury cathode . This sodium then reacts with water to produce NaOH. Another process is called the Diaphragm Cell process, where it uses a steel cathode and a porous diaphragm. The third is the most cost-effective and environmentally friendly process. It is called the Membrane Cell process, and it is similar to the Diaphragm Cell process, just that it produces a higher quality of sodium hydroxide.

Flow chart on how to make sodium hydroxide using the Mercury Cell process:


Diluted saltwater (brine)

Raw brine


Hydrochloric acid


Water Chlorine

Amalgam Mercury


Caustic solution


Flow chart on how to make sodium hydroxide using the Diaphragm Cell process:

Salt Water

Raw salt


Chlorine Heat Chlorine





All flow charts based on the ones at:

Flow chart on how to make sodium hydroxide using the Membrane Cell process:

Water Salt

Diluted Brine

Raw salt

Hydrochloric acid

Purified brine Caustic solution

Caustic solution


An older process used to make sodium hydroxide is the LeBlanc process, which is a process that heats sodium chloride with sulphuric acid to make hydrochloric acid and sodium sulphate. The sodium sulphate is mixed with crushed limestone (calcium carbonate) and coal (carbon) then fired . The coal oxidizes, and the chemical reaction leaves behind a solid mixture of sodium carbonate and calcium sulphide. The sodium carbonate is fired to make sodium oxide and carbon dioxide. The sodium oxide is then



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