C3.3 Why do we need chemicals such as alkalis and chlorine and how do we make them?


Long before industrialisation, alkalis were used in everyday life. Alkalis are very important chemicals as they NEUTRALISE acids to make salts. Traditional sources of alkali included BURNT WOOD or STALE URINE. Some uses of the uses of alkalis include:

  • Neutralising acidic soil
  • Producing chemicals that bind natural dyes to cloth
  • Converting oils and fats into soap
  • Producing glass

With increased industrialisation, and more demand for alkaline – based products, there was a shortage of alkali in the 19th Century. As a result, people looked for other ways of processing alkali.

THE LeBlanc PROCESS – The first processes for manufacturing ALKALI from SALT and LIMESTONE using COAL as a fuel caused pollution by releasing large volumes of an acid gas (HYDROGEN CHLORIDE) and creating great heaps of waste that slowly released a toxic and foul smelling gas (HYDROGEN SULFIDE).

Industrialists have a responsibility to try to minimise the pollution caused by chemical processes. Sometimes the problems can be solved by converting the waste pollutant into a USEFUL CHEMICAL. For example in this case, by dissolving hydrogen chloride gas in water you can make hydrochloric acid:

Hydrogen Chloride + Water → Hydrochloric acid

Alternatively, a more useful method was to OXIDE the hydrogen chloride to form CHLORINE.

Hydrogen Chloride → Chlorine + Hydrogen

The properties of compounds are different from those of the elements from which they are made.

CHLORINE is used to kill microorganisms in domestic water supplies and as a bleach. Chlorinated drinking water protects against illnesses including:

  • Typhoid fever
  • Dysentery
  • Cholera

CHLORINATION of water began in the early part of the twentieth century, having a dramatic effect of reducing many waterborne diseases and so increasing public health. However there may be disadvantages of consuming water that has been treated with chlorine including possible health problems from traces of chemicals (THM’s) formed by reaction of chlorine with ORGANIC MATERIALS in the water.

Chlorine is produced by the ELECTROLYSIS OF BRINE (sodium chloride solution):


Passing an electric current through the brine causes a chemical change to take place. This forms new products – CHLORINE, SODIUM HYDROXIDE and HYDROGEN.



Environmental impacts of the ELECTROLYSIS OF BRINE on a large scale must be considered.

  • The process uses a lot of electricity that is mainly produced by the burning of fossil fuels.
  • Metal must be in contact with the solution of brine – a metal commonly used is mercury which is toxic and some mercury escapes into the solution and into the environment.
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