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12th March 2005
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Schools GCSE Bitesize Revision
Chemistry  useful products from rocks

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Home / Chemistry / Useful products from rocks / Chemicals from salt

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The chlor-alkali industry

You need to know the products formed by the electrolysis of concentrated sodium chloride solution, and simple laboratory tests for them.

The electrolysis of brine
Brine is concentrated sodium chloride solution. If an electric current is passed through it, hydrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode. A solution of sodium hydroxide forms. These three products have important uses in industry.

You might have expected sodium metal to be deposited at the negative electrode. But sodium is too reactive for this to happen, so hydrogen is given off instead.

image: electrolysis of sodium chloride solution

Laboratory tests
The table shows the common laboratory tests for the products from the electrolysis of brine.

Substance Test and result
hydrogen gas a lighted splint pops
chlorine gas damp litmus paper is bleached white
sodium hydroxide solution universal indicator solution turns purple
| Intro 1 2 3 Print
To sterilize means to make something free from micro-organisms such as bacteria.
Electrolysis is the decomposition (separation or break-down) of a compound using an electric current.
A product is a substance formed in a chemical reaction.
Electrodes are conductors used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode.
A catalyst changes the rate of a chemical reaction without being changed by the reaction itself.

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