Page 18 - Curriculum Visions Dynamic Book

Making oxygen using electricity
Water contains 90% oxygen by mass.
But the oxygen and hydrogen atoms are
so tightly bound that it is difficult to imagine liquid water composed of elements that are normally found as gases.
To produce oxygen from water, a large amount of energy has to be applied in order to break the bonds between the oxygen and the hydrogen atoms. This is usually done with an electric current. In the demonstration on this page, water is split into its two elemental forms in an apparatus called Hoffman’s voltameter. An industrial version of this electrical method is used for producing very pure oxygen.
Hoffman’s voltameter
This equipment uses the principle of electrolysis; that is, by passing an electric current through a liquid, the compound can be forced to decompose (break up) into its building blocks.
It consists of three tubes. Each of the outer tubes has an electrode connected to an electrical supply.
 Hoffman’s voltameter consists of a direct current power supply and a set of three tubes connected together. The liquid in the tubes is water containing a chemical indicator to make it easier to see what processes are occurring and a dissolved substance (in this case sodium sulphate) that acts as an electrolyte and allows the current to flow. The change in colour of the indicator helps to identify the changes that take place.
The apparatus is filled with sodium sulphate solution containing an indicator (in this case Universal Indicator). Before the direct current is applied the whole solution is green.
The apparatus is held in place with a retort stand.
Direct current power supply.
 Within a few minutes of applying the direct current the indicator in the solution begins to change colour, showing that reactions are taking place at each electrical terminal. The solution turns red at the
terminal where oxygen is discharged and a deep blue where the hydrogen is discharged.
negative terminal
positive terminal
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