Electrolysis involves the decomposition of an ionic compound by the passage of an electric current, when molten or in aqueous solution. During electrolysis, positively charged ions move to the negative electrode (cathode) and negatively charged ions move to the positive electrode (anode). At these electrodes, the ions either gain or lose electrons. The loss of electrons is known as oxidation and the gain of electrons is known as reduction, and this can be remembered by using the mnemonic, OILRIG (Oxidation is Loss, Reduction is Gain). Once the ions have lost or gained their respective electrons, they are no longer charged and are known as elements.

Another important detail to note is that electrolysis can only be conducted when the ionic compounds are molten or in aqueous solution. This is because the electrically-charged ions are fixed when ionic compounds are solid, meaning that they cannot move and hence cannot conduct electricity. When the compounds are molten or in aqueous solution, the ions are able to move and carry charge.

Let’s take a look at a recent IGCSE past paper question on electrolysis:

The student’s results are shown in the table:

Part (i): To calculate the increase in mass at the negative electrode we will do the following calculation:

Increase in Mass = Mass of electrode After - Mass of electrode Before. This calculation 8.25g - 7.95 g should give us 0.30 g.

Part (ii): Two reasons why the increase of mass of the negative electrode was less than expected could be that the copper did not stick to the electrode, resulting in a lower mass weighed. Additionally, during the washing and drying process, some copper may have been lost. A final reason that could be considered is that the positive electrode was impure, meaning that the negative electrode would not be able to convert these impurities to pure copper, and therefore cannot contribute to the increase in mass.

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