Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.
For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or molten (melted). For example, if electricity is passed through copper chloride solution, the copper chloride is broken down to form copper metal and chlorine gas.
Here is what happens during electrolysis:
- Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
- Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.
At the negative electrode
At the negative electrode, positively charged ions gain electrons. This is reduction, and you say that the ions have been reduced.
Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:
- the metal will be produced if it is less reactive than hydrogen
- hydrogen will be produced if the metal is more reactive than hydrogen
At the positive electrode
At the positive electrode, negatively charged ions lose electrons. This is oxidation, and you say that the ions have been oxidised. The table summarises some of the elements you should expect to get during electrolysis.