Electrolysis

Home learning focus

Learn about the process of electrolysis in both molten and aqueous solutions.

This lesson includes:

  • a learning summary
  • one video
  • two activities

Learn

Watch this video from GCSE Chemistry where Fran Scott demonstrates how electrolysis works.

Fran Scott explains the science behind electrolysis

Electrolysis is the breaking up of an electrolyte using an electric current to form elements.

Electrolytes are ionic compounds that can be either:

  • molten (heated up to become a liquid)
  • aqueous (dissolved in water)

In both states, the ions in the electrolytes are free to move around.

Molten electrolytes

When an electric current is passed through a molten electrolyte:

  • the negatively charged ions will be attracted to the positive anode
  • the positively charged ions will be attracted to the negative cathode

  • metals will form at the cathode

  • non-metals will form at the anode

Example: Electrolysis of molten lead bromide

At the cathode:

  • Pb²⁺ ions gain electrons to form metal lead (Pb)

At the anode:

  • Br⁻ ions lose electrons and combine to form bromine gas (Br₂)
Electrolysis of molten lead (II) bromide

Aqueous electrolytes

In an aqueous electrolyte solution there is also the presence of water molecules as well as the dissolved ions.

When electricity is passed through the solution, the water molecules break up to form H⁺ and OH⁻ ions.

Example: Electrolysis of aqueous sodium chloride

The following ions are present in this solution:

H⁺
OH⁻
Na⁺
Cl⁻

Each one of them competes to be attracted to either the cathode or anode.

At the cathode:

  • sodium is more reactive than hydrogen
  • therefore hydrogen ions build up at the cathode and form hydrogen gas

At the anode:

  • chloride ions lose electrons and form chlorine gas
  • if chlorine ions (halides) weren't present, then oxygen gas would have formed from the OH⁻ ions instead
The product formed at the cathode will depend on where it sits in the reactivity series.

Oxidation and reduction

Oxidation and reduction is what happens when ions either gain or lose electrons at an electrode.

  • oxidation is the loss of electrons
  • reduction is the gain of electrons

This means that:

  • reduction happens at the cathode
  • oxidation happens at the anode

Remember OIL RIG:

Oxidation Is Loss; Reduction Is Gain

Practise

There are lots of ways to try out your science skills.

Activity 1

Electrolysis match and draw

Match and draw the electrolysis terms with this downloadable worksheet from Beyond.

Either print it out or write your answers on a piece of paper.

Electrolysis match and draw

Activity 2

Electrolysis quiz

Test what you know about electrolysis with this quiz.

There's more to learn

Have a look at these other resources around the BBC and the web.

GCSE Chemistry
14-16 Chemistry
Bitesize Daily lessons