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Electrolysis decomposes an ionic substance using direct current. The electrolyte must contain mobile ions, so it is molten or dissolved. Positive ions move to the negative cathode, where they gain electrons and are reduced. Negative ions move to the positive anode, where oxidation occurs. Electrons flow through the external circuit and ions carry charge through the electrolyte. Solid ionic compounds cannot normally be electrolysed because their ions are fixed in place.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Predict electrode products
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Where does reduction occur in electrolysis?
Cathode. Positive ions gain electrons there.
After watching 2: Why must an electrolyte be molten or dissolved?
Its ions must move. Mobile ions carry charge.
What you'll be able to do
Electrolysis decomposes an ionic substance using direct current. The electrolyte must contain mobile ions, so it is molten or dissolved. Positive ions move to the negative cathode, where they gain electrons and are reduced. Negative ions move to the positive anode, where oxidation occurs. Electrons flow through the external circuit and ions carry charge through the electrolyte. Solid ionic compounds cannot normally be electrolysed because their ions are fixed in place.
A molten binary compound supplies only its own ions, so the metal and non-metal products are usually straightforward. Aqueous solutions also contain water-derived species. In standard GCSE predictions, hydrogen forms at the cathode unless the metal is less reactive than hydrogen. At the anode, halide solutions often give the halogen; otherwise oxygen is commonly formed with inert electrodes. Concentration and electrode material can matter, so use the conditions stated in the problem.
Copper sulfate with inert electrodes gives copper at the cathode and oxygen at the anode; the solution’s copper-ion concentration decreases. With copper electrodes, the anode dissolves and replaces deposited copper ions, enabling purification or plating. An electrode mass change can support the electron-transfer explanation. Test gas products rather than identifying them from bubbles alone. Connect electrical energy cost, product purity and electrode consumption when evaluating an industrial process.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Predict products from aqueous sodium chloride with inert electrodes under ordinary brine conditions.
Hydrogen at the cathode and chlorine at the anode.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.