Electrolysis of aqueous solutions: Required practical activity 9: investigate what happens when aqueous solutions are electrolysed using inert electrodes. This should be an investigation involving developing a hypothesis.

The idea

You plan and carry out an investigation into electrolysis of aqueous solutions using inert electrodes, developing a hypothesis. You set up a circuit with a DC supply, inert electrodes (such as graphite), and an aqueous electrolyte. You observe and record products at each electrode, then explain them using ion discharge rules. For example, electrolysing copper(II) chloride solution may deposit copper at the cathode and produce chlorine at the anode, while electrolysing sodium chloride solution may produce hydrogen at the cathode and chlorine at the anode. You test your hypothesis by comparing observations with predictions based on reactivity and concentration. You assess risks and control variables to ensure valid results.

What earns the marks

• Develops a hypothesis linking the ions present in the aqueous solution to the products expected at each inert electrode.
• Sets up a safe electrolysis circuit using a DC power supply, inert electrodes, and an aqueous electrolyte, ensuring electrodes do not touch.
• Records observations at both electrodes, such as gas bubbles, colour changes, or metal deposits, and identifies the products where possible.
• Explains cathode products by linking to the reactivity series and the discharge of positive ions, including hydrogen from water when appropriate.
• Explains anode products by considering negative ion discharge and, where relevant, the effect of concentration on which halogen or oxygen is produced.
• Evaluates the hypothesis against evidence, suggesting improvements or further tests to increase confidence in conclusions.

Common mistakes and how to put them right

• Assuming the positive electrode is the cathode.

  Put it right: The cathode is the negative electrode where reduction occurs; the anode is the positive electrode where oxidation occurs.

• Predicting that reactive metals such as sodium are produced at the cathode from aqueous solutions.

  Put it right: Hydrogen is usually produced instead because water is reduced more readily than sodium ions.

• Ignoring concentration effects at the anode.

  Put it right: In concentrated halide solutions, the halogen may form; in dilute solutions, oxygen may form instead.

• Using reactive metal electrodes rather than inert ones.

  Put it right: Inert electrodes such as graphite or platinum must be used so they do not react and affect the products.

Exam tips

• State your hypothesis clearly and link it to the ions present in the solution, not just to the name of the solution.
• Describe observations precisely, for example 'bubbles of gas at the cathode' or 'pink-brown deposit at the cathode', rather than vague statements.
• When explaining products, refer to the reactivity series and the relative ease of discharge of ions, including hydrogen and hydroxide ions from water.
• Include a risk assessment and identify the independent, dependent, and control variables in your plan.
• Use the terms 'inert electrodes', 'electrolyte', 'cathode', and 'anode' correctly throughout your answer.