Electrolysis

Connect a piece of copper to a battery and a complete circuit: the copper warms up slightly, and nothing else happens. Connect a solution of potassium iodide into the same kind of circuit instead, and something genuinely new occurs — a chemical change. Bubbles of hydrogen gas appear at one electrode, purple iodine is produced at the other, and neither substance was there before. That contrast is the whole business of this chapter: electricity passing through some substances just makes them warm, while electricity passing through others tears them apart into new substances. The process that does the tearing is electrolysis, and by the end of the lesson you will be able to predict its products for any compound the exam throws at you.

Every objective of record for this lesson is a chemistry-only specification statement — assessed in Paper 2 (2C) only; none of them appears on Paper 1 (1C), and none is shared with Science Double Award:

• understand why covalent compounds do not conduct electricity (spec 1.55C);
• understand why ionic compounds conduct only when molten or in aqueous solution (spec 1.56C);
• know that anion and cation name negative and positive ions respectively (spec 1.57C);
• describe experiments to investigate electrolysis, using inert electrodes, of molten compounds (including lead(II) bromide) and aqueous solutions (including sodium chloride, dilute sulfuric acid and copper(II) sulfate) — and predict the products (spec 1.58C);
• write ionic half-equations for the electrode reactions and classify them as oxidation or reduction (spec 1.59C);
• practical: investigate the electrolysis of aqueous solutions (spec 1.60C).

The lesson runs in four movements after this map. First, the conduction rulebook — which structures carry a current and which refuse, and why, ending in the definitions of electrolysis itself and the apparatus it needs. Second, the molten-compound case study: molten lead(II) bromide, every observation explained, and the redox language (oxidation, reduction, OILRIG) that grades the reactions at the two electrodes. Third, the aqueous case studies — sodium chloride, copper(II) sulfate and dilute sulfuric acid solutions — where water itself joins the competition and changes the products, closing with the prediction rules that work for every solution. Fourth, the quantitative experiment: measuring how the volume of gas produced depends on the current, and reading a proportional relationship off a graph. Work through the sections in order — each movement's equipment feeds directly into the next.