Edexcel GCSE Chemistry Topic 3: Chemical Changes Exam Questions

About Edexcel GCSE Chemistry Topic 3: Chemical Changes

Topic 3 of Edexcel GCSE Chemistry covers acids and electrolysis. You learn how acids and alkalis behave, measure pH, describe neutralisation, and prepare soluble salts and insoluble salts by precipitation using solubility rules. At Higher tier you explain strong and weak acids and the relationship between pH and hydrogen ion concentration. The second half of the topic is electrolysis of molten and aqueous ionic compounds, including the purification of copper. Topic 3 is assessed on Paper 1 and includes core practicals on pH change during neutralisation, preparing copper sulfate crystals and electrolysing copper sulfate solution, so method questions are very common.

What you need to know

• Acids and alkalis Acids form H+ ions in water; alkalis form OH- ions. Indicators: litmus, methyl orange and phenolphthalein. Neutralisation: H+ + OH- → H2O. A base is any substance that reacts with an acid to form salt and water only; an alkali is a soluble base.
• Strong and weak acids (Higher tier only) Strong acids fully dissociate in water; weak acids partially dissociate. A strong acid can be dilute, and a weak acid can be concentrated. A decrease of one pH unit means a tenfold increase in H+ concentration.
• Reactions of acids Acid + metal → salt + hydrogen; acid + metal oxide or hydroxide → salt + water; acid + metal carbonate → salt + water + carbon dioxide. Core practical: investigate the change in pH on adding powdered calcium hydroxide or calcium oxide to dilute hydrochloric acid.
• Preparing salts Soluble salts from insoluble bases: add excess base to acid, filter and crystallise (core practical: hydrated copper sulfate crystals from copper oxide). Soluble salts from alkalis are made by titration. Insoluble salts are made by mixing two solutions to form a precipitate, which is filtered, washed and dried.
• Solubility rules All common sodium, potassium and ammonium salts are soluble; all nitrates are soluble; common chlorides are soluble except silver and lead; common sulfates are soluble except lead, barium and calcium; common carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium.
• Electrolysis Electrolytes are molten or dissolved ionic compounds. Cations move to the cathode and anions to the anode. In aqueous solutions, hydrogen forms at the cathode if the metal is more reactive than hydrogen, and oxygen forms at the anode unless halide ions are present. Higher tier only: write half equations. Core practical: electrolysis of copper sulfate with inert and copper electrodes; copper electrodes are used to purify copper.

Common mistakes in Topic 3: Chemical Changes

• Using excess acid when preparing a salt from an insoluble base. Use excess base so all the acid reacts, then filter off the leftover solid.
• Mixing up strong and concentrated. Strength depends on the degree of dissociation, not the amount of acid in a given volume.
• Saying all carbonates are soluble. Most are insoluble except sodium, potassium and ammonium carbonates.
• Saying electrons travel through the electrolyte. Ions move through the electrolyte; electrons flow in the external circuit.
• In copper purification, forgetting that the impure copper anode loses mass while the pure copper cathode gains mass.

Try three Topic 3: Chemical Changes exam-style questions