Seal the flask, fill the cylinder, start the clock. Steeper means faster.
Measuring the gas made in a reaction is one of the most common rate experiments in IGCSE Chemistry Paper 6. Here a strip of cleaned magnesium ribbon reacts with dilute hydrochloric acid, and the hydrogen is collected over water in an upside-down 100 cm³ measuring cylinder. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge IGCSE Chemistry 0620 Paper 6 (Alternative to Practical). It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 0620/63 May/June 2024, (hydrogen from magnesium and ethanoic acid collected over water at two temperatures) · official past papers
Practise next: 0620/61 October/November 2023, (time to collect 100 cm³ of carbon dioxide at six temperatures) · official past papers
Practise next: 0620/63 May/June 2025, (naming the conical flask and gas syringe; why a water bath is used) · official past papers
Examiner reports used: 0620 June 2024 (Paper 63), June 2025 (Papers 61, 62 and 63), November 2023 (Paper 61), November 2024 (Paper 63) and March 2023 (Paper 62), with the June 2024 and November 2023 mark schemes.
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What a full-mark diagram shows
Sealed flask
A conical flask with a bung and delivery tube. Not a beaker, and not left open while the gas is made.
Collect and measure
A measuring cylinder full of water, upside down in a trough of water, with the delivery tube under its mouth (or a gas syringe).
Fair test
Same volume of acid, same length of cleaned magnesium ribbon, same starting temperature. Only the concentration changes.
Timing and table
Stopwatch started as the bung goes in, volume read every 30 s, times in seconds, all volumes to the same number of decimal places.
Habits the examiner reports keep asking for
Clean magnesium ribbon with sandpaper to remove the oxide layer; water does not remove it.
Say “more gas in the same time”, or “steeper line”, not just “more gas”, to show a faster reaction.
Keep the reactants apart until the flask is sealed: a small tube inside the flask, or a divided flask, tipped to start.
Use a water bath to keep the temperature steady, not insulation, which keeps the heat of the reaction in.
Show your working on the graph with vertical and horizontal lines when you read off a value.