The sodium thiosulfate and acid reaction keeps coming back as Question 1 of 9701 Paper 3. By hand, run FA 1 and water from two burettes so the total volume never changes, stand the beaker on the printed cross, pour in the acid and start the stopwatch. You decide when the cross has gone. Choose your own further volumes, calculate the rates, draw a best-fit line and decide the order with respect to thiosulfate. Every attempt uses a different rate. Allow about 40 minutes.
Checked: Cambridge 9701 Paper 3, February/March 2023 to May/June 2026. Only the papers in our collection are listed, so the real number may be higher. Question papers are available from Cambridge.
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Headings with units, burette volumes to 0.05 cm³ and times to the nearest second, the two set experiments plus three volumes of your own made up to the same total, volumes and times that match your apparatus, rates as 1000 / time, and two accuracy marks for the ratio of your first two times.
A straight line of best fit through your points of rate against volume of FA 1, and the anomalous point (if there is one) identified.
The order with respect to thiosulfate, justified by whether your line passes through the origin, then a rate read from your line and turned back into a time.
Why the total volume and the depth over the cross stay the same, the effect of a warmer room, the percentage error of the measuring cylinder, and an improvement that removes the judgement of when the cross disappears.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.