Testing an Antacid Tablet
A student investigates how quickly an antacid tablet neutralises stomach acid. She uses dilute hydrochloric acid as a model for stomach acid and measures the time taken for the pH to rise from 1.5 to 4.0 after adding a tablet. In Trial 1 she uses a whole tablet in 50 cm³ of acid at 20 °C and records 140 s. In Trial 2 she crushes the tablet and uses the same acid volume and temperature, recording 55 s. In Trial 3 she uses a whole tablet in 50 cm³ of acid warmed to 40 °C and records 70 s. She claims that crushing the tablet and warming the acid have equal effects because both roughly halve the time. She also notices the solution fizzes during each trial.
1. Assess the claim that crushing and warming have equal effects. Refer to collision theory and the fizzing in your answer.
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The claim is based only on time and ignores mechanism and control. Crushing increases surface area, so more acid particles contact the solid per second; the reaction rate rises and the time to reach pH 4 falls from 140 s to 55 s. Warming raises particle kinetic energy, so more collisions exceed the activation energy; time falls from 140 s to 70 s. Both roughly halve the time, but that does not prove equal effects. The two changes act differently, and the trials are not matched: Trial 3 changes temperature while Trial 2 changes surface area, so each is compared with Trial 1 separately. The fizzing is carbon dioxide from the carbonate reacting with acid, confirming a chemical reaction rather than simple dissolving. A fair comparison would test each variable at several levels and keep all others fixed.
2. The student wants a reliable conclusion about which factor matters more. Describe how she should redesign the investigation and state one limitation that remains.
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She should run a controlled series: vary surface area (whole, halved, crushed) at fixed 20 °C and fixed acid volume and concentration; then vary temperature (20, 30, 40 °C) with whole tablets and the same acid. Each condition should be repeated at least three times and a mean time calculated, so random variation is reduced. The same pH end point and the same stirring method must be used throughout. Only then can the size of each effect be compared fairly. A remaining limitation is that the tablet mass and acid concentration may vary between batches, and pH change is only an indirect measure of reaction rate; measuring gas volume collected over time would give a more direct rate. Also, stomach acid is not exactly dilute hydrochloric acid, so results are a model, not a clinical prediction.