Plan a fair test: variables, repeats and errors sim
Plan an investigation into acid concentration and reaction time, choose your apparatus, then run it. Your choices set the scatter. Repeat, spot the anomaly, take means, draw your own best-fit line, conclude and evaluate, all marked against an exam mark scheme.
Practical skills: planning an investigation, handling results and evaluating (magnesium and hydrochloric acid) · covers the GCSE, IGCSE, O-Level topic
What the sim shows
A fair test changes one thing (the independent variable), measures one thing (the dependent variable) and keeps everything else the same (the control variables), each in a stated way. Here the acid's concentration is changed, made up from 2.0 mol/dm³ acid and water to 50 cm³, and the time for 2 cm of magnesium ribbon to react is measured. Your choice of apparatus sets how much your results scatter: a stopwatch reads to 0.01 s but your reaction time and your judgement of the end still matter; a desk clock reads only whole seconds; a burette measures the acid to ±0.05 cm³, a measuring cylinder to ±0.5 cm³. Controls you leave out add scatter too. Repeat each value, leave out any anomalous result, and take the mean; half the range of the repeats is the uncertainty. Because rate is about 1/time, in this model a graph of 1/time against concentration is a straight line through the origin: the rate is proportional to the concentration (real school results often curve a little). The timings come from the library's own rate law (the same as the gas-syringe and metals-with-acid figures): k = 6 × 10⁻⁵ mol/s per mol/dm³ for 0.072 g of ribbon at 20 °C, Ea = 51 kJ/mol, so 2 cm takes about 150 s in 1.0 mol/dm³ acid and 75 s in 2.0. Model limits: the ribbon is taken as 0.012 g per cm; in this rate law the time hardly depends on the ribbon's length, but it is kept the same because it changes how much acid is used and the heat given out; one anomalous result is planted on purpose; time moves only when you press the button.
- Conical flask on a white tile: 50 cm³ of acid
- Stopwatch or desk clock; burette or measuring cylinder
- Magnesium ribbon, ruler and sandpaper
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