Interpret gas collection and disappearing-cross experiments for AQA and Edexcel.
Read the chemistry notes and worked example
Think through the method
Gas-volume and disappearing-cross methods measure different observable changes. State the endpoint and keep the measurement method consistent.
For the AQA concentration series, keep total volume and other factors consistent while changing concentration. For the specified Edexcel cross variant, compare different starting temperatures.
A shorter time to the same obscuring endpoint suggests a faster reaction under those controlled conditions. You can compare 1 ÷ time between trials. This does not give a rate in mol/s unless you also know how much substance reacted.
Worked example
Two controlled cross trials take 80 s and 40 s. The second reaches the same visual endpoint sooner.
You still need evidence that the viewing conditions and endpoint judgement were comparable.
Exam-board practical requirements
AQA 8.2.5 / RP5
Specification printed pages 105–106.
concentration effects
gas-volume method
colour or turbidity method
hypothesis
Edexcel 7.1
Specification printed pages 60.
hydrochloric acid and marble chips gas volume
sodium thiosulfate and hydrochloric acid cross method
cross experiment at different temperatures
time against temperature graph
Use this to revise the method. You'll still need to carry out the practical at school.
Copy these details if you'd like to report a problem. Your answers and history aren't included. Nothing is sent automatically.
Write an explanation
Explain why a gas-volume experiment and a disappearing-cross experiment cannot be interpreted as identical measurements of rate.
Compare your explanation with the points below. The text you type isn't marked, saved or sent anywhere.
Compare with the example
Check your explanation for
Name what you measure in each method.
State when you stop timing or which time interval you use. Say what must be kept the same.
Explain how a gas leak or judging the cross differently could affect the result.
For Edexcel, retain the specified temperature-series graph.
Example answer
Gas collection measures volume at different times. Use the change in volume to calculate a mean rate for a chosen time interval. The cross method measures how long the mixture takes to become cloudy enough to hide a cross. Keep the viewing conditions and depth of liquid the same. A leak makes the collected gas volume too low. Judging the cross differently changes the recorded times. Edexcel compares cross times at different starting temperatures.
An answer to improve
Both measure time, so a shorter time always means more final product.
Improve your answer
Separate final amount from rate, and name the endpoint and limitation of each method.
Save your progress
Your progress stays in this browser. Turn on saving if you want to keep it when you leave the page.