Chemical problem-solving competition

Chemical clock

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Solutions
• Solution X is mixture of potassium iodide 6 g/L, and sodium thiosulphate 7.5 g/L
• Solution Y is 2M sulphuric acid
• Solution Z is 20 volume hydrogen peroxide
• Starch

Demo
Mix 20 ml X, 10 ml Y, 10 drops starch, and 30 ml Z in that order.
Students see a colour change to blue/black after a certain length of time. You can set this up as a trick and snap your fingers exactly at the right time.

Student activity
Then set the problem to vary amounts of X, Y and Z to slow the reaction down.

Give students 1 hour to work out as many patterns as they can. Explain that after one hour they will be told a time between 30 seconds and 2 minutes, and will be given 15 minutes to use their previous findings to get as close as possible to the chosen time.

After 15 minutes judging occurs in relation to accuracy.

Students can then be given a further period of time to produce a poster that reflects all aspects of their work including results, graphs, findings, evaluation, etc.

If set as a competition
In deciding a winner, judging of presentation of findings forms an equal part to accuracy of results.

Last updated: 31 January 2008