It’s a race against time to put your chemistry knowledge to the ultimate test

Make this experiment a source of competition in any learning space, and see if students can find the mix of solutions that creates an exact 70s time delay in reaction.

This experiment should take one hour.

Need more support?

This collection presents learners with problem-solving challenges. The iodine clock reaction is included in our Rates of reaction video, 16-18 years, along with supporting resources, including illustrated technician notes, learner instructions, questions, worksheets and more.


Materials per group

These solutions are better made up fresh, not more than 24 h before they are required.

  • Solution A: 2.1 g potassium iodate(V) is dissolved in 1 dm3 deionised water followed by the addition of 10 cm3 of 1 mol dm–3 sulfuric acid.
  • Solution B: 4 g of soluble starch is made up into a paste with a little cold water and this is added to 1 dm3 boiling deionised water. 0.9 g of sodium hydrogensulfite and 10 cm3 of 1 mol dm–3 sulphuric acid are added to the cooled solution. (Sodium hydrogensulfate is available as a solution from some suppliers.)

Equipment per group

  • Beakers, 100 cm3, x2
  • Beakers, 250 cm3, x2
  • Measuring cylinder, 25 cm3
  • Measuring cylinders, 100 cm3, x2
  • Measuring cylinders, 250 cm3, x2
  • White tile
  • Stirring rod
  • Stop-watch
  • The use of burettes allows more accurate measurements to be made.
  • Safety glasses
  • Graph paper
  • If this problem is used in a competition then a large display digital clock can heighten the excitement at the final stage. 

Health, safety and technical notes

  • Wear eye protection. 
  • Read our standard health and safety guidance here.
  • This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete. 
  • Potassium iodate(V) is harmful if swallowed and a skin/eye/respiratory irritant. The solution is of low hazard. See CLEAPSS Hazcard HC080.
  • Sulfuric acid, 1 mol dm–3 is a skin/eye irritant. See CLEAPSS Hazcard HC098a.
  • Sodium hydrogensulfate causes serious eye damage. The solution is of low hazard. See CLEAPSS Hazcard HC092.


This problem can be approached as a competition, in which case the time for experimenting should be limited. Sufficient time should be left at the end for judging.


The competition may be made more difficult by limiting the volume of sock solutions available to each competitor.


This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.

Collect up the blue solutions. Add some thiosulphate (solid or solution) with stirring, until the solution is no longer blue. Then wash to waste.