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Skip to 0 minutes and 4 seconds In these beakers, there is a solution of hydrogen peroxide. I am going to mix this with a solution containing potassium iodide, starch and sodium thiosulphate. This is a great introduction to getting students to think about rates of reaction, how we can make the clock reaction speed up or slow down?

Skip to 0 minutes and 43 seconds This week we are going to look at how practical’s can support progression for students in chemistry, both in practical work and theoretical knowledge. We are going to look specifically at the rates of reaction topic and progression from qualitative to quantitative experiments and how we can get our students to understand the underlying concepts that effect rates of reaction. This topic is often taught at the same time as equilibria in chemistry and we have to be careful that students do not confuse rate with equilibria, as we are often discussing the same factors such as temperature and pressure. Some of the classic ways of carrying out practical activities are not always the best approaches.

Skip to 1 minute and 23 seconds Large quantities of reagents, lots of waste and a potentially hazardous practical can often be made quicker, cheaper and with better results, while reducing risk- and we will highlight these improved methods.

Progression and rates of reaction

Welcome to week 2. This week, we will look at practicals to support an understanding of rates of reaction in chemistry. The underlying ideas of what speeds up or slows down a chemical reaction are not too difficult for students to grasp, but applying their scientific understanding in a practical context can be a challenge.

We will also consider how planning for progression within practical science is just as important as progression within subject knowledge and understanding.

Hooking your students in

In the first video this week we have a quick way to get students attention: the Iodine Clock reaction. There are many different versions of this, but essentially there is a reaction which liberates iodine and immediately reacts with starch indicator to change from colourless to blue-black.

By changing the concentrations of the reactants, we can make the “clock” change colour in different times. Here we diluted ‘solution A’ with different amounts of distilled water (to keep the final volume the same). With practice, you might be able to turn this into a very effective visual trick, with the solutions turning at the click of a finger!

This can be an ideal practical to show to students, and then ask them to pose questions and make suggestions as to why the solutions change colour at different times. This practical can be used to introduce the idea of rate as the phrase used for how fast, or slow, a chemical reaction proceeds.

Getting students engaged, and not just giving them an explanation at the start, can help them pose enquiry questions, which the practicals will help them develop answers to.

Our solutions are:

  • Solution A: Dissolve 0.25g of sodium thiosulfate in 100ml of 1% starch solution
  • Solution B: Dissolve 13.50g of potassium persulfate in 1000ml of solution
  • Solution C: 0.1M potassium iodide (16.6g in 1000ml water)

Add 5ml of solution A to 50ml of solution B in a beaker. Then when ready, add 50ml of solution C into the beaker. You can vary the concentrations of solutions B and C to change the rate of this reaction. Dispose used solutions via dilution down the foul water drain. If your drains lead to a septic tank or direct discharge into a watercourse, consider local controls on disposal of solutions.


Before we look at progression with further examples, share your preferred experiment when teaching rates of reaction. Is it the most effective for learning, or just a wow demonstration? Does it achieve both?

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This video is from the free online course:

Teaching Practical Science: Chemistry

National STEM Learning Centre