In the conversation below, two teachers discuss ways of helping students understand the effect of changing the concentration of a component of an equilibrium mixture on that equilibrium.

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How might you help students to understand what happens to the rates of forward and reverse reactions when the concentration of one of the components is changed?

Ask students about the change in concentrations of reactants and products. Then ask them how this will affect the rate of forward and reverse reaction. The key point to get over is that equilibrium will be re-established when the rates of forward and reverse reactions are equal again.

Quite a lot of students struggle to understand the effect of change of pressure on gaseous equilibria. A good way to help them is to demonstrate the effect of changing pressure on a gaseous equilibrium using a nitrogen dioxide/dinitrogen tetroxide mixture in a gas syringe. Students can interpret the change in intensity of the brown colour of the mixture in terms of movement of the equilibrium position. This can be linked with the number of molecules in reactants and products.

What are the advantages of demonstrating a reaction that is complex to set up and carry out?

Students can focus their attention on what they see rather than how to perform the experiment.

When might you decide on the questions to ask students during the demonstration?

It's better to think of the questions before you start the demonstration. Questions such as 'What is happening in terms of forward and reverse reactions?' help make the link between what students can see and what is happening on a molecular scale.

How long should you wait for a student to answer a question you've asked during the demonstration?

Take your time. Give the student adequate time in which to provide an answer. If no answer is forthcoming, try asking another student and then ask the original student to build upon that answer. Try to build up a culture where all answers are valued and where students are encouraged to suggest explanations, whether they turn out to be correct or not.