Pure and impure substances: diffusion in terms of the particle model
Alongside traditional demonstrations of diffusion, we've found some more unusual practical activities and added a few helpful tips.
Visit the practical work page to access all resources and lists focussing on practical work in secondary science: www.nationalstemcentre.org.uk/sciencepracticals
Particles
Chapter five of this resource provides a series of student experiments designed to reinforce the concept of diffusion. There are both teacher notes and student worksheets here to use although teachers may want to produce their own updated student worksheets based on these.
Activity two (coloured crystals in solid gelatin) is particularly recommended since it is so visible. The experiment works equally well in petri dishes of agar where it is probably easier to observe and where holes can be made with straws instead of cork borers. However, the other three activities are worth doing too and taken together this would be a good hands on lesson for students to experience diffusion for themselves.
Perfumes and Smelling
Perfume is often used to teach diffusion since if you can smell it then the particles must have travelled to your nose. If you have plenty of time, you might like to work through this whole sequence of these activities where students extract their own scents and develop their literacy skills by writing for a teenage magazine.
However you could base a just single lesson around this resource, finishing off a topic on diffusion and the particle model with students using steam distillation to extract the smelly molecules in orange peel. Students will enjoy developing their practical skills and teachers could challenge them to explain how they can smell their resulting product.
Usefully two versions of the student sheets are provided. Choose the version most suitable for how you want to run the lesson and the ability of your students.
Diffusion with jelly cubes
In this experiment from the Nuffield Foundation cubes of alkaline cubes of agar impregnated with indicator gradually change colour once they are placed in acid solution.
The experiment can be used exactly as it stands as an investigation into the relationship between diffusion rate and size. It will extend and reinforce work that students have already done on diffusion and help them to develop their numeracy and graph drawing skills and would make a very good investigation. Towards the bottom of the page you’ll find a downloadable student worksheet.
However, you could use a cut down version of this resource just to demonstrate diffusion by preparing cubes all of the same size and allowing students to observe the colour change when placed in acid.
Diffusion in liquids
Diffusion occurs when particles can move and it’s good to remind students that this means it will happen in liquids as well as gases. In this lovely experiment students will really enjoy observing the bright yellow lead iodide (as used in the yellow lines on roads) that forms when lead nitrate and potassium iodide meet and react.
The student worksheet isn’t really necessary since the experiment is so straightforward, but teachers are advised to show students how to set the experiment up and talk through what they might expect to happen before they start work. It’s worth spelling out to students that they should take care not to disturb or jolt the experiment once it is running.