Making fizzy "bath bombs" is one way to explore which ingredients in the product are the ones with the exciting effects and which ones are there to add colour and fragrance.
To make a bath bomb you will need citric acid, baking soda, food colouring and nice smelling oils, such as coconut oil. Making the bath bomb needs a bit of practice, so it is worth experimenting before you do this activity with your students.
Take a dry mixing bowl and add one part of citric acid to three parts baking soda. If you use one cup of citric acid, and three cups of baking soda you will make enough mixture for about eight bath bombs.
The children can then choose the colour they want their bath bombs to be, and which fragrant oil they would like to use. These oils can be expensive, but you can add a few drops of the fragrant oil to olive oil to make it go further.
Mix half a teaspoon of oil and a quarter of a teaspoon of food colouring in a jar and quickly combine it with the dry mixture using a spoon or gloved hands. Once mixed, the ingredients should easily stick together. Make one or two round shapes from the mixture and place on an oiled piece of greaseproof paper. Leave in a dry place for 24 hours to dry. Add the bath bomb to a large bowl of water and watch it fizz as the acid reacts with the baking soda to form carbon dioxide.
You can investigate this further in the classroom. Test each of the ingredients by adding them to water and observing what happens. Then put pairs of ingredients together and add water to see which combination causes the reaction.
As with all experiments you need to take proper precautions with your students, and make sure that they wear gloves when mixing the ingredients.