9 Dec 2018 Primary Science, STEAM / STEM


If you’re looking for a great activity to do with your class or with children at home, why not try these fizzing bath bombs? Wrap them up for a lovely fragrant Christmas gift!


You will need:

  • Bicarbonate of soda
  • Citric acid
  • Fragrance (essential oil or food flavouring)
  • Food colour
  • Disposable gloves (optional)*
  • Molds, e.g. silicone molds, ice cube trays, bun cases
  • Mixing dish, cup and desert spoon


  1. Measure 4 desert spoons of bicarbonate of soda and 2 desert spoons of citric acid into a bowl and mix well. This will make one large or 4 smaller bath bombs.
  2. In a cup, mix together the wet ingredients. If you have essential oils add 2 drops. Alternatively use food flavourings such as lemon, vanilla or coconut; you will need to add about 1/2 teaspoon. Add 4-6 drops of food colour, or more if you want a deeper colour.
  3. Add the wet ingredients to the dry. Now you will need to work quickly, as you will see the mixture starts to fizz as the reaction occurs. Mix well, using the back of the spoon to distribute colour and fragrance thoroughly.
  4. Depending on how much colour and fragrance you used, you may need to add a little water. Preferably ‘spritz’ this in gradually. The texture you are looking for is of sand barely starting to bind together. Too wet and the mixture will react too soon and begin to bubble.
  5. Using the spoon, push the mixture well into the mold. Compact it well and leave to dry overnight.
  6. Carefully remove the bath bomb. Wrap it attractively for a lovely hand-made Christmas gift!

*If you have sensitive skin or cuts on your hands, the citric acid may cause a slight stinging. Wear gloves or wash hands well after touching the mixture.

What’s the science?

The chemistry behind this involves mixing dry and wet materials to make a gas. When the bath bomb is dropped into the bath water, the bicarbonate of soda (an alkali) and the citric acid react and produce a gas – carbon dioxide.