Christmas Science – Making bath bombs

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Making a Christmas present using science

Get your students, or children at home, to make bath bombs which can be given as Christmas presents.

 

Making bath bombs

Bath bombs produce an enjoyable fizz when placed into a bath. They contain sodium bicarbonate and citric acid. When added to water, the acid reacts with the carbonate:

acid + carbonate ⇒ salt + water + carbon dioxide

The carbon dioxide is a gas which forms the bubbles in the water.

You will need:

  • A large mixing bowl
  • Suitable moulds to shape the bath bombs. You can use silicone ice cube moulds or even polystyrene cups
  • A garden sprayer containing cold water
  • 300g bicarbonate of soda
  • 100g citric acid
  • 10ml fragrance or essential oil of your choice (take into account allergies and medical conditions of the potential recipient)
  • Food colouring
  • Edible glitter (optional but makes the bath bombs nice and Christmas-y!)

 

Method

Mix the citric acid and bicarbonate in the bowl. Mix in some glitter, if using.

Add a few drops of food colouring and mix to evenly distribute the colour.

Mix in the fragrance.

A little at a time, spray water from the bottle and continuously mix until the mixture starts to hold together when lightly squeezed in your hands. Be careful not to add too much water, or they won’t work very well.

Pack the mixture into the moulds. You should have enough for around 5 large bath bombs or 10 small ones.

Leave to dry for around 30 minutes.

 

You can now wrap up your bath bombs in coloured cellophane. Add a label which explains how the bath bomb works, for a lovely Christmas present with a scientific twist!

 

Questions to ask the class

What evidence is there that a chemical reaction is happening when the bath bomb is added to the water?

Name the gas being produced.

Why do you think water is needed for the reaction to take place? (it allows the solids to dissolve so the ions can move)

How could we measure the rate of the reaction?

What variables affect how long the bath bomb lasts? How could you investigate this? (you could set up a whole rates of reaction investigation based around bath bombs made by the class. They could change the temperature of the water, the ratio of citric acid to bicarbonate of soda, the shape of the bath bomb etc).

 

Further Festive Science

For more chemistry-inspired fun that can be done at school take a look at the article Christmas science – The chemis-tree.