Create your own bath bombs at home with this step by step guide from science charity the Royal Institution (Ri) and L’Oréal UK & Ireland, headline partners of this year’s hands-on Science Zones at Butlin’s.

Weigh out 40g of Sodium Bicarbonate into a mixing bowl.

Add 20g of Citric Acid to your bowl.

Add three drops of the essential oil and mix well. If you are using any of the optional ingredients then add these now.

Bath bombs fizz and break up when dropped into water, releasing all the smells and colour. The fizzing is caused by the ingredients in the bath bomb reacting together in the bath water to produce a harmless gas – carbon dioxide.

The main ingredients are a weak acid – citric acid – and a weak alkali - sodium bicarbonate. When they react together, they ‘neutralise’, meaning that they create a product that is neither acid nor alkali, just like water!

How to make your own bath bomb

Ingredients to make one bath bomb:

40g Sodium bicarbonate powder (baking soda) 20g Citric Acid powder Liquid colour (check that the colour does not stain your skin before using) Fragrance or essential oil Witch hazel (can be substituted with water)

Equipment:

Mixing bowl Whisk Scales Shape mould Tile or similar surface Baking paper Spray bottle for witch hazel/water

The science:

Instructions:

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Use the whisk to gently mix and press out the lumps.

Add roughly four drops of a liquid colour (or two drops of two different colours!) and mix well.

You want a fine powder consistency.

If you add too much liquid, the bath bomb will start to fizz.

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Discover loads more hands-on science activities to try at home atrigb.org

Add three sprays of witch hazel to the mixture and mix well with the whisk or your hand. If you use water you want to use as small an amount as possible as the neutralisation reaction will start.

Cover a surface with baking paper and turn out the bath bomb. Turn the bath bomb out as close to the surface as possible to avoid breakages. Lift the mould to leave the bath bomb behind on the tile.

Leave bath bomb for around one hour to dry and solidify.

Scoop small handfuls of mixture into the mould and pack tightly with your fingers or thumbs. Keep adding and packing mixture until the mixture is level with the top of the mould.

You want the mixture to clump when compressed. A consistency of slightly wet sand is ideal.

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If the mixture is too dry or not packed in enough, the bath bomb may crumble. Tip the bowl and try again, but repeatedly packing the powder makes it increasingly difficult to create a solid bath bomb.

The longer the bath bomb is left, the more solid it will be. This can be sped up by warming up to 50oC.

Feel free to play around with your bath bombs and experiment with colours, smells and other ingredients like glitter or dried flowers (though do consider the clean up after your bath!).

The recipe can be scaled up, just make sure you have a 2:1 ratio of sodium bicarbonate to citric acid.

Be creative!

Images: Katherine Leedale

Our partnership:

The Ri and L’Oréal UK & Ireland have been working together for over 16 years. A highlight of this partnership is the L’Oréal Young Scientist Centre, a modern laboratory in the Ri’s historic home in central London where we offer young people of all ages a fascinating insight into the reality of life as a scientist or engineer.