Butcher, baker, bath bomb maker

Before you begin

• You could run this activity as one of two or more bases. This allows everyone to work together and save resources. If you’re doing more than one experiment, make sure you have enough adult supervision for each one and that all the necessary equipment is ready.
• Bath bombs need time to dry before they can be tested. The group could prepare their own in one session and take them home to try out themselves. You could prepare some beforehand to show them what’ll happen and explain the science while they’re waiting for theirs to dry.
• Double-check to see whether anyone has an allergy or sensitivity to any of the ingredients.
• Let everyone know about this activity beforehand and encourage everyone to bring something along that they can use for a mould. Silicon trays or cupcake cases work well, as do yoghurt pots or plastic biscuit cutters. Bring some along for those who can’t find anything.

Make your bath bombs

1. Everyone should split into pairs or small groups. Give each group a ‘Recipe card (Brilliant bath bombs)’ and all the ingredients and equipment they’ll need.
2. There are instructions to tick off on the recipe card. Start by weighing out and adding the bicarbonate of soda, citric acid, corn flour and Epsom salt to a mixing bowl. Whisk the ingredients together.
3. Use teaspoons and tablespoons to measure out the correct quantities of oil, essential oil and food colouring in a cup or beaker. Mix these together too.
4. Slowly add the oil mixture to the dry ingredients bowl. Do this a little at a time and keep mixing.
5. When all of the oil has been added, add a few drops of water so that the mix clumps together and keeps its shape when pressed.

When water is added, the mixture should fizz up a bit. Mix it quickly. If the fizzing up continues after everything’s mixed together, then the mixture is too wet. You’ll need to start again from step 2, and use less water this time around!

6. Take the mould and add any decorations to the bottom of it. Pack the mixture from the bowl on top and press it down into the mould. Smooth the top with a teaspoon.
7. Put the filled mould somewhere it can dry for the next couple of hours (or pack them up to take home).
8. While waiting for bath bombs to set, everyone should come together and discuss the science of bath bomb reactions. Everyone will need a bath bomb to test. They can either use their own if you made them in a previous session, or a pre-prepared example.
9. Fill a sink or bowl with water and take turns dropping in bath bombs. See if anyone knows why the bath bomb reaction takes place.

The fizzing when the bath bomb hits the water happens because of a chemical reaction taking place between the bicarbonate of soda, citric acid and water. It’s an ‘acid-base’ reaction, where the sodium bicarbonate (from the bicarbonate of soda) is a weak base and the citric acid a weak acid.

10.  Everyone should say what they can see, hear and smell while the bath bomb reacts.

The acid-base reaction makes a lot of bubbles – these bubbles are carbon dioxide, like you see in fizzy drinks. The bath water’s not for drinking, though! If anyone’s bomb contained scented oils, they’re released into the air with the carbon dioxide bubbles. Bath bomb recipes usually include corn flour as a dry ‘filler’. This can be used to control the reaction – it can slow down the rate that the sodium bicarbonate and citric acid dissolve, so that the fizzing sensation in your bath lasts longer.