Fizzing bath bombs

by Fellowship Agency March 29, 2019

Bring some science into Mother’s Day with this fun bath bomb activity. Explore the properties of a mixture, the process of dissolving, and the formation of a gas.

Learning Outcomes

You will be able to:

  • Describe the properties of a mixture, the process of dissolving and the formation of a gas.

You will need:

  • 1 x Deep Gratnells (F2) tray, two-thirds full of warm water
  • 1 x Shallow Gratnells tray
  • 1 x 30 section Gratnells insert
  • 1 Cup bicarbonate of soda (aka baking soda or sodium bicarbonate)
  • 1/2 Cup lavender scented Epsom salt (or if you use unscented you may wish to add a few drops of essential oil for scent)
  • 1/2 Cup Cornflour (aka Corn Starch)
  • 1/2 Cup Citric Acid
  • 3 tsp water
  • 4 tsp of vegetable oil
  • 4-6 drops of soap colouring or food colouring (optional)
  • Bath bomb moulds, or a 12 bun/muffin tin or large ice cube trays sufficient for each participant to make one bath bomb – we used a Gratnells 30 section insert
  • 4-5 x Wooden spoon (optional)
  • 1 x Set of measuring spoons from 1 cup to 1 teaspoon
  • 1 x Small beaker
  • Nitrile gloves – one pair per participant
  • Cellophane bags or ribbon for packaging
  • Pre-printed or home-made labels – one per bomb

This recipe will make 10 bath bombs if using a 30-section tray insert as a mould. To make more bath bombs, simply multiply up the quantity of ingredients used, or do batches of different colours or scents across the class. Increasing the quantity of the ingredients could be done as a maths activity by the students before you begin.

What to do:

  • Explore the properties of each bicarbonate of soda, Epsom salt, cornflour, and citric acid separately, look at particle size, smell and texture. You could do this by placing each ingredient in a separate, labelled, shallow Gratnells tray and rotating the trays around your classroom before you add them to the mixing tray.
  • Using the amounts specified in the list provided, pour the bicarbonate of soda, Epsom salt, cornflour, and citric acid into the deep tray.
  • Use gloved hands or wooden spoons to permanently mix the dry ingredients. You have now created a mixture, which cannot be separated.
  • Mix the water and colouring in a small beaker.
  • Slowly pour the liquid, a little at a time, into the tray of dry ingredients and mix well, working out any lumps. You can do this as a class demonstration or move around and repeat this step with each group, allowing them to see what is happening and for each group to have a go at mixing. You may not need to use all the liquid.
  • The students can see the reaction starts to take place and bubbles of gas are created.
  • The mixture should start to develop the consistency of moon sand, i.e. it holds its shape when compressed but breaks apart easily. That is the stage you are looking to get to, you do not want the mixture to be wet.
  • Grease the inside of each mould with one drop of vegetable oil.
  • Pack the mixture into the individual sections of the tray insert.
  • Leave the bath bombs overnight to harden, or place in the fridge or freezer for one hour. Or, if you have used a bun/muffin tin, you can pop the tray into a low (50-80 degree Celsius) oven for 30 mins.
  • Pop the hardened bombs out of their moulds, package up in cellophane bags with ribbons and add your label.
  • Place your finished bath bombs into a shallow Gratnells tray and take a photograph to evidence your work.
  • Any left-over mixture in the trays could be used to demonstrate the reaction, just add warm water and observe or, make some bath bombs up in advance to use as a demonstration at the end of this activity.
  • To test your bath bombs, fill a deep Gratnells tray 2/3 of the way to the top with warm water. Drop a bath bomb in and observe what happens. You could use a stopwatch to time the length of the reaction.

What is happening:

Keywords: Mixture, Solid, Liquid, Gas, Reaction.

Each ingredient in the bath bombs has different properties, explore their texture, smell and particle size before combining them.

When the bath bombs come in to contact with water a chemical reaction takes place. The two key ingredients are bicarbonate of soda and citric acid, they react with the water to form carbon dioxide, a gas, which forms as the bubbles you will observe. This is an acid-base reaction. Bicarbonate of soda is a weak base and citric acid is a weak acid. The other ingredients are not reactive in water and act as a filler, to help pack out the bath bomb and slow the reaction down so it happens in 3 to 4 minutes, rather than seconds.

Bicarbonate of soda has the chemical formula NaHCO3. In water, it quickly dissolves and the positively charged sodium (Na+) breaks away from the negatively charged bicarbonate (HCO3-). At the same time, the citric acid also dissolves and a single hydrogen ion (H+) separates from the rest of the molecule. The H+ and the HCO3– mix and quickly undergo a series of reactions. One of the end products of these reactions is carbon dioxide (CO2), a gas. The gas forms bubbles in the water, causing the characteristic bath bomb fizz. Any scent or essential oils incorporated into the bath bombs are released into the air with the carbon dioxide, creating a pleasant aroma.

Other things to try…

  • Try using unscented Epsom salts and replace the scent with essential oils, experimenting to find the scent combination you like the most.
  • Experiment with your bath bomb recipe, decreasing the amount of citric acid and bicarbonate of soda and increasing the amount of cornflour and vice versa, what is the impact of the change?
  • Test your bath bombs in hot and cold water, what is the difference in the rate of reaction? How will you measure this and keep all other experimental factors constant to ensure a fair test?
  • What other ingredients could you add to your bath bombs? Flower petals, such as rose or lavender, or chopped herbs such as rosemary, could work well.
  • Share your bath bomb tray photographs on social media using #WhatsInMyTray #MothersDay, tag your mum and @Gratnells

Health & Safety

As with all Gratnells Learning Rooms What’s In My Tray activities, you should carry out your own risk assessment prior to undertaking any of the activities or demonstrations. All the ingredients used in this activity are safe to handle without gloves, but as some children may have particularly sensitive skin, use of spoons or gloves are suggested as a precaution.