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Bath fizzers

 
 

Overview

Materials per group

  • Latex-free gloves
  • Goggles and lab coats
  • Bowl or wide-topped beaker, at least 500 ml capacity
  • Mixing utensils (forks, whisks or spatulas)
  • At least one plastic pipette (5 ml capacity)
  • 500g of sodium bicarbonate powder (from either chemical or baking supplier)
  • 250 g of citric acid monohydrate (available from chemical suppliers or can be ordered from some high street chemists)
  • Spray bottle, at least 250 ml
  • Witch hazel solution (available from chemists and some supermarkets; sold as a ‘natural’ antiseptic)
  • Food colouring
  • Food flavouring and/or fragrance oils (e.g. vanilla essence, lavender oil)
  • Scales

Optional

  • Glitter for decoration
  • Scoops and/or moulds (e.g. biscuit cutters)
  • Pack of paper cake cases (approx. 100 cases)

Practicalities and safety

  • This activity works well if students are in groups of three or four.
  • As with any activity using chemicals, check that members of your group do not have specific allergies.
  • We advise wearing protective clothing (lab coats or aprons), disposable gloves and goggles.
  • Some teachers have found it beneficial to do a demonstration first so that students understand exactly what they are doing.
  • Making the bath fizzers will take approximately 20 minutes.
  • The amounts of materials are for guidance and need not be followed exactly.
  • Once bath fizzers have been made there is a chance that they will grow, so make them slightly smaller than needed.
  • Ensure that you can store the bath fizzers until your next session as they will need about 24 hours to dry out fully.
  • If you want to make bath fizzers with different colours or smells you will need to separate the mixture between step 3 and step 4.
  • In step 7 you should add only a couple of squirts of witch hazel otherwise the mixture will start to react and ‘grow’. If this happens do not add any more liquid and mix well to encourage evaporation.
  • If the mixture has partially reacted, the fizzers will still work, but not as well.

Discussion

  • What makes the best bath fizzer?
  • What is the best recipe for bath fizzers?
  • Are there any considerations that need to be taken into account before a product like this is sold?
  • Why is it safe to have acid in a mixture that people use in the bath?
  • Do bath fizzers clean you?
  • What is the chemical reaction in the bath fizzers – why does it only happen in water?

Extensions

  • The groups could consider packaging for their bath fizzers – this could include wrapping the bath fizzers in tissue or net curtain, or making boxes. 
  • You could hide toys or dried flowers in the centre of the bath fizzer, so that when it dissolves you get a surprise.
  • The bath fizzer could be used to look at different themes with different age groups; for example you could use green food colouring to make a cauldron effect.
  • You could link this activity to ‘young enterprise’ where you make the fizzers as a product to sell within the school.
  • You could extend this activity by setting each group the task of working out the optimum way to make bath fizzers, whilst considering investigation methodology, e.g. it is important to change only one variable at a time, ensuring a fair test, recording actions and the results.
  • Additional investigations could include the best proportion of sodium bicarbonate to citric acid, the best way to add the colour and fragrance, the best mixing method (utensils, hands, shaking, etc.), the best way to mould the bath fizzers, and the best packaging to make the fizzers look good and protect them until use. 
  • In another session you could test the bath fizzers, and choose which one students think is the best from the different investigations above. From the results of this you could create the ultimate bath fizzer recipe.
  • You could visit different shops that sell bath fizzers and find out how theirs are made and what they put in them.
  • If you are using this as a science club activity you may want to sell the bath fizzers to provide financial support for science club sessions.

The science – an introduction

Bath fizzers fizz in water because the sodium bicarbonate reacts with the citric acid. This releases bubbles of carbon dioxide and produces a salt and water:

3NaHCO3 + C6H8O7 --> C6H5Na3O7 + 3CO2 + 3H2O
(sodium bicarbonate) + (citric acid) --> (sodium citrate) + (carbon dioxide) + (water)

The reaction only starts when the two dry chemicals dissolve in the water i.e. it is the solutions reacting. However, when you add any liquid, such as food colouring, to the mix, then your mixture may begin reacting early. You can use the activity to introduce the idea that dry chemicals and solutions can behave differently.

Witch hazel is a suitable liquid to use when moulding the fizzers as it evaporates quickly. The witch hazel solution only minimally dampens the dry ingredients for a short time, conserving the ‘fizz’ for when you want it in the bath!

 

Links to everyday life

Bath bombs or bath fizzers are sold in many cosmetics shops around the world. They are therapeutic during bathing because of the essential oils that they contain and are also used for cleansing.

Effervescent vitamin C tablets use a reaction to create ‘fizz’ in a similar way, to disperse the contents evenly throughout a glass of water.