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Soap challenge

 
 

Overview

Materials needed per group

  • One bar of cheap white soap (not clear soap)
  • One mixing bowl or jug, of at least 500 ml capacity
  • One cheese grater, preferably with several different grating plates
  • Warm water from a hot tap or kettle
  • Food colouring
  • Mixing utensils (a variety of forks, spoons and spatulas if possible)
  • Plastic tray or large plate
  • Pipette
  • Latex-free gloves or washing-up gloves

Optional

  • Food flavouring (e.g. peppermint) or fragrance oils (e.g. rose); food flavouring is cheaper but also gives an aroma
  • Glitter
  • Decorative cake cases, silicone ice trays, baking trays or cookie cutters
  • Decorative packaging, e.g. cellophane, cardboard, paper, ribbon

The activity

There are many ways you could choose to run this activity, but the format suggested below will spread over about two sessions.

  • In groups students could work out the optimum way to complete part of the method. You could examine and discuss investigation methodology – e.g. it is important to change only one variable at a time, ensuring a fair test, recording actions and the results. Investigations could include the best base soap, best size grater, the optimum temperature for the warm water and the optimum amount of colour and fragrance to add.
  • In the next session each group should test the soaps that they made and choose which they think is the best. Using all the individual group results they should compile the ultimate soap recipe.

Practicalities and safety

  • Hypoallergenic facial soap is best if you have students with sensitive skin or allergies. We also tried a range of facial and bath soaps, branded soaps as well as supermarket own brands, and found no difference in performance between them. So we advise you to go for the cheapest!
  • Use latex-free protective gloves to avoid staining hands with the colouring and to protect skin from prolonged exposure to soap ingredients.
  • If you choose to use essential oils, seek advice from the salesperson on which are suitable for children/sensitive skin. Certain oils are also harmful to people with some medical conditions. If in doubt use food flavouring instead.
  • The finished soap should be left to dry for 2–3 days in a cool dry place, after which it will be ready to use or put inside a package.
  • If you want different soap bars with different colours you should separate the mixture and add the different colours at step 3 of the instructions.
  • If you want different soap bars with different smells you should separate the mixture and add the different aromas at step 4.

Discussion

  • What makes the best soap?
  • How does soap clean skin?
  • If we use coloured soap why does the colour not come out on our skin?
  • When making soap what considerations do you think you have to take into account?
  • What other materials behave like soap when they are heated?
  • How do the properties of soap change throughout this activity?

Extensions

  • You could make one really large soap bar by making the mixture with 4–5 plain white soap bars and putting the mixture into a plastic food container. Pop the large soap bar out of the container, and when it has set slice it like a cake. You could even make layers of different colours for a stripy end result.
  • Visit a soap shop for inspiration.
  • Your club could use this activity as a youth enterprise project incorporating STEM and sell the soap as a product. The money made from this could also contribute to funding your science club.
  • Make ‘real’ soap using caustic soda and oils. This involves a spectacular exothermic reaction and illustrates the fascinating process still used to make an everyday item, but there are significant safety requirements.
  • Students could make packaging for their soap and test it to see whose is best at protecting the soap inside.
  • You could add some extra complexity to the activity – e.g. ask the students to make soap on a rope or soap with a toy inside.

Links to everyday life

Soap

Evidence suggests people have been making soap since 2800 BCE. Over the years and with new scientific discoveries approaches to making soap have changed, as have its uses. It has now become an everyday item rather than a luxury product.

Bar of 'Sunlight Soap', c.1890-1914.
Bar of 'Sunlight Soap' c.1890-1914 

Chocolate

Chocolate behaves in a similar way to soap: when you heat it the bonds between the molecules change so that it becomes more like a liquid; when it is left to cool down it returns to its original form.

A chocolate factory at work, Liebig trade card, early 20th century.
A chocolate factory at work, Liebig trade card, early 20th century.

The science – an introduction

Soap is made up of sodium or potassium salts of fatty acids. It is an alkali that also contains glycerol.

When hot water is added to the soap, the heat of the water affects the bonds between the soap molecules, allowing them to move more easily and making the soap mouldable. This is a reversible change and is a result of the type of bonds within the soap. As the soap mixture cools down the bonds begin to return to their original form, giving it a more solid structure which solidifies further when the soap is left to dry, as the water content slowly evaporates. You can melt grated soap over a heat source rather than adding hot water, but we recommend the method above as it is easier and safer in the classroom. There are other materials that behave in a similar way, including chocolate.