Few people think of soap as a chemical and yet soap making was one of the first pieces of applied chemistry to benefit society. It is a great topic for students to investigate since it provides an authentic excuse to play with bubbles, as well as the opportunity to find out more about the history of science. There are lots of activities, using simple materials, which can be modified for children of any age.
Justus von Liebig (1803-73), the chemist whose name is immortalised by the eponymous condenser, had strong views about soap. According to Liebig, the "wealthiest and most highly civilised (country) will consume the greatest weight of soap". Attitudes have changed since the time of Elizabeth I, who famously said she would bathe but once a month. At that time, most people made their own soap or did without.
The ingredients needed to make soap are widely available: an oil or fat and an alkali. By the 9th century, soapmaking had become an industry, especially in southern France and Italy where there was an abundance of olive oil. Plant alkalis were extracted from ashes.
Soap was also made using animal fats, such as tallow (from sheep or beef dripping) or lard from pig meat. By the 18th century, tallow chandlers and soap boilers would call door to door buying tallow and other waste fats.
These were contaminated with meat and other food waste and the fat had to be rendered (purified) before soap-making could begin. If soap is made from unrendered fat, the finished product smells very bad.
Activity 1Alkali from plants (KS3) You will need:
plant or wood ashes from a fire;
beaker, funnel, filter paper;
Boil the ashes with water for 10 minutes, filter and test the pH of the filtrate. The extract will be alkaline. Most plant ashes produce solutions containing potassium carbonate or potassium hydroxide, both of which produce alkaline solutions.
Plants that grow near the shoreline give the corresponding sodium compounds. The commonest salt in the sea is, of course, a sodium compound.
Although the sea contains a variety of salts of calcium, magnesium, potassium and others, sodium chloride is the dominant one. Some plants can selectively accumulate particular salts from soils, for example potassium compounds.
Extension: Develop flame tests to identify the alkali metal present in compounds in the ash. Sodium gives a strong yellow flame and potassium a weaker lilac colour.
Older students can safely prepare soap from olive oil, but the dangers of using sodium hydroxide should be emphasised. There is a good reason why it was called caustic (burning) soda.
Activity 2Preparing soap (KS45)
You will need:
olive oil or lard;
beaker with watch glass to cover it;
sodium hydroxide solution (about 2M ).
Mix about 20ml oil with 60ml sodium hydroxide solution in the beaker. Place the watch glass on top to reduce evaporation and boil very gently for 30 minutes. Leave to cool, then add 30g salt to salt-out (precipitate) the soap. Separate the soap for testing.
Extension: Investigate the solubility of the soap in water and see if the solution is neutral or alkaline. Is this a problem for those with sensitive skin? Does the soap lather easily with distilled water and tap water?
As demand increased, especially after the removal of soap duty in 1853 after two centuries of taxation, the production of vegetable alkalis could not keep up.
There are natural sources of alkalis, such as those in the soda lakes of East Africa but the supply problem was solved by the invention of two major chemical processes. These were the Leblanc process and, later, the Solvay process. Both produced soda ash, the material we now know as sodium carbonate.
Sodium carbonate crystals (washing soda) is available in shops. It is a good degreasing agent, useful for cleaning the intractable grease left in frying pans and barbecues.
Sodium carbonate (KS34)
You will need:
fresh washing soda crystals;
dilute hydrochloric acid;
medium size test tubes;lindicator paper;
for the extension activity: lbalance reading to 0.1g Dissolve some washing soda crystals in water. Test the pH of the solution.
Add a few drops of acid - effervescence releases carbon dioxide gas.
Extension: sodium carbonate crystals contain 10 molecules of water of crystallisation. On exposure to air for a few days, the crystals effloresce, losing some of the water. Devise an experiment to determine how many molecules of water of crystallisation are lost, using a balance. (The answer is that nine of the 10 molecules of water of crystallisation are lost.) Bubbles and foams
Soaps reduce the surface tension of water. You can use a range of soaps, home-made or commercial products, to create some interesting effects.
Bubble machine (KS23)
You will need:
soap or liquid soap;
baking soda (sodium hydrogen carbonate )
10 per cent solution of aluminium sulphate or just use some vinegar;
Mix warm water and soap in a jar. Add some baking soda (try varying the amount each time). Start the bubble machine by adding aluminium sulphate solution. Vinegar is generally less successful.
Extension: add food dyes for coloured foams, for example you can use a tall jar and fill with bubbles. When full, try viewing the foam using a bright light or a torch in a darkened room.
An overhead projector can be used to show surface tension effects to the whole class.
Activity 5Bubble mix (KS23)
You will need:
pack of bubble mix and the circular holder used to blow streams of bubbles;
wire loop, about 3cm diameter, and cotton thread;
Materials from Activity 3 above to produce carbon dioxide gas.
(a) Place a beaker on the projector stage. Add sodium carbonate (washing soda) and acid to fill the beaker with dense carbon dioxide gas. Blow some bubbles into the beaker where they float and move around on the more dense gas below.
(b) Loosely tie cotton thread across the wire loop. Dip the loop into bubble mix to give a continuous soap film. Hold the loop above the projector stage and, using the point of a pencil, pierce the soap film on one side of the thread only. The thread is pulled into an arc.
Extension: try different thread patterns, including ones that involve complete loops.
When soap-making was a cottage industry, it was difficult to get reproducible results. The main problem was the strength of the alkali.
Unless this was consistent, the soap could be a failure since it would contain much unreacted oil or fat and would not solidify.
In the American colonies before independence, an unusual method was used to overcome this difficulty. The alkaline solution, known as lye, was tested using a potato or an egg. If the object floated, the alkali was strong enough for soap-making.
There are some obvious problems, not least the variability of potatoes and eggs. A modified method provides a good investigation. Use a solution of salt in place of the alkali as this is safer.
The task is to find a reliable way to measure how much salt is dissolved in different solutions by floating objects in them.
You will need:
potato, straws and plasticine;
tall narrow jar or cylinder.
Prepare some standard salt solutions of known concentrations. Float a slice of potato or a weighted straw in each solution. The weights do not have to be equal. You can draw a correlation graph of the level at which the straw floats in each solution. The straw is a simple hydrometer, a device that floats at a level determined by the density of the liquid supporting it. If using potato slices, watch out for changes in the potato when it is in contact with the salt solution.
Soap Bubbles by CV Boys (1911), has been republished by Dover.
The internet is full of information about soap and its history. Try "soap-making history" in Google. This produces pages of websites. Also, visit the following pages:
Ray Oliver teaches science at St Albans Girls' School