What it shows
Fats and oils are esters of glycerol and fatty acids. Heating coconut oil with sodium hydroxide solution breaks these esters into glycerol and the sodium salts of the fatty acids, which is soap. A small batch sets into a hard bar that is tested for leftover alkali before it is ever used.
The science
Coconut oil is mostly triglycerides, esters formed from glycerol and fatty acids such as lauric acid. Sodium hydroxide hydrolyses these ester links, a reaction called saponification, releasing glycerol and sodium laurate and similar salts, which are soap. The amount of lye is calculated from the saponification value of coconut oil so that a little oil is left over and no free alkali remains. Curing lets the last of the reaction finish and water evaporate, giving a hard, mild bar. Soap molecules have a water-loving ionic head and an oil-loving hydrocarbon tail, which is why they lift grease.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Coconut oil, 100 g
- Sodium hydroxide pellets, 17 g, weighed and dissolved in 35 ml water by the teacher
- A steel or heat-proof glass bowl, a steel spoon and a thermometer
- A water bath or a hot plate (teacher only)
- A small plastic box or silicone mould lined with paper
- Turmeric indicator or pH paper
- Gloves, goggles and a long-sleeved shirt
Safety
For this project: Sodium hydroxide is strongly caustic and this project must be done only in the school lab with the teacher weighing, dissolving and handling the lye. The pellets are always added to water, never water to pellets, because the mixture gets very hot. Everyone wears gloves and goggles, keeps vinegar nearby to rinse any splash on skin, and no one touches the fresh soap bare-handed. The bar is not used on skin until it has cured four weeks and its pH test shows it is mild.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
The teacher dissolves the sodium hydroxide in the water in a steel container, stirring until clear, and lets it cool to about 40 degrees Celsius.
Warm the coconut oil in the bowl on the water bath until it is fully liquid and about 40 degrees Celsius.
The teacher pours the lye solution slowly into the oil while a student stirs steadily with the steel spoon.
Keep stirring for 15 to 30 minutes until the mixture thickens so that a drizzle from the spoon leaves a trace on the surface.
Pour the thick mixture into the lined mould, cover with a cloth and leave it undisturbed for two days.
Wearing gloves, unmould the soap and cut it into bars, then stand them on paper in an airy place.
Cure the bars for four weeks, turning them weekly, and weigh one bar each week to track water loss.
Wet a bar after curing and touch the lather to turmeric paper or pH paper; a reading of about 9 to 10 shows it is normal soap, while a higher reading means it must cure longer or not be used.
What you should see
Hard white bars that lather in water and clean oil from a greasy plate, losing a few percent of mass during curing, with a lather pH of about 9 to 10 on the test strip.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is saponification?
The reaction of a fat or oil with an alkali such as sodium hydroxide to form soap and glycerol.
2Why is the lye added to water and not the reverse?
Dissolving releases a lot of heat; adding water to pellets can boil and spit caustic solution.
3What is the by-product of soap making?
Glycerol, which stays in home-made soap and makes it gentle.
4How does soap remove grease?
Its molecules have an oil-loving tail that enters the grease and a water-loving head that lets water carry it away.
Take it further
Make a second small batch with potassium hydroxide, if the teacher allows, and compare the soft soap it gives with the hard sodium soap.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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