What it shows
Soap forms insoluble scum with the calcium and magnesium in hard water and lathers poorly, while a synthetic detergent forms soluble calcium salts and lathers almost as well in hard water as in soft. Measuring foam height for both cleaners in water of increasing hardness makes the difference visible and numerical.
The science
Soap molecules are sodium salts of long-chain fatty acids. In hard water the fatty acid anions pair with calcium and magnesium ions to form insoluble stearates and palmitates, the scum, so the soap is wasted and cannot foam until the ions are used up. Detergents such as sodium lauryl sulphate or alkylbenzene sulphonates have a sulphate or sulphonate head group whose calcium salts remain soluble. They therefore keep their cleaning and foaming action in hard water, which is why detergents replaced soap for laundry in hard-water regions.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A plain bathing soap and a liquid or powder detergent
- Distilled or RO water, 1 litre
- Epsom salt (magnesium sulphate) from a chemist, or calcium chloride from the school lab, to make hard water
- Six identical test tubes or narrow bottles with stoppers
- A measuring cylinder, a ruler and a balance
- Labels and a notebook
Safety
For this project: Epsom salt and calcium chloride are not for eating; wash hands after use. Nothing is heated.
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.
Make 1 percent solutions of soap and detergent in distilled water, 1 g of each in 100 ml.
Make three water samples: distilled (soft), distilled with 0.5 g Epsom salt per litre (moderately hard) and with 2 g per litre (very hard).
Put 10 ml of soft water in a tube, add 1 ml of soap solution, stopper and shake twenty times.
Measure the foam height with the ruler immediately and again after two minutes, and look for cloudiness or scum.
Repeat with the moderately hard and very hard water, using fresh tubes.
Repeat all three waters with the detergent solution instead of soap.
Run each combination twice and average the foam heights.
Plot foam height against hardness for both cleaners on one graph and describe the two lines.
What you should see
Soap gives tall foam in soft water that collapses to a scummy film in very hard water, while detergent foam height drops only slightly across the three samples, giving two clearly different curves.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does soap fail in hard water?
Its fatty acid anions form insoluble calcium and magnesium salts, the scum, instead of lathering.
2Why does detergent still work?
The calcium salts of its sulphonate or sulphate group are soluble, so it stays active.
3Is soap or detergent more easily broken down by nature?
Soap and straight-chain detergents are biodegradable; branched-chain detergents are not.
4How did you make water hard?
By dissolving magnesium sulphate, Epsom salt, which adds magnesium ions.
Take it further
Add washing soda to the very hard water before testing soap and show that the foam height recovers.
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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