What it shows
Hard water contains dissolved calcium and magnesium salts that react with soap to form scum before any lather can form. Counting the drops of a standard soap solution needed to get a lasting lather ranks tap, borewell, RO, rain and bottled water from softest to hardest.
The science
Soap is the sodium salt of a fatty acid. In water containing calcium or magnesium ions, the soap first reacts with those ions to form insoluble calcium and magnesium stearate, seen as grey scum. Only when all the hardness ions have been used up can extra soap stay in solution and form a stable lather. The drops of soap needed before lather lasts is therefore proportional to the hardness of the sample. This is the idea behind the classical Clark soap test for hardness.
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, 2 g, grated
- Distilled or RO water, 200 ml, for the soap solution
- Five water samples of 10 ml each: tap, borewell or hand pump, RO or bottled, rainwater, and distilled water as a control
- Five identical small bottles with caps or test tubes with stoppers
- A dropper or a 1 ml syringe without needle
- A measuring cylinder or a 10 ml syringe
- A notebook and labels
Safety
For this project: Nothing hazardous, but samples from ponds or drains must not be used; an adult must collect borewell water. Wash hands after handling samples.
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.
Dissolve the grated soap in the distilled water, stirring until clear, to make the standard soap solution.
Measure exactly 10 ml of the first sample into its bottle and label it.
Add one drop of soap solution, cap the bottle and shake hard for ten seconds.
Look for a lather that covers the whole surface and lasts for two minutes; if it collapses, add another drop and shake again.
Record the total drops needed for a lasting lather; distilled water should need only one or two.
Repeat for each of the other samples, rinsing the dropper between them.
Run each sample twice and take the average drop count.
Rank the samples in a bar chart from softest (fewest drops) to hardest (most drops) and note any scum seen.
What you should see
Distilled and RO water lather with one or two drops, rainwater is nearly as soft, tap water needs a few more, and borewell water typically needs many drops and shows grey scum, giving a clear hardness ranking.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What makes water hard?
Dissolved calcium and magnesium salts.
2Why is scum formed before lather?
Soap reacts with calcium and magnesium ions to form insoluble stearates until those ions are used up.
3Why is distilled water the control?
It has no hardness ions, so it shows the minimum drops needed for lather.
4Name a problem caused by hard water at home.
Scale in kettles and geysers, and more soap needed for washing.
Take it further
Boil a portion of the hardest sample, cool it and retest; the drop count falls if its hardness was mostly temporary.
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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