What it shows
Temporary hardness, caused by calcium and magnesium bicarbonates, is removed by boiling, which deposits them as carbonate scale. Permanent hardness from sulphates and chlorides is not. Testing a sample before and after boiling, and after adding washing soda, shows which kind of hardness it has.
The science
Calcium bicarbonate dissolved in water decomposes on heating into insoluble calcium carbonate, water and carbon dioxide, so the calcium leaves the solution as scale. Sulphates and chlorides of calcium and magnesium are stable to heat and remain dissolved, so boiling cannot remove permanent hardness. Washing soda supplies carbonate ions that precipitate calcium and magnesium as carbonates from both kinds of hardness, which is why it softens any hard water. Comparing soap drop counts measures how much hardness each treatment removed.
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.
- Borewell or hard tap water, 200 ml
- A saucepan and stove or a lab beaker and burner (adult or teacher only)
- Standard soap solution (2 g soap in 200 ml distilled water)
- Washing soda (sodium carbonate), a quarter teaspoon
- Three small bottles with caps
- A dropper and a 10 ml syringe
- A clean steel spoon to inspect for scale
Safety
For this project: An adult or teacher must do the boiling and handle the hot pan. Let the boiled water cool fully before testing. Washing soda is mildly caustic to eyes; wash hands.
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.
Measure 10 ml of the untreated sample into bottle one and find its soap drop count for a lasting lather.
Ask the adult to boil 100 ml of the sample gently for ten minutes, then let it cool.
Look for a white film on the pan or spoon; this is calcium carbonate scale.
Measure 10 ml of the cooled boiled water into bottle two and find its drop count.
Dissolve a pinch of washing soda in 100 ml of untreated sample, stir, let any cloudiness settle, and pour off the clear part.
Measure 10 ml of this treated water into bottle three and find its drop count.
Repeat each count once more and average the results.
Tabulate untreated, boiled and washing soda treated counts, and work out what fraction of hardness was temporary.
What you should see
The boiled sample needs fewer soap drops than the untreated one, the difference being the temporary hardness, while the washing soda sample needs the fewest of all; a white scale appears on the boiled pan.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What causes temporary hardness?
Dissolved calcium and magnesium bicarbonates.
2Why does boiling remove it?
Bicarbonates decompose on heating to insoluble carbonates, which settle as scale.
3Why can boiling not remove permanent hardness?
Calcium and magnesium sulphates and chlorides do not decompose on heating.
4How does washing soda soften water?
Its carbonate ions precipitate calcium and magnesium as insoluble carbonates.
Take it further
Scrape the scale from the pan, add a drop of vinegar and watch it fizz, confirming it is a carbonate.
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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