The Hard Water Lab
Make stubborn water lather with one spoon of washing soda
What this lesson covers
Why it matters
Two buckets drawn from the same village: in one, soap whips into rich foam at a touch; in the other it only smears into grey scum. The culprit is invisible — dissolved calcium and magnesium salts — and one spoon of washing soda can arrest it.
The idea in plain words
Balance the softening step: washing soda traps the calcium as insoluble CaCO₃ and leaves harmless, soluble NaCl behind in the water.
Predict first
Soap rubbed into a sample of well water refuses to lather and leaves grey scum floating on top. What does this prove about the water?
No lather plus scum is the classic soap test for hardness. Soap swaps its sodium for the calcium and magnesium in the water, forming insoluble scum until those ions are removed — so the soap is wasted.
- It is hard — it contains dissolved calcium or magnesium salts — correct
- It is dirty — mud particles stop soap from foaming
- The soap has gone bad
- The water is too cold for soap to work
What you do
Add washing soda to the hard water and watch the calcium fall out as chalky white calcium carbonate: CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl. Filter off the precipitate, rub in soap again — now it lathers freely.
Check yourself
Two identical-looking samples A and B are given. Soap lathers at once in A but forms scum in B. Your conclusion?
The sample that lathers easily with ordinary soap is soft water. Scum means dissolved calcium or magnesium salts are stealing the soap — that sample is hard.
A hard water sample is boiled thoroughly and filtered, yet soap still refuses to lather in it. Why?
Temporary hardness (bicarbonates) breaks down on boiling and filters away as CaCO₃. Permanent hardness (chlorides and sulphates of Ca and Mg) survives boiling — that is exactly what makes it permanent.
Why is washing soda a more complete cure for hardness than boiling?
Whatever the calcium or magnesium is paired with — bicarbonate, chloride or sulphate — washing soda pulls the metal ions out as insoluble carbonate. Filter it off and soap lathers freely.
- A is soft water, B is hard water — correct
- A is hard water, B is soft water
- Nothing can be concluded without boiling both samples
- Its hardness is permanent — chlorides and sulphates of calcium and magnesium are not removed by boiling — correct
- Boiling always softens water, so the soap must be faulty
- Boiling turned the dissolved salts into new, harder salts
- It precipitates calcium and magnesium ions as insoluble carbonate, removing BOTH temporary and permanent hardness — correct
- It works only on temporary hardness, just like boiling
- It reacts with the soap to make it foam harder