Take about 10 mL of distilled water (or rainwater) and 10 mL of hard water (from a tubewell or hand pump) in separate test tubes.
Add a couple of drops of soap solution to both.
Shake the test tubes vigorously for an equal period of time and observe the amount of foam formed.
In which test tube do you get more foam?
✅ Answer: In the test tube containing distilled water.
In which test tube do you observe a white curdy precipitate?
✅ Answer: In the test tube containing hard water.
Note for the teacher: If hard water is not available in your locality, prepare some hard water by dissolving hydrogen carbonates/ sulphates/chlorides of calcium or magnesium in water.