Measuring hardness

Soft water lathers well when mixed with soap, but hard water does not. Both hard water and soft water lather well with soapless detergents, eg washing-up liquid.

To investigate hardness in water:

  1. Put a known volume of water into a boiling tube.
  2. Add soap solution, one drop at a time, and shake the mixture for 10 seconds.
  3. Record the number of drops of soap needed to make a lather. The greater the number of drops needed to make a stable lather, the harder the water.
  4. To separate temporary and permanent hard water samples, boil the water.
  5. Repeat steps two and three with the boiled samples.
Diagram showing how soap lathers in water when added 1 cm3 at a time.

Typical results

Water sampleVolume of soap before boiling (cm3)Volume of soap after boiling (cm3)Conclusion
A22Soft water
B152Temporary hard water
C1515Permanent hard water
D1510A mixture of temporary and permanent hard water