Measuring hardness
Water that contains very little or no calcium ions or magnesium ions. Soft water forms a permanent lather with soap easily. lathers well when mixed with soap, but Water that contains magnesium and calcium ions, which react with soap. This makes it difficult to produce a lather and causes 'scum' to form. does not. Both hard water and soft water lather well with soapless detergents, eg washing-up liquid.
To investigate hardness in water:
- Put a known volume of water into a boiling tube.
- Add soap solution, one drop at a time, and shake the mixture for 10 seconds.
- 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.
- To separate temporary and permanent hard water samples, boil the water.
- Repeat steps two and three with the boiled samples.
Typical results
| Water sample | Volume of soap before boiling (cm3) | Volume of soap after boiling (cm3) | Conclusion |
| A | 2 | 2 | Soft water |
| B | 15 | 2 | Temporary hard water |
| C | 15 | 15 | Permanent hard water |
| D | 15 | 10 | A mixture of temporary and permanent hard water |
| Water sample | A |
|---|---|
| Volume of soap before boiling (cm3) | 2 |
| Volume of soap after boiling (cm3) | 2 |
| Conclusion | Soft water |
| Water sample | B |
|---|---|
| Volume of soap before boiling (cm3) | 15 |
| Volume of soap after boiling (cm3) | 2 |
| Conclusion | Temporary hard water |
| Water sample | C |
|---|---|
| Volume of soap before boiling (cm3) | 15 |
| Volume of soap after boiling (cm3) | 15 |
| Conclusion | Permanent hard water |
| Water sample | D |
|---|---|
| Volume of soap before boiling (cm3) | 15 |
| Volume of soap after boiling (cm3) | 10 |
| Conclusion | A mixture of temporary and permanent hard water |