Hard and soft water: the soap test
Shake soap with soft and hard water to compare lather and scum, then soften hard water by boiling and with washing soda.
Core concept
Hard water contains dissolved calcium and magnesium salts that stop soap lathering
Topic: Water and the environment
Rainwater is soft, but water that flows through limestone rock dissolve: To mix into a liquid so completely that you can no longer see the pieces. All key words calcium and magnesium salts and becomes hard. In hard water, soap reacts with these salts to form a sticky white scum instead of a lather, so more soap is wasted. Temporary hardness is caused by hydrogencarbonates and can be removed by boiling. Permanent hardness is caused by sulfates and chlorides and is not removed by boiling, but washing soda (sodium carbonate) removes both kinds.
What you will learn
- Use the soap test to tell soft water from hard water.
- Distinguish temporary and permanent hardness.
- Explain how boiling and washing soda soften water.
Before you start
- Know that some salts dissolve in water.
Materials
- Distilled water (soft)
- Temporary hard water and permanent hard water
- Soap solution
- Washing soda (sodium carbonate)
Equipment
- 6 test tubes with stoppers in a rack
- Heat source
Procedure
- Add soap solution to Tubes A, B and C and shake each one. Compare the lather.
- Boil Tube D (temporary hard water) and Tube F (permanent hard water) for 5 minutes, then test both with soap.
- Add washing soda to Tube E (permanent hard water), shake, then test it with soap.
How this simulation is simplified
- Shaking is shown as stirring.
- Boiling for 5 minutes removes all the temporary hardness in this model.