When water passes through or over mineral deposits such as limestone, the levels of certain ions present in the water increase greatly and cause the water to be classified as hard water. This term results from the fact that calcium or magnesium ions in water combine with soap molecules, forming a sticky scum that interferes with soap action and makes it “hard” to get suds. Soft water, such as rainwater or melted snow, is mostly free of these ions. Soft water, when mixed with soap will form suds. A Conductivity Probe can be used to test for ions in water. Hard water, because of the ions it contains, gives a high conductivity reading. Soft water gives a low conductivity reading.


  • Use soap to find water hardness.
  • Use a Conductivity Probe to find water hardness.
  • Find the hardness of the water in your school.
  • Apply what you learn as you do a water-hardness study.