Is the Water Hard or Soft?
Introduction
Homeowners frequently hear from telemarketers peddling everything from magazines to vacuum
cleaners. Telemarketers for water purification systems seem especially common lately. They offer
free testing of household water in hopes that they can show that an expensive water softener
should be purchased. A consumer advocate wishing to help homeowners test their own water has
asked for your group’s help in developing a technique for testing water hardness at home.
To determine if a home really needs an expensive new water softener, you should address several
questions. How do you know whether your water is hard or soft? How hard is it? Which
chemicals work to soften the water? As a group, your team will investigate the above concerns.
The technique you develop might be endorsed by a consumer advocate company.
Goals
As you complete this investigation you will:
1. Develop a method for determining the hardness of a water sample.
2. Use this method to measure the temporary and permanent hardness of the water.
3. Use this method to measure the relative effectiveness of various chemicals as water softeners.
4. Report your results as directed by your instructor
Materials
0.00100 M CaCl2 solution, which has a hardness of 100 ppm
Soap solution
Water sample
Homeowners frequently hear from telemarketers peddling everything from magazines to vacuum
cleaners. Telemarketers for water purification systems seem especially common lately. They offer
free testing of household water in hopes that they can show that an expensive water softener
should be purchased. A consumer advocate wishing to help homeowners test their own water has
asked for your group’s help in developing a technique for testing water hardness at home.
To determine if a home really needs an expensive new water softener, you should address several
questions. How do you know whether your water is hard or soft? How hard is it? Which
chemicals work to soften the water? As a group, your team will investigate the above concerns.
The technique you develop might be endorsed by a consumer advocate company.
Goals
As you complete this investigation you will:
1. Develop a method for determining the hardness of a water sample.
2. Use this method to measure the temporary and permanent hardness of the water.
3. Use this method to measure the relative effectiveness of various chemicals as water softeners.
4. Report your results as directed by your instructor
Materials
0.00100 M CaCl2 solution, which has a hardness of 100 ppm
Soap solution
Water sample
Distilled water sample
Borax, Na2B4O7(s)
Sodium chloride, NaCl(s)
Sodium carbonate (washing soda), Na2CO3(s)
Sodium phosphate, Na3PO4(s)
Sodium sulfate, Na2SO4(s)
Plastic droppers
Balances
Graduated cylinders, burets, pipets, or other volume measuring devices
Other supplies by request
Borax, Na2B4O7(s)
Sodium chloride, NaCl(s)
Sodium carbonate (washing soda), Na2CO3(s)
Sodium phosphate, Na3PO4(s)
Sodium sulfate, Na2SO4(s)
Plastic droppers
Balances
Graduated cylinders, burets, pipets, or other volume measuring devices
Other supplies by request
Getting Started
Water that contains the impurities Ca2+, Mg2+, and sometimes Fe3+, along with the anions Cl–,
SO42–, and HCO3–, is called hard water because the metal ions cause an undesirable precipitate of
soap scum when soap is added. For example, some soaps contain compounds of the anion,
C17H35CO2–. This anion can react with calcium ions present in hard water to make an insoluble
compound:
Water that contains the impurities Ca2+, Mg2+, and sometimes Fe3+, along with the anions Cl–,
SO42–, and HCO3–, is called hard water because the metal ions cause an undesirable precipitate of
soap scum when soap is added. For example, some soaps contain compounds of the anion,
C17H35CO2–. This anion can react with calcium ions present in hard water to make an insoluble
compound:
Lathering will not occur until the metal ions associated with water hardness are consumed. The
formation of a permanent lather can be used as a measure of the hardness of the water. When
developing your method for determining hardness, however, be sure to consider whether suds
would form with the first addition of soap in the absence of any hardness. You might begin by
comparing the hardness of tap water and deionized or distilled water. A solution of 0.00100 M
calcium chloride is provided. This solution has a hardness of 100 part per million. This solution
can be used as a hard water standard to quantify your results.
Hardness is considered to be permanent if it remains after water is boiled. Hardness that is
removed by boiling is called temporary hardness. The temporary hardness is caused by
bicarbonate compounds, which are converted to insoluble carbonate compounds by heating:
Since the calcium is removed as a carbonate precipitate, it is no longer present as free calcium
ions in solution. Permanent hardness is caused by chloride or sulfate salts, which are not changed
by heating. When testing the use of various solid chemicals as water softeners, be sure to compare equal amounts of the solids and make sure that the solids are completely dissolved.
Report
Submit a report as directed by your instructor. It may be returned to you for corrections if it is not
acceptable. Your discussion should address the different types of hardness and the relative
efficiency of the various salts as water softeners. You should also compare various watersoftening
systems, such as the new magnetic systems and older methods such as ion exchange,
distillation, and reverse osmosis. Useful information about water softening can be found on the
internet. Compare the costs of the different systems. Could you use inexpensive chemicals to
soften the water?
source: (Brooks Cole Laboratory Series for General Chemistry) Richard Bauer, James Birk, Doug Sawyer - Laboratory Inquiry in Chemistry, 2009
