Which Addition to Hard Water would Reduce its Conductivity, Washing Soda or Rain Water? (page 2)

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Author: Michael Calhoun


Hard water refers to the presence of dissolved ions, mainly of calcium Ca2+ and magnesium Mg2+ which are acquired through contact with rocks such as limestone and dolomite which contain calcium and magnesium. Because of these and other metallic ions that are present in hard water it can conduct and electric current. 

Solutions that contain this capability of conducting electricity are called electrolytes. Because the electrical current is transported by the ions in solution, the conductivity increases as the concentration of ions increases or decreases when the ionic concentration decreases. While hard water is not a health concern, excessive amounts of water hardness can cause lime or scale build-up in pipes and reduce the effectiveness of soaps and laundry detergents.  

Washing soda (sodium carbonate) is used in homes for the purpose of softening hard water. When soaps or detergents are added to hard water, no lather is formed, which in turn affects the cleaning ability of the soap. This is because the calcium and magnesium ions present in hard water react with soaps and detergents and changes their properties and prevents formation of lather. When washing soda is added the calcium and magnesium ions in hard water react with the carbonate ions from the sodium carbonate and a precipitate of calcium carbonate results. This can be seen in the equation.

As can be seen in the chemical reaction equation ions are still present after the addition of washing soda therefore an electric current can still flow through the now “soft” water which will be detected when the conductivity is measured. 

Compared to the hard water and water treated with washing soda, rainwater has lower conductivity. The addition of rain water to hard water will diluted the concentration of ions and thereby lowers the conductivity of the hard water.  

Electrical conductivity can be measured using a conductivity measuring device consisting of two metal electrodes usually spaced 1 cm apart (thus the unit of measurement is microSeimens or milliSeimens per centimeter). A constant voltage is applied across the electrodes resulting in an electrical current flowing through the solution to be tested. Since the current flowing through the water is proportional to the concentration of dissolved ions in the water, the electrical conductivity can be measured. The higher the dissolved ion concentration, the more conductive the sample and hence the higher the conductivity reading.

Digital photos can be taken during the experimenting process and the following websites offer down loadable images that can be used on the display board:

Research Questions

  • What is hard water and how is it produced?
  • What is conductivity?
  • Does the addition of washing soda change hard water from conductor of electricity to a non conductor?
  • Which has lower conductivity, hard water treated with washing soda or hard water treated with rainwater?
  • If a hard water sample was taken from a natural body of water (lake, river, or stream) before it rained and after it has rained would there be a noticeable change in conductivity?
  • What is the control for this investigation? 
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