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Double-Replacement Reactions

Ions of two compounds exchange places to form two new compounds

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Harder Than It Looks

Harder Than It Looks

Credit: Laszlo Ilyes
Source: http://www.flickr.com/photos/laszlo-photo/4093575863
License: CC BY-NC 3.0

In ads for soaps and detergents, you may hear them refer to hard water. You can also buy water softeners if hard water is a problem for you. Of course, the water is not literally hard (unless it’s frozen). In these cases, the hardness that is being referred to is the presence of certain metal cations. By themselves, these cations are not problematic, but they can react with certain anions, particularly those found in soaps, to form solid layers of scum on your showers, sinks, and dishes.

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  • The most common ions that lead to hard water are the alkaline earth metals Mg2+ and Ca2+. These form nearly insoluble salts with ions such as carbonate (which forms when CO2 dissolves in water) and stearate (which is a fatty carboxylate ion found in soaps).
  • Credit: Graeme Maclean
    Source: http://www.flickr.com/photos/gee01/5585613560/
    License: CC BY-NC 3.0

    The build up due to hard water must be cleaned regularly [Figure2]

  • Not only does hard water cause unsightly soap scum, it can also lead to the deposition of minerals like calcium carbonate on the interior walls of water pipes. If enough of a buildup is formed, the flow of water will be slowed or stopped.
  • Learn more about hard water in the following video: 


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With the links below, learn more about physical and chemicalchanges that can take place in hard water. Then answer the following questions.

  1. What type of reaction (combination, decomposition, etc.) is the formation of soap scum? The chemical equation for this reaction is as follows:
  2. If you boil hard water and then condense the resulting steam in a different container, the liquid water that you obtain is soft. Where did the dissolved ions go? Why aren’t they transferred into the new container along with the water?
  3. We have seen that CaCO3 will form an insoluble precipitate, but Ca(HCO3)2 is much more soluble in water. Based on this observation, what are some types of household items that could be used to dissolve a CaCO3 buildup?
  4. Particularly hard water might contain over 100 mg/L of dissolved calcium and magnesium ions. Would this water be safe to drink? Compare this value to the recommended daily intake (RDI) for these minerals at the following link: http://www.netrition.com/rdi_page.html.

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Image Attributions

  1. [1]^ Credit: Laszlo Ilyes; Source: http://www.flickr.com/photos/laszlo-photo/4093575863; License: CC BY-NC 3.0
  2. [2]^ Credit: Graeme Maclean; Source: http://www.flickr.com/photos/gee01/5585613560/; License: CC BY-NC 3.0

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