Features

• A crystal of an ionic compound like NaCl (table salt) or CaCl2 (calcium chloride) is composed of positively charged ions (cations) and negatively charged ions (anions) that are attracted to each other and hence stay together. When the ionic compound dissolves in water, these ions dissociate.
  
  

Function

• Hard water has lots of calcium and magnesium cations. Removing the calcium and magnesium ions makes the water "soft," but the soft water still contains other ions, such as sodium and chloride. Pure water, on the other hand, contains only molecules of water.
  
  

Considerations

• Water is so good at dissolving ionic compounds that it is seldom, if ever, pure in nature. It is possible to purify or deionize water by removing all of the dissolved ions; in general, however, this is unnecessary. Soft water is preferable to hard water because the mineral content of hard water can form scale on pipes and appliances, which is why many homes that rely on well water use a water softener.