Hard & soft water. Water is said to be hard because it contains
  dissolved calcium salts, most frequently calcium bicarbonate,
  which form insoluble salts with soap, so that it's hard to get
  a lather. Such waters are usually neutral or mildly alkaline.
  Heating, especially boiling, changes calcium bicarbonate to
  the carbonate, which builds up as insoluble scale in kettles,
  water baths etc. The scale is often discoloured by precipitated
  salts of other metals such as iron or manganese.

  Soft water lathers easily - you need much less soap to get 
  yourself and your clothes clean than with hard water. Pure
  water is, of course, soft. Soft tap water is often mildly
  acidic due to dissolved CO2, and some other solutes such as 
  iron salts that may be present. Soft tap water doesn't form
  scale in kettles, pies etc and for many purposes can be used
  instead of distilled or otherwise purified water. 
  
                                      John Kiernan
                                      London, Canada.
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On Tue, 31 Oct 2000, Cheryl Crowder wrote:

> Sharon - If you mean an alkaline pH of the tap water, we have been using it 
> since I starting working here.  Our tap water has a pH of 8.3.  We have 
> found it does not hurt staining - in fact, hematoxylin blues more readily 
> because of the alkaline pH.  We use it in the water baths, for everything 
> where we use tap water and have noticed no differences in staining from 
> that and deionized water.  Cheryl