Why should distilled water and not tap water be used to prepare solutions?

Water bath is used for uniform and steady heat transferring by heating water in it. If distilled water is not used during heating process all the salts present in normal water will settle down on the surface causing scaling and decreasing heat transfer.

Distill water is free from salts and compound which form scale like calcium carbonate etc. Water-bath with distilled water will work with efficiency and low maintenance.