Vicki Wagner, age 11, of Victoria; B.C., for her question:

What makes water hard and soft?

The falling rain is soft water. It is fresh and pure because little, if anything is dissolved in it. It may gather a little dust and soot on the way down. It may also dissolve a little carbon dioxide from the air. But when it reaches the ground, rain water is as pure as natural water can be. We call it soft water and it combines easily with. soap to make bubbles and get out the dirt.

As it runs over and through the ground, water dissolves chemicals from soil and rocks. It dissolves calcium from limestone and becomes hard water. The calcium in hard water fights with the soap. Instead of making useful bubbles soap combines with the calcium to form scum.

Certain chemicals remove the calcium from hard water and make it sink to the bottom. A little soda does the trick. Then, when we put in soap with no calcium to fight, the soap goes to work just as though the hard water were soft water.