John Kolach, age 13, of Albany, New York, for his question:

Why is some water hard and some soft?

In nature, the softest water is falling raindrops. It forms from misty cloud moisture, which often distills from gaseous water vapor in the air. At this stage, the rain water may be free from other chemicals and impurities. It is pure or soft water, the sort of water that mixes with the soap to form the foamy white lather that gets out the dirt. But water is a great solvent, always eager to dissolve almost every other chemical it touches.

I)issolved chemicals turn soft water into hard water. Some may be dissolved as rain falls through polluted air. But far more are gathered when running water seeps through rocky minerals in the ground. Most hard water refuses to form soapy foam. Instead of washing the laundry, it leaves a gummy scum around the tub. Some of the hardest water gathers its invisible chemicals as it seeps through laywers of chalky limestone. When alkaline sodas are added to this water, they precipitate the hard chemicals and the water becomes soft enough to wash the laundry.