Hard Rain, Soft Suds
By John Cowens

To read John Cowens' related article "Hard Water, Soft Water" click here

Hard water (left) produces no suds, but soft water (right) lathers up.

In the May 2004 issue of Teaching K-8, I explained the difference between soft water and hard water, and described methods that homeowners use to soften their hard water.

Included was an activity in which students made hard water "scum." Here are two more aqueous activities – one in which students will soften hard water, and one that compares how soap reacts with hard water and soft water.

Softening hard water
As rain water passes through the ground, it usually dissolves some of the minerals in the soil, creating permanent hard water. The calcium sulfate that causes the water's hardness can be removed with Arm & Hammer's "All Natural Super Washing Soda" (sodium carbonate).

When the washing soda is added to the hard water, calcium carbonate and sodium sulfate are formed. Calcium carbonate is insoluble and settles to the bottom as a fine white powder. The sodium sulfate remains dissolved in the water but doesn't interfere with the cleansing action of soap.

If magnesium sulfate is also present in hard water, lime must be used in addition to the washing soda. You can demonstrate the action of washing soda in softening water by doing the following experiment:

Materials:

• plaster of Paris


• water


• clear tall 16 oz. (.47 L) drinking glass or clear plastic jar


• quart-sized (.95 L) jar


• teaspoon


• washing soda (Arm & Hammer's "Super Washing Soda")*

*NOTE: This product is found in the detergent or cleaning aisle of grocery stores. It's made of sodium carbonate or soda ash and is a mixture of common salt, ammonia, carbon dioxide and lime. It has a pH of 11 (caustic/alkaline). Don't confuse this with baking soda, which is sodium bicarbonate, with a pH of 8.1

Procedures:

1. To make hard water, add a teaspoon of dry, powdered plaster of Paris to a quart (.95 L) of water. Stir well and let stand overnight.
2. Pour half of the clear hard water into a tall 16 oz. (.47 L) clear glass. Dissolve a teaspoon of washing soda in the second glass of water and add some of this solution to the hard water in the first glass.
3. A cloudy solution of calcium carbonate (powdered chalk) will form immediately. If the glass is left undisturbed, the chalk will settle to the bottom and the water will be softened. To prove you made soft water, use it, with ordinary soap, to wash your hands.

Sudsing ability of a detergent in hard and soft waters
Materials:

• distilled water


• epsom salts (magnesium sulfate)


• two clean two-liter plastic soft-drink bottles with screw caps (labels removed)


• bottle of liquid dishwashing detergent (do not use automatic dishwasher soap)

Procedures:

1. Pour two pints (1 L) of distilled water into each of the empty soft-drink bottles.
2. Add two teaspoons (10 mL) of epsom salts to one of the bottles and stir until the epsom salts dissolve.
3. Add 3-4 drops of liquid dish detergent to both bottles, seal the bottles, and shake them for 5-8 seconds.

The bottle without epsom salts produces a large amount of suds in the form of tiny bubbles. Without the liquid soap, the bubbles would instantly lose their holding power, known as "film," and collapse.

To read John Cowens' related article "Hard Water, Soft Water" click here

John Cowens teaches science at Fleming Middle School in Grants Pass, OR.