Tap water in many parts of the country contains minerals that can interfere with the cleaning ability of detergents. That's why water softeners are popular in these locations. Water softeners remove these minerals. In this experiment, you will make "hard" water from distilled water, which contains no minerals, and is therefore "soft." You can then compare the sudsing ability of a detergent in soft and hard water.
For this experiment you will need:
Pour 250 milliliters (1 cup) of distilled water into each of the empty soft-drink bottles. Add 5 milliliters (1 teaspoon) of epsom salts to one of the bottles. Swirl the bottle until the epsom salts dissolve. Add several drops of liquid dish detergent to both bottles. Seal the bottles with their caps. Shake both bottles. A large amount of suds will form in the bottle without epsom salts. Far fewer suds will form in the bottle containing the epsom salts.
The suds formed in this experiment are made of tiny bubbles. The bubbles are formed when air is trapped in a film of liquid. The air is trapped when it is shaken into the water. The film of liquid surrounding each bubble is a mixture of water and detergent. The molecules of detergent form a sort of framework that holds the water molecules in place in the film. If there were no detergent, the bubbles would collapse almost as soon as they are formed. You can see what this would look like by repeating the experiment, but leaving out the detergent.
This experiment will not produce suds if detergent for a dishwashing machine is used. (Try it and see.) No suds are formed because automatic dishwasher detergent is formulated so that it does not form suds. Suds create problems in a dishwasher. They interfere with the movements of the washing arms, and they are difficult to rinse off of the dishes.
The minerals that make water hard usually contain calcium and magnesium. In this experiment, you made water hard by adding epsom salt, which is magnesium sulfate. Calcium and magnesium in water interfere with the cleaning action of soap and detergent. They do this by combining with soap or detergent and forming a scum that does not dissolve in water. Because they react with soap and detergent, they remove the soap and detergent, thereby reducing the effectiveness of these cleaning agents. This could be overcome by adding more soap or detergent. However, the scum that is formed can adhere to what is being washed, making it appear dingy.
Water can be softened in a number of ways. An automatic water softener connected to water supply pipes removes magnesium and calcium from water and replaces them with sodium. Sodium does not react with soap or detergents. If you don't have an automatic water softener, you can still soften laundry water by adding softeners directly to the wash water. These softeners combine with calcium and magnesium, preventing the minerals from forming a soap scum.
| For additional information, see CHEMICAL DEMONSTRATIONS: A Handbook for Teachers of Chemistry, Volume 3, by Bassam Z. Shakhashiri, The University of Wisconsin Press, 2537 Daniels Street, Madison, Wisconsin 53704. |