Appendix 4: How Soap Removes Dirt
Purpose: To observe the action of soap on the surface of water.
Materials: shallow container with cold water (jar lid, small plastic bowl, petri dish), dropper, paper clip, and a bar of soap.
Procedure: 1) Fill a clean shallow container with cold water. The container should be 5-10 cm in diameter and 1-2 cm deep, preferably a lid from a jar, or plastic bowl. Ensure that the container is clean and does not have any soapy residue. Make sure you are eye level with the top surface of the container. Fill the container slowly to the top using a small graduated cylinder or dropper.
2) Take a clean paper clip about 3 cm long and gently place ease it onto the rim of the container and slowly level it on the surface of the water to make it float. It must be done slowly and gently, or else the paper clip will sink. Surface tension will hold the paper clip on the surface of the water.
3) Gently dip one end of a bar of soap into the water, in a spot that is farthest away from the paper clip. Try not to make any waves because then you will break the surface tension. Notice as the soap dissolves that the surface tension will also change and eventually the paper clip will drop to the bottom of the container.
Analyze Questions:
1) What effect did soap have on the surface tension? Explain.
2) Water alone is not good at removing dirt because surface tension prevents water molecules from surrounding dirt particles. How do you think soap improves the ability of water to remove dirt?
3) Temperature is the measure of the amount of energy of a substance. What effect would you expect a high temperature to have on the water molecules and therefore on the surface tension of the water?
Extension: Use warm water or even hot water and repeat the procedure, observing the difference, if any in the surface tension. Use caution with the hot water, and remember slowly and gently.
Vocabulary: surfactant, surface tension, polar, nonpolar, hydrophilic, hydrophobic, attract, repel.
(Adapted from Prentice Hall Science Explorer, Interdisciplinary Explorations, 2001)
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