“Mad Science” Mini-Lessons
10. Surface tension and detergents
Surface tension; solubility
Water, the purer the better; small basins, one for each student group; liquid detergent; soap; hydrophobic powder (cinnamon, ground pepper, lycopodium or other pollen); small paper clip, needle, or fine wire for each student group; forceps if available, forks if not; waxed dental floss
If time permits, try using soap in all of these procedures where detergent is called for.
Floating paper clip
- Place clean water in a clean basin to about 5 cm depth. Make sure that there is no crud in the beaker, and no oily scum on the water. Everything should be as clean as possible.
- Make sure that your paper clip, needle, or wire is clean and dry. Gently hold it with forceps, scissors, or a fork and lower it to the surface of the water. Release it so that it floats on the surface of the water. If it does not float, recover the paper clip, carefully clean and dry it, and try again.
- Look at the floating paper clip, both from above and with your eyes level with it. What do you see?
- Now place a tiny amount of detergent on the end of a toothpick and touch it to the surface of the water some distance away from the paper clip. What happens?
- Recover the paper clip, rinse it off, and carefully dry it. Try to make it float on the surface of the water again, as you did before. What happens?
- Can you make another new, dry paper clip float on the surface? What must you do to accomplish this?
- When you are finished, wash the basin well with detergent and water, and place it in the drainer to dry. Similarly wash the paper clips, rinse well, and dry.
- Fill the bowl with clean water. Float a loop of waxed dental floss on the surface of the water. The loop should be open, but oblong rather than round. () The entire loop should lie on the surface of the water.
- Touch a drop of detergent to the water surface inside the loop. What happens?
- Dump out the water and rinse the bowl and the floss loop with clean water. Refill the bowl with clean water. Float the floss loop on the surface, this time making it as round as possible.
- Touch a drop of detergent on the end of a rod to the water surface outside the loop. What happens?
- Clean up when you are done.
- Fill the bowl with clean water. Sprinkle ground pepper (cinnamon or lycopodium powder will also work) onto the surface of the water. The pepper should be evenly distributed over the entire surface of the water.
- Touch a drop of detergent to the water surface near the center of the bowl. What happens?
- Clean up when you are done.
Questions to consider
- What did the surface of the water look like when the paper clip was floating on it?
- What causes a buoyancy force on an object submerged in a fluid?
- Can the (upward) buoyancy force on the paper clip be as great as the paper clip’s (downward) weight?
- When the paper clip was floating on the surface of the water, what force kept it from sinking?
- What did adding a small amount of detergent do to the surface of the water?
- How did detergent keep the paper clip from floating on water?
- What forces acted on the dental floss on the water surface?
- What made the dental floss and pepper move on the surface of the water?
The science details
Water molecules are strongly attracted to each other. When water contacts another material to which it is not so strongly attracted, such as air or oil, it forms a layer at the surface that is under tension, like a sheet of stretched rubber. This layer is strong enough to support light objects that are heavier than water, such as a paper clip or an insect.
Soaps and detergents are surfactants (short for surface active agents) stabilize the surface, so that it is not under such tension. A small amount of detergent added to the surface of water is pulled to a thin film by the retreating surface tension. You can see this by the movement of items floating on the water surface.
Copyright © 2004, Richard Barrans
Revised: 21 December 2016; Maintained by Richard Barrans.