Surface Tension Grabbers
1991 Woodrow Wilson Biology Institute
Surface tension is a very important biological concept. The myriad of biota that interact at the air-water interface are vital components of aquatic and marine ecosystems.
Despite the significance of surface tension, it is a concept that students find difficult to understand. The following are a few quick activities that allow students to comprehend what surface tension is. Doing some of these activities prior to discussions about life at the air-water interface will greatly enhance student understanding of the topic.
- Fill a gas bottle with water. Place an uncentered burner screen over the bottle. Hold the screen in place with your fingers and invert the bottle on to the ring of a ring stand. Ask the students why the water does not run out.
- Cut a circle of transparent plastic from a can lid. Punch a hole in the center with a paper punch and trim the disc to fit the top of a flat topped reagent bottle. Fill the bottle with water and place the disc on top. Invert the bottle while holding the disc in place with your fingers. Carefully remove your fingers from the disc, it should remain in place and there should be no leakage. The inverted bottle can be held by hand or placed in an iron ring on a ring stand for support. Ask one of your students to try inserting a colored match stick into the paper punch hole. If they are careful, it should float up with little or no loss of water. It is a good idea to have a catch pan under the bottle to take care of unsuccessful attempts. If problems occur, try using different types of plastic for the disc. Some teachers use the plastic from the tops of fast food drink cups.
- Obtain plastic berry baskets from the produce section of your grocery store. The ones that work best have square rather than diamond shaped holes in the bottom. In a large beaker or fish tank demonstrate to your students that the basket will float if placed carefully on the surface. Push the basket under to show that it sinks when not supported by surface tension. Have one of your students try to float the basket, they will not be able to easily float the basket because it will be wet. After the basket dries they will be able to float it again easily. (discuss).
- Mix Joy or Dawn dish washing liquid with water in a 1 to 10 ratio. Place the mixture in containers and have students blow under the surface of the liquid with a straw to see who can blow the largest bubbles. Have them add a little glycerine to their solutions to see if there is and difference in the size of the bubbles produced. (Be sure to use eye protection and other safety procedure during this process).
- Have your students fill a cup with water until the water bulges over the rim. Float a small object such as a cork fragment on the surface. It will float at the center of the cup. Tell the students to remove some other water from the cup and repeat the cork float. The cork will now always float to the side. (discuss)
- Take single common staples and instruct your students to bend them into squares. Students should be able to float the squares on the surface of a container of water if they place them on the surface carefully.
- Try repeating the activities in 1, 2, 3, and 6 after adding a small amount of dish detergent to the water. Discuss how a spill of detergents or similar chemicals into pond or stream could effect surface tension dependent organisms such as water striders, mosquito larvae, diving spiders, and others. Discuss what would happen to the food chain in the contaminated ecosystem.