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Lesson 11:

Surface Tension

Concepts conveyed | Materials | Procedure | Benefits | Resource

Concepts conveyed:

The purpose of this demonstration is to illustrate the effects of surface tension and its importance in our daily lives.

Materials:

(for one surface tension kit)

  • 1, 3 x 5 in. index card
  • 1 pair of scissors
  • 1 baking dish (or other container with dimensions that are about 10 in. on a side or in diameter)
  • liquid dish detergent

Procedure:

Ask for two student volunteers or instruct the students to form small groups of two to six. Each pair or group should receive a surface tension kit. From an index card, have one of the students cut out a boat. It should have a pointed end and a straight end, and the straight end should have a square notch cut out of the middle of it for the soap to go (See Figure 1).


Figure 1

Place the boat gently on the water in the dish, and ask the other student to pour a little detergent into the notch at the end of the boat. Observe what happens. If student volunteers are involved, try the experiment again with a new boat and two new students, until everyone has had a chance to see what has taken place. Explain surface tension to the students and discuss the relevance of surface tension, as it relates to soap in our lives.

Surface Tension:

Surface tension is the measure of the inward forces that must be overcome in order to expand the surface area of a liquid. Water has a high surface tension because of its strong hydrogen bonds. Soap is made up of long hydrocarbon chains, which are hydrophobic (repel water). One end of each hydrophobic chain is hydrophilic (attracts water). When the soap enters the water, it spreads out so that the hydrophobic ends extend away from the water’s surface and the hydrophilic ends are in the water. This phenomenon causes the water’s surface tension to decrease by disrupting the arrangement of the water molecules at the surface, expanding the surface area inside the notch and propelling the boat forward.

The different ends of the soap molecule are also an important part of our daily lives. The hydrophobic end attracts grease and dirt, while the hydrophilic end attracts the water. Thus, the soap allows the grease and dirt to be washed away by water. Without the attractive forces of water and soap, this world would be a much dirtier place.

Benefits:

  • This demonstration gives students a fun way to experience surface tension.
  • The demonstration can be used as a means to check attendance. The students can pass the boats around within their group, sign them, and turn them into the instructor.

Resource:

  • Soap Power. Obtained from the Website, Helping Your Child Learn Science. http://www.ed.gov./pubs/parents/Science/soap.html United States Department of Education home page, 1996.
  • Chemistry the Central Science; Brown, T. L., LeMay, H. E., & Bursten, B. E., Eds.; Prentice-Hall: Englewood Cliffs, NJ, 1991; p 375.