Investigating a Property of Liquids

Developer:

Great Ideas from DuPont Teaching Fellows
Compiled by the Kinston, NC
DuPont Communication Team
March, 1994

Charles Scaife
Department of Chemistry
Union College
Schenectady, NY 12308

Grade Levels:

K through 2

Discipline:

Physical Science

Goals:

  1. Determine how many drops of water are required to cause a cup to overflow with water.
  2. Develop counting and recording skills.
  3. Work as a team to solve a problem.


Background:

The structure of water molecules makes water act in some interesting ways. The following activity shows you one of the wonders of water, namely its ability to cling to itself and to pile up on a surface. This investigation builds upon the concept of surface tension. Surface tension allows water to rise considerably above the top of a cup without spilling. The youngsters predict how many drops of water it will take to cause a cup to overflow with water. Believe it or not, it takes about 200 drops. Similar experiments are discussed by other authors (1, 2, 3).



Materials:

For each group of four students

4 Plastic cups

1 Plastic Beral pipet to function as a medicine dropper

1 Magnifying glass

Paper towels

1 15-cm Ruler

1 Sponge

Water

Explanation:

Because of the way hydrogen and oxygen atoms are joined within a water molecule, water molecules are attracted to or pulled toward one another. This attraction between two identical molecules is called cohesion.

There is a difference in this attraction among water molecules at the surface of water and in all the rest of the water. A water molecule at t molecules on the surface can pile up and form a kind of "skin" on the surface of the water. This results in what scientists call surface tension. Surface tension helps a drop of water hold its shape. It also lets some insects walk around on the surface of the water without sinking.

Questions:

  1. Did each member of your group require the same number of drops before water spilled from the cup? What were some of the reasons for this?
  2. What is the shape of the "skin" of water just before the cup overflows?
  3. Could you add something to the water in the filled cup that would make the water spill over when fewer drops have been added?
  4. How would you modify the plastic dropper so that you could release smaller drops and thus get more drops on the cup before it overflowed?


References:

  1. WonderScience, Fun Physical Science Activities for Children and Adults to Do Together, Volume 7, Number 1, January, 1993, American Chemical Society/American Institute of Physics, p. 7.
  2. Alfred E. Friedl, Teaching Science to Children: An Integrated Approach, 2nd ed., McGraw-Hill, New York, NY, 1991, pp. 26-29.
  3. Tik L. Liem, Invitations to Science Inquiry: Supplement to First and Second Edition, Science Inquiry Enterprises, Chino Hills, CA, 1991, p. 44.


Procedure:

PREPARING FOR THE EXPERIMENT



In this experiment you will use a plastic medicine dropper. Note that you take water into the dropper by squeezing the plastic bulb, placing the tip of the dropper well under the surface of water in a cup, and releasing your squeeze to let water into the dropper. You let drops out of the dropper by gradually squeezing the plastic bulb.

Divide into groups of four students each.

PERFORMING THE EXPERIMENT



One student fills one plastic cup with water until it appears to be level with water across the top of the cup as vp as viewed with the magnifying glassiewed with the magnifying glass by a second student. A third student partially fills a second cup with water. Before the fourth student begins the experiment that student practices using the dropper over the second cup of water until drops can be let out only one at a time.

The fourth student partially fills the plastic dropper with water from the second cup, and holds the plastic dropper vertically about 1 centimeter above the filled cup in order to place drops of water on the filled cup one by one. (If you arenŐt sure how high 1 centimeter is, check the ruler nearby.)

RECORDING YOUR RESULTS



On a sheet of paper nearby, write the first and last name of each member in your group and the number of drops of water that each was able to get onto the cup before it overflowed.


Go to Easy and Interesting Science Experiments for the Classroom Grades K-12 ONLINE

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Last revision to this page on March 23, 1998

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