How Many Drops Can You Pile on a Penny?
Introduction:
Have you ever seen how water striders can walk across the surface of a pond, or how a mosquito can land on a
puddle of water without sinking? Although the strider’s weight pushes down on the water, forces within the water
create a “skin” that supports the strider on the water’s surface. This phenomenon is called surface tension.
The forces within water that are responsible for surface tension originate at the molecular level. As you know, a
water molecule is polar. The oxygen atom has a partial negative charge, and each hydrogen atom has a partial positive
charge. As a result, electrical attractions occur between the oxygen atom of one molecule and the hydrogen atom of
another molecule, as illustrated in the figure below. These intermolecular attractive forces are called hydrogen bonds
(H-bonds).
Surface tension can be affected by substances dissolved in water. If a substance interferes with hydrogen
bonding, the surface tension of water decreases. If a substance enhances hydrogen bonding, the surface tension of the
water increases. In this investigation, you will observe the effect of several solutes in water. The ionic or molecular
structures of the solutes you will be using are illustrated below. You will use your knowledge of molecular geometry and
the behavior of molecular dipoles to make predictions about the effects of these solutes on the surface tension of water.
In part A, you will investigate the surface tension of water by seeing how many drops of water can be piled on
top of a penny. You will observe the shape and behavior of the water as surface tension holds the drops of water
together. In part B, you will design your own experiment to determine the effects of three substances on the surface
tension of water: liquid detergent, sodium chloride, and sodium carbonate.
Pre-Lab Questions:
Read the entire laboratory investigation and then answer the following questions.
1.
Explain how the intermolecular forces between water molecules act to create the “skin” or surface tension o
upon which water striders can walk.
2. What causes water molecules to be polar?
3. Predict whether the nature of detergent molecules would increase or decrease hydrogen boding in water.
Explain your reasoning.
4. What is viscosity? How is viscosity related to the surface tension of a liquid?
Problem:
What effects do chemical have on the surface tension of water?
Materials:
Part A
Micropipette
Clean, dry penny
Tape water
Paper towel
Part B
3 micropipettes
10mL saturated sodium chloride solution
3 beakers, 100mL
10mL saturated sodium carbonate solution
Graduated cylinder, 10mL
Tap water
10mL liquid detergent solution
Safety:
Wear your goggles and lab apron at all times during the investigation. Sodium carbonate is irritating to the skin. Do not
let this material come in contact with your skin.
Procedure:
Part A
1. Obtain one micropipette and a penny.
2. Fill the micropipette with water.
3. Place drops of water on top of the penny. Be sure to hold the micropipette vertically. Do not touch the penny or
the water that is accumulating there.
4. Count the number of drops you can add before the water flows off the penny. Record the total.
5. Dry the penny and repeat steps 3 and 4 two more times, or until you get consistent results.
Part B
1. You have three solutions: sodium carbonate, sodium chloride and 1% liquid detergent. Your task is to design
and carry out an experiment to determine their effect on the surface tension of water. Write your plans in your
science notebook and have them approved.
2. Design a procedure for your test that includes (a) a control, (b) steps that will ensure that the results are reliable
(or reproducible, and (c) your predictions of the effect of each solute.
3. Create a table that will organize your data clearly.
4. Obtain the necessary materials and run the experiment.
5. Clean up your work area and wash your hands before leaving the laboratory.
Observations:
Part A
Number of drops of water observed:
Trial 1 ____
Trial 2 ____
Trial 3 _____
Average _____
Critical thinking:
1. Compare the average number of drops placed on your penny with the results of your classmates. What might
account for any differences?
2. What happened when the water finally flowed off the penny? Explain in terms of the chemical and physical
forces involved.
3. People usually think of water as being wet. If water alone is used to wash clothing, however, it doesn’t
penetrate the fabric very well. How does the addition of detergent make water “wetter”, that is, more capable
of penetrating the pores of fabric?
Conclusion:
1. Do the results of your investigation agree with your hypotheses?
2. State 1 question or project for further investigation that surfaced as a result of your investigation.