What’s special about detergent? You know, of course, that liquid detergent is great for washing
dishes. But did you know that you can use liquid detergent to stage a unique boat race?
StuFF You’ll Need• paper clip
• toothpick
• liquid dish-washing detergent
• cup or glass filled with water
StuFF You’ll Need• paper clip
• toothpick
• water
• scissors
• glue
QuestionHow does detergent affect things floating on water?
1. Fill in the Question, Research, and Hypothesis sections of your lab report.
2. Float the paper clip on the surface of the water in the cup. To do this, very gently and slowly lay the paper clip on the water’s surface. (If you are having trouble making it float, cut a small bit of paper towel, lay the paper clip on top of the towel and float that on the water’s surface. Then carefully slide the towel out from under the paper clip.)
3. Dip the toothpick in the detergent. Place one drop of detergent into the water. Record your observations in the Data section of your lab report.
1. Glue the boat pattern on page 5 onto the index card. Carefully cut out the cardboard boat.
2. Fill the pan with water (or you can plug up your sink and fill it with water).
3. Gently place your boat in the water so it floats.
4. Dip the toothpick into the liquid detergent.
5. Place one drop of detergent into the triangular hole in the boat’s stern (back) and watch what happens to the boat! Record your observations in the Data section of your lab report.
6. Wash and dry the pan completely each time you repeat this experiment. Don’t forget to replace your soggy boat with a fresh, dry one. Use another pattern and index card to make more boats.
Water striders can actually walk on water. Sometimes people dump detergent in water. What do you think happens to a water strider when detergent is dumped in the water where it lives?
Lab Report
Extra CreditHave a soap boat race with a friend. Float two boats side-by-side in a pan. Drop two different types of detergent into the stern holes of each boat at exactly the same time and see which detergent propels the winning boat.
AnswersQuestionWhen the detergent is added, the paper clip sinks and the cardboard boat zooms forward.
Real World ExampleDetergent causes water striders to sink just as the paper clip sinks. Water striders eat mosquito larva and other insects. In what ways do you think detergent spilled into our waterways affects our ecosystem?
ExplanationWater molecules “stick” together. The positive hydrogen atoms on one water molecule are attracted to the negative oxygen atom on the next water molecule. When they stick together at the surface, this is called surface tension. Paper clips don’t really float (they are more dense than water). But by laying one carefully on the water’s surface, you can get it to rest on the surface because the water’s surface tension doesn’t allow the paper clip to pass through.
Detergent breaks the surface tension of water. When the detergent was added to the cup with the paper clip, the soap molecules squeezed in between the water molecules, pushing them farther apart. Because of the greater separation distance between water molecules, their attractive force (surface tension) is weaker. The paper clip was then able to pass through the surface and sink to the bottom. Picture a grapefruit being supported by
a dry paper towel. If you wet the paper towel, its ability to support the weight is weakened, and the fruit falls through. Wetting the paper towel is similar to breaking up the surface tension with detergent so the paper clip falls through.
The cardboard boat floats on water. Before the detergent was added, surface tension pulled on the boat in all directions equally so it didn’t move. When detergent was added to the back, the surface tension broke apart locally as the water molecules separated. The force pulling the boat was uneven in the forward direction, forcing the boat to move forward.
The detergent dissolves in the water very quickly, breaking up the surface tension everywhere in the water within seconds. For this reason, you must clean out the pan and put in fresh, detergent-free water to repeat the experiment.
