Hands-on Activity: Wet Pennies

Contributed by: Engineering K-PhD Program, Pratt School of Engineering, Duke University

Summary

Students conduct a simple test to determine how many drops of each of three liquids—water, rubbing alcohol, vegetable oil—can be placed on a penny before spilling over. Because of their different surface tensions, more water can be piled on top of a penny than either of the other two liquids. However, the main point of the activity is for students to come up with an explanation for their observations about the different amounts of liquids a penny can hold. To do this, they create hypotheses that explain their observations, and because middle school students are not likely to have prior knowledge of the property of surface tension, their hypotheses are not likely to include this idea. Then they are asked to come up with ways to test their hypotheses, although they do not need to actually conduct these tests as part of this activity.
This engineering curriculum meets Next Generation Science Standards (NGSS).

A color photo of a water strider insect on the surface of water with leaves floating nearby.
The same phenomenon that lets a water strider walk on water also allows students to place an astonishing number of drops of water on a penny.
copyright
Copyright © Robert Suter, Vassar College http://faculty.vassar.edu/suter/1websites/bejohns/mateselection/files/female.htm

Engineering Connection

Chemical engineers apply their understanding of natural scientific properties, such as surface tension, as they design experiments to create new materials and products that behave as they desire.

Pre-Req Knowledge

Students should be able to calculate the average of four numbers less than 50.

Learning Objectives

After this activity, students should be able to:

  • Give an example of a hypothesis that is based on an observation of a natural phenomenon.
  • Give an example of an experiment designed to address a specific hypothesis.

More Curriculum Like This

How Many Drops?

In this lesson and its associated activity, students conduct a simple test to determine how many drops of each of three liquids can be placed on a penny before spilling over. The three liquids are water, rubbing alcohol, and vegetable oil; because of their different surface tensions, more water can ...

Middle School Lesson
Students as Scientists

Through two lessons and their associated activities, students do the work of scientists by designing their own experiments to answer questions they generate. Through a simple activity involving surface tension, students learn what a hypothesis is—and isn't—and why generating a hypothesis is an impor...

Middle School Unit
Surfactants: Helping Molecules Get Along

Students learn about the basics of molecules and how they interact with each other. They learn about the idea of polar and non-polar molecules and how they act with other fluids and surfaces. Students acquire a conceptual understanding of surfactant molecules and how they work on a molecular level. ...

Factors Affecting Friction

Based on what students have already learned about friction, they formulate hypotheses concerning the effects of weight and contact area on the amount of friction between two surfaces.

Middle School Lesson

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Make observations and measurements to identify materials based on their properties. (Grade 5) Details... View more aligned curriculum... Do you agree with this alignment?
  • Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Decide whether two quantities are in a proportional relationship, e.g., by testing for equivalent ratios in a table or graphing on a coordinate plane and observing whether the graph is a straight line through the origin. (Grade 7) Details... View more aligned curriculum... Do you agree with this alignment?
  • Summarize, represent, and interpret data on a single count or measurement variable (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Interpret and evaluate the accuracy of the information obtained and determine if it is useful. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Decide whether two quantities are in a proportional relationship, e.g., by testing for equivalent ratios in a table or graphing on a coordinate plane and observing whether the graph is a straight line through the origin. (Grade 7) Details... View more aligned curriculum... Do you agree with this alignment?
  • Summarize, represent, and interpret data on a single count or measurement variable (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Understand the properties of matter and changes that occur when matter interacts in an open and closed container. (Grade 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Explain the effects of forces (including weight, normal, tension and friction) on objects. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Understand the bonding that occurs in simple compounds in terms of bond type, strength, and properties. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
Suggest an alignment not listed above

Materials List

  • 3 small beakers (50-100 ml) or plastic cups (about 4 to 8 ounces) per group
  • 1 disposable pipette per student, plus a few extras
  • 1 penny per student
  • rubbing (isopropyl) alcohol, two 16-ounce bottles
  • vegetable oil, one 32-ounce bottle
  • paper towels, several per student
  • water

Introduction/Motivation

This activity engages students' attention very quickly and thus requires little or no introduction. Simply divide the class into teams of four students each, provide the materials and written instructions for students, and let them proceed with the activity.

Vocabulary/Definitions

hypothesis: A tentative explanation for a fact or set of observations, which can be tested objectively.

Procedure

Before the Activity

  • Gather materials and make copies of the Wet Pennies Handout.
  • As indicated in the student instructions, each team needs three containers (beakers or cups) of liquid: one containing water, one containing rubbing alcohol, and one containing vegetable oil. For each liquid, use about 2 ounces (about one-half inch of liquid height). Label the containers.

With the Students

  1. Divide the class into groups of four students each.
  2. Pass out the materials and handouts. Emphasize the need to follow the instructions carefully, especially in regard to the order of the liquids they test on their pennies.
  3. After the experiment is done, lead a concluding class discussion, as described in the handout and Assessment section. Ask the Investigating Questions.

Attachments

Troubleshooting Tips

Some students may get the mistaken idea that the water-dropping activity is some sort of contest that they can win by getting the most drops, of any liquid, on the penny. Try to avoid giving students this idea, and if they come up with it on their own, explain that the point is for them to be able to fairly and accurately compare the numbers of drops of each liquid a penny can hold -- not to compare how many drops each student can get a penny to hold. To fairly and accurately compare the number of drops of each liquid a penny can hold, students need only try to use the same dropping technique (same squeezing pressure on the pipette bulb, same height it is held above the penny) for each liquid.

Investigating Questions

  • How many drops of water fit on your penny? Was this number similar to the number other members of your group got? What do you think could cause differences between the numbers you got? (Students might respond that individuals have different dropping styles [speed, height they hold the pipette above the coin, etc.] and that newer pennies seem to have a more distinct rim, which could allow more water to stay on the penny. They might also speculate that differences between the pipettes could cause different sized drops to form.)
  • Did you get different averages for the three liquids? If so, which liquid allowed you to put more drops on the penny?
  • Are your results consistent with those of other groups? In what ways are they the same or different?

Assessment

Concluding Discussion: As outlined in the Wet Pennies Handout, lead a class discussion so students can share and compare results and conclusions. Ask the Investigating Questions. Listen to students' discussion contributions and answers to gauge their level of comprehension.

Activity Extensions

Students may want to immediately test the hypotheses they develop as part of this activity. If time allows, some of the ideas they are likely to generate can be quickly and easily tested, and it is a good idea to encourage their enthusiasm by letting them do so. See the Lesson Closure and Lesson Extension Activities sections of the How Many Drops? associated lesson or more information.

Contributors

Mary R. Hebrank, project writer and consultant

Copyright

© 2013 by Regents of the University of Colorado; original © 2004 Duke University

Supporting Program

Engineering K-PhD Program, Pratt School of Engineering, Duke University

Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.

This activity was originally published, in slightly modified form, by Duke University's Center for Inquiry Based Learning (CIBL). Please visit http://www.biology.duke.edu/cibl for information about CIBL and other resources for K-12 science and math teachers.

Last modified: September 7, 2017

Comments