Hands-on Activity Measuring Surface Tension

Learning Objectives

After this activity, students should be able to:

• Describe how the combination of adhesive and cohesive forces causes water to rise in a thin tube (capillary action).
• Discuss the role of capillary action in measuring surface tension.
• Practice using scientific notation and calculating experimental error.

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.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

For the introductory teacher demonstration:

• a can of paint, motor oil or other fluid
• a container into which you can pour the above fluid

Each group needs:

• 1 ring stand with support ring attached
• capillary tubes of various diameters; suggested sources: McMaster-Carr http://www.mcmaster.com/ or Vitrocom http://www.vitrocom.com/
• large Petri dish
• twist-ties
• water and dark food coloring
• ruler
• eye protection (glasses or goggles) for each student
• Measuring Surface Tension Worksheet, one per person

Worksheets and Attachments

Pre-Req Knowledge

Ability to manipulate algebraic equations, and calculate averages and standard deviations.

Introduction/Motivation

(Open a can of paint or motor oil or other fluid, and pour into a container or otherwise demonstrate its properties.)

What are some important properties of paint (or whatever liquid you are using)? What do we want to accomplish with the paint? What does it need to be able to do? (Encourage students to think about what would happen if paint spread like plain water on a wall. The surface tension is too high and you cannot get the paint to spread evenly. On the other hand, if the surface tension is too low, the paint layer will be too thin and the surface under the paint will show through. If any students have painting experience—artistic or home improvement or hobbies—encourage them to describe any problems they had that are related to this surface tension discussion.)

An adhesive force occurs when molecules of different types are attracted together. This occurs between water and glass, as the charged particles of water are attracted to the glass more than they are to each other. If particles prefer to be near glass than other water molecules, then what do you think will happen when water is near glass? (Answer: it will be attracted to the glass!)

For liquids used in coatings, their surface tensions are extremely important to get just right so they work correctly. One of the most accurate ways to measure surface tension is to use capillary action, the ability for a fluid to climb a thin tube. In this lab activity, we will use capillary tubes in order to measure the surface tension of water. Capillary action is caused the adhesive forces between the glass and water are greater than the water's cohesive forces. Because the water is attracted to the glass, it will begin to rise up the wall of the glass tube.

Procedure

Background

These procedures are written with advanced students in mind. They can be modified for use in a general class.

Before the Activity

• Gather materials and make copies of the Measuring Surface Tension Worksheet.
• Organize lab stations.
• Mix food coloring with water and distribute to each station.
• If lab time is limited, attach the capillary tubes to the ring stands above the Petri dishes before class (see Figure 1).

  

With the Students

1. Divide the class into groups and send them to the lab stations.
2. Direct students to follow the worksheet lab instructions, recording data and answering questions.
3. Have students carefully attach the capillary tubes to the support rings on ring stands in increasing order of their inner diameters.
4. Have groups each place a large petri dish beneath the capillary tubes and lower the tubes so that their ends are just above the bottom of the dish (see Figure 1).
5. Have students fill the Petri dishes with dyed water and watch the water rise in each tube. Expect the water to reach its maximum height quickly (2-3 minutes).
6. Have groups measure the height of the water in each tube and record their data.
7. Have each student calculate the surface tension found experimentally for each tube.
8. Have each student calculate the average surface tension overall and the standard deviation.
9. The accepted value of the surface tension of water in air at 20 °C is γ = 0.073 J/m². Have students compare their measured values for surface tension with the accepted values and speculate on why the values are different.
10. Assign students to individually research a substances for their surface tensions and applications, as described in the Assessment section. Follow-up in a later class period to share findings.

Assessment

Pre-Activity Assessment

Brainstorming: In small groups have students agree upon three applications in which knowing the surface tension of a product is important in quality control. (Anywhere a liquid needs to adhere to a surface in a particular way will depend on surface tension. Examples include: Painting cars or aircraft, cleaning with soap and water, and applying sunscreen or moisturizer.)

Activity Embedded Assessment

Activity Questions: Students demonstrate their thought processes during the activity by answering the questions on the Measuring Surface Tension Worksheet. They extend their learning to the real-world problem of measuring the surface tension of a liquid to ensure proper application in an industrial process. Gauge student understanding by circulating the classroom and asking how they answered specific worksheet questions.

Exploring Further:

Post Assessment: Ask students the following questions to assess their understanding:

• What type of forces cause capillary action? (Answer: Electrical forces between molecules of water, known as adhesive forces.)
• What type of forces resiste capillary action? (Answer: Cohesive forces and gravity.)
• What would happen if the adhesive forces between water and the glass were even stronger? (Answer: Water in a glass tube would rise further up the sides of the glass tube.)

Exploring Further: Assign students to individually research the surface tensions reported online for various substances, and report on applications for which these substances are used.

Safety Issues

• Have students wear eye protection since glass is used in this experiment.
• Handle the capillary tubes with care! The tubes used in this experiment have a thick outer wall, but the glass is still fragile and can be easily broken if not treated gently.

Troubleshooting Tips

• Important: The measurement of surface tension depends greatly on: 1) the purity of the water, and 2) cleanliness of the capillary tubes. (See pg. 6 in Capillarity and Wetting Phenomena by Pierre-Gilles de Gennes, et al., for a short discussion.) Students' results should still agree within an order of magnitude with the accepted value, but will probably be less than the accepted value.
• To see the dyed water in the thinnest tubes, dye the water very dark. If students still have trouble seeing the water, suggest they place a piece of white paper directly behind the capillary tubes.
• Capillary tubes are fragile. Depending on students and time constraints, you may want to set up the capillary tubes and large Petri dish (without water) before class.
• Clean capillary tubes as soon as possible after class by submersing them in water and then blowing air through them.
• On occasion, water rises significantly different amounts, depending on which side of the capillary tube is placed in the water. If possible, check tubes before class or number the capillary tubes given to each group and note if any seem to behave oddly.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This digital library content was developed under an NSF CAREER Award (CBET- 08-46705) and an RET supplement (CBET-10-09869). However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.