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Lesson: Capillarity—Measuring Surface Tension

Quick Look

Grade Level: 12 (10-12)

Time Required: 30 minutes

Lesson Dependency:

Subject Areas: Chemistry, Physical Science, Physics

Photo shows a glass tube held from above and positioned vertically into a container of water below it.
The surface tension of a liquid can be found by measuring how high a liquid climbs a capillary tube.
Copyright © Michael Krahe, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Oberfl%C3%A4chenspannung_Kapillareffekt.jpg


Students are presented with a short lesson on the difference between cohesive forces (the forces that hold water molecules together and create surface tension) and adhesive forces (the forces that causes water to "stick" to solid surfaces. The interaction between cohesive forces and adhesive forces causes the well-known capillary action. Students also use the associated activities to introduce examples of capillary action found in nature and in our day-to-day lives.
This engineering curriculum aligns to Next Generation Science Standards (NGSS).

Engineering Connection

Many industrial processes depend on the accurate measurement of surface tension. When an object is painted or coated, the coating surface tension must be carefully maintained to produce the desired thickness without creating uneven patches. The strength and effectiveness of detergents are also partially determined by surface tension. One accurate method of measuring surface tension is through capillary action. The height water rises in a thin tube is related to the surface tension of the climbing liquid. Besides providing a method for measuring surface tension, capillary action itself has many different applications. Capillary action in part determines the behavior of ground water in the soil, which makes it important to civil and environmental engineers in understanding the stability of buildings and roads as well as the environmental impact of human development. Petroleum engineers use their understanding of capillary action in the extraction of crude oil from its rock reservoirs. And, understanding capillary action in the transport of fluids in animals is important in biomedical engineering.

Learning Objectives

After this lesson, students should be able to:

  • Describe the differences between cohesive and adhesive forces.
  • Explain how a meniscus is formed.
  • Describe how the combination of adhesive and cohesive forces causes water to rise in a thin tube or other small space (capillary action).
  • Explain how capillary action is used.

Educational Standards

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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 Performance Expectation

HS-PS1-3. Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. (Grades 9 - 12)

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This lesson focuses on the following Three Dimensional Learning aspects of NGSS:
Science & Engineering Practices Disciplinary Core Ideas Crosscutting Concepts
Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

Alignment agreement:

The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.

Alignment agreement:

Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.

Alignment agreement:

  • Knowledge gained from other fields of study has a direct effect on the development of technological products and systems. (Grades 6 - 8) More Details

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Pre-Req Knowledge

A basic understanding of cohesive forces (attraction of liquid molecules to each other) and surface tension.


(In advance, have handy a glass of water, food dye/coloring, and white paper towels, to use as described below.)

(Hold up a beaker or glass of water colored with food dye.) Imagine you've been outside during a hot July day. You come inside and pour a big glass of water. (Place glass on counter top.) Now let's say you start talking to your parents or a brother or sister and... (knock the glass over to spill some of the water with food dye.) What are you going to do now? (Expect someone to suggest using a paper towel to clean it up.)

(Place one edge of a white paper towel into the colored water spill. Let the water climb into the paper towel while you talk.) Will someone in front who can see what is happening to the towel describe it for those in the back? (Expect a student to describe how the color is creeping up the towel.) Why is this happening? (Give students some time to answer. Expect a student to mention the spaces or holes in the towel.) Good. What else can you think of that has holes and absorbs water? (Various answers are possible. One example is a sponge).

Photo shows a woman using paper towels and a spray bottle of liquid to clean a wall mirror.
The ability of paper towels to absorb liquids is an example of capillary action.
Copyright © 2004 Microsoft Corporation, One Microsoft Way, Redmond, WA 98052-6399 USA. All rights reserved.

(Lift the paper towel out of the liquid and show the class how the liquid has climbed.) What if I tried to do this with a plastic bag, would that absorb the water? Why not? Let's list some materials that can absorb and some that cannot. We'll see what each group have in common. (With students' help, create two lists and look for the similarities within each group and the differences between them.)

One very accurate method of measuring surface tension is through capillary action, since the height water rises in a thin tube is related to the surface tension of the climbing liquid. For many industrial processes, an accurate measurement of surface tension important — such as when painting or coating an object (coating a pan with oil from a spray can, applying solder to form electrical connections on a circuit board, applying ink using inkjet printing, aircraft painting, etc.), so the surface tension of the coating is carefully maintained to produce the desired thickness without creating any uneven patches. The strength and effectiveness of detergents are also partially determined by surface tension.

Besides providing a method for measuring surface tension, capillary action itself has many different applications. Can you think of any examples of capillary action in the real-world? (Take suggestions from students.) What about: cleaning with a sponge, how the soil in your yard absorbs rain water, wax rising up a wick, or how your T-shirt gets wet when you sweat? What about some applications that engineers might be involved with? Well, capillary action in part determines the behavior of ground water in the soil, which makes it important to civil and environmental engineers in understanding the stability of buildings and roads as well as the environmental impact of human development. Petroleum engineers use their understanding of capillary action in the extraction of crude oil from its rock reservoirs. And, understanding capillary action in the transport of fluids in animals is important in biomedical engineering.

We've already learned about surface tension and the forces that hold water drops together. Today we are going to discuss why water sticks to other things and even climbs up some objects. We will see that these phenomena provide us with a method of accurately measuring surface tension.

Lesson Background and Concepts for Teachers

Cohesive and Adhesive Forces

In a side view color diagram, arrows show forces on water molecules.
Figure 1. Within the body of water, cohesive forces acting on a water molecule pull in all directions and so cancel out. On the water's surface, the cohesive forces pull the water molecule inwards.
Copyright © Roland.chem, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Wassermolek%C3%BCleInTr%C3%B6pfchen.png

In any liquid, intermolecular forces cause the liquid molecules to be attracted to each other. These forces that pull liquid molecules towards each other are known as "cohesive" forces. In the body of a liquid, a molecule is surrounded by other molecules in all directions, so the attractive forces cancel and the molecule feels no overall force (see Figure 1). On the surface of the liquid/air interface, however, a molecule of the liquid feels the attractive forces of the other molecules from within the liquid, but none from the outside. This causes the outer layer of the liquid to act like a stretched membrane and minimize the surface area. We call this elastic membrane-like behavior surface tension.

Two images: (left) Side-view photo of a meniscus in a graduated burette of colored water. (right) A sketch mimics the photo to show the concave curve of a meniscus and identify its contact angle as θ.
Figure 2. The adhesive forces between the water and the glass cause water molecules to cling to the glass walls and create the well-known shape of the meniscus.
Copyright © (left) PR Haney and (right) Karl Hahn, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Meniscus.jpg http://commons.wikimedia.org/wiki/File:ConcaveMeniscusContactAngle.png

Besides cohesive forces, "adhesive" forces also exist; they cause water molecules to try to "stick," or "adhere," to solid surfaces. The most common example of the effect of adhesive forces is the meniscus that is commonly seen when using graduated cylinders. The water molecules respond to an attractive "adhesive" force pulling them towards the glass walls. Near the walls of the cylinder, the adhesive force pulling the water towards the glass walls is stronger than the cohesive force pulling the water molecules together. This attractive force pulls the water up the sides of the glass tube against the downwards pull of gravity (see Figure 2). Refer to the Exploring Capillary Action activity to have students replicate these effects with a hands-on design and test procedure.

Two images: (left) Photo shows a vial of mercury with an inverted (convex) meniscus. (right) A sketch mimics the photo to show the convex curve and identify its contact angle as θ.
Figure 3. Different than water, mercury creates a meniscus that is bowed upward.
Copyright © (left) Flickr user Auntie P, Flickr and (right) Karl Hahn, Wikimedia Commons http://www.flickr.com/photos/auntiep/17135145/ http://commons.wikimedia.org/wiki/File:ConvexMeniscusContactAngle.png

Conversely, liquid mercury is repelled rather than attracted to glass. The mercury tries to minimize its contact with the glass walls. This causes an upward bowing of the mercury liquid against the downwards pull of gravity as it tries to maximize the contact between mercury atoms and minimize the contact with the glass wall (see Figure 3).

Capillarity is the combined effect of cohesive and adhesive forces that causes water and other liquids to rise in thin tubes or other constricted spaces. Inside a thin glass tube, the adhesive force, the attraction between the water and the glass wall, draws water up the sides of the glass tube to form a meniscus. The cohesive force, the attraction of the water molecules to each other, then tries to minimize the distance between the water molecules by pulling the bottom of the meniscus up against the force of gravity.

Line drawing shows water rising highest in the narrowest of three capillary tubes.
Figure 4. Water rises to different heights, depending on the diameter of the capillary tube.
Copyright © 2010 Jean Stave, Duke University

In large-diameter tubes, this rise in the water level is unnoticeable. However, in very small-diameter tubes, the cohesive forces can draw the water upwards appreciable distances (see Figure 4). The water continues to climb up the tube until the downward force of gravity on the water equals the upwards force caused by the surface tension.

Line drawing of a spherical meniscus with angles and radius indicated.
Figure 5. The height, h, that water will rise due to capillary action is related to the contact angle, θ, and the radius of the tube, a.
Copyright © cutler, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Spherical_meniscus.PNG

A simple relationship determines how far the water is pulled up the tube (see Figure 5). The force upwards due to the surface tension is given by the following relationship:

Equation for the upwards force due to surface tension = γ (2πa) cos θ.

In this relationship, γ is the liquid-air surface tension at 20o C, 2πa is the circumference of the tube, and θ is the contact angle of water on glass, a measure of the attraction of the liquid to the walls. The opposing force down is given by the force of gravity on the water that is pulled above the reservoir level.

Equation for the downwards force due to gravity = ρg (hπa^2)

Here, ρ = 1000 kg/m3 is the density of water, g = 9.8 m/s2 is the acceleration due to gravity, and (hπa2) is the volume of the water in the column above the reservoir.

Measuring Surface Tension

One method to measure the surface tension of a liquid is to measure the height the liquid rises in a capillary tube. By setting the two forces above equal, we find the surface tension to be:

Equation to find the liquid-air surface tension. γ = (ρga/2) (h/cos θ)

For pure water and clean glass, the contact angle is nearly zero. In a typical high school lab, this may not be the case, but θ is small and we assume that cos θ is close to 1.

Equation to find the liquid-air surface tension, with cos θ as 1. γ ≈ (ρga/2) h

Note that students must convert ρ, g, a and h into SI units before entering them into the equation. The SI unit for surface tension is J/m2 (or N/m).

The accepted value of the surface tension of water in air at 20o C is γ = 0.073 J/m2. However, you must use pure water and extremely clean glass to get this result. Usually, the measured surface tension is at least half of this number. Students can practice measuring surface tension with a hands-on experiment in the Measuring Surface Tension activity.

Associated Activities

  • Exploring Capillary Action - Students observe glass-water menisci and explain the shape in terms of adhesive forces. Using capillary tubes, they see water climbing due to capillary action. Then, teams design and test "capillary siphons" that can be used to filter water.
  • Measuring Surface Tension - Students use capillary action to measure surface tension. They find the average surface tension and calculate the statistical error.


meniscus: The convex or concave upper surface of a column of liquid, the curvature of which is caused by surface tension. (plural: menisci) Source: Dictionary.com.

surface tension: The property of the surface of a liquid that allows it to resist an external force. This property is caused by cohesion of like molecules and explains many of the behaviors of liquids. Source: Wikipedia, May 2011.


Pre-Lesson Assessment

Discussion Questions: Ask the students and discuss as a class:

  • Who can remind the class what we have already learned about surface tension?
  • When you water a plant, no matter where in the pot you pour in the water, the water reaches all of the roots. How does it do this?
  • Why does a paper towel absorb water, while a piece of plastic does not?

Post-Introduction Assessment

Problems to Check for Understanding: Ask students to work individually (or in pairs or small groups) to answer the following two problems. Review and discuss answers as a class (or have students compare answers) in order to gauge their level of understanding before moving on to conduct the associated lab activity.

  1. Estimate, in cm, how high water will reach by capillary action if the tube is the diameter of a human hair, 150μm (1.5 x 10-4 m), if we assume the surface tension of water in air is 0.073 J/m2. (Answer: 19.86 cm. See solution if necessary.)
  2. Explain in 3-4 short sentences why water is able to move up a thin tube.

Lesson Summary Assessment / Homework

Expand on What You Have Learned: Either in class or at home, assign students to investigate a real-life example of capillarity in action. Use the examples mentioned in the Introduction/Motivation section or others provided by the teacher. Require that each student prepare a poster and/or three-minute class presentation that includes at least four pictures to illustrate the example, and answers to the following questions:

  1. How would you describe your capillarity example?
  2. Why is your topic an example of capillarity?
  3. What are the practical uses or effects of your example?


Adamson, Arthur W., et al. Physical Chemistry of Surfaces. New York, NY: Wiley, 1997, p. 16-19.

Brown, Theodore, et al. Chemistry: The Central Science. 9th edition. Upper Saddle River, NJ: Pearson Education, Inc., 2003. (General information on surface tension and capillary action.)

JRank Science & Philosophy Science Encyclopedia. "Capillary Action." Science.jrank.org. Accessed June 2010. http://science.jrank.org/pages/1182/Capillary-Action.html

Mike. "Tree Physics 1: Capillary Action, the Height of Trees, and the Optimal Placement of Branches." Posted July 2009. Npand.wordpress.com. Accessed August 2009. (Derivation of water height in capillary tubes). http://npand.wordpress.com/2008/08/05/tree-physics-1/

Robinson, Clay. "Capillary Action." Last updated January 27, 2009. Accessed August 2009. (Includes discussion of capillary action in soil). http://www.wtamu.edu/~crobinson/SoilWater/capillar.html

Smith, S. E. "What is Capillary Action?" Accessed June 2010. http://www.wisegeek.com/what-is-capillary-action.htm

Stein, Becky. "Capillary Action." Last updated August 8, 2009. Chemwiki.ucdavis.edu. Accessed July 2010. http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Cohesive_And_Adhesive_Forces/Capillary_Action

Other Related Information

Browse the NGSS Engineering-aligned Physics Curriculum hub for additional Physics and Physical Science curriculum featuring Engineering.


© 2013 by Regents of the University of Colorado; original © 2010 Duke University


Jean Stave, Durham Public Schools, NC; Chuan-Hua Chen, Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University

Supporting Program

NSF CAREER Award and RET Program, Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University


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.

Last modified: December 10, 2020

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