Surface Tension

Either do the following demonstration for students or show them the video Water’s surface tension 

Materials

• 1 clear plastic cup
• Water
• 1 standard size paper clip
• 1 large paper clip

Expected Results

The paper clip should rest on the surface of the water. This may take a couple of tries.

Ask students:

• Why do you think a paper clip, which is more dense than water, can stay on the surface of water?
  Remind students that paper clips are more dense than water and would normally sink. Help students realize that the paper clip in the demo stayed on the surface of the water because of something having to do with the water molecules at the water’s surface, called surface tension.

Project the image Water Strider and Molecule. 

Point out how the surface of the water seems to bend but not break under the water strider’s legs. Tell students that what they see is another example of water’s surface tension.

Ask students:

• Why do you think water has such a strong surface tension?
  Encourage students to think about what they already know about the strong attraction between water molecules. Students should remember that water molecules are very attracted to each other and this attraction is the basis for surface tension. This will be explained in more detail later in this lesson.

Give each student an activity sheet.

• Lesson 5.2 Student Activity Sheet PDF | DOCX | Google Doc
• Lesson 5.2 Activity Sheet Answers PDF | DOCX | Google Doc

Download the student activity sheet, and distribute one per student.

All Downloads

The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity sheets are formative assessments of student progress and understanding. A more formal summative assessment is included at the end of each chapter.

Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.

Tell students that there is another phenomenon that is caused by water’s surface tension. It is water’s ability to fill beyond the top of a container.

Question to Investigate                                                                                    

How much water can you add to a full test tube?

Materials for Each Group

• Water
• Dropper
• Test tube
• Penny
• 2 paper towels

Expected Results

While looking from the side, students will see that the water forms a dome on the top of the test tube and on the penny.

Ask students:

• What did the water look like as you added it to the top of the test tube and the penny?
  The water makes a “dome” or “hill” of water above the top of the test tube and penny.

Project the image Why Water Beads.

Explain to students that water’s surface tension is based on the attractions between water molecules at the surface and the water molecules in the rest of the water. A water molecule beneath the surface feels attractions from all the molecules around it. But the molecules at the surface only feel attractions from the molecules next to them and beneath them. These surface molecules are pulled together and inward by these attractions. This inward pull has the effect of compressing the surface molecules which form a tight arrangement over the water’s surface. This tight arrangement at the surface is called surface tension.

The inward pull from the attractions of the molecules results in the smallest possible surface for a volume of water, which is a sphere. This is why water forms a round drop or dome at the top of the filled test tube and on the surface of a penny.

Ask students:

• How could we compare the surface tension of water and alcohol?
  Students may suggest placing an equal number of drops of each liquid on wax paper, overfilling a test tube, or comparing the number of drops that can be added to the top of a penny.

Note: Even though there are many ways to compare the surface tension of water and alcohol, the procedure written below compares each liquid on the surface of a penny. In order to make this as fair a test as possible, students should place each liquid on two similar pennies that are either both “heads” or “tails.” They should also be sure to add single drops of each liquid slowly and carefully.

Question to Investigate

Which has a greater surface tension, water, or alcohol?

Materials for Each Group

• 2 pennies
• 2 droppers
• Water
• Isopropyl alcohol (70% or higher)
• Paper towel

Expected Results

The water beads up on the penny and the alcohol spreads out flat. Many more drops of water can be added to the penny than drops of alcohol.

Project the image Water and Alcohol.

Review that water molecules are polar and that they are very attracted to each other. Point out that alcohol molecules are polar in only one area, making them somewhat attracted to each other. They are not as attracted to other alcohol molecules as water is to other water molecules.

Water and Alcohol on Pennies

Explain that water’s attraction pulls itself together into a tight arrangement. Explain that alcohol molecules don’t have a structure that is as good as water’s for attraction to itself. Alcohol molecules only have 1 O–H bond and they have some C–H bonds that are pretty nonpolar. There is not as strong an attraction between them as there is between water molecules.

The shape of the water molecule and its polarity at the top and the bottom give water molecules lots of opportunities to attract. Almost anywhere two water molecules meet they can be attracted to each other.

But alcohol has a different size and shape and has its polar part on one end. Alcohol molecules can meet at areas where they would not attract as strongly. The water is more attracted to itself than to the metal of the penny. The alcohol is a bit less attracted to itself, so it spreads more on the penny.

Question to Investigate

How does detergent affect water’s surface tension?

Materials

• Dish detergent in cup
• 2 pennies
• Dropper
• 2 toothpicks
• Paper towel

Expected Results

Touching the water with the toothpick causes the surface of the water to be pressed down and bend. Touching the water with the toothpick and detergent causes the water to collapse and spill off the penny.

Project the image Water and Detergent.

Explain that detergent is made from molecules that have a charged end and a longer uncharged end. The detergent molecules spread out over the surface of the water with the charged end in the water and the uncharged end sticking out. The water molecules at the surface are attracted to the charged end of the detergent molecules. As the surface water molecules are attracted outward, this acts against their inward attraction that was creating the surface tension. This reduces the surface tension, and the water does not hold its round shape and thus spills.

Ask students:

• If water absorbs into a paper towel but does not absorb into wax paper, what does that say about the polarity of paper and wax paper?
  The molecules that make up paper are probably polar, and the molecules that make up wax are probably nonpolar.

Project the animation Water on Paper Towel.

Explain that paper towel and other paper is made from cellulose. Cellulose is made from repeating molecules of glucose that are bonded together. The glucose molecule has many O–H bonds, which are polar. Polar water molecules are attracted to polar cellulose.

Project the animation Water on Wax Paper.

Tell students that wax is made from paraffin, which is repeating carbon–hydrogen bonds. The C–H bond is not very polar, so water is more attracted to itself than to the wax. This causes the water to bead up on wax paper.