Cohesion and Adhesion

Cohesion and Adhesion

Middle School

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Learning Objectives

• Define cohesion and adhesion and understand how they are different.

• Explain how cohesion creates surface tension with real-world examples.

• Describe how cohesion and adhesion work together to cause capillary action.

• Identify examples of cohesion, adhesion, and surface tension in everyday life.

Key Vocabulary

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Cohesion

The tendency of water molecules to stick together due to forces of attraction between them.

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Adhesion

An attraction between molecules of different substances, such as water and glass.

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Surface Tension

A measure of how difficult it is to stretch or break the surface of a liquid.

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Capillary Action

The movement of water within narrow spaces, which often defies the force of gravity.

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Meniscus

The curve in a liquid's upper surface, created by its attraction to the container walls.

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The Water Molecule

• A water molecule (H₂O) has two hydrogen atoms and one oxygen atom.

• A water molecule is polar, giving it a positive and negative end.

• This polarity helps water molecules bond with each other and other substances.

Multiple Choice

What makes a water molecule 'polar'?

• It has a slightly positive end and a slightly negative end.

• It is made of two different elements.

• It is a liquid at room temperature.

• It contains three atoms in total.

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Cohesion: Water Molecules Sticking Together

• Cohesion is the attraction between molecules of the same substance.

• Water molecules have a strong cohesive attraction to each other.

• This is why water tends to form beads or droplets.

• For example, raindrops are spherical because of cohesion.

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Multiple Choice

Which of the following is the best example of cohesion?

• A paperclip floating on water.

• Water moving up a plant stem.

• Water spreading out on a glass surface.

• Water forming spherical raindrops.

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Adhesion: Water Sticking to Other Things

• Adhesion is the attraction between molecules of different kinds of substances.

• ​This is why water molecules will stick to surfaces like glass or fabric.

• This property is responsible for causing these other surfaces to become wet.

• Water clinging to a glass is a perfect example of adhesion at work.

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Multiple Choice

When water sticks to the window after it rains, which property is being demonstrated?

• Adhesion

• Polarity

• Cohesion

• Surface Tension

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Capillary Action and the Meniscus

Capillary Action

• Capillary action is the movement of water up narrow tubes, working against the force of gravity.

• Adhesion makes water stick to the tube, while cohesion pulls other water molecules up with it.

• This is how plants pull water from the soil all the way up to their leaves.

The Meniscus

• A meniscus is the curved surface of water you see when it is in a container.

• It forms because water's adhesion to glass is stronger than the cohesion between water molecules.

• This stronger attraction to the glass causes the water at the edges to "climb" the sides.

Multiple Choice

How do adhesion and cohesion work together to allow water to move up the stem of a plant?

• Adhesion sticks water to the stem, and cohesion pulls other water molecules up.

• Adhesion makes the water heavier, and cohesion makes it float.

• Cohesion prevents water from sticking to the stem, and adhesion moves it.

• Cohesion sticks water molecules together in a ball, and adhesion pushes the ball up.

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What Is Surface Tension?

• Surface tension measures how hard it is to stretch or break a liquid’s surface.

• This property is a direct result of cohesion between the liquid's molecules.

• Surface molecules are pulled inwards, creating a strong, flexible 'skin' on the water.

• This allows water striders to walk on water and paperclips to float.

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Multiple Choice

What property of water allows a small paperclip to float on its surface?

• Adhesion

• Its chemical formula, H₂O

• Capillary Action

• Surface Tension

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Common Misconceptions

Misconception	Correction
Cohesion and adhesion are the same thing.	Cohesion is attraction between same substances; adhesion is between different substances.
Surface tension is a separate, independent force.	Surface tension is not a separate force but a result of cohesion.
A meniscus forms because of gravity.	A meniscus forms because adhesion is stronger than cohesion.

Multiple Choice

A student observes water forming a dome on top of a penny. Which property of water is the primary reason for this?

• Cohesion

• Polarity

• Capillary Action

• Adhesion

Multiple Choice

How does a paper towel absorb a water spill effectively?

• By using static electricity to attract the water.

• Through adhesion and cohesion working together to pull water into the towel's fibers.

• Through surface tension alone, which lifts the water off the table.

• By chemically changing the water into a solid.

Multiple Choice

If you add soap to water, it breaks down the surface tension. What would likely happen to a water strider trying to stand on this soapy water?

• It would float higher.

• Nothing would change.

• It would sink.

• It would be able to move faster.

Multiple Choice

Analyze the forces at play when water in a narrow glass tube forms a concave (curved down) meniscus. Why does this happen?

• Gravity pulls the center of the water down more than the edges.

• The adhesive forces between water and glass are stronger than the cohesive forces in water.

• The cohesive forces between water molecules are stronger than the adhesive forces to the glass.

• Surface tension pushes the edges of the water up the glass.

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Summary

• Cohesion is water sticking to itself, while adhesion is water sticking to other things.

• Cohesion and adhesion cause capillary action in plants and the meniscus in containers.

• Surface tension results from cohesion at the water's surface, letting small objects float.

• All of water's unique properties are because its molecules are polar.

Poll

On a scale of 1-4, how confident are you about explaining the difference between cohesion and adhesion?

• 1

• 2

• 3

• 4