Key Concepts
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Chemistry -
Cohesion -
Adhesion -
Surface Tension
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Introduction
You have probably noticed water droplets often appear to seek out other water droplets becoming a bigger and bigger blob, like rain on a window. Or maybe you have noticed little bugs standing on water. How do they do that? It all has to do with the property of water known as surface tension. In today’s experiment we are going to explore just what a powerful force that can be.
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Background
Surface tension can explain why small insects can walk on water and why water droplets tend to form a spherical shape. It all has to do with the way water molecules like to stick to other water molecules.
A water molecule is made up of one oxygen atom and two hydrogen atoms. These atoms connect by sharing electrons. Due to the forces that connect the hydrogen atoms to the oxygen atoms, a water molecule tends to have one side that is little more positive and another side that is a little more negative. This causes the negative side of one molecule to attract to the positive side of another molecule. Thus, water molecules push and pull themselves into a certain arrangement. This equilibrium is where all the positives and negatives balance each other. In the middle of the water those forces are pulling in all equal directions. But on the surface, something a little different happens. On the edge of the water, along the surface, water tends to arrange so that the molecules on the surface get pulled towards the rest of the water molecules. This pulling creates a thin layer of water molecules that link together creating the surface tension we will see in this experiment.
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- An adult helper
- An eye or medicine dropper (pipette if you have one)
- A penny
- A glass of water
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Preparations
- Fill a drinking glass around half full of water from the faucet
- Find an eye dropper
- Squeeze the rubbery part of the eyedropper, place the tip in the water. Ease the pressure on the rubbery part and watch the water go into the dropper
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Procedure
- Place a penny face side up on the table in front of you.
- Make a prediction. How many droplets of water can you drop on a penny before it spills over the side?
- Try it. Use the dropper to carefully place droplets of water on a penny one at a time. Be sure to count how many drips you drop. Was it more or less than you predicted?
- Try it again. Dry off your penny and repeat. Do you think it will be more, less, or about the same?
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Observation and Results
What allows us to get such a large dome of water on top of the penny? Water has a high surface tension. This happens because water molecules like to stick together. This property is called cohesion. Water also likes to stick to other things, like the penny. This property is call adhesion. What causes the dome to finally collapse? Gravity! During the time of the experiment, gravity is pulling, trying to flatten the water out and pull it down to the table. Gravity succeeds at collapsing the dome when the weight of the water becomes too great. That means the force of gravity pulling on the water is greater than the cohesion force pulling the water together. This helps us understand why water bugs can stand on water. The force of gravity pulling down on the bug is less the force of the water molecules sticking together, so the bug can stand on the water without breaking the surface tension of the water. -
Clean Up
Be sure to clean up when you are done. Pour the liquids back in the sink and rinse them down the drain. Put your tools back in cupboard or drawer they came from after you have washed them. -
More to Explore
There are lots of ways to extend this experiment and better understand surface tension. Do you think all liquids have the same surface tension? What if you tried milk, orange juice, or vegetable oil? Would you get the same results? What if you mixed things into your water, will that change the surface tension of the water? Try adding soap or salt to your water and see if that affects your results.
- Does the temperature of the water matter? Try again with hot and cold water.
- What happens when you mix in little soap gently into your water?
- Does the type of liquid matter? Try different liquids like oil, milk, or juice.
- What other variables could you experiment with?
Remember you only want to change one variable at a time so you can better understand what is causing your results to change.
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Safety First & Adult Supervision
- Follow the experiment’s instructions carefully.
- A responsible adult should assist with each experiment.
- While science experiments at home are exciting ways to learn about science hands-on, please note that some may require participants to take extra safety precautions and/or make a mess.
- Adults should handle or assist with potentially harmful materials or sharp objects.
- Adult should review each experiment and determine what the appropriate age is for the student’s participation in each activity before conducting any experiment.
Next Generation Science Standard (NGSS) Supported - Disciplinary Core Ideas
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.
Physical Science (PS) 1 – Matter and Its Interactions
Grades K-2
- 2-PS1-1. Different kinds of matter exist and many of them can be either solid or liquid.
Grades 3-5
- 5-PS1-1. Matter of any type can be subdivided into particles that are too small to see, but even then, the matter still exists and can be detected by other means.
Grades 6-8
- MS-PS1-4. Gases and liquids are made of molecules or inert atoms that are moving about relative to each other.
- MS-PS1-4. In a liquid, the molecules are constantly in contact with others.
Grades 9-12
- HS-PS1-1. Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons.
- HS-PS1-3. The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.
PS2 – Motion and Stability: Forces and Interactions
Grades K-2
- K-PS2-1. Pushes and pulls can have different strengths and directions.
- K-PS2-2. Pushing or pulling on an object can change the speed or direction of its motion and can start or stop it.
Grades 3-5
- 3-PS2-1. Each force acts on one particular object and has both strength and a direction.
Grades 6-8
- MS-PS2-1. For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction.
Grades 9-12
- HS-PS2-1. Newton’s second law accurately predicts changes in the motion of macroscopic objects.
Grades K-2
- K-PS2-1. When objects touch or collide, they push on one another and can change motion.
Grades 3-5
- 3-PS2-1. Objects in contact exert forces on each other.
- 5-PS2-1. The gravitational force of Earth acting on an object near Earth’s surface pulls that object toward the planet’s center.
Grades 6-8
- MS-PS2-4. Gravitational forces are always attractive. There is a gravitational force between any two masses, but it is very small except when one or both of the objects have large mass.
Grades 9-12
- HS-PS2-6. Attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and transformations of matter, as well as the contact forces between material objects.
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