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Title: Wet Pennies- Surface Tension and Surfactants
Principle(s) Investigated: Cohesion, surface tension, surfactant.
Standards : CA State Standards:
8.3.e. Students know that in solids the atoms are closely locked in position and can only
vibrate; in liquids the atoms and molecules are more loosely connected and can
collide with and move past one another; and in gases the atoms and molecules are
free to move independently, colliding frequently.
8.9.b. Evaluate the accuracy and reproducibility of data.
8.9.c. Distinguish between variable and controlled parameters in a test.
NGSS:
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended
structures.
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the
substances interact to determine if a chemical reaction has occurred.
Materials: Pennies
Pipettes
Paper Towels
Liquid Hand Soap
Procedure:
Prediction A:
Predict the number of water drops that will sit on the surface of a penny without spilling over. Record this number on the spreadsheet.
Part A: Control group
1. Use water and a paper towel to clean off the surface of your penny.
2. Rip off a chunk of paper big enough for the penny to sit on and put the penny in the middle.
3. Use a pipette to place as many drops of water onto the penny (1 at a time). As soon as any of the water runs
off onto the paper towel, stop adding drops.
4. Record the number of drops for that trial and dry off the penny.
5. Repeat steps 1-4 three more times.
6. Calculate the average number of drops per trial.
Prediction B:
Predict the number of water drops that will sit on the surface of a penny, coated with soap, without spilling over. Record this number on the spreadsheet.
Part 2: Experimental group
1. Use water and a paper towel only to clean off the surface of your penny.
2. Use your finger to spread a thin layer of liquid soap onto 1 side of the penny.
3. Rip off a chunk of paper towel big enough for the penny to sit on and put the penny in the middle with the soap
facing up.
4. Use the pipette to place as many drops of water on the penny (1 at a time). As soon as any of the water runs
off onto the paper towel , stop adding drops.
5. Record the number of drops for that trial and dry off the penny.
6. Repeat steps 1-5 three more times.
7. Calculate the average number of drops per trial.
Data Collection:
Student prior knowledge: Water is composed of 2 Hydrogen atoms and 1 Oxygen atom.
Water forms individual droplets on a flat surface.
Explanation: Give a thorough explanation of the experiment or demonstration. Your explanation should be written to give your fellow teachers a solid understanding and include greater detail than what you might provide for your secondary students. Make certain to include equations whenever pertinent.
This experiment is designed to help students improve their skills of observation, accurate recording of data, and identification of independent and responding variables. The lab is set up as a discrepant event, where students hypothesize the number of drops that will fit on their penny, then test their hypothesis and find the average number of drops per trial. Then, they add the surfactant and repeat the process. They compare their results to the control and determine what caused the difference between the number of drops.
Water molecules are attracted to other water molecules (kind of like little magnets.) The oxygen atoms have a (-) negative charge, and the hydrogen atoms have a (+) positive charge. This makes the oxygen of one water molecule attracted to the hydrogen atoms of other water molecules. This is called cohesion.
This is why the water can stack up into a large “bead” on top of the penny. The water molecules are holding onto each other and not letting each other fall off the penny! When water molecules hold onto each other like this and form a skin it is called surface tension. A water spider can sit on top of pond water because its weight isn’t more than the surface tension of the water can hold.
But what if we could get rid of the cohesion?
Soap molecules have a front which is attracted to water molecules and a back that repels them. This means that the soap gets in the way and keeps water molecules from grabbing onto each other.
Questions & Answers: Give three thought-provoking questions and provide detailed answers.
1. How does the surface tension created by the water molecules affect the shape of the water droplet?
The water droplet shape, a dome, allows the water molecules to stack on top of each other on the penny. The outer layer of water molecules exerts so much cohesive force between the molecules, it holds the weight of the rest of the water within the droplet. The outer 'skin' of molecules will expand upwards first, and then out towards the edge until the weight of the water inside is greater than the cohesive attraction between the water molecules.
2.
Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.
1) Mercury used in the thermometer: The mercury used in the thermometer does not stick to the wall of the capillary tube because of the surface tension. Had it stuck to the walls the measurement of temperature would not have been correct.
2) Separation of oil and water: The separation of oil and water is caused due to the difference in surface tension of the two liquids. There is also separation of the fuel and lubricating oil in the engine due to their surface tension. Had they mixed together, the fuel would have lost its combustion properties while lubricating oil would have lost its viscosity and lubrication properties.
3) Soap bubbles: Ordinarily the bubbles in water are unstable, but when surfactants are introduced in water its surface tension is reduced by factor of three or more. In such water the bubbles can remain stable; hence lots of bubbles are seen in such liquids.
4) Oil spills: The oil that spills from tankers or oil rigs float along the surface of the water. With the BP spill in the Gulf Coast, an oil dispersant, a mixture of several chemical surfactants, was sprayed on the surface of the oil and water. This disrupted the surface tension of the water, causing the oil to sink underneath the surface.
Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.
Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.
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