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Title: Investigating the properties of adhesion + cohesion via water droplets
Principle(s) Investigated: Surface tension, adhesion, cohesion
Standards:
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.
PS1.A: The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.
Materials: -water dropper -pennies -water -paper towels -data sheet (quickwrite) -50mL beakers
Procedure:
0) Explain the concept of molecular adhesion and cohesion, and that surface tension is a special name we give to water molecules exhibiting cohesion.
1) Assign groups of three or four. Each group needs two droppers and one data recorder - these can be interchanged during each trial. Each group is to drop as many water drops as they can on a penny and count the total number of single drops until the water breaks surface tension
2) Distribute materials to each group (one penny, one dropper, one paper towel)
3) Begin the experiment. Each group will count the number of drops until the water spills over the edge
4) Record drops in quickwrite. After three trials, calculate average.
Student prior knowledge: We should have covered how to record data in excel and how to calculate an average. This experiment can either confirm information covered in a prior discussion on the properties of water, or to serve as an exploratory experiment to begin the discussion of water properties.
Explanation:
There are two properties at work in this experiment: cohesion and surface tension. Cohesion is the attraction of like molecules to one another. In this case, the like molecules are the H20 molecules in the water drops. Surface tension is a special term we use to describe the cohesion between water molecules.
Water’s cohesion and surface tension are special because of hydrogen bonds. Hydrogen bonds are formed by the hydrogen atoms of one molecule being attracted to the oxygen atoms of another molecule.
The cohesion and surface tension of water becomes apparent when the drops of water you add to the penny reach the penny’s edge. Once the water has reached the edge, you begin to see a bubble or dome of water forming on top of the penny. The bubble shape is a result of the water molecules clinging to one another in an optimal shape (just like the bonds on the surface of a blown bubble).
Questions & Answers:
1) The effect we observed is due to the very distinct nature of how water molecules interact with each other. Would a substance other than water allow the penny to hold differential amounts of total drops? Why or why not? Give an example.
Most likely, yes. The phenomena we experience is due to the specific nature of water molecules. Their hydrogen bonding creates a high surface tension force that is able to withstand much more pressure than initially thought. Soapy water would disrupt the hydrogen bonding strength, as would adding additional substances to water (sugars, salts, etc).
2) What are some of the variables that could be changes to change the amount of drops that can fit on the coin?
Coin material (nickle, penny, quarter, etc), cleanliness of the coin, liquid poured onto the coin, consistent coin dropper, size of drops.
3) What molecular feature of water allows it to experience a high degree of surface tension?
Hydrogen bonding. Each hydrogen bond on the surface of the water is stronger than those within the sphere of water because instead of creating bonds with additional water molecules, the surface molecules are creating bonds with air.
Applications to Everyday Life:
One simple phenomenon is that animals can take advantage of a high degree of surface tension to skim along the waters surface.
Capillary action is driven by surface tension. Plants require capillary action to drive water up the roots. Evaporation at the leaves drives osmotic pressure to draw water up the plant or tree to the shoots and leaves. Our eyes use capillary action to draw away tear fluid produced in the eyes.
Bubbles are also possible because of the properties of water! Each water molecule wants to operate in as much space as possible, thus forming a sphere so that each water molecule is maximizing the distance to it's neighbor while still maintaining a hydrogen bond with its neighbors.
Photographs: Surface tension!
Videos: Penny dropper lab
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