Background
Water is a polar molecule, which means it has a positive end and a negative end. The oxygen end (blue) is like the negative pole and the hydrogen end (gray) is like the positive pole. Just like magnets, the positive end of one water molecule will attract the negative end of another water molecule resulting in the properties of adhesion, cohesion, and surface tension.
- Adhesion: the attraction of water molecules to other substances
- Cohesions: the attraction of water molecules to other water molecules
- Surface tension: the attraction of surface water molecules to other surface water molecules resulting in a surface “skin.”
Because of water’s unique molecular properties, its observable physical properties impact our daily lives in ways we don’t often notice. This activity provides you with visual and quantitative evidence of the phenomena of adhesion, cohesion, and surface tension in water. You will compare these properties of water for deionized water and deionized water with Dawn dish detergent added to make sense of how water’s polar nature determines its physical properties.
Standards
This activity addresses the following AP® Biology concepts:
- SYI-1 Living systems are organized in a hierarchy of structural levels that interact.
- SYI-1.A Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.
- SYI-1.A.3 The hydrogen bonds between water molecules result in cohesion, adhesion, and surface tension.
Materials
Materials (per group)
- Foam tray
- Paper towels
- 2 100-mL beakers
- Dawn dish detergent
- 1 9-oz cup for wastewater
- 2 plastic straws
- 1 10-mL plastic graduated cylinder
- 2 pipets
- 2 clean pennies
- Petri dish
- 1 piece of #2 mechanical pencil lead
- Forceps
- Distilled water
Preparation and Procedure
Procedure
Preparation
- Line the foam tray with paper towels. You will perform all activities on the foam tray.
- Label one beaker “Dawn solution” and the other “Distilled water.”
- Prepare a Dawn dish detergent solution.
- Place 60 mL of water in a 100-mL beaker.
- Add 10 drops of Dawn dish detergent to the water in the 100-mL beaker.
- Mix the solution.
- Fill a 100-mL beaker with distilled water.
Activity 1: Adhesion
- Place the straw vertically in the beaker of deionized water until it reaches the bottom.
- Place a finger over the top of the straw.
- Remove the straw. Place the tip of the straw into the graduated cylinder and remove your finger, releasing the water into the cylinder.
- Repeat 2 more times.
- Record the total amount of distilled water transferred to the graduated cylinder.
- Dispose of the distilled water into your waste cup.
- Take the second straw and repeat the procedure with the Dawn solution.
- Record the total amount of solution transferred to the graduated cylinder.
- Dispose of the Dawn solution into your waste cup.
- Rinse out the graduated cylinder.
- Collect data from the other groups.
- Calculate the mean from the group data.
- Explain how your observations demonstrate the concept of adhesion.
Activity 2: Cohesion
- Place a clean pipet in the distilled water beaker and one in the Dawn solution.
- Using the pipet, slowly place individual drops of distilled water on one penny’s surface until no more can be added. Record the total number of drops of distilled water on the surface of the penny.
- Observe and sketch the shape of the water on the penny.
- Repeat the procedure with the second penny and the Dawn solution. Record the total number of drops of Dawn solution on the surface of the penny.
- Explain how your observations demonstrate the concept of cohesion.
- Collect data from the other groups.
- Calculate the mean from the group data.
Activity 3: Surface Tension
- Pour distilled water into the top or bottom of the petri dish until it is between 1/3 to 1/2 full.
- Break the pencil lead in half.
- Using the forceps, gently place the pencil lead on top of the water’s surface. It may take 3 or 4 tries to get the lead to stay on the surface.
- Observe and sketch the surface of the water around the pencil lead.
- Remove the pencil lead and dry it off.
- Repeat steps 1–5 with the Dawn solution.
- Observe and sketch the surface of the Dawn solution around the pencil lead.
- Place all leftover water and Dawn solution in your waste cup and dispose of it as your teacher directs.
- Explain how your observations demonstrate the concept of surface tension.
Results
Activity 1: Adhesion
Students should see that they are able to transfer the distilled water but are able to transfer little to no Dawn solution. In the Dawn solution, the soap disrupts the bonds that allow the water to adhere to the side of the straw. Because of this, the solution is unable to be transferred. Note: Some cohesion plays a role in the water remaining in the straw.
Activity 2: Cohesion
Students should see that more drops are able to remain on the penny with distilled water than the penny with Dawn solution. In testing, there were about half as many drops able to remain on the Dawn solution penny. This is because, in the Dawn solution, the soap disrupts the bonds that allow the water molecules to stick to each other through cohesion. Note: Some adhesion plays a role in the water sticking to the penny but does not account for the total amount of water.
Activity 3: Surface Tension
Students should see that the pencil lead is able to float on the surface of the distilled water but is not able to float on the surface of the Dawn solution. The soap in the Dawn solution disrupts the bonds between the water molecules that create surface tension.
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