Properties of Water: High Surface Tension (Teresa Gonzalez)

Title: The Water Drop Experiment

Principle(s) Investigated: Water cohesion. Hydrogen bonding. The strength of weak bonds.

Standards: HS-PS-1-3: 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..

Materials:

• Pipettes
• Pennies
• Paper towels
• Beakers
• Detergent
• Food dye (optional)
• Quickwrite

Procedure: Intro video. Teacher will give a demo. Students will get the supplies. They will place a paper towel down on the desk and place a penny on top of the towel. There will be two beakers of water, one labeled #1 and the other labeled #2. Students will use their pipettes to collect water from the beaker and then slowly place the same size of water drops onto their penny. It is important that this is done slowly. The students will continue to add water until it spills over the penny. Students will do three trials of each labeled beaker and calculate the average. Students will record the average on the google form provided.

Student prior knowledge: Students should know that water molecules are made of one (1) oxygen atom and two (2) hydrogen atoms. These atoms have slight positive (H) and slight negative (O) charges when part of a water molecule.

Explanation: The students will be in groups of two or three. Each group will have two beakers, labeled 1 and 2. Beaker 1 will just be plain water (dyed if so desired) and Beaker 2 will be water with some soap/detergent mixed in. Students will have already made a prediction in their quickwrite based on the teacher's demo. Students will do the three trials of each beaker, find the average number of drops, and then report the average of each on the google form. Hopefully the ensuing graph will show that beaker 1 required more drops before spilling over than beaker two. This should lead to a discussion about why the water itself is so strong and why the soap made such a difference.

Questions & Answers:

1. What is implied about hydrogen bonding if the detergent is able to break apart water’s surface tension so easily?
   1. The fact that adding soap can break down water’s surface tension means that hydrogen bonds are weak bonds.
2. Why does the soap break down the water's surface tension?
   1. Part of the soap molecule is hydrophilic (attracts water) and other parts of the soap molecule are hydrophobic (repel water) and those interactions are enough to break the surface tension of the water.
3. So why do we use soap to wash clothes?
   1. Since the soap breaks down the water’s high surface tension, it allows for the water to sink deeper into the material of the clothes, which allows it to clean out more dirt. Without the soap, the water would stick together more and get to less of the dirt!

Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.

IMPORTANT NOTE: Embed a Youtube Video that illustrates the procedure and results. This video should reside in your YouTube account and should be viewable by all. You may use a video in another persons YouTube channel if the video is better quality than what you can make.