Properties of water, part 3

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Objective

Students will observe the normal surface tension of water and attempt to break the water's surface tension using various substances.

Big Idea

Some substances can break the surface tension of water and others cannot.

Warm-up

5 minutes

Warm-Up:  “Have you ever watched a rain drop on a windshield or window?  Describe what happens as the raindrop travels down the window pane.” 

Allow 2-4 students to respond to the warm-up question.  Act as a facilitator for the discussion.  Make sure students are able to tie the description to what they've learned about water properties, part 1 and part 2. 

If students are able to describe what occurs but are not able to identify the property of water that they are describing, use effective questioning to help the students make the connection.   Ask, “So, you’ve seen a water drop become bigger and bigger as it travels down a window pane.  What property of water allows this to occur?” Ensure that the discussion ends with the correct identification of the properties of water that are displayed by rain drops clearly communicated. 

Make sure that the misconception about water “growing” is addressed if it arises from the student responses.  Remind students that growth is an aspect of living things and that rain drops do not “grow”.  It’s always good to periodically review the characteristics of living things as it sometimes takes students several conversations about the characteristics of life to fully comprehend the concepts and dispel long held misconceptions.

Introduce New material

15 minutes

Show students an image of surface tension at the molecular level.  Point out how the water molecules form stronger bonds at the surface level. Present a few examples of surface tension in order to build on students’ understanding of the concept.  These examples also provide students real-world applications of the concept of surface tension:

Walking on water: Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

Floating a needle: A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.

Don't touch the tent!: Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

Clinical test for jaundice: Normal urine has a surface tension of about 66 dynes/centimeter but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but will sink if the surface tension is lowered by the bile.

Surface tension disinfectants: Disinfectants are usually solutions of low surface tension. This allows them to spread out on the cell walls of bacteria and disrupt them.

Soaps and detergents: These help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

Washing with cold water: The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unneccessary.

Why bubbles are round: The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes.

Surface Tension and Droplets: Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer.

Source: USGS

 

Guided Practice

15 minutes

Distribute copies of a water properties concept map in plastic sheet protectors to students, along with dry erase markers.  Display the same concept map on a LCD projector.  Note: Placing the concept map in plastic sheet protectors and using dry erase markers allows for repeated usage throughout the day without paper use.

Explain that students are to use the given set of words or phrases once to complete the concept map.  Set the timer and allow students 10 minutes to work independently on the concept map. Encourage students to cross out words as they used them on the map.  This practice will help students maintain a better sense of what words they’ve used so that they don’t use the same words more than once.

Encourage students to first try to answer as many questions as they can without accessing their notes instead of immediately using their notes.  This helps students rely more on their own understandings.  Use of notes is not discouraged but during a review activity,notes should be accessed only after the student has attempted to respond to the questions without use of notes.

Note: To amp up the rigor, give students 5 mins to try to memorize as much info as they can and/or quiz each other.  Then have students put notes away and give 10 mins to complete without notes.  Studies have shown that this “quizzing” helps students to retain information longer.  

Walk around the room to observe students as they work.  Notice how many students seem to be able to complete the map without using their notes and how many students need to reference their notes.  Look at which parts of the concept map are left blank more frequently.  Make a mental note to review the related concepts to the parts of the concept map that were more frequently left blank when reviewing responses with the class.

At the end of the timed work period, review the correct responses to each of the parts of the concept map.  Using information gained from walking around while students were working, conduct a brief review of concepts that students were not able to readily identify on the concept map.

Independent Practice

25 minutes

Inform students that they will work with a lab partner to observe the normal surface tension of water and attempt to break the water’s surface tension using various substances.  

Display the lab procedures  on an LCD projector as you summarize the lab activity. Ask students to predict and jot down on their papers which substances they think will likely break the surface tension of the water. 

Read the lab background and spiral back briefly to discuss both adhesion and cohesion. Distribute copies of the Surface tension lab to each group of two students.  Send one member from each group to retrieve the lab materials.  Always review safety practices and lab expectations before beginning a lab. Also, it’s best to have all substances prepared and ready in labeled cups before the students arrive.  This reduces time spent pouring substances  and labeling cups.

After summarizing the lab procedure, ask a student to quickly re-state what they are expected to do in the lab.  This serves as a check for understanding that allows for correction of any misunderstandings before students are released to perform the lab. Walk around and observe students at work1.  This serves as a formative assessment of what students at work2 and what they are able to do correctly.

Point out that the lab conclusions are written responses.  Remind students to use the RACE writing strategy that was taught previously in responding to each of the conclusion questions:

  1. What is surface tension?
  2. Which substance(s) are best at breaking the surface tension of water?
  3. Water is a polar molecule.  What does this mean?
  4. Why do you think that some substances can break the surface tension of water and others cannot?  Use related vocabulary in your response.
  5. How are cohesion and adhesion alike?
  6. How are cohesion and adhesion different?
  7. What was the significance of using wax paper to conduct the lab?

The lab questions are varied in rigor, some requiring basic recall skills, while others require critical thinking. When reviewing the student responses, look for those students are struggle with the critical thinking questions and plan specific activities to build their ability to think critically. The student work sample that is attached shows that the student was able to recall the information that was taught and connect it to the lab observations.

Ensure that students know how long they have to complete the lab. Set and display the timer on the LCD projector so that students will be able to monitor their progress.  Walks around the room as students are working to ensure that safety procedures are being followed and students are on task.  Be available to answer questions, but only after students have utilized their own resources, first. 

Create a standard for seeking assistance like, “Ask three(students), before me.”  This alleviates the students seeking validation of every step that they take in the performance of a lab, instead of asking a peer who likely knows the answer.

The student work sample demonstrates a working knowledge of the academic vocabulary, an understanding of water's polarity and other properties.

Close

5 minutes

Discuss the lab finding with the class. Ask students if their prediction about which substances would break the surface tension was correct or not. . Listen for students’ ability to explain their prediction in terms of what they know about polarity.  Make sure that students are able to identify that polar substances did not break the surface tension of the water, while non-polar substances did.