Exploring IMFs via Physical Properties

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Objective

SWBAT deduce the relative strengths of intermolecular forces in four liquids from their bulk properties.

Big Idea

Surface tension and viscosity provide measurable evidence of the strength of a substance's intermolecular forces.

Background

This lab is one that I wrote using an old favorite (drops of liquid on a penny) and trying to find a way to test viscosity simply.  Due to traveling classrooms, some students performed this lab prior to the Evaporation of Alcohols lab, others perform this experiment first.

The necessary materials per lab table (2 groups) are:

  • 8 pennies
  • Paper towels
  • 4 disposable Beral pipets
  • 3 50 mL beakers
  • 1 100ml erlenmeyer flask with stopper (for the acetone)
  • Acetone, Isopropyl alcohol, vegetable oil, fake blood (bought on sale after Halloween and shared with our forensic science teacher)
  • 2 Small whiteboards
  • 2 Whiteboard markers
  • 2 Rulers
  • Masking tape
  • 2 Stopwatches (I have students use cell phone apps)

This lab aligns to the following components of the NGSS:

  • 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.
  • Science and Engineering Practice 3: Carrying out investigations.
  • Science and Engineering Practice 4: Analyzing and interpreting data.  Students will look for patterns in their data and graph the results to help discover patterns.
  • Science and Engineering Practice 6: Constructing explanations.  Students will use their evidence from their lab results to explain the differences in the strength of intermolecular attractions between the different materials.

Introduction- Lab Safety

6 minutes

When students enter the room, I ask them to get out their Surface Tension and Viscosity lab directions and the accompanying filmstrip.

While students are getting materials out, I move from the front to the back of the room into the lab.  When students are ready I inform them that we will not need goggles for this lab, but because the liquids can stain clothing, they will need to wear aprons.  I ask if there are any questions about the surface tension procedure.

Students tend to be shy about asking questions, so I tell them to lay a piece of paper towel on the table under the pennies to help avoid making too large of a mess.  I also show them how the beakers and pipets are labeled with the initials of the chemical as they work.  I point out that the acetone is in a stoppered flask due to its strong smell, and ask them to keep the stopper on it whenever they are not using acetone.

Next I show them the basic setup of the viscosity lab, with a starting line on the whiteboard and the whiteboard taped at an angle to the wall.  I tell them that if they have questions after setting it up, to call me over and check with me.

At this point, I invite them back to the lab to start working.

Surface Tension Lab

15 minutes

When students enter the lab, they orient themselves to the materials, figuring out what each chemical is, and where their materials are. Students will be adding drops of various liquids (distilled water, 70% isopropyl, vegetable oil, or fake blood) to the top of a penny to count how many drops will hold together on the penny.

Some groups will choose to work one penny at a time, others will have each partner take a penny and a chemical.  One question that arose in each class this year was if it matters which way the penny is facing.  I responded "Why would it matter?" and students said "The liquid might stick better on the tails side because of the creases in the building."  I told them to pick a side, but to be consistent on which side they use.

As I monitor the room, I am picky about them keeping the stopper on the acetone flask due to the odors from it.  I encourage the students, and help them keep on task.  When students finish collecting data, I encourage them to switch to the viscosity lab, since they can analyze data in or out of class.

This video shows a table that coordinated their work, with the far side working on the surface tension portion and the near side doing the viscosity portion of the lab.

Viscosity Lab

20 minutes

For the viscosity lab, I have students tape the whiteboard at an angle to our computer monitor stands.  In the past, a ring stand has worked just as well.

Students take the ruler and draw a starting line 1cm from the top of the board.  Then they add a drop of the liquid and time how long it takes to flow down the board.  Some of the chemicals will not flow all the way down the board, so I have the students measure how far they went as an indication of their ability to flow.

The data from this portion of the lab was particularly inconsistent this year.  Therefore, as I referenced in the reflection of the Background portion of the lesson, I am going to get 30mL burets and have students fill them to 15mL with the liquid.  Then they will time how long it takes for 5mL of liquid to flow out.  This should be a much more accurate measurement of viscosity, and provide much better data for the students.  Unfortunately for this year, we did not have the materials to make it work.

Drawing Conclusions/Closing

9 minutes

When students finish the labs and return to the front, they need to average their two data trials and create their bar graphs.  Some students struggle with how to find the average, or how to make the bar graph.  I provide calculators to help with the averages, and put a sample set of axes on the board, showing how to put each chemical on the X-axis, but not how they should label their Y-axis.

This student work from the surface tension lab is pretty decent given the space provided.  Everything is clearly labeled, and the trend is obvious from both the graph and the data table.

This student did not find the averages, but made a double bar graph instead.  Notice the lack of rankings here, because without the averages the student wasn't sure which trial to use when ranking.  This led to a nice individual discussion about the importance of following directions and asking for help when unsure how to do the mathematics of chemistry.

I remind students that when they are done with the graphs, to rank their substances from highest to lowest for the property.  This is a copy of student data for the viscosity lab portion:

Unfortunately, due to the inconsistencies of the viscosity lab portion, some students struggle to see a relationship between the surface tension data and the viscosity data.  The student above was able to determine the relationship, but didn't extend to the micro-level attractions between molecules as the cause of the pattern.

For all students, the acetone is consistent in having both the lowest surface tension and the lowest viscosity.  I encourage students to only write about what they find to be consistent, and if there is none, to think about why that might have been-- whether there were errors in how they performed the lab, or if something is incorrectly setup in the lab for them to find accurate results.

If students finish their conclusion prior to the end of the period, they turn in their paper.  If not, they take it home to finish.

In reading through the labs, students understood how surface tension related to the strength of the forces, but the viscosity was much more difficult.  I believe this stems directly from the inconsistencies in the data and hope it will be remedied with the changes in the lab for next year.