Summary

In this lesson students explore the viscosity of different mixtures. They measure viscosity of different mixtures and then have the opportunity to design a mixture to attain a particular viscosity. Students will relate their lab experience to the application of viscosity in the real-world as they consider the differences in viscosity between different types of paints.

Grade Level

Middle School

NGSS Standards

• 2-PS1-1: Plan and conduct an investigation to describe and classify different kinds of materials by their observable properties.
• 2-PS1-2: Analyze data obtained from testing different materials to determine which materials have the properties that are best suited for an intended purpose.
• MS-ETS1-2: Evaluate competing design mixtures using a systematic process to determine how well the meet the criteria and constraints of the problem.
• MS-ETS1-3: Analyze data from tests to determine similarities and difference among several design mixtures to identify the best characteristics of each that can be combined into a new mixture to better meet the criteria for success.
• MS-ETS1-4: Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be reached.

Objectives

By the end of this lesson, students should be able to

• Explain the meaning of viscosity and describe it as a physical property.
• Compare the viscosity of different mixtures.
• Design a mixture with the intention of increasing or decreasing its viscosity.
• Analysis results and observations to determine factors that may influence viscosity.

Chemistry Topics

This lesson supports students’ understanding of

• Mixtures
• Physical Properties
• Viscosity
• Observations
• Solution
• Solute & Solvent

Time

Teacher Preparation: 20 minutes

Lesson:

• Engage: 10 minutes
• Explore: 40 minutes
• Explain: 10 minutes
• Elaborate: 10 minutes
• Evaluate: 20 minutes

Materials (Teacher Demonstration)

• Water
• Molasses
• Spoon or dropper
• Vertical surface (ex: white board)

Materials (per lab group)

• 5 small disposable paper cups
• Scissors
• Bucket or bin to capture liquid
• 100ml graduated cylinder (or measuring cup)
• Water
• Sugar (approximately ½ cup)
• Salt (approximately ½ cup)
• Flour (approximately ½ cup)
• Fiber (approximately ½ cup), Metamucil or Citrucel works well
• Disposable metal pan or sheet pan
• 5 spoons
• Measuring Spoons/cups (optional)
• Marker
• Ruler
• Timer
• Stack of books or blocks
• Sharp object such as a nail (teacher use only)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Do not consume lab mixtures, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

• Engage: Start this lesson with a teacher-led demonstration. Place a glob of molasses on a vertical surface such as a white board where students can see it and ask them to predict how long it will take for the molasses to reach the bottom. Then, place some water on the same board next to it and ask them the same question. Explain the definition of viscosity as the resistance to flow, and ask the students which substance showed a greater viscosity. Brainstorm a list of different liquids and mixtures with a wide range of viscosities and seek student ideas on what might make liquids have different viscosities.
• Have students act out a highly viscous fluid versus one that has a low viscosity, particularly to spark discussion about sources of resistance to flow:
  • Stronger attractions between the molecules cause the molecules to cling together and become more viscous.
  • Longer chains that allow the molecules to become ‘tangled’ in each other, resisting flow and increasing viscosity.
• Continue the above discussion by having students act out how changes to a fluid might affect viscosity:
  • Increasing temperature will speed up the molecules, weakening the attraction between the molecules and thus reducing viscosity.
  • Adding a viscosity modifier that increases viscosity, including:
    • Volume exclusion modifiers that absorb liquid from the mixture, crowding the molecules into a smaller volume.
    • Associative modifiers that attach themselves to the molecules of the mixture and link them all together.

• Explore: Students will complete the lab activity as described on the Student Handout for “How Fast Does it Flow?” They will learn how to measure the relative viscosities of liquids using a homemade viscosity cup. They will then use this knowledge to test ideas about how to increase viscosity in order to change the flow rate.
• For this lab you can use pointed cups such as those used for Sno-Cones (as shown in the photo) or regular small paper drinking cups. If you are using a flat bottomed cup, poke a hole near the edge of the bottom of the cup so students can tilt it to allow all of the liquid to flow out.
• You may wish to put the holes in the viscosity cup in advance. Experiment with holes of different sizes to find one that will suit your available class time and level of student patience. I found that poking a hole using a standard nail worked well for smaller, thinner cups, but sturdier cups required that we cut off the tip to allow flow.
• Food coloring can help students see the water better so as to more accurately time its flow in Part A of the student procedure. Using food coloring is optional, but if you decide to include it, ask students to mix the food coloring and water in a separate cup before measuring and pouring it into the viscosity cup.
• If you want to create a larger time separation between water and molasses in Part 1 of the student procedure, place the molasses in a refrigerator for an hour before using or substitute honey for molasses.
• For Part B & C of the student procedure:
  • Do not dispose of fiber mixtures in the sink, as they can clog the drain.
  • This could be completed outside to minimize clean-up.
  • If you need to slow down the pace of the mixture traveling down the pan, simply tilt the pan away from vertical until the desired pace is achieved.

• Explain: After completing this activity, students will complete several Analysis questions where they will state factors that affect viscosity in a fluid, compare the viscosity of fluids in a quantitative manner and identify methods to change the viscosity of a mixture as desired.

• Elaborate: Conclude this activity with a class discussion after completing the student activity. Also, the conclusion question on the student handout addresses this as well. Prompt a discussion about any or all of the following situations in which it might be desirable to increase or decrease the viscosity of a mixture.
  • Ask students what painters do when their paint is too thick. Also consider finger paint, nail polish, and other pigmented mixtures. Students may note that sometimes painters use water to thin their paints, while at other times it is a different material (acetone, turpentine), depending on whether the paint is water based (latex) or oil based.
  • Ask students to consider spray paint versus paint applied by a brush. What must be true of their relative viscosities?

• Evaluate: This is completed in Part C of the student lab procedures. An alternative to this could be to ask students to prepare a mixture for a viscosity race amongst groups. For example students could be tasked with designing a mixture with the slowest possible rate and then have a reverse race (whoever is last over the finish line wins!).

• Sample Data: Although there will be variation depending on brands of ingredients used as well as the temperature of each, molasses tends to have a viscosity roughly twice that of water.
• In student procedure Part B, students will find that salt and sugar don’t change the viscosity of water, flour does a bit (although it tends to also be lumpy), and soluble fiber dramatically increase the viscosity when mixed into water.

• Disposal of lab materials: Students will collect all of their samples, including molasses as well as the mixtures, directly into their bucket. Depending on the age of the students you may need to keep them away from the sink and alter the disposal instructions in the student handout. In these cases, when the lab is completed, the teacher can carefully pour off the liquid and then dispose of the solid in the trash.