Summary

In this activity, students are introduced to molarity at the particle level. Students will activate their prior knowledge by demonstrating their understanding of concentration by preparing several Kool-Aid drinks, and then applying that information at the particle level to various models.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• Scientific and Engineering Practices:
  • Developing and Using Models
  • Using Mathematics and Computational Thinking

Objectives

By the end of this activity, students should be able to:

• Explain the meaning of molarity in reference to the particle concentration in a solution.
• Use models to accurately represent the meaning of molarity.
• Determine the molarity of a solution using the molarity equation.

Chemistry Topics

This activity supports students’ understanding of:

• Solutions
• Molarity
• Concentration
• Particle level

Time

Teacher Preparation: 30 minutes

Activity: 50 minutes

Materials (per group)

• 1 small packet of Kool-Aid Powder
• 2 cups
• Water
• Molarity card sort cards
• Calculator

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 solutions, even if they’re otherwise edible products.
  

Teacher Notes

• Find out more about this Activity in an article the November 2017 issue of Chemistry Solutions.
• I use this activity as the opening to a solutions unit. First, I have my students make concentrated and dilute Kool-Aid solutions.
  • Put students in small groups.
  • Give each group a packet of Kool-Aid powder, water, and two cups.
  • Without any additional instruction, tell students to make a cup of highly concentrated Kool-Aid and a cup of Kool-Aid that is weak in concentration. Typically, they easily do so.
• Students can communicate that the dark colored Kool-Aid is strong in concentration while the light colored Kool-Aid is weak. This is the macroscopic representation. I then ask the students to draw diagrams of Kool-Aid and water in the two cups (on a whiteboard, in a notebook, etc.). A typical student response is shown in the first image.
  
• I then ask students to add “particles” of Kool-Aid to their drawings. This typically results in the sketch being improved, such as shown in the second image.
  
  • If you want to allow students to drink their Kool-Aid solutions, be sure to do so in a classroom rather than a lab environment. If this activity is done in the lab, students should not drink the solutions.
    
• Next, instead of giving the equation for molarity and having students calculate concentrations, students work with the particulate representation of molarity and look for patterns on their own so that they construct their own knowledge about determining concentrations of solutions.
• To accomplish this, share the student instructions available in the downloadable presentation as well as downloadable sets of cards.
• These cards will have to be prepared (printed and cut) prior to the activity. Each group will need a set of the cards from at least pages 1-6. You may want to print them on various colors of paper/cardstock. You could also laminate them so they can be used for multiple years. The card set includes cards with particle diagrams of beakers containing various concentrations of solutions, and cards that indicate values for moles of solute, liters of solution, and molarity.
  • Pages 7-10 of the activity cards set include cards indicating values for kilograms of solvent, molality, and particle diagrams labeled in kg rather than L that can be used, along with the moles cards from page 1, to represent molality (moles of solute per kilogram of solvent) rather than molarity. One example of a molality question is given on the final slide of the presentation, and you could add more if you would like.
  • Not every card in the set is necessarily used to answer the problems in the presentation. You can add more questions, change the values provided, have students come up with values that match the particle diagram cards, etc. See the extension options below.
    
• Each dot in the particle diagram of the solution represents one mole of solute. Students should be able to find the matching particle diagram card, the correct mole value card, and associated liters of solvent card that defines the given concentration.
• Use the presentation to direct the groups with each task and progress through the activity. While doing this, be sure to check in on groups to ensure understanding and progress.
• Once students are proficient with identifying concentration with the picture, moles, and liters cards, I asked students to divide the fraction of moles/liters. Many students already make the connection of the fraction of moles/liters and begin to use the molarity cards without knowing what the “M” stands for on the molarity card. There are a variety of combinations of cards that could be used to describe most concentrations.
• Possible extensions for this activity:
  • One way to increase the difficulty of this activity is to have students find all possible combinations of moles and liters for their given molarity value (rather than just 1 solute and 1 solution card), and/or to find all of the particle diagrams that work for a given molarity value.
  • Have students select random cards for moles of solute and liters of solution and use them calculate the molarity, or provide another molarity value, and then have them draw one or more corresponding particle diagrams that are not already in the card sort.
  • Have students select one of the particle diagram cards that they have not used yet and provide one or more sets of values (moles of solute, volume of solution, and molarity) that would match that particle diagram. (Allow students to come up with their own values rather than restricting them to the values that are on the printed cards.)
    
• After this activity, students were then asked to calculate concentrations via a traditional worksheet. (A few example problems are given on slides 8-10 of the presentation and could be done as a class or individually. You could also have students draw a particle diagram similar to the models they have been using throughout this activity.)
• Possible answers for the card sorts and the example problems are provided in the “notes” section of the PowerPoint presentation, as well as the “Presentation – Possible Answers” pdf.