Summary

In this activity, students will use a colligative properties simulation to investigate the effects of different solutes, and different amounts of those solutes, on the boiling point and freezing point of a solution. Students will see particle-level animations of boiling and freezing with different types and amounts of solutes, as well as graphical representations of the results of each trial.

Grade Level

Middle School and High School

NGSS Alignment

This activity will help prepare your students to meet the following scientific and engineering practices:

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

Objectives

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

• Identify trends in boiling point elevation and freezing point depression for different solutes and different concentrations using simulation data.

Chemistry Topics

This activity supports students’ understanding of:

• Solutions
• Solutes and Solvents
• Concentration
• Colligative Properties
• Boiling Point Elevation
• Freezing Point Depression

Time

Teacher Preparation: minimal
Lesson: 60-90 minutes

Materials

• Computer, tablet, or phone with internet access
• Student handout
• Variety of colors of pens/pencils (for sketching particle diagrams, graphing different data sets)
• https://teachchemistry.org/classroom-resources/colligative-properties

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• The simulation can be found at the following link (note that students can access the simulation without an AACT login):
  • https://teachchemistry.org/classroom-resources/colligative-properties
• This simulation introduces students to the colligative properties of boiling point elevation and freezing point depression using 1 mol, 2 mol, and 3 mol amounts of three solutes dissolved in water: sucrose (C₁₂H₂₂O₁₁, a molecular compound), sodium chloride (NaCl, an ionic compound), and magnesium chloride (MgCl₂, an ionic compound).
  • It is up to the teacher when to introduce the definition of the term “colligative properties.” It is recommended that students complete this simulation activity first, and once they have determined that the number of particles in solution (regardless of their identity) determines the effect on the boiling point/freezing point, then the term can be introduced as a succinct way to describe this phenomenon. (Students may ask about the title of the simulation, “Colligative Properties” – you can tell them that it is not necessary for them to know this term to complete the activity, and this will be something they discuss afterwards.)
  • Similarly, to avoid spoiling their efforts to solve the problem presented in the activity, the terms “boiling point elevation” and “freezing point depression” should not be introduced until after they run their experimental trials in the simulation. Read more about the “Explore-before-Explain” approach to science vocabulary in this article and podcast.
• It would be beneficial for students to know about the differences in solubility between ionic and molecular compounds prior to using this simulation. The Solubility Animation could be used to show how ionic compounds (like salt) dissociate into cations and anions when they dissolve, but polar molecular compounds (like sugar) remain as one unit. In both cases, there are attractions between the water molecules and the solute particles. This video goes into it in more depth if you wish to provide students with more detail on the dissolving process.
  • Note that in the Colligative Properties simulation, the solutions are already formed, so it does not show the actual process of the particles dissolving.
• In the “particulate views” in the Colligative Properties simulation, students will notice that some of the water molecules are attracted to the dissolved solute particles and that they sometimes “stick” together. This represents the solvation of the solute particles. In reality, there would be numerous water molecules surrounding each solute particle, but we just included one to avoid overcrowding the particulate view and giving the impression of solid formation.
• When analyzing the particulate views throughout the simulation, it is important to note that 1 mole of a substance is represented by 1 particle. Since the ionic solutes dissociate, the particulate views show more than 1 particle in solution. For example, the 1 mol sucrose solution has 1 particle (red, oblong), the 1 mol sodium chloride solution has 1 Na⁺ particle (yellow) and 1 Cl^– particle (green), and the 1 mol magnesium chloride solution has 1 Mg²⁺ particle (purple) and 2 Cl^– particles (green). These are scaled up proportionally for the 2 mol and 3 mol solutions. Encourage students to record these accurately when they are asked to sketch these particle diagrams, as they will be important for the questions at the end of the activity.
  • Note that the different oblong shape of the particle representing the sucrose molecule is used because it is a molecular compound and is a much larger collection of atoms than the individual ions in the other compounds. This could be a good opportunity to discuss how the symbolic particles in this model are used to represent differences and similarities between substances.
• For the freezing point depression trials, the beakers containing the solutions are sitting in ice baths. These ice baths are symbolic representations showing that the solutions are being cooled, as an actual ice bath would only go down to 0°C (unless a solute was added to them as well!). If an observant student notices this discrepancy, you can tell them that there is also a solute in the ice bath, or that it is symbolic and it just indicates that the solutions are being cooled down.
• A lot is happening visually in each trial – the temperature is changing, the particles are moving in the animations, and the data is being graphed in real time. Encourage students to use the “Re-Run Experiment” button often so they notice all the different pieces of information being provided by the different parts of the simulation.
  • This is particularly true in the freezing point depression particulate views – as soon as the water molecules start to stick together, the temperature stops decreasing, indicating that the freezing process has started. This may be hard to notice on the first viewing, so students can use the “Re-run Experiment” button to watch the same trial multiple times.
• The freezing point depression particulate views show hexagonal crystals forming as the water molecules in the solution begin to freeze. (Only 2 crystals are shown, but over a longer time, more and more of the water molecules would eventually form these crystals.) This is a good opportunity to remind students that water freezes in these hexagonal structures with a small gap in the center, which is why ice is less dense than liquid water and floats on top of the liquid, rather than being more dense and sinking underneath the liquid like most materials would do.
• This simulation activity asks students to compare the results of adding 1 mol of each solute so they can make a direct comparison between the same amount of molecular compound and the two ionic compounds. They will notice in the particulate views that the molecular compound (sucrose, C₁₂H₂₂O₁₁) does not dissociate and remains as one particle, and the ionic compounds dissociate into two ions (sodium chloride, NaCl) and three ions (magnesium chloride, MgCl₂). Student should see that this difference in dissociation causes a proportional difference in the changes in boiling and freezing points given the same number of moles of solute.
  • The van’t Hoff factor: This term is not included in the simulation but may be appropriate to introduce to more advanced students. The van’t Hoff factor is the number of moles of particles formed when dissolved per mole of solute.
  • Example: For molecular compounds such as C₁₂H₂₂O₁₁, which do not dissociate, the van’t Hoff factor (i) is generally 1. For NaCl, i = 2 (Na⁺ and Cl^–), and for MgCl₂, i = 3 (Mg²⁺ and 2 Cl^–).
  • The difference in van’t Hoff factors means that a given amount of NaCl (i = 2) added to the water will produce twice as much of a change in boiling or freezing point as the same amount of C₁₂H₂₂O₁₁ (i = 1). The same amount of MgCl₂ (i = 3) will produce three times the change compared to C₁₂H₂₂O₁₁.
• In the analysis section, students are asked to graph data from the various trials. You could have students create their graphs in Excel (or other graphing software) and attach a copy when they submit the activity if you prefer.
• Related classroom resources from the AACT Library that may be used to further teach this topic:
  • Lab: Changing Water’s Boiling Point
  • Lesson Plan: The Hot and Cold of it All
  • Lesson Plan: Fuel Line Antifreeze

Optional extension:

• If you wish to dive deeper into a quantitative understanding of colligative properties with an advanced group of students, you could introduce them to the formula for calculating the change in boiling (and freezing) point with the addition of a solute:  (substitute “F” for “B” subscripts for freezing point). They will have a general understanding of the van’t Hoff factor, I, from the simulation, and the concentration, m, is expressed in molality (mol of solute/kg solvent) rather than molarity since the volume of a liquid will change slightly as density changes with temperature. K_B (or K_F for freezing point) is the boiling point elevation (or freezing point depression) constant for a given solvent. For water, K_B = 0.512 °C/m and K_F = -1.86 °C/m. Students could use the results of this activity to calculate K_B and K_F for water. They could follow the instructions in the analysis section of the AACT lab Changing Water’s Boiling Point. This lab would also make a good hands-on follow-up activity to the simulation.