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LAB in Phase Changes, Molecular Motion, Heat, Freezing Point Depression. Last updated November 16, 2021.


Summary

In this lab, students conduct a micro-scale investigation to explore how various solutes affect the freezing point of water. Because of the small volume of liquid used, results are visible within minutes. Students observe what happens to the liquids as they are cooled and use their observations to infer what is going on at the particle level. They will use the results to explain the familiar phenomena of why we salt our roads and sidewalks in the winter and why freshwater lakes and ponds freeze over more easily than saltwater oceans in the winter.

Grade Level

Middle School and High School

NGSS Alignment

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

  • MS-PS3-4: Plan an investigation to determine the relationships among the energy transferred, the type of matter, the mass, and the change in the average kinetic energy of the particles as measured by the temperature of the sample.
  • MS-PS3-5: Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object. 
  • 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.
  • Scientific and Engineering Practices:
    • Developing and Using Models
    • Analyzing and Interpreting Data
    • Planning and Carrying Out Investigations
    • Constructing Explanations and Designing Solutions
    • Engaging in Argument from Evidence

Objectives

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

  • Explain how adding energy to and removing energy from a system affects particle speed and spacing.
  • Describe the heat transfer and change in kinetic energy during a phase change.
  • Determine how the addition of solutes affects the freezing rate of water.

Chemistry Topics

This lesson supports students’ understanding of:

  • Solutions
  • Freezing Point Depression
  • Heat/Energy Transfer
  • Phase Changes
  • Kinetic Molecular Theory
  • Properties of Matter

Time

Teacher Preparation: 15 minutes
Lesson: 60 minutes

Materials (per group)

  • Ice, ~1 cup
  • Water, ~60 mL
  • Table salt, ~25 grams
  • Table sugar, ~5 grams
  • Spoon (or another tool for stirring)
  • 25 mL graduated cylinder, 3 needed
  • Small cups or beakers for making solutions, 3 needed
  • Digital balance
  • Snack-size zip-close bag, 3 needed
  • Small binder clips, 3 needed
  • Rectangular, 4-cup plastic container or tray (such as a takeout container)
  • Tape for labeling
  • Permanent marker

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 touch the ice and salt mixture with bare hands.

Teacher Notes

  • Advanced Preparation:
    • Make or purchase a large bag of ice.
    • If you have a shorter class period, you can make the salt and sugar solutions for students.
    • You can also have students complete the pre-lab during the previous class period or for homework.
  • Background:
    • All matter is made up of tiny particles that are constantly moving. The temperature of a substance is a measure of its average kinetic energy. As the temperature increases, particles gain more energy, move more quickly and spread further apart. Salt is commonly used for melting snow and ice. Salt lowers the temperature at which water freezes and makes it difficult for the water to bond into rigid crystals of which ice is made. As the air temperature drops, the efficiency of salt melting the ice also decreases.
  • Lab Set-Up Photos:
  • Extensions:
    • Discuss the difference between the dissolution of ionic vs. covalent substances and its effect on freezing point.
      • Explore the pros and cons of sodium chloride vs. other deicing materials (ex: calcium chloride, beet juice).
      • Carry out a similar investigation that instead focuses on boiling point.

    For the Student

    Lesson

    Pre-Lab Questions

    1. Go to the PhET Simulation, States of Matter: Basics and click on “states”.
      URL: https://phet.colorado.edu/sims/html/states-of-matter-basics/latest/states-of-matter-basics_en.html
    2. Use the default blue particles for all parts of this simulation. Play around with the simulation, then answer the questions. In the chart below, describe the characteristics, such as particle spacing and movement, of this substance as a solid, liquid, and a gas. Make sure to also include a picture for each state.
    Solid
    Liquid
    Gas
    Picture
    Observations
    1. Set the simulation back to solid. Now play around with heating (adding energy to) and cooling (removing energy from) the container. What happens to particle speed and particle spacing as you heat them? As you cool them? Why do you think this happens?
    2. Consider the following and record your reflection before beginning the lab: Why do you think we salt our roads and sidewalks in the winter? Why do you think freshwater lakes and ponds freeze over more easily than saltwater oceans in the winter?

    Purpose

    In this lab, you will explore how adding a substance to water affects its ability to freeze and use the results to explain the phenomena posed in the pre-lab.

    Materials (per group)

    • Ice
    • Water
    • Table salt
    • Table sugar
    • Spoon (or another tool for stirring)
    • 25 mL graduated cylinder
    • Small cups or beakers for making solutions
    • Digital balance
    • Snack-size zip-close bag
    • Small binder clips
    • Rectangular, 4-cup plastic container or tray
    • Tape for labeling
    • Permanent marker

    Safety

    • Always wear safety goggles when handling chemicals in the lab.
    • Wash your hands thoroughly before leaving the lab.
    • Follow your teacher's instructions for clean-up of materials and disposal of any chemicals.
    • Do not touch the ice and salt mixture with bare hands.

    Procedure

    1. Prepare the 3 solutions required for this experiment: Using a graduated cylinder, measure the water into beakers or small cups, add the solute required. Stir each solution very well (until the solutes are completely dissolved).
      • Solution 1: 20 mL water
      • Solution 2: 20 mL water + 5 g table salt
      • Solution 3: 20 mL water + 5 g table sugar
    2. Obtain 3 small plastic bags. Label one bag “water”, another bag “salt + water”, and the last bag “sugar + water”.
    3. Carefully add 20 mL of solution 1 to the bag labeled “water”, 20 mL of solution 2 to the “salt + water” bag, and 20 mL of solution 3 to the “sugar + water” bag. Work with a partner to securely seal the bag so that there’s no air in the bag.
    4. Hold the bag with water by the top (zipper side) so that the liquid remains at the bottom of the bag. Fold the top of the bag over the crease of the zipper several times. Put a binder clip over the folds so the bag stays folded. The bag will look like a tube of liquid (see teacher example for reference). Repeat for the other 2 bags.
    5. Add enough ice to your container or tray so that it covers a single, flat layer.
    6. Add about 20 grams of salt on top of the ice.
    7. Place the 3 sealed bags into the container so that the liquid in each bag is surrounded by the ice as much as possible.
    8. Gently rock the container side to side. Check on the individual bags every 30 seconds. Record your observations in the data table.

    *Note: you may also gently squeeze the contents of each bag to make observations.

    Prediction

    What do you think will happen to the liquid in each bag when placed in a cold environment? Why?

    Data Table (record observations)

    Water
    Salt + Water
    Sugar + Water

    Summary

    1. Write a 1-2 sentence summary of how adding salt and sugar to water affects its ability to freeze. Make sure to state which substance has a greater impact.
    2. Draw arrows to show the direction of heat transfer when the 3 bags were placed on ice. Is heat transferred into or out of the liquid in each bag?
    1. Describe the change in kinetic energy of the liquid particles when the 3 bags were placed on ice.
    2. Use the particle diagram representations below to answer the following question.

    Water
    Sugar or Salt Dissolved in Water

    Based on the results from this activity, how do you think adding salt or sugar to water affects the ability of water to freeze? Make sure you talk about energy and movement of water molecules in your response.

    Application

    Use the results from this activity to explain the following phenomena. Make sure you discuss energy and movement of water molecules in your responses.

    1. Freshwater lakes and ponds freeze over more easily than saltwater oceans in the winter.
    2. Why putting salt on the roads and sidewalks in the winter is useful.