Summary

In this lesson, students investigate how the freezing point of water changes when salt is added. Many students know that water freezes at 0°C, and many believe that all substances freeze at 0°C. The investigations in this lesson help students understand why salt is spread on roads in cold and snowy conditions.

Grade Level

Elementary School, Middle School

NGSS Alignment

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

• 5-PS1-1: Develop a model to describe that matter is made of particles too small to be seen.
• 5-PS1-2: Measure and graph quantities to provide evidence that regardless of the type of change that occurs when heating, cooling, or mixing substances, the total weight of matter is conserved.
• 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
• Cross-Cutting Concepts:
  • Cause and Effect: Mechanism and Explanation
  • Systems and System Models
  • Energy and Matter: Flows, Cycles, and Conservation
  • Stability and Change

Objectives

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

• Describe how the addition of salt affects the freezing point of water.
• Measure amounts of water (solvent) and salt (solute) carefully to prepare solutions.
• Conduct an experiment to show that salt lowers the freezing point of water and use the results to explain why salt is spread on snowy roads in winter.

Chemistry Topics

This lesson supports students’ understanding of the following topics in chemistry:

• States of matter
• Solutions
• Solubility
• Solute/solvent
• Freezing point
• Freezing point depression

Time

Teacher Preparation: 15 minutes
Lesson:
Part 1: 45 minutes
Part 2: 20 minutes on day one, put samples in freezer overnight, 30 minutes on day 2

Materials

Part 1:
For each group:

• 2 paper towels
• 2 ice cubes
• Table salt in a small cup

Part 2:

• Freezer
• Thermometer

For each group:

• 3 clear plastic cups, 8 oz.
• 3 plastic spoons
• Permanent marker
• Measuring spoon (Tablespoon)
• Measuring cup (¼ or ½ cup)
• Water
• 3 thermometers that can measure below -20°C

Extension materials for each small group:

• Various types/brands of ice melt (or other solute) and materials requested by students (similar to what was used in parts 1 and 2 to perform student-designed experiments

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.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

• To get the most from this lesson, it is important that students discuss their results and how their findings relate to the real-life problems being investigated before they move on to the next experiment.
• As a model for using salt on icy roads in the winter, it would be best to run this experiment in winter-like cold conditions rather than the room temperature conditions of the classroom. However, because of convenience and practicality, the experiment is done in the classroom with ice cubes as the closest model that can reasonably and efficiently be used in most school settings.
• This lesson can be coupled with another AACT resource, Pothole Science, that also examines winter road phenomena.
• The chemistry phenomenon that is explored in this lesson is called freezing point depression. More background information on this topic is provided below:
  • The temperature at which water turns from liquid to solid or from solid to liquid is known as the freezing point or melting point. (They happen at the same temperature: freezing happens when thermal energy – heat – is removed from the system, melting happens when thermal energy is added to the system).
  • Pure water at normal atmospheric pressure has a freezing/melting point of 0°C (or 32°F). When a solute such as salt is added to water, a solution forms. The solution has a lower freezing point than the solvent which means that its temperature must be lowered to less than o°C in order for it to freeze. In the case of salt and water, saltwater freezes (and melts) at a lower temperature than fresh water.
  • The lower freezing point occurs because of the interaction between the solute and solvent. In the solution, water molecules interact with other water molecules but also with the particles of the solute. The solute particles interfere with water’s ability to crystalize which requires the solution to be at a lower temperature to freeze.
  • Roads are salted in winter to melt ice or prevent water from freezing on the road. However, if temperatures are significantly lower than 0 ^oC, this method won’t prevent roads from freezing.

Lesson Introduction

• Before starting the student activities, engage the class on the topic of snow and how it affects road conditions. This could be done as a class or small group discussions using talk moves, a think-pair-share activity, or other similar strategy. Prompt students to think about some or all of the following questions:
  • In what ways can driving be dangerous during the winter?
  • What happens to wet roads and sidewalks during the winter?
  • What can happen overnight?
    • Guide them to think about the effects of weather on roads and driving – roads get slippery, snowy, and icy. Melted snow/ice can refreeze overnight if temperatures dip back below freezing. Students may associate inclement winter weather with school closings.
• Next, discuss what they’ve seen road crews do to keep people safe on slippery roads. Ask them:
  • What do you see people doing to help prevent icy roads?
  • What kind of equipment or vehicles have you seen being used? What were they doing?
    • Students should suggest snowplows scraping snow and slush off roads, trucks spreading salt and/or sand. (Note that sand serves a different purpose than salt – it provides traction on icy roads, rather than affecting the freezing point the way salt does.) They may also make the connection to shoveling and salting sidewalks for the same reason.
• After the discussion, consider showing students a video (2:12) of snowplows on the road: http://www.youtube.com/watch?v=7PwJ2rSKeWM

Hands-on Activities

• In part 1, students model what happens when salt is spread on icy roads and sidewalks using ice cubes and table salt: salt lowers the freezing point of water, which causes the ice with salt on it to melt faster than the ice with no salt.
  • Expected results in the photo below show that the salted ice cube (on the bottom) melts faster than the unsalted ice cube.

• In part 2, students model what happens to salted slushy roads when temperatures drop below freezing again: because of the lower freezing point of salt water vs. pure water, salty water on the roads doesn’t refreeze as easily as pure water would. Salt water solutions are put in a freezer overnight to model this phenomenon.
  • The actual amount of ice that forms in the salt solutions and the temperature of the samples depends on the temperature of the freezer and the concentrations of the solutions. Students should note that the pure water sample froze solid, the solution with less salt may have some ice formation, and the solution with more salt should have the least ice formation.
  • The temperature of all three cups should be about the same as each other and the freezer if they have been left in the freezer long enough to reach thermal equilibrium with the freezer.

Extension Opportunities

• One extension activity is included in the student document. Other activities that could help deepen students’ understanding of this phenomenon include:
  • Have students measure the temperature of cups of ice/water from part 2 when placed at room temperature over time (after they are taken out of the freezer). Graph results and compare the different concentrations.
  • Have students design and carry out an investigation to determine the minimum amount of salt needed to prevent water from freezing at a particular temperature.
  • Connect to Math:
    • In part 1, have students measure the mass of each ice cube (removed from its paper towel) and record the change in mass at each time interval when they record other observations. Students can then calculate the change in mass (and/or percent change in mass) per minute or hour for the salted and unsalted ice cube to compare the rate of melting mathematically.
    • In part 2, have students calculate the percentage of salt by volume in the solutions that they made. Note that they will first need to convert units from tablespoons to cups or cups to tablespoons.
  • Connect to Reading and Writing:
    • Have students research the impact of salting roads on the environment and write an essay to communicate their results. How does the salt affect grass and other plants near the road? Animals? Local streams? Bodies of water father away? Are the benefits of salting roads worth the risks to the environment?
    • Have students imagine that the budget for road salt has been cut: the city will no longer buy any road salt. Students can then either:
      • Write a short story in which they imagine what a winter in a cold city would be like without road salt, or
      • Write a persuasive letter to the city council against or in support of this decision.
  • Connect to Social Studies:
    • Have students research and communicate about the use of salt in their area. What kind of salt is used in your state to clear roads during the winter months? Where does the salt come from? How is it transported to your area?