LEARN NC

CareerStart lessons: Grade eight

5.9 Winter advisory: The effect of salt on the freezing point of water

By Tammy Johnson and Martha Tedrow

Provided by CareerStart

Essential question: What effect does salt have on the freezing point of water?

Learning outcomes

Students will investigate how salt lowers the freezing point of water and how it can be used by road crews to keep the roads safe for drivers.

Teacher planning

Materials needed

  • Lab materials (for each student):
    • Safety goggles
    • Salt (sodium chloride)
    • Teaspoon
    • 2 beakers
    • Stirring rod (spoon)
    • 2 Thermometers
    • Ice
    • Cold water
    • Timer
  • Student lab worksheet

Time required for lesson

One class period (45-50 minutes)

Teacher background

The following background information has been adapted from the Federal Highway Administration’s “Effective Anti-Icing Program” web page.

De-icing and anti-icing

During winter advisories, wet and dry salt compounds are used by city street divisions as preventative and reactive agents.

These two distinct snow and ice control strategies make use of chemical freezing-point depressants: de-icing and anti-icing. They differ in their fundamental objective. Whereas anti-icing operations are conducted to prevent the formation or development of bonded snow and ice for easy removal, de-icing operations are performed to break the bond of already-bonded snow and ice. De-icing is familiar to most agencies since it has been the most widely used strategy in the past. The design of de-icing operations as a bond-breaking operation stems from its timing: it is commonly initiated only after 25 mm (1 in) or more of snow has accumulated and bonded to the road.

Preventative salting

Before a storm begins, an aqueous solution of sodium chloride and/or calcium chloride can be used as salt brine on highways. It is only spread on dry road surfaces. The solution sticks to the road surface until it reacts with moisture. As soon as the snow falls, the brine reacts with the precipitation causing a quick exothermic heating and melting. The new solution created will refreeze at the temperature of the salt brine, which is 5 degrees Fahrenheit (calcium chloride) and 15 degrees Fahrenheit (sodium chloride). This results in less “Black ice” and frozen roads.

Anti-icing brine is used by most municipalities before moisture falls from the sky. It is more cost-efficient and environmentally friendly.

De-icing reactive agents

After a winter storm has begun, dry salt and abrasives are added to the top of existing snow. Most de-icing agents bounce on the surface causing various scattering and distribution. Abrasive agents act only to provide friction. The moisture in the snow pack will react with chemical salt agents causing a slow snow melt. Both of these reactive agents are considered environmentally toxic and costly.

Activities

  1. Have students brainstorm for 5-8 minutes about why they think the city or county road crews use salt on the roads in the winter before a snow or ice storm.
  2. Have students brainstorm why the crews spread salt brine on the dry roads as an anti-icing precaution. (Remember to point out the roads must be dry or the liquid brine will wash off the road before it starts to snow.)
  3. Discuss why rock salt is used as a de-icing agent after the snow has fallen or if the snow was preceded by heavy rain. (Rock salt will mix with the precipitation and create helpful friction to keep cars from slipping.)
  4. Discuss the role of city/county road crews in the community before, during, and after a winter storm. How would a working knowledge of chemistry be beneficial?
  5. Inquiry activity introduction: Ask the students how adding salt affects the freezing temperature of water. Hand out the lab worksheet and tell students they will complete a lab that will help them answer that question.

Student lab instructions

Lab safety note: Students should wear safety goggles and be warned not to touch the sides of the beaker as the temperature drops.

Note: Student lab instructions also appear on the lab handout.

  1. Fill two beakers half-way with cold water. Label one beaker “A” (the variable beaker) and the other “B” (the control beaker).
  2. Add ice to the cold water in both beakers, filling them to the top.
  3. Record the temperature of each beaker and the time on the lab worksheet.
  4. Predict what will happen as salt is added: Will the temperature of the water rise or fall as more salt is added?
  5. Add 1 teaspoon of salt to beaker A and stir. After 3 minutes record the temperature of each beaker.
  6. Add another teaspoon of salt to beaker A and stir. Record the temperature. [The temperature will continue to fall well below the freezing temperature of water.]
  7. Continue to add salt to beaker A every 3 minutes for up to 15 minutes, stirring constantly.
  8. Record the final temperature readings for each beaker. Create a line graph to plot the change in temperature over time.

Teacher note: As salt is added to the beaker of ice water, the temperature will fall well below the freezing temperature of water. This is why salt brine and/or rock salt are added to roads and highways by road crews before a snowstorm. It keeps the freezing temperature well below 32 degrees Fahrenheit or 0 degrees Celsius.

Follow-up

  1. Have the students answer the analysis questions on the lab worksheet:
    1. What happened as the salt was added to the ice water?
    2. Why do you think the water temperature fell?
    3. If the salt brine lowers the freezing point of water, then what would be the benefit of applying the solution to municipal streets before a snowstorm?
    4. If a winter storm begins before the salt brine solution can be applied, would it be beneficial to apply it directly to the snow or ice? Why or why not?
    5. Salt brine is considered an anti-icing agent, and rock salt is a de-icing agent. How are anti-icing and de-icing agents different? Which do you think is more effective? Why?
    6. Career connection: Environmental scientists are concerned that the salty brine and salt slush created by winter road treatments are killing plants and other foliage on the side of the road, and ultimately affecting animal habitats. Chemical engineers have been given the task of finding a suitable replacement for rock salt that is more environmentally friendly. Think of some alternative substances that could be used. Explain why you have selected each new alternative. Then, select one of the new agents you have proposed and explain how it may be more “kind” to the environment.
  2. Wrap up with a discussion of the careers of chemical engineers, environmental scientists, and hydrologists. For more information on these careers, see the Bureau of Labor Statistics’ Occupational Outlook Handbook:

Extension

Record change of temperature over time on a spreadsheet. Use a projector and computer and integrate technology by placing the data into a Microsoft Excel spreadsheet. Teachers can also draw a line graph on a chalkboard.

North Carolina Curriculum Alignment

Science (2005)

Grade 8

  • Goal 1: The learner will design and conduct investigations to demonstrate an understanding of scientific inquiry.
    • Objective 1.04: Analyze variables in scientific investigations:
      • Identify dependent and independent.
      • Use of a control.
      • Manipulate.
      • Describe relationships between.
      • Define operationally.
    • Objective 1.05: Analyze evidence to:
      • explain observations.
      • make inferences and predictions.
      • develop the relationship between evidence and explanation.
    • Objective 1.06: Use mathematics to gather, organize, and present quantitative data resulting from scientific investigations:
      • Measurement.
      • Analysis of data.
      • Graphing.
      • Prediction models.
  • Goal 4: The learner will conduct investigations and utilize technology and information systems to build an understanding of chemistry.
    • Objective 4.05: Identify substances based on characteristic physical properties:
      • Density.
      • Boiling/Melting points.
      • Solubility.
      • Chemical reactivity.
      • Specific heat.
    • Objective 4.06: Describe and measure quantities related to chemical/physical changes within a system:
      • Temperature.
      • Volume.
      • Mass.
      • Precipitate.
      • Gas production.