Summary

In this lab, students will consider why salt is used to aide in snow clearing and to help keep icy roads safe. They will investigate how salt ‘melts’ ice and determine the best type of salt to do so.Additionally, students will explore the advantages and disadvantages of the various different types of salt.

Grade Level

Middle or High School

NGSS Standards

• MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
• 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 particles as measured by temperature of the sample.
• MS-PS3-3: Apply scientific principles to design, construct, and test a device that either minimizes or maximizes thermal energy transfer.

Objectives

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

• Develop a testable question and set up a procedure to find an answer to that question.
• Analyze data and express the results of that data.
• Discuss the meaning of freezing point and freezing point depression.
• Identify phase changes, and indicate that creating a mixture can change the melting/freezing point of a substance.

Chemistry Topics

This lab supports students’ understanding of

• Solutions
• Freezing Point Depression
• Mixtures
• States of Matter
• Phase Changes
• Freezing Point
• Melting Point

Time

Teacher Preparation: 15-20 minutes

Lesson: 90 minutes

Materials (per group)

• 1-2 Tbsp per substance:
  • Sand
  • Table salt
  • Rock salt
  • Calcium chloride
• Thermometers
• Measuring spoons
• Ice cubes, one large plastic container or ice chest
• Small plastic containers, tubs or cups with wide openings, approx. 5
• Plastic spoons for each substance

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.

Teacher Notes

• Teacher may need to guide students in the process of designing a controlled experiment with testable questions (For example: What substance melts ice faster? What temperature is the ice as it starts to melt? Does the amount of substance effect how fast the ice melts?) Also, the teacher may need to review independent and dependent variables, such as: substance used, time to melt, temperature when melted, etc.
• Engage: Ice and snow pose obstacles to drivers throughout the winter season. Show students portions of the following video clip. Discuss with students the problems caused by driving on roads covered with ice and elicit why transportation departments want and need to clear ice off of roadways.
• Explore: Draw on students’ past experiences with snow and ice on the roads and make a list of how students have seen those roads made safe for travel. (Student responses should include snow plows, rock salt, sand, brine, ice melt, etc.)
• Discuss how each of these methods remove the ice/snow. When discussing the rock salt, elicit from students’ responses why they think the rock salt is used. Most responses will include that the rock salt ‘melts’ the ice.
• Working with partners or small groups, have students determine ways they could design a test either focused on a particular salt type or comparing salt types (use student handout). For example, students could set up an investigation to determine which substance ‘melts’ the ice faster, or they could set up an experiment to see if the amount of a particular type of salt effects how fast the ice melts. The teacher will look for students to set up experiments that have testable questions, identify independent and dependent variables, establish set up procedures and create data tables.
• Once students have established the parameters of their testable question, the teacher will have students obtain necessary supplies and conduct their experiments. Students will report their data and conclusions to the class.
• Explain: Record student group results on whiteboard. Discuss with class the different types of tests that were completed, and what questions were addressed by each group. Hopefully, some groups have made the connection that the temperature of the ice has actually dropped below the freezing point when it is combined with a salt (freezing point depression).
• Show students the ACS video ‘How Does Salt Melt Ice?’ up to the 1:22 minute mark.
• Students should understand that salt doesn’t melt ice but lowers the freezing temperature of the water so that the ice doesn’t stay in solid form.
• Evaluate: Using class results or their individual results (see conclusion section on student handout), students should write a Claim-Evidence-Reasoning statement that addresses how salt ‘melts’ the ice and snow from the roadways and use data to support it .Their reasoning statements should explain the science behind the ‘melting’. Additionally you could require students to also draw a model that explains what happens to the molecules as the salt mixes with the ice.
• Extension: If time permits, allow student groups time to research the impact that rock salt and sand have on both the environment and the ‘budget’. Have students research information on the internet and use what they have learned to develop a solution to ice and snow on the roadways that balances safety, environmental concerns and budgetary constraints.