Exploring Freezing Point Depression using Freeze Pops

Jayme Fast, Springfield Public School, Springfield, MN based on activity by Diane Brudelie, Martin County West High School, Sherburn, MN.
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This lab is designed to familiarize students with freezing point depression. When a nonvolatile solute (such as salt) is added to a solid, it causes the point of freezing to drop. Compounds that break into ions when dissolving are better at lowering the freezing point than substances that do not separate into particles because the added particles disrupt the ability of the water to form crystalline ice. The more particles there are, the greater the disruption and the greater the impact on particle-dependent properties such as the freezing point. In this lab, students will add salt to ice. They will record the time and temperature data and interpret their results. They will then compare their data to that of their classmates.

Learning Goals

- Students will work as a group to determine which time interval is most suitable for their data collection.
- Students will collaborate with other groups to gather and arrange data pertaining to the temperature of each groups system.
- Upon completion of the initial exercise, students will use their lab experience to explain the concept of freezing point depression and relate their findings to other colligative properties.
- Students will relate their findings to everyday situations such as using salt to clear icy side walks and to determine which material is best suited for use by highway departments to de-ice roads and bridges

Context for Use

This lab activity is used to introduce colligative properties in solutions, concentrating primarily on the effects of nonvolatile solutes on freezing point. This activity is used in a general chemistry class consisting of juniors and seniors in high school and it is completed before any notes are given on the subject. This lesson usually takes 2 50 minute class periods. The first lab day is spent collecting data and the second is spent sharing and analyzing data and drawing conclusions. The class usually has an excellent discussion following the completion of the lab. Topics usually brought up during the discussion include what happened, why, suggesting alternative materials to test, and applying it to other scenarios such as boiling point.

Subject: Chemistry:General Chemistry:Thermodynamics:Heat
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12)

Description and Teaching Materials

In this activity, students are given a freeze pop, a sealable container, a predetermined amount of salt and a thermometer. They follow lab instructions to make their system. I usually give them 15 minutes to collect data; the data collection interval is determined by the group. Also, as they collect temperature data, they need to make observations, paying particular attention to the consistency of their freeze pops and the ice in their container. From start to finish, including opening lab instructions and clean – up, this takes a class period.

I use this activity as an introduction to colligative properties. The students have not had any background information pertaining to this topic. They have, however, talked about interaction of solute and solvent particles when making a solution. They are also familiar with polarity, ionic and covalent bonds.

The second day of class is spent sharing data. A large data table is projected on the board and lab groups enter their data to share with each other. Each group now has a large collection of data to help them evaluate the effect of varying amounts of salt on the temperature of the ice/water. A significant portion of the second day is spent discussing what really happened during the activity. This discussion happens before the students write their conclusion for the lab. They're graded on the content of their conclusion. If they benefited from the group discussion, it will be evident in their conclusion as this is their chance to tell exactly how much they understand about what happened. Please see the lab hand out associated with this activity in the "attachments" section. Student Hand Out for Freezing Point Depression Lab (Microsoft Word 31kB Aug16 09)

Teaching Notes and Tips

This lab is a perennial favorite of my students'. It has not produced any safety issues in the past. Be aware, however, that the floor will become wet in places and as the countertops are cleaned off they will develop a salt crystal residue. Be very patient with your students when discussing the concepts at the end as they will struggle to find a way to describe what happened in their own words.


As mentioned before, the students are graded on their conclusion. The class discussion takes place before the conclusion is written so it is expected that they should have been actively involved with the discussion. If they did, their conclusion will basically be a summary of the discussion, in their own words.


- – Evaluate the explanations proposed by other by examining and comparing evidence
- – develop possible solutions to an engineering problem and evaluate them using conceptual and mathematical models.
- – Select and use appropriate numeric, symbolic, pictorial, or graphical representation to communicate scientific ideas, procedures, and experimental results.
- – Relate exothermic and endothermic reactions to temperature and energy changes
- 9.C. – Describe dynamic processes by which solutes dissolve in solvents
- 9C. – Use kinetic molecular theory to explain how changes in energy content affect the state of matter.

References and Resources