Teach Freezing Point Depression with a Tasty Activity!
Demonstrate the colligative property freezing-point depression and make edible ice cream at the same time. This safe, inexpensive, and engaging experiment works well for a science night, open house, kids’ group, or anywhere fun and science meet. Using table salt, reduce the temperature of a mix of ice and liquid water to below the freezing point of water. After 5 minutes, open the cup for a tasty treat.
Three-Dimensional Learning and NGSS
This activity is appropriate for high school physical science and supports incorporation of the science practices of planning and carrying out investigations, analyzing and interpreting data, using mathematics and computational thinking, and constructing explanations. The activity builds toward the performance expectations below.
- HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative positions of particles (objects).
- HS-PS3-4: Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperatures are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).
- Individual Packet (sealed cup) of Liquid Nondairy Creamer
- 16-oz Deli Cup and Lid
- Water (tap, deionized, or distilled)
- Sodium Chloride, about 50 g
This activity is based on Carolina ChemKits® Ice Cream and Freezing Point Depression Kit.
Teach freezing-point depression, a colligative property, and yield a delicious result. Students use sodium chloride to lower the freezing point of ice and freeze a single serving of nondairy instant creamer. Varying the amount of sodium chloride gives unexpected (but still tasty!) results. Students measure the temperature of the ice bath over time and graph their results to determine the freezing-point depression. This activity covers the nature and real-life applications of colligative properties, freezing-point constant, and the van’t Hoff factor. The experiment is clean, simple, and much easier to manage than other methods of making ice cream. Plus, the only consumable items are ice and creamers.
No special safety precautions are required in using these materials. It may be helpful to perform this activity in a non-laboratory setting so as not to confuse students about the basic rule of not eating or drinking anything in the lab.
Preparation and procedure
- Pour the 50 g of sodium chloride (salt) into the deli cup. If you do not have a scale, simply pour a layer of salt from 0.5 to 1 cm deep.
- Place the unopened creamer cup in the deli cup.
- Fill the deli cup with ice
- Fill the deli cup with water, leaving just enough room to attach the lid.
- Place the lid on the cup.
- Swirl the cup on a flat surface for 5 minutes to mix.
- Remove the creamer and, without opening it, check for firmness either by squeezing the sides or shaking the cup near your ear. If you feel or hear liquid sloshing, place the creamer back in the deli cup and swirl for 5 more minutes.
- Open and enjoy!
Freezing-point depression is an example of a colligative property. A colligative property of a solution is one that depends on the number of solute particles but not on the solute’s identity. For example, the freezing point of an aqueous 1 molal sodium chloride solution is the same as the freezing point of a 1 molal potassium chloride solution because sodium chloride (NaCl) and potassium chloride (KCl) dissociate into the same number of solute particles, thus lowering the freezing point of the solvent (water) the same amount. Calcium chloride (CaCl2) lowers the freezing point to a greater extent because it dissociates into 3 ions (1 calcium and 2 chloride) rather than 2. Other colligative properties include boiling-point elevation and osmosis and diffusion.
In this experiment, sodium chloride is added to ice and lowers the freezing point of water from 0 to –10 °C. With its surroundings reduced to this temperature, the nondairy creamer freezes. Packing the creamer cup in ice, water, and salt and then swirling to maintain temperature evenly through the cup results in ice cream. Enjoy this tasty application of a colligative property.
The creamer itself consists of water as a solvent and contains particles of various types. While its own freezing point is not depressed to the extent of the ice-water-salt mix, it is depressed. Your students may enjoy developing experiments to find the freezing point of the creamer or to compare the freezing point among creamers with different ingredients.