By Bruce Wilson
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.
National Science Education Standards
- Grades 9-12: Structure and Properties of Matter
- Individual Packet (sealed cup) of Liquid Non-Dairy Creamer
- 16-oz Deli Cup and Lid
- Water (tap, deionized, or distilled)
- Sodium Chloride, about 50 g
- Taster Spoon
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 .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 min 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 non-dairy creamer freezes. Packing the creamer cup in ice, water, and salt and then swirling to maintain temperature evenly through the cup results in ice creamer. 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.
To view more activities like this one, click here.