Did you know that you can make ice cream in a bag with just ice, salt and a few other ingredients? Salt is the ingredient that lets you chill the mixture to the right temperature to make a tasty frozen treat. The salt acts to lower the temperature at which a solution freezes. This is known as freezing point depression. You may not be familiar with the term, but you are probably aware that salt is used on icy roads to keep them from freezing in the winter, just like antifreeze is used in cars to prevent radiators from freezing or overheating in extreme weather.
Our newest Making YOU the Scientist activity explores the concepts of freezing point depression and phase changes. The freezing point of water is zero degrees celsius. In the first part of the activity you will lower the freezing point of water to below zero degrees celsius using ordinary table salt (sodium chloride). The salt will depress or lower the freezing point of the water.
Freezing point depression is a colligative property of water. Colligative properties are the physical changes that result when a solute is added to a solvent. This experiment uses water as the solvent and salt as the solute. You could also use alcohol or other types of salts to lower the freezing point. Colligative properties do not depend on the type of solute that is added to the solvent, but on how many particles of the solute are added to it.
In the second part of the experiment, you will further explore the phase changes that take place when room temperature water is added to your salty ice water mixture. Phase changes occur when energy is added to or removed from a substance. When energy is removed from a substance, the particles of the substance begin to move more slowly and when the freezing point is reached they will stop moving and the substance will change from a liquid to a solid.
Watch the video to see what happened when we placed a test tube with tap water into our beaker of salty ice water. To further explore the concepts of freezing point depression and phase changes, check out my full write-up for this experiment in my TpT store.