Sometimes, when we add a compound to water or ice, it can change the freezing temperature of the liquid. The process of lowering the freezing temperature of a liquid is called freezing point depression.
This is a concept that those who live in areas with very harsh winters see in action very often. Consider a day when the temperatures are far below freezing. For these areas, large portions of salt are typically used to keep roads from icing and making conditions unsafe for drivers. This is an everyday example of freezing point depression! The salt changes the temperature at which the water freezes, and cars can continue to drive on the road safely.
At High Touch High Tech, we have an experiment called Ice Cube Fishing that utilizes this scientific concept while creating a fun game for children to better understand the concept. During the experiment, string is added to a cup of ice, and salt is poured in the cup. The salt lowers the freezing point of water causing the ice to begin to melt. As the ice melts it becomes colder and the string freezes to the ice, making it possible to “catch” many pieces of ice with the string.