Source: Adding Salt to Ice in a Cooler | stuffexplained
Why do people sometimes add salt to ice in a cooler for drinks or beer? It’s a simple question. There are tons of articles and videos online but I couldn’t find a comprehensive explanation of why it works.
Some Basics
It’s important to first understand what melting point and freezing point mean. Melting point is a property of solids. The melting point is simply the temperature at which the solid turns into a liquid. For example, the melting point of normal ice is 0°C at standard pressure.Freezing point is a property of liquids. The freezing point is simply the temperature at which the liquid turns into a solid. Thus the freezing point of normal freshwater is 0°C. The value of the melting and freezing points is the same (ie. both are 0°C for water). So as you can see, melting point and freezing point are kind of “mirrors” of each other. It doesn’t make sense to talk about the freezing point of a solid (since it’s already solidified).