When temperatures dip, liquid water forms crystals and becomes a slippery solid. This transformation generally occurs when water reaches 0 degrees Celsius (C), turning wet roads into veritable skating rinks. In response, highway workers often use salt to make icy roadways drivable.
Salt makes winter driving safer by lowering the temperature at which water freezes. While pure water has a 0-degree freezing point, a solution that's 10 percent salt and 90 percent water, for example, has a freezing point of minus 6 C; a 20 percent solution drives the freezing point down to minus 16 C.
Salt changes the temperature at which water changes from a liquid to a solid in essence by diluting a collection of water molecules with the salt's sodium and chlorine ions. This change in composition makes it harder for water molecules to join together and form crystals. As a result, colder temperatures are required to form ice crystals from the water molecules remaining in solution.
Just as it does on our roadways, salt melts the ice cube used in this ZOOM video segment. Small puddles of liquid form on top of the ice cube as the salt dissolves and lowers the water's freezing point. As this happens, though, the water from the ice cube and the surrounding water in the glass dilute the salt. This dilution has the effect of moving water's freezing point back toward 0 degrees Celsius. When this happens, ice crystals reform, trapping the string and joining the string and ice cube together.
