How salt prevents ice--it's not what you think

During the winter, it is common to put salt on icy roads in order to prevent the formation of ice. It would be reasonable to assume that salt has ice-melting properties, or that the reaction between ice and salt produces heat that melts the ice. However, the actual explanation requires a bit more chemistry. Salt […]

Elizabeth Meade
30th November 2020
During the winter, it is common to put salt on icy roads in order to prevent the formation of ice. It would be reasonable to assume that salt has ice-melting properties, or that the reaction between ice and salt produces heat that melts the ice. However, the actual explanation requires a bit more chemistry.

Salt is commonly known by the chemical formula NaCl, or sodium chloride. When NaCl is mixed with water (H2O), the Na cations (positive ions) and Cl anions (negative ions) mix with the H cations and OH anions of the H2O to form a new solution. This solution causes the freezing point of the water to lower--that is, any solution that is above that freezing point will not freeze. Therefore, it is harder for ice to form unless it is cold enough to be below the new freezing point. The salt lowers the freezing point by altering the rate at which water molecules attach to the ice, causing the point at which ice and water molecules are switching places at the same rate (the freezing point) to decrease.

There are limitations, however: once enough salt has been added to lower the freezing point to 0 degrees Celsius, the water will have reached its limit as to how much salt it can dissolve, and the freezing point can not be further lowered.

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AUTHOR: Elizabeth Meade
Science sub-ed and Chemistry major. Avid reader. Chaos theorist. Amateur batrachologist and historian. Rock fan. Likes cybersecurity and cooking.

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