Why Does Rock Salt Make Ice Melt Slower?

Features

• Water is a unique substance, and it owes many of its important properties to hydrogen bonds, which are chemical bonds between the hydrogen atoms in water and other atoms. In liquid water, molecules are moving constantly and forming brief hydrogen bonds while they move. This gives water molecules the ability to stick together, and makes water more cohesive than many other liquids. When the temperature cools, water molecules slow down, and stable bonds form ice at 32 degrees F.

Function

• How fast water molecules move depends on temperature; higher temperatures mean faster-moving molecules. At 32 degrees F, the melting point of ice and the freezing point of water are at equilibrium, so even though some ice is melting, other water molecules are freezing. This means the proportion of ice and liquid water remains approximately equal. Adding salt affects the balance between liquid water and solid ice, and lowers the temperature of the solution, which is why the ice melts slower.

Effects

• Adding salt (or any substance) to water affects the freezing point of the water because fewer water molecules are available to form bonds with the ice and be "captured" themselves and frozen. In order for ice to melt, the hydrogen bonds in the ice most be broken. Breaking hydrogen bonds requires energy. In this case, heat is drawn from the solution when the bonds in melting ice are broken, and because of this, the temperature of the entire solution drops. Salt increases the amount of energy required to break bonds, and makes the temperature drop more. A lower temperature in the solution means that ice melts more slowly.

Significance

• People who make homemade ice cream take advantage of these properties of salt and ice water. Because adding salt to ice water makes the solution much colder than ice water alone, ice cream can be frozen more quickly and uniformly using a rock salt and ice water solution.

Considerations

• To people accustomed to driving on salted roads during winter, it may seem counter-intuitive that salt slows the melting of ice in water. On snowy roads, salt is used to keep roads free of ice. This is because, as seen above, adding salt means that the temperature must be lower for water to freeze. On roads, adding salt means that melting snow will stay liquid instead of turning to ice and causing hazardous conditions.