How Ice Melts

Clearing Roads and Making Ice Cream Have A Lot in Common. Both occur because of something called freezing point depression. When ice and water come in contact molecules from the ice melt into the water and molecules of water freeze onto the surface.

When the temperature is 0°C or 32°F, the rate of freezing is the same as the rate of melting so the amount of ice vs. water won't change unless conditions change in a way that favors one processes over the other. If the temperature drops, the molecules move slower. Slower-moving molecules are more easily captured by the ice so freezing occurs at a greater rate than melting. Heating the mixture makes the molecules move faster melting is favored.

Adding a foreign substance to the system can also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some thing that is not water. When these “something else” molecules dissolve in the water they take up space so that the number of water molecules are diluted. These “something else” molecules do not fit well into the arrangement of molecules during the freezing process.

Since there are less water molecules for the forming ice to snag because many of the molecules are made of “something else” and because the “something else” is not water it’s molecules don’t fit into forming crystals, therefore melting is favored.

The freezing rate is decreased and the melting rate stays unchanged - so there is more melting than freezing. The resulting temperature of the mixture becomes lower - this is called freezing point depression and this is how salt melts ice.

If the temperature of a highway is below around 15^OF adding salt has little effect. Why? Because solid salt doesn’t mix with solid ice any more than a chair mixes with a desk. There has to be something to be dissolved in. There has to be liquid water present in order for a dissolving process to start.

Will any foreign substance cause freezing point depression?

Yes! As long as it dissolves in water. Sugar, alcohol, or other salts will also lower the freezing point and melt the ice. Salt is used on roads and walkways because it is inexpensive and readily available.

Sand and cat litter do not dissolve in water. While they may improve traction by increasing friction, they do not cause freezing point depression and therefore do not melt ice.

The idea that large molecules cause more freezing point depression than smaller molecules is not born out by experimental testing.

Does the amount of salt or other foreign material matter?

Yes - the higher the concentration of salt, the greater the freezing point depression – up to a point.

A 10% salt solution freezes at 20°F (-6°C), while a 20% salt solution freezes at 2°F (-16°C).

How ice melts page 2 >


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How Ice Melts Clearing Roads and Making Ice Cream Have A Lot in Common. Both occur because of something called freezing point depression. When ice and water come in contact molecules from the ice melt into the water and molecules of water freeze onto the surface. When the temperature is 0°C or 32°F, the rate of freezing is the same as the rate of melting so the amount of ice vs. water won't change unless conditions change in a way that favors one processes over the other. If the temperature drops, the molecules move slower. Slower-moving molecules are more easily captured by the ice so freezing occurs at a greater rate than melting. Heating the mixture makes the molecules move faster melting is favored. Adding a foreign substance to the system can also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some thing that is not water. When these “something else” molecules dissolve in the water they take up space so that the number of water molecules are diluted. These “something else” molecules do not fit well into the arrangement of molecules during the freezing process. Since there are less water molecules for the forming ice to snag because many of the molecules are made of “something else” and because the “something else” is not water it’s molecules don’t fit into forming crystals, therefore melting is favored. The freezing rate is decreased and the melting rate stays unchanged - so there is more melting than freezing. The resulting temperature of the mixture becomes lower - this is called freezing point depression and this is how salt melts ice. If the temperature of a highway is below around 15^OF adding salt has little effect. Why? Because solid salt doesn’t mix with solid ice any more than a chair mixes with a desk. There has to be something to be dissolved in. There has to be liquid water present in order for a dissolving process to start. Will any foreign substance cause freezing point depression? Yes! As long as it dissolves in water. Sugar, alcohol, or other salts will also lower the freezing point and melt the ice. Salt is used on roads and walkways because it is inexpensive and readily available. Sand and cat litter do not dissolve in water. While they may improve traction by increasing friction, they do not cause freezing point depression and therefore do not melt ice. The idea that large molecules cause more freezing point depression than smaller molecules is not born out by experimental testing. Does the amount of salt or other foreign material matter? Yes - the higher the concentration of salt, the greater the freezing point depression – up to a point. A 10% salt solution freezes at 20°F (-6°C), while a 20% salt solution freezes at 2°F (-16°C). How ice melts page 2 >
Melting Ice with Icenator Liquid Deicer


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How Ice Melts

Melting Ice with Icenator Liquid Deicer