Why does salt melt ice faster?

Written by allison michelle
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Why does salt melt ice faster?
Adding salt to ice helps it melt. (Getty Creative)

For people who survive icy winters, the process of sprinkling salt onto the roads and sidewalks is a familiar one. Even just a light sprinkling of salt, combined with the wetness of the snow or ice, can cause the ice to disappear. This reaction is caused when the salt mixes with the water, which lowers the freezing point of the water.

Freezing balance

Ice is formed when the temperature of water reaches 0 degrees. According to General Chemistry Online, if water and ice are placed near each other, the amount of water and ice will not change. This balance is maintained at 0 degrees Celsius. The key point is that while some ice will melt and water will freeze, it is always the same amount of each.

Disruption of balance

When some of the water is replaced with salt, the molecules are no longer the same. According to Primer Magazine, the salt disrupts the balance of the water melting and ice freezing. The salt replaces the water in the equation. Since the salt does not bond easily with the ice, there is then more ice melting than water freezing. The salt then keeps any of the melted ice from refreezing.

Freezing and melting rates

While before the rate of freezing molecules were at equilibrium with the rate of melting molecules, the addition of salt has shifted this process. Now, since salt crystals are being captured instead of water molecules, the amount of ice melting is now higher than ice freezing, according to Worsley School's website. The salt is now mixing with the ice and forming a substance known as eutectic mixture.

Freezing point depression

To re-establish freezing, it is necessary to lower the temperature below the melting point of water, according to General Chemistry Online. When the freezing point of the water has been lowered due to being mixed with another substance, this is called a freezing point depression, according to General Chemistry Online. The greater the concentration of a foreign substance in the water, the higher the freezing point depression will be, according to Worsley School. It is now harder for the ice to refreeze since the freezing point of a saltwater solution is -21.1 degrees Celsius.

The use of sodium

Since a foreign substance mixing with the water molecules lowers the freezing point of the ice, you could essentially mix other substances, such as sugar and alcohol, with ice to cause it to melt as well. Salt is easier to use on large projects such as roads for several reasons. First, it is more readily available, according to Primer Magazine. Secondly, the amount sugar you would need to melt the ice at the same rate as salt is six times the amount of salt, according to Worsley School.

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