Why Does Rock Salt Make Ice Melt Slower?
Plain water freezes at 32 degrees F, but a mixture of water and rock salt must be much colder to freeze. This means that adding rock salt to ice water will also cause the ice to melt more slowly than it would in plain water. A mixture of ice and water is a dynamic system. Some of the ice is always melting, and at the same time, some water molecules are being "captured" by the ice and freezing. The way rock salt affects the balance between these two states (solids and liquid) in water has practical applications for drivers and ice cream lovers.
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Features
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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
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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.
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Effects
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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
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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
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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.
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References
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- Photo Credit Image by Flickr.com, courtesy of lost in translation) (Gordana Adamovic-Mladenovic
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Comments
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kraemer06
Jul 02, 2010
Ice 'CAN' melt slower in saltwater, but it can also melt faster. It depends on the experiment. This article makes some correct statements, but overall it is incorrect. The article basically states that more ice melts in saltwater causing the water to become cooler, causing the ice to melt slower. This doesn't make any logical sense and may confuse readers. The author states: "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." The above statement is INCORRECT! Salt does not increase the amount of energy required to break bonds. "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...
