Salt is added to ice to lower its freezing point.
The Science Behind It: Freezing Point Depression
Adding salt to ice causes a phenomenon called freezing point depression. Here's how it works:
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Pure Water's Freezing Point: Pure water freezes at 0°C (32°F).
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Salt Dissolves: When salt (sodium chloride) is added to ice, some of it dissolves into the thin layer of liquid water that exists on the ice's surface.
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Ions Interfere: The dissolved salt separates into sodium (Na+) and chloride (Cl-) ions. These ions interfere with the water molecules' ability to form the organized crystalline structure of ice.
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Lower Freezing Point: To compensate for this interference and allow the water to freeze, the temperature must be lower than 0°C (32°F). The more salt you add, the lower the freezing point becomes, up to a certain concentration.
Practical Applications
This principle is used in several applications:
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Ice Cream Making: Salt is added to the ice surrounding the ice cream mixture. This lowers the temperature of the ice bath, allowing the ice cream to freeze.
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De-icing Roads: Salt is spread on roads to prevent ice from forming or to melt existing ice. This makes driving safer in cold weather.
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Keeping Drinks Cold: A mixture of ice and salt water can keep drinks colder than ice alone. This is because the saltwater mixture has a lower temperature than ice alone.
Limitations
It's important to note:
- Effectiveness Limit: Adding too much salt won't make the ice colder; there's a limit to how much the freezing point can be lowered.
- Temperature Range: Salt is most effective when temperatures are near freezing. It becomes less effective as temperatures drop significantly below freezing.
In summary, salt lowers the freezing point of water, allowing ice to become colder than its typical freezing temperature. This is utilized to facilitate the freezing of ice cream, de-ice roadways, and achieve colder temperatures for preserving items such as drinks.
