Why is sodium chloride added to the ice when cooling down mixture?

Sodium Chloride, commonly known as table salt, is added to the ice when cooling down a mixture for several important reasons:

1. Lowering the freezing point: Sodium chloride acts as a freezing point depressant, meaning it lowers the temperature at which water freezes. When salt is added to ice, it disrupts the formation of ice crystals, causing the ice to melt at a lower temperature. This results in a colder freezing mixture that can more effectively cool down other ingredients.

2. Increasing heat transfer: The addition of salt to ice increases the rate of heat transfer between the mixture and the surroundings. Salt enhances the thermal conductivity of the ice, allowing heat to be transferred more efficiently from the mixture to the ice, resulting in faster cooling.

3. Preventing re-freezing: Adding salt to ice helps prevent the mixture from re-freezing after it has been cooled down. The dissolved salt in the water lowers the freezing point, ensuring that the mixture remains in a liquid state even at temperatures below the freezing point of water.

4. Extending the cooling time: Since the presence of salt lowers the freezing point of water, it takes longer for the ice to completely melt. This extended cooling time allows the mixture to stay cold for a more extended period, making it ideal for activities such as chilling beverages or keeping food cool.

5. Eutectic mixture formation: When sodium chloride is added to ice in a specific ratio (approximately 23.3% salt by weight), it forms a eutectic mixture. This mixture has a unique freezing point, lower than that of either salt or ice alone, making it even more effective in lowering the temperature.

Overall, adding sodium chloride to ice when cooling down a mixture enhances the cooling process by lowering the freezing point, increasing heat transfer, preventing re-freezing, extending the cooling time, and sometimes forming a more efficient eutectic mixture.