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My question is this, In European countries, they use NaCl or KCl to melt ice during the winter season. In Asian Countries, they use NaCl to keep the ice without melting. (Ex- Ice cream boxes, Beer Boxes)

I asked my chemistry teacher about this and his answer was, "when you add ions to the ice, it's melting point goes (freezing point of water) to -9 to -19 Celsius (depending on the amount of ions). Reason behind that is, when water become ice it's water molecules re-arrange to specific shape(link for the video is available at bottom). after we put ions to it, ions disturb that shape and they come in between water molecules. If water want to be ice again it should be like -9 Celsius because of that ions disturbing them to rearrange as a solid."

And I also asked my Physics teacher about this and his answer was, When we put salt into water, it's temperature goes down to somewhere around $-6^oC$ so that it takes more time to come to the 0 Celsius point and begin melting

Link to the videos are as follows,

I would be very thankful if you can give me a correct explanation for this question!!!

Thank you!

(English is not my mother tongue, I'm sorry if for any inconveniences.)

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When you mix water and NaCl together, you create a solution which melting temperature is lower than 0°C (say around -10°C, depending on the amount of salt.

When you put salt on a frozen road during winter, the air temperature can be between 0 and -10 degrees, in this case the mixture of ice and salt melts, because the exterior temperature is above the melting point of the mixture ice/salt : adding salt on the road "makes ice melt", if you want. If the exteroir temperature is extremely low, like -30°C, adding salt won't solve your problem.

On the other hand, to keep stuff like ice cream at a really low temperature, you might want to have a material which stays in equilibrium at a temperature lower than 0°C. This is the case of a frozen mixture of salt and ice, which melting point can be around -10°C.

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