How can sodium chloride melt ice or keep it frozen?

Question

In European countries, they use $\ce{NaCl}$ or $\ce{KCl}$ to melt ice during the winter season. In Asian Countries, they use $\ce{NaCl}$ to keep the ice without melting, for example in ice cream and beer boxes.

I asked my chemistry teacher about this and his answer was,

when you add ions to the ice, its melting point goes (i.e. freezing point of water) to -9 to -19 degree Celsius, depending on the amount of ions. The reason behind that is, when water becomes ice its water molecules re-arrange to specific shapes (see the link for the video at bottom). After we put ions to it, ions disturb that shape as they come in-between the water molecules. If water wants to be ice again, it should be like -9 degree Celsius because of these ions disturbing them to rearrange as a solid.

I also asked my Physics teacher about this and his answer was,

When we put salt into water, its temperature goes down to somewhere around $\pu{-6 ^\circ C}$, so that it takes more time to come to $\pu{0 ^\circ C}$ and begins melting

Link to the videos are as follows,

• Salt melts ice (YouTube)
• Salt reduces the temperature of ice (YouTube)

Could you give me a correct explanation for this question?

Answer 1

The apparent contradiction is resolved as follows: in both cases, addition of salt makes ice melt faster, which is important in the street and negligible in a beer box (or an old style cold bath used in the lab). But in doing so, it also causes the temperature of the ice and its surroundings to go well below $0^\circ\rm C$, which is important in the lab and negligible in the street.

Answer 2

The main difference is, that in order to melt the ice, you add the salt to the ice itself, where it lowers the melting point and thus (if the ambient temperature is only just below zero). Melts away the ice from the road.

On the other hand to keep ice cream cold, you add ice+salt around the ice cream that you want to prevent from melting.

This has three effects that help you cool your drink/ice cream faster to a lower temperature:

1. The melting point of the ice around your ice cream is below zero. This means that the temperature on the outside of your ice cream box cannot rise above zero until all the ice on the outside is molten and the temperature of your ice cream is kept below zero for that duration. The reason for this is, that it takes energy to break up the structure of solid ice. This is the same effect that keeps your coke at zero degrees until the last of your ice cubes has melted (and is the reason why adding more ice than necessary just reduces the effective volume of your container and does not help to keep your drink cool).

2. As your teacher correctly pointed out, when salt dissolves in water, the temperature of the water is lowered (because of the entropy vs. enthalpy of dissolving crystalline salt). However, as pointed out in the comments, this effect is small compared to the others.

3. Your drink/ice cream is cooled faster because the very cold salty water (ideally with some ice cubes still swimming in it) around the bottle transports the heat away from your drink faster than some ice cubes touching the bottle would.

Answer 3

Adding salt or any solute causes freezing point depression as one of the colligative properties of the material.

In the cold climate, when snow is left to accumulate it forms a slippery ice which is dangerous. Workers utilize the freezing point effect and add salt to snow to melt avoiding the danger of the slippery ice.

In the hot climate, when ice in the ice box is left it is going to melt. People utilize the freezing point (or melting point) depression effect of salt to ice to melt it at the depressed melting (or freezing) point (below the usual $\pu{0 ^\circ C}$). This can be described as a preemptive protective step to the ambient temperature effect, since the ambient temperature will melt the ice at a relatively high point (the usual $\pu{0 ^\circ C}$). This means a high rate of heat transfer and corresponding rapid melting to the remainder of the bulk of the ice, the salt-water will cover the ice with a layer of relatively cold water (below the usual $\pu{0 ^\circ C}$) which confers a slow rate of heat transfer and corresponding slow melting.