Can anybody please explain to me? So salt lowers the freezing point of ice right? When you put salt on your ice to make it colder, is there any heat transfer between the two?
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No, there is not. If you add salt to ice there is no temperature change. If the ice is 0C pre-salt, it remains 0C post-salt.
What happens is that salty water doesn’t freeze until, say, -10C. So your now 0C salt+ice mixture, being greater than -10C will melt to liquid water. If it’s exactly 0C outside, that’s it, you have 0C salty water.
If the temperature continues to drop to -2C, -4C, -8C the salty water just gets colder. Once the temperature reaches -10C the water will freeze once again and turn to ice.
Yes and no. Basically, in the conditions where you’d normally be doing this, the ice has a surface coating of liquid water that’s constantly melting and refreezing. Phase transitions bind up energy – when the water freezes, it releases some energy, and when it melts, it sucks up some energy. Adding salt makes this salt water, which has a lower freezing point – meaning that the water can’t re-freeze. The ice continues to melt, though, which sucks up more energy, lowering the temperature of the ice.
There is a heat transfer, of course, as the ice cools the salt down (and therefore warms itself up). However, that heat transfer has nothing to do with why the whole thing gets colder.
The real reason is that, when ice melts, it gets colder. Energy is absorbed. This is true for most phase transitions. It’s why, when you put 0 degree celsius ice into water, the water cools down to 0 C but the ice does not raise in temperature. The heat of the water that *would* raise the temperature instead is consumed melting the ice. This is known as the “heat of fusion”, and it is a large amount of heat. Iirc, it’s even larger than the heat requires to raise water from 0C to 100C.
Normally of course, ice doesn’t get *colder* as it melts, since then it would stop melting. Instead, the ice stays at the same temperature, sucking heat out of its environment as it melts.
The salt moves the goal posts. It lowers the freezing point of water, *forcing* it to keep melting even as it gets colder. This allows it to suck heat out of itself and still keep melting – it keeps getting colder.
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