system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, Why is Molten Salt(?) Used for heat transfers?
gpt
1. Relatively large heat capacity: on the order of 1 J/gK
2. Relatively dense: on the order of 2 g/cc
3. Very good working range: depending on your choice you get a few hundred degrees of working temperature between "melts" and "boils/otherwise breaks"
4. Relatively cheap
5. It turns out that a LOT of compounds are technically salt

When you're designing a heat transfer system, you want something that can carry a lot of heat per unit volume, that is relatively cheap, and that works over the range of temperatures you need.  Liquids work well (in comparison to gases, which don't usually hold as much heat in a given volume), and if you're looking for a liquid that works at 500C -- pretty much all the results you'll find will be different salts.