Melting of ice With Salt

Does frozen saltwater of a particular salt concentration melt over a range of temperatures, or does it melt at one sharp temperature? It is like this, I froze saltwater of 20% concentration of salt by mass and studied its melting with an thermal imaging camera and found that the melting temperature is not constant over time. Is there something wrong?
- Dawn (age 37)
Singapore

Great question! We've given a closely related answer at https://van.physics.illinois.edu/qa/listing.php?id=1722, https://van.physics.illinois.edu/qa/listing.php?id=1722.

First, some background. The almost all of salt doesn't really become part of the ice. It sits around in pockets outside the ice. That salt lowers the melting point of the ice it contacts to -21.1°C.  Until almost all the ice has melted, there will be a little leftover undissolved salt. Thus most of the ice will melt at -21.1°C, at least in thermal equilibrium. Once all the salt has dissolved, the concentration of salt in the water will go down as more ice melts. At 20% salt by weight the ice should finish melting at ~ - 18°C, where I've used the approximation that the freezing point depression is linear in salt concentration. At any rate, you will find that the melting occurs over a little range of temperatures, unlike for pure water.

Mike W.

(published on 09/14/2018)