Nib asked
Why does salt melt ice and make it colder?
Salt doesn't add any heat: it lowers water's freezing point, and the ice then melts by stealing heat from the mix.
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the puzzle
Salt melts ice on roads, yet salt isn't warm. Stranger still, ice mixed with salt gets colder than ice alone, down to about minus twenty-one Celsius.
2two jobs
So salt isn't heating anything. Two things happen: salt lowers water's freezing point, and then the melting ice steals heat from everything around it.
a common mix-upSalt does not warm or burn the ice. It lowers the freezing point, and melting then absorbs heat.
3a balance
At zero Celsius, pure ice and water are in balance: water molecules leave the ice as fast as others join it. Nothing changes.
4salt tips it
Add salt and it splits into sodium and chloride ions among the water. They crowd the surface, fewer water molecules join the ice, and melting wins.
5heat theft
Melting needs energy, about 334 joules for every gram of ice. It takes that heat from the mix itself, so the mix gets colder and colder.
6the limit
It stops at a limit. At 23 percent salt by mass the mix bottoms out near minus 21.1 Celsius. Below that, salt can't melt more ice, so road salt fails in deep cold.
7the whole idea
So, the whole picture: salt in the water blocks refreezing, melting wins, melting steals heat, and the mix cools until it hits its new, lower freezing point.
Salt never warmed the ice; it just made freezing harder, so melting stole the heat and the whole mix grew colder.