Salt melts ice by lowering the freezing point of water.
Pure water freezes at about 32 degrees Fahrenheit, or 0 degrees Celsius, under ordinary conditions. Dissolve salt in the water and the solution has to become colder before it freezes.
The effect is called freezing-point depression.
Salt has to dissolve first
A block of ice may look completely solid, but under normal conditions there can be a very thin liquid layer at its surface, and melting can occur wherever heat is available from the surroundings.
When salt reaches liquid water, it dissolves into charged particles called ions.
Those dissolved particles interfere with the conditions under which water molecules organize into the ordered crystal structure of ice.
Now the liquid solution can remain unfrozen below the normal freezing point of pure water.
Why does that make existing ice melt?
Imagine ice sitting at a temperature where pure water and ice could normally coexist.
Add salt to the liquid at the surface and suddenly that salty water’s freezing point is lower than the current temperature.
To reach a new balance, more ice melts into liquid water. That additional water dissolves more salt, creating brine.
As long as conditions allow, the process continues.
So the salt is not a tiny heater.
It changes the phase-change conditions.
Why do road crews spread salt before storms?
Applying brine before snow or freezing rain can help prevent ice from bonding tightly to pavement.
That makes later plowing and removal easier.
After ice has already formed, solid salt can still help once it dissolves into available liquid water and creates a lower-freezing-point solution.
Traffic and sunlight can help create and spread that slushy mixture.
Does salt work at any temperature?
No.
As temperatures fall, ordinary sodium chloride becomes less useful for practical deicing. There are limits to how much salt can dissolve and how far it can lower the freezing point.
Other deicers, including calcium chloride and magnesium chloride, can work under colder conditions, although cost and environmental effects also matter.
This is why a road crew cannot solve every winter storm by simply ordering another mountain of table salt.
Why does salt and ice get colder when making ice cream?
The same principle is used in old-fashioned ice cream making.
Salt lowers the freezing point of the water around the ice. More ice can melt even though the mixture is already cold.
Melting requires energy, which is taken from the surroundings. The salty ice-water mixture can become colder than ordinary melting ice, allowing it to pull heat from the ice cream mixture and freeze it.
The road treatment and the dessert machine are using the same chemistry for very different career paths.
Is road salt harmless once winter is over?
No.
Dissolved sodium and chloride can move with runoff into soil, groundwater, streams, and lakes. High chloride concentrations can damage freshwater ecosystems, vegetation, infrastructure, and drinking-water sources.
That environmental cost is one reason transportation agencies work on application rates, brines, timing, and alternative deicers instead of treating salt as unlimited and harmless.
Why does sand help if it does not melt ice?
Sand mainly provides traction. It gives tires and shoes a rougher surface to grip.
Salt changes the freezing behavior of water. Sand changes the physical grip on the surface.
They can be used together, but they are doing different jobs.
So salt does not defeat winter by being warm.
It changes the rules water has to follow before it can freeze.
A related explanation is Why does metal feel colder than wood?, which looks at another familiar effect from the same general system.
And that’s the short explanation.
Related explanations: Why does metal feel colder than wood?, Why are there rocks on train tracks?, and Why does Earth have seasons?