A truck sprinkles salt on the icy road — but what does the salt do?
Brrr, it is a frosty morning, and the road is covered in hard, slippery ice. Here comes the salt truck, sprinkling tiny grains of salt all over it. And guess what — the salt is just as cold as the ice, not warm at all! Now lots and lots of salt is landing on the ice. Ready to guess… will the ice melt into a puddle, or stay frozen and still?
After you watchA truck sprinkles salt on the icy road — but what does the salt do?
The short answer
When you sprinkle salt on ice, the ice melts into water — even though the salt is cold, not warm. The cold salt mixes into the thin wet layer on the ice and stops the water from freezing solid again, so the ice keeps melting and turns into a puddle.
Try this next
- What if you sprinkle salt on one ice cube and nothing on another? Guess which one turns into a puddle first, then watch both cubes side by side and see which one melts faster.
- What if you put the salty ice in the freezer? Predict whether the puddle freezes back or stays wet, then leave it a while and check what happened.
The whole story
How it works
Ice always has a tiny bit of melted water on its surface, and water keeps trying to freeze back onto it. When salt lands on the ice, it dissolves into that thin wet layer and gets in the way, so the water cannot freeze solid again as fast. Melting keeps going while freezing falls behind, so the ice slowly turns into a puddle. No warmth is added at all — the salt is just as cold as the ice it sits on.
What people get wrong
Lots of kids think salt must be warm or hot to melt the ice, like it burns it. It does not. A grain of salt is exactly as cold as the ice. Salt melts ice by getting in the way of the water freezing back, not by adding any heat.
The catch
Salt is great for clearing an icy road or sidewalk so nobody slips, and just a small sprinkle works. But on a really, really cold day even salt cannot keep up, and the ice stays frozen. Salty water can also hurt plants and rust metal, so we only use it where we need it.
Questions kids ask
Is the salt warm? Is that how it melts the ice?
No. A grain of salt is just as cold as the ice it lands on. It melts the ice by getting into the thin wet layer and stopping the water from freezing back solid, not by adding any warmth.
Why do trucks put salt on icy roads?
Salt turns the slippery ice into wet water so people and cars do not slide. Just a small sprinkle melts a big patch, which keeps roads and sidewalks safer in winter.
Does salt always melt the ice?
Almost always — but on a really, really cold day, even salt cannot keep up, and the ice stays frozen. That is when roads need a different helper.
Talk about it
- Before we sprinkle, guess: will the ice melt or stay frozen? Why do you think so?
- The salt is just as cold as the ice, so where do you think the melting comes from?
- Why do you think the road crew uses salt instead of sugar from the cupboard?
For grown-ups
This is freezing-point depression, a colligative property — it depends on the number of dissolved particles, not the heat. At 0 °C ice and liquid water swap molecules at equal rates (dynamic equilibrium). Dissolved salt dilutes the liquid film touching the ice, lowering the rate at which water molecules redeposit while melting continues, so net melting wins and the freezing point drops. Road salt loses its effectiveness below roughly -18 °C (0 °F).
Keep going
What else makes you wonder?
- What else could you sprinkle on ice to see if it melts — and how would you find out?
- The salt is cold but the ice still melts, so where do you think the melting comes from?
- If salt melts ice, why do you think the cold, salty sea near the very top of the world can still freeze over?