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Now you know how salt melts ice!

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Try it yourself ↓

How does salt melt ice?

▶ 4:52 video · Watch on YouTube

  1. 0:00 Start
  2. 0:39 The balancing act
  3. 1:28 Salt gets in the way
  4. 2:17 Where salt gives up
  5. 3:12 Ice cream in a bag
  6. 3:55 The price of salt

Your turn

Tip the balance yourself

Play with salt and ice!

Each lab is one step from the video. Change things and see what happens.

Each lab isolates one part of the story. Molecule counts, the freezing-point curve and the bag temperatures are simplified models.

Tap the buttons and slide the sliders.

1

Salt the icy road

Salting a road

Salt the ice!

Tap the icy patches to salt them. Watch them turn to slush.

Salt each icy patch and watch it turn to slush, while the air stays below freezing.

Tap the ice. Salt makes slush!

▶ Watch this part
2

The balancing act

Dynamic equilibrium at 0 °C

Warm or cold?

Slide the temperature. Which wins: leaving or joining?

Shift the temperature and watch melting and freezing rates move apart.

Slide it. Does the ice grow or shrink?

▶ Watch this part
3

Salt gets in the way

Freezing point depression

Add salt!

Add salt pinch by pinch at −2 °C. When does leaving win?

Add ions at −2 °C and watch the attachment rate fall.

Tap to add salt. The ice melts!

▶ Watch this part
4

Where salt gives up

Practical and absolute limits

Drag the thermometer down. Where does rock salt stop helping?

Lower the road temperature past −9 °C (15 °F) and −21 °C.

▶ Watch this part
5

Ice cream in a bag

A salt–ice freezing mixture

Make ice cream!

Add rock salt and shake. How cold does it get?

Add salt and watch the ice–brine mixture settle below 0 °C (rough model).

Add salt. Shake! Brrr!

▶ Watch this part
6

The price of salt

Smart salting

Be kind with salt

Pick how the town salts its roads.

Compare heavy salting, smart salting and sand (illustrative bars).

Pick one. Less salt is kinder to streams.

▶ Watch this part

How does salt melt ice?

Salt is not hot. But it can still turn ice into wet slush!

1. Ice has a wet top

Ice is made of tiny bits of water, much too small to see. They hold on tight in neat rows. On top of the ice there is a thin wet layer.

2. Let go, grab on

All the time, some tiny bits let go of the ice, and others grab back on. When the same number go and come back, the ice stays the same.

3. Salt gets in the way

Salt mixes into the wet layer. The salt bits get in the way, so fewer water bits can grab back on. More let go than come back, so the ice melts.

4. Extra cold!

Ice with salt can get colder than ice in plain water. With a grown-up, you can use ice and salt in a bag to make ice cream!

Be safe: Ice is slippery, even with salt on it, so walk slowly. Never eat road salt. Too much salt can hurt plants and a dog's paws, so wipe paws after a winter walk.
Did you know? The sea is salty, so it has to get colder than a pond before it freezes.

How is snow made? · Why is the sea salty? · How does a fridge work? · ← More explainers

How does salt melt ice? The short answer

Salt doesn't heat ice. It lowers the freezing point of water, so ice that would normally stay frozen just below 0 °C (32 °F) melts instead.14 Chemists call this freezing point depression.1

Here's the trick. Near its melting point, even a little below 0 °C, ice has a very thin film of liquid-like water on its surface.12 Water molecules are constantly breaking free from the ice into that film, while others lock back onto the ice. At 0 °C, the two happen at the same rate, so the ice stays the same size.12 Salt dissolves into the film and its particles get in the way of the molecules trying to lock back on.28 Molecules keep leaving, fewer rejoin, and the ice melts.

What does salt do to ice, molecule by molecule?

  1. The film. Even a little below freezing, the surface of ice carries a thin layer of liquid water, sometimes called a premelting or quasi-liquid layer. Scientists have known about it for more than a century.12
  2. The balancing act. Molecules at the ice surface are always swapping places: some melt away, some freeze back. Warm it up and melting wins; cool it down and freezing wins.2
  3. Salt dissolves. Salt crystals dissolve into the film, making salty water called brine. In water, table salt (sodium chloride) splits into sodium ions and chloride ions.18
  4. The particles get in the way. The dissolved particles make it harder for water molecules to line up into the orderly ice crystal.28
  5. The balance tips. Now the ice can't freeze the brine back at 0 °C, but the brine can still melt the ice. Result: less ice.1

Freezing point depression is a colligative property: it depends on how many particles are dissolved, not what they are.67 That's why sugar lowers the freezing point too.3 The molecule toy in the player is a simplified model: the counts and rates are made up to show the idea, not measured.

Does salt need water to melt ice?

Yes. Salt only works once it's dissolved in liquid water.4 Salt dropped on dry, very cold ice waits for a bit of melting (from sunshine, tyres or a warmer road) to get started. Salt needs some liquid water to dissolve in, so on dry, very cold ice it works slowly or not at all.24 That's why many cities spray salty water (brine) on roads before a storm, so ice can't bond to the road in the first place.48

At what temperature does salt stop working?

The more salt dissolves, the lower the freezing point goes.38 But there are limits:

WaterFreezes at about
Fresh water0 °C (32 °F)3
Ocean water−1.8 to −2 °C (about 28.5–28.8 °F)311
Rock salt on a real roadstarts losing effectiveness near −9 °C (15 °F)94
The saltiest sodium chloride brine−21.1 °C (−6 °F): the absolute floor3

Below about 15 °F, road crews switch tools. Calcium chloride and magnesium chloride split into three ions each instead of two, and work at lower temperatures, but they cost more.189 Sand doesn't melt anything; it just gives tyres and boots grip.4 The curve in the thermometer toy is smoothed between these real anchor points, so treat the in-between values as rough.

more dissolved particles → lower freezing point

For chemistry students: for dilute solutions, the drop in freezing point is proportional to the concentration of dissolved particles, with a constant Kf = 1.86 °C per molal for water.7 Real brines at high concentration drift from that simple rule.

Why does salt make ice colder?

Melting soaks up heat. When salt makes ice melt, the energy is drawn from the ice, the brine and things touching them, so they cool down.5 Plain ice and water settle at 0 °C, but with salt the ice-and-water mix settles at a lower temperature.5 NOAA puts it neatly: salt doesn't decide how fast ice melts; it decides at what temperature it melts.3

Try it at home: ice cream in a bag

This is the same science, used to make something colder. Do it with an adult.3

  1. In a small zip bag, mix about half a cup of milk (or cream), 2 teaspoons of sugar and a dash of vanilla. Seal it tight, pushing out the air.
  2. Fill a large freezer zip bag about half full of ice. Add a few big spoonfuls of rock salt.
  3. Put the small bag inside the big one and seal it.
  4. Wear gloves or wrap it in a towel: the bag gets far colder than ice alone. Shake and squish until it thickens (NOAA’s class version allows about half an hour for the whole activity).
  5. When the milk has thickened, rinse the salt off the small bag before opening it. Enjoy!

Want an experiment? Make one bag with no salt. NOAA's classroom lesson says the bags with the most salt should freeze first, and a bag with no salt shouldn't freeze at all.3 The temperatures in our bag toy are a rough model, not measurements.

Is road salt bad for the environment?

Salt makes winter roads safer, and it has been used on US roads since at least the 1930s.8 More than 20 million tons are used every year.1 But it has downsides:

  • Streams: in 19 northern US streams studied by the USGS, chloride levels roughly doubled from 1990 to 2011.10 Chloride can harm fish and other water life.1
  • Cars and bridges: salt speeds up rust and corrodes bridges and other structures.89
  • Plants: salty runoff can dry out and kill roadside plants and change the soil.19
  • Pets: rock salt and ice melters can give dogs sore paws; rinse paws after winter walks.13

That's why many towns now aim for "smart salting": using just enough, and spraying brine before storms so less salt does more.9

Salt, ice and the ocean

The same rule explains why the sea freezes later than lakes. Ocean water has to cool to about −1.8 °C before it freezes.11 When sea ice forms, the growing crystals push most of the salt out into the water around them.11 Curious where all that salt came from? See why the ocean is salty. And for why ice floats at all, try why does ice float.

Misconceptions

"Salt heats up the ice." No. Salt doesn't work by heating the ice. Melting actually pulls heat in, so salted ice gets colder.5
"Salt melts ice at any temperature." Rock salt fades near 15 °F (−9 °C), and no sodium chloride brine stays liquid below −21 °C.39
"Salt works instantly on dry ice." It has to dissolve in liquid water first.4
"Sand melts ice too." Sand only adds grip. It doesn't change the freezing point.4
"Road salt is a special chemical." It's the same sodium chloride as table salt, just less pure and brownish-grey.4

Keep exploring

Snowflakes start as tiny ice crystals high in the clouds: how does snow form. And a fridge moves heat around using a liquid that evaporates and soaks up heat, a cousin of the ice-cream-bag trick: how does a refrigerator work.

FAQ

How does salt melt ice, in simple words?

Ice near its melting point has a very thin film of liquid water on top, and salt can also start its own tiny puddles. Salt dissolves into that film and makes salty water (brine). The salt particles get in the way of water molecules trying to freeze back onto the ice, while molecules keep breaking free just as fast. More leave than join, so the ice melts, even a few degrees below freezing. Chemists call this freezing point depression.

Does salt make ice colder?

Yes. Melting soaks up heat, so when salt makes ice melt, the ice and salty water can get colder than 0 °C (32 °F), the temperature where melting plain ice stays. That's why a bag of ice and rock salt can freeze milk into ice cream.

Can I put table salt on ice to melt it?

Yes. Table salt and rock salt are the same chemical, sodium chloride; rock salt is just coarser and less pure. Either works best a little below freezing, not in deep cold.

How quickly does salt melt ice?

It depends on the temperature, how much salt you use and whether there is liquid water for it to dissolve in. Just below freezing, wet salt starts working within minutes. On very cold, dry ice it takes much longer, and below about 15 °F (−9 °C) rock salt barely helps.

At what temperature does salt stop melting ice?

On real roads, rock salt starts to lose its effectiveness at about 15 °F (−9 °C). In theory the saltiest salt water freezes at −21 °C (−6 °F), so below that no amount of sodium chloride will melt ice. Colder places use calcium chloride or magnesium chloride, or sand for grip.

What kind of salt melts ice best?

Calcium chloride and magnesium chloride work at colder temperatures than rock salt because each splits into three particles in water instead of two. They cost more and can be harder on cars and concrete. Rock salt is the cheapest and most common.

What makes ice melt fastest?

Adding warmth is usually the quickest: warm water, warm air, sunshine, or running water over it. Of the things you sprinkle on, salts work well just below freezing, especially when the salt is wet. Sand doesn't melt ice at all; it only adds grip.

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Sources

  1. Encyclopaedia Britannica, Why Does Salt Melt Ice?
  2. Scientific American (Science Buddies), Salt on Ice
  3. NOAA JetStream, Learning Lesson: We All Scream for Ice Cream
  4. University of Richmond (Prof. Julie Pollock), Salt doesn't melt ice: here's how it actually makes winter streets safe
  5. McGill University Office for Science and Society, Salt melts ice but also makes ice cream. Why?
  6. OpenStax Chemistry 2e, 11.4 Colligative Properties
  7. Purdue University Chemistry, Freezing Point Depression
  8. PBS NewsHour, How Does Salt Battle Road Ice?
  9. Minnesota Pollution Control Agency, Smart Salting
  10. USGS, River chloride trends in snow-affected urban watersheds (Corsi et al., 2014)
  11. National Snow and Ice Data Center, Science of Sea Ice
  12. PNAS commentary, Melting the ice one layer at a time
  13. ASPCA, Dog Grooming Tips (winter paw care)