What If? Lab
What if salt did not melt ice?
In winter, people sprinkle salt on icy sidewalks. The ice melts! What if salt did not melt ice? Let's find out with Olly.
Salt water freezes at a lower temperature than plain water. That's why salt melts icy roads and why salty ice can freeze ice cream. Imagine salt that did nothing to ice and follow what might change.
Dissolved salt lowers the freezing point of water, a colligative property that depends on how many particles are dissolved, not what they are. This thought experiment removes that effect and explores icy roads, the chemistry of freezing-point depression and homemade ice cream.
The big question
Ice on the sidewalk is slippery. People sprinkle salt on it.
Soon the ice turns to slush. What if salt did not melt ice?
On a frosty winter morning, trucks may spread salt on the roads and people shake salt onto icy steps. Before long, the ice turns to wet slush, even though the air is still below freezing.
That's strange when you think about it. So what if salt did not melt ice? This lab is a thought experiment. We can't change how salt behaves, but we can follow the clues, and one of them tastes delicious.
OpenStax Chemistry notes that solutions freeze at lower temperatures than pure liquids, which is exploited in de-icing with salt, calcium chloride or urea on roads and sidewalks, and in antifreeze for car radiators.
This thought experiment asks what would change if dissolved salt no longer lowered water's freezing point. It uses the question to explore how freezing-point depression works, where its limits lie and why an ice-and-salt mixture can freeze cream.
Explore the idea
Make a prediction
You sprinkle salt on ice. What happens?
You sprinkle salt on ice on a freezing day. What happens to the ice?
What happens to ice at −5 °C when salt is applied?
- It melts, even below freezing
- It freezes harder
- Nothing happens
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Established fact
Salty water freezes colder than plain water.
Dissolved salt lowers the freezing point of water, which is how salt melts ice below 0 °C.
Freezing-point depression is proportional to solute-particle concentration (OpenStax).
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Established fact
When it's too cold, salt can't melt ice.
Salty water can only get so cold before it freezes, so below about −21 °C (−6 °F) salt can't melt ice.
Sodium chloride brine freezes no lower than about −21 °C (at 23 percent salt), setting the lower limit for salt de-icing (U.S. Federal Highway Administration).
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An idea scientists discuss
Salt that doesn't melt ice is pretend.
A world where salt did nothing to ice is an idea, worked out from how salt behaves today.
Consequences of removing freezing-point depression are reasoned; real solutions always show it.
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Established fact
Salty water freezes when it is colder than plain water.
Solutions freeze at lower temperatures than pure liquids. That's why salt is used to melt ice on roads and sidewalks.
Solutions freeze at lower temperatures than pure solvents, exploited in de-icing with salt, calcium chloride or urea (OpenStax Chemistry 2e).
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An idea scientists discuss
Without salt's trick, sidewalks might stay icy.
If salt didn't melt ice, icy roads and sidewalks might stay slippery for longer.
Without freezing-point depression, chemical de-icing would fail; road safety would depend on mechanical removal and abrasives. A reasoned idea.
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Established fact
Road crews sometimes mix different salts.
Calcium and magnesium chloride are often mixed with cheaper rock salt. They are a bit less harsh on metal and lower the freezing point more.
Group 2 chlorides are mixed with cheaper sodium chloride because they are somewhat less corrosive and, yielding three particles per formula unit, give a larger depression (OpenStax).
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Established fact
When it's very, very cold, salt stops working.
Below about −21 °C (−6 °F), salt can't melt ice at all. Road crews find it works too slowly below about −9 °C (15 °F).
Sodium chloride brine has its lowest freezing point, about −21 °C (−6 °F), at 23 percent salt; highway agencies find salt impractically slow below about −9 °C (15 °F) (U.S. Federal Highway Administration).
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An idea scientists discuss
We might need lots more shovels.
Without salt's help, people might need more plows, shovels and sand to keep roads safe.
Mechanical clearing and abrasives would carry the whole load; their adequacy is a matter of speculation here.
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Established fact
Salt gets in the way of water joining the ice.
Salt doesn't stop molecules leaving the ice, but it means fewer water molecules stick back on, so the ice melts.
Dissolved salt leaves the detachment rate unchanged but lowers the attachment rate, mainly through the lower water concentration in the liquid (Pelton, in Scientific American).
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Established fact
Sea water freezes when it is colder than lake water.
Seawater freezes at a lower temperature than fresh water, so polar seas can stay liquid below 0 °C.
Seawater freezes below 0 °C, so the Arctic and Antarctic oceans remain unfrozen at temperatures below 0 °C (OpenStax).
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An idea scientists discuss
Maybe the sea would freeze more easily.
If salt didn't lower the freezing point, seawater might freeze at 0 °C like fresh water, and cold seas might ice over more easily.
Seawater freezing at 0 °C might enlarge sea ice, with knock-on effects this simple idea cannot predict.
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Established fact
Ice with salt gets colder than plain ice.
Ice cubes with salt melted more and felt much colder than plain ice. Only the salty ice froze the cream.
In the Science Buddies activity, the ice-salt bag fell several degrees below freezing and made ice cream; the plain-ice bag left the ingredients fluid (Scientific American).
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Established fact
Dissolving salt makes water a little colder.
Stir salt into water and the water gets a little colder, because dissolving salt takes in heat.
Overall, dissolving salt absorbs heat: separating the salt into ions takes more energy than hydration releases (Pelton, in Scientific American).
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An idea scientists discuss
Without salt's trick, we'd need a freezer.
Without salt's trick, people would need a freezer or another way to get colder than ice to make ice cream.
Ice-salt baths were the classic way to go below 0 °C without refrigeration; losing them would remove that option. An inference.
How salt melts ice
Ice always has a tiny bit of water on top. The salt mixes into that water.
Salty water stays runny when it's cold. So more ice melts.
Even solid ice has a very thin film of liquid water on its surface. When you sprinkle salt on it, the salt dissolves in that film. Salty water has a lower freezing point than plain water, so it stays liquid even when it's colder than 0 °C (32 °F).
That salty water melts the ice it touches, which makes more water, which dissolves more salt, which melts more ice. The more salt dissolves, the lower the freezing point goes.
Arthur Pelton of the University of Montreal, writing in Scientific American, explains that salt first dissolves in the film of liquid water always present on an ice surface, lowering its freezing point below the ice's temperature. Ice in contact with the brine melts, creating more liquid, which dissolves more salt, and so on.
The higher the salt concentration, the lower the freezing point, but only up to a limit. The U.S. Federal Highway Administration gives the lowest freezing point of sodium chloride brine as about −21 °C (−6 °F), at 23 percent salt, so salt cannot melt ice below that temperature; highway agencies find it too slow to be practical below about −9 °C (15 °F).
Switch salt's trick off
Pretend salt does nothing to ice. Icy roads might stay icy.
People might need other ways to stay safe, like spreading sand.
If salt did not melt ice, sprinkling it on a frozen sidewalk would do nothing. Roads might stay slippery for longer, and people might rely more on plows, shovels and sand for grip.
Other chemicals that melt ice work the same way as salt, by lowering the freezing point. So if that trick stopped working, they might stop too. These are ideas, worked out from how salt behaves today.
Freezing-point depression depends only on the total concentration of dissolved particles, not on their identity. So a world where salt did not lower the freezing point of water would, consistently, be one where calcium chloride, urea and glycols failed too.
The consequences, from icier roads to unusable antifreeze, are reasoned speculation. Real chemistry offers no such world: any dissolved solute lowers the freezing point of water.
Why salty water freezes colder
At the edge of ice, water bits join the ice and leave it.
Salt gets in the way of joining. So salty water needs to be colder to freeze.
Where ice and water meet, there's a busy two-way traffic. Some water molecules break away from the ice, and others stick on. At 0 °C, both happen at the same rate.
Salt crowds the liquid, so fewer water molecules are ready to stick onto the ice, but the ice keeps losing molecules as before. More leave than join, so the ice melts. To make it freeze, you have to cool it further. That's why salty water freezes at a lower temperature.
At an ice-water interface, molecules continually detach from the ice and attach to it. At the freezing point the rates balance. Dissolved salt does not change the detachment rate but lowers the attachment rate, mainly because the concentration of water molecules in the liquid is lower, so the melting point falls (Pelton).
The depression ΔTf is proportional to the molal concentration of solute particles. Calcium and magnesium chlorides dissociate into three particles per formula unit, versus two for sodium chloride, and so give a larger depression.
Salt at work
Salt helps on icy roads. It also helps make ice cream!
Ice mixed with salt gets extra cold. It can freeze milk into ice cream.
Road crews use rock salt, sometimes mixed with calcium chloride, which lowers the freezing point even more. But salty water can only go so low: below about −21 °C (−6 °F), salt can't melt ice at all, and road crews find it works too slowly below about −9 °C (15 °F).
Salt helps in the kitchen too. Old-fashioned ice cream makers pack ice and rock salt around the cream. The salty ice melts and gets colder than plain ice, cold enough to freeze the cream. In tests with plain ice instead, the cream stayed runny.
Rock salt is often mixed with calcium or magnesium chloride, which are somewhat less corrosive and give a larger freezing-point depression. Because these ionic salts speed up metal corrosion, aircraft are de-iced with heated glycols instead.
In a Science Buddies activity published by Scientific American, ingredients shaken in a bag of ice and salt froze into ice cream, while those in plain ice stayed fluid: the salt-ice mixture fell several degrees below 0 °C. Seawater also freezes below 0 °C, which keeps Arctic and Antarctic seas liquid at sub-zero temperatures.
Amazing facts
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Salt makes water a tiny bit colder.
Stir salt into a glass of water and test it with your finger: the water gets a little colder, because dissolving salt takes in heat.
Dissolution of salt is net endothermic: breaking it into ions absorbs more heat than hydration gives off (Pelton, in Scientific American).
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Airplanes are not de-iced with salt.
Salt helps metal rust, so airplanes are de-iced with special heated liquids called glycols instead.
Ionic de-icers hasten metal corrosion, so aircraft are sprayed with heated glycols, which also prevent new ice forming (OpenStax).
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Fish in icy seas don't freeze.
Fish in Arctic and Antarctic seas have body fluids that stay liquid below 0 °C, just like the salty seawater around them.
Like seawater, the body fluids of cold-blooded animals in polar oceans remain unfrozen below 0 °C (OpenStax).
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Salt only works when it's not too cold.
Below about −21 °C (−6 °F), even the best mix of salt and water freezes, so salt can't help.
The depression has a floor: sodium chloride brine freezes no lower than about −21 °C, at 23 percent salt (U.S. Federal Highway Administration).
Words to know
- freezing point
- the temperature at which a liquid turns into a solid
- dissolve
- to mix into a liquid so completely that the pieces spread out and disappear from view
- solution
- a mixture in which one substance is dissolved evenly in another, like salt in water
- brine
- water with a lot of salt dissolved in it
- de-icing
- removing or melting ice, for example from roads or airplanes
- freezing-point depression
- the lowering of a liquid's freezing point when something is dissolved in it
Discovery challenge
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What does salt do to the temperature water freezes at?
What does dissolved salt do to water's freezing point?
How does dissolved salt affect water's freezing point?
- Lowers it
- Raises it
- Nothing
Show the answer
Lowers it
It lowers it. Salty water must get colder to freeze.
It lowers it, so salty water stays liquid below 0 °C.
It depresses it, in proportion to the concentration of dissolved particles.
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Where does salt start melting the ice?
Where does salt first dissolve when it lands on ice?
In what does salt first dissolve on an ice surface?
- In the thin film of water on top of the ice
- Inside the middle of the ice
- In the air above the ice
Show the answer
In the thin film of water on top of the ice
In the tiny bit of water on top.
In the very thin film of liquid water always on the ice.
In the liquid film present on the ice surface (Pelton).
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When is it too cold for salt to melt ice?
Below about what temperature can salt no longer melt ice?
Approximately what temperature limits sodium chloride de-icing?
- About −21 °C (−6 °F)
- About 0 °C (32 °F)
- About 10 °C (50 °F)
Show the answer
About −21 °C (−6 °F)
About −21 °C. That's very, very cold!
About −21 °C (−6 °F), where even the best mix of salt and water freezes.
About −21 °C (−6 °F), the lowest freezing point of sodium chloride brine, at 23 percent salt.
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Why use salt with ice to make ice cream?
Why is salt mixed with the ice when making ice cream in a bag?
What does salt contribute to an ice-cream bath?
- Salty ice gets colder than plain ice
- Salt makes the ice cream salty
- Salt keeps the ice dry
Show the answer
Salty ice gets colder than plain ice
Salty ice gets extra cold.
The ice-salt mix gets several degrees colder than freezing, cold enough to freeze the cream.
It lowers the mixture's temperature below 0 °C, below what plain ice can reach.
Experiments to try
- Watch What If? Lab What if sugar would not dissolve? When sugar dissolves, it doesn't disappear. Its tiny molecules spread out among the water molecules, so you can't see them anymore. Warm water holds more sugar, and the sugar can even grow back into crystals. Imagine sugar that would not dissolve and follow what might change.
- Watch What If? Lab What if soap did not exist? Water on its own slides right off greasy hands. Soap is different: each soap molecule has one end that holds on to grease and another end that holds on to water, so the grease, dirt and germs rinse away. Imagine a world without soap and follow what might change.
Explore next
- Watch What If? Lab What if sugar would not dissolve? When sugar dissolves, it doesn't disappear. Its tiny molecules spread out among the water molecules, so you can't see them anymore. Warm water holds more sugar, and the sugar can even grow back into crystals. Imagine sugar that would not dissolve and follow what might change.
- Watch What If? Lab What if soap did not exist? Water on its own slides right off greasy hands. Soap is different: each soap molecule has one end that holds on to grease and another end that holds on to water, so the grease, dirt and germs rinse away. Imagine a world without soap and follow what might change.
- Watch Try it What If? Lab What if a balloon floated very high? As a balloon floats higher, the air around it gets thinner and pushes on it less, so the gas inside spreads out and the balloon swells. In our model, a perfectly stretchy balloon launched at sea level is about 14 times its starting size by the time it reaches 20 kilometers.
- Watch What If? Lab What if apples never turned brown? When an apple is cut, a natural chemical in it mixes with oxygen from the air, and the fruit turns brown. Lemon juice and the fridge slow it down, and some apples are now grown to brown less. Imagine apples that never browned and follow what might change.
- Watch What If? Lab What if bubbles could last forever? A soap bubble's skin is a thin sandwich of soap and water, and it pops when the water drains or dries away. Scientists in France made a bubble that lasted 465 days. Imagine bubbles that lasted forever and follow the clues: thin films, drying out and frozen bubbles.
- Watch What If? Lab What if cakes had no baking powder? Look closely at a slice of cake and you'll see it is full of tiny holes. Baking powder makes bubbles of a gas called carbon dioxide, and those bubbles puff up the batter as it bakes. Imagine baking with no baking powder and follow what might happen.
- Watch What If? Lab What if colors never faded? Leave a bright poster in a sunny window and, over weeks, its colors grow pale. Light slowly changes the colored molecules in dyes and inks. Imagine a world where colors never faded, and follow the clues: sunlight, paints and blocking ultraviolet.
- Watch What If? Lab What if concrete could heal its own cracks? Concrete is strong, but it cracks, and water creeping into the cracks can rust the steel inside. Scientists are testing concrete that seals its own cracks, using sleeping bacteria or special lumps of lime. Follow the clues to see what self-healing concrete might change.
For parents & teachers
What children will learn
- Dissolved salt lowers the temperature at which water freezes.
- Salt melts ice by dissolving in the thin film of water on the ice surface.
- Salt cannot melt ice below about −21 °C (−6 °F), and it works too slowly to help much below about −9 °C (15 °F).
- A mixture of ice and salt gets colder than plain ice and can freeze ice cream.
Recommended ages: 7 to 17 · About 20 minutes
Talk about it
- Where have you seen salt used on ice? Did it work right away?
- Why might seawater stay liquid when a pond nearby freezes?
- If salt did not melt ice, what else could people do to stay safe on icy days?
Try at home or in class
- Put two ice cubes on separate plates. Sprinkle a pinch of table salt on one. Check every five minutes: which melts faster? (Do not taste the ice.)
- Make ice cream in a bag with a grown-up: seal milk or half-and-half with a little sugar and vanilla in a small bag, put it in a bigger bag with ice and table salt, and shake for five minutes wearing oven mitts or with the bag wrapped in a towel, because salty ice gets very cold. Try a second bag with ice and no salt and compare.
Key vocabulary: freezing point, dissolve, solution, brine, de-icing, freezing-point depression
Where these facts come from
- Chemistry 2e, 11.4 Colligative Properties , OpenStax
- Why do we put salt on icy sidewalks in the winter? , Scientific American
- Manual of Practice for an Effective Anti-Icing Program (FHWA-RD-95-202, 1996), freezing points of brines , U.S. Federal Highway Administration
- Scrumptious Science: Making Ice Cream in a Bag (Science Buddies) , Scientific American
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.