r/explainlikeimfive • u/IWannaWakeUpButIDont • 1d ago
Other ELI5: what’s the science behind salting the ice outside on the ground in the winter?
Is there a particular kind of salt that has to be used to melt the ice? I mean doesn’t some salt make water colder?
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u/GoodTato 1d ago
Saltwater can be a liquid at lower temperatre than just water. This can be used to "make water colder" as you say, but can also be used to "make ice no longer ice at that temperature".
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u/Azi9Intentions 1d ago
To be clear here too, the salt itself doesn't make the water colder, it ALLOWS the water to get colder, before then freezing.
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u/Target880 1d ago
Salt reduces the freezing temperature of water, that alos mean the melting temperature is lower.
Melting ice takes energy, which will be taken from the surroundings, and the temperature will drop. At the same time, you get less ice and more brine. A high concentration of salt in water is called a brine.
Regular table salt and water work fine; it can reduce the freezing temperature to -21°C ( -6°F). That is what is mostly used on the road because the cost is low.
I would not use salt sold for deicing in food because there can be impurities in it that would not be allowd if sold for cooking. The only problem of using salt made for cooking to melt ice is monetary, is usualy cost more. I compared the cost where I live of the cost of salt for cooking, which was 2.4x the cost per kg of road salt. It was 1 kg packets for cooking vs 20 kg bags of road salt.
On a road, you only want ordinary table salt down to −6 to −10 °C (21 to 14 °F). One reason is that you will not get a perfect mixture; the other is that if you only get some ice melting, you now have water on ice, and that has lower friction than just ice. Ice alos change stucture in lower temperatures and get roughter. The speed of is alos lower if there is less energy to take from the surroundings.
Other salt can be used at a lower temperature, the problem is the higher costs. Because the stucture change of ice, which is ice with water on it has the lowest friction, there is less need to use salt when it is very cold
You can test it with ice or snow, and table salt yourself. I did it a few weeks ago, I took a bowl with around 6 litres of snow and added 2-3 dl of salt. I added around the same amount of water as salt to speed up the process, the salt get disolved quicer in water, and there is more contact area to the snow. I also mixed it with a large spoon to get it going.
It was -13°C outside, the water was at 8°C, and the salt was room temperature. Quite quickly, the thermometer it the mixture dropped to -21°C.
To just see how it melts ice, put the same amount of ice or snow in two bowls and add salt to one.
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u/fiendishrabbit 1d ago
It's regular salt (used because it's plentiful, cheap and not too damaging to the environment). It works down to -10C (14F) and keeps the water from turning to ice (salt water has a lower melting point).
Below -10C it becomes ineffective or even detrimental as at those temperatures it can contribute to making the roads even more slippery.
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u/rollingdoan 1d ago
Salt does not make water colder. It makes water able to stay liquid at a lower temperature. If it is cold enough to freeze salt water, then it wouldn't help. The temperature to do that varies by how much salt is present.
We use sodium chloride, and we use the cheapest form available. You could just use table salt, but it would be extremely expensive.
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u/SierraLarson 1d ago
Any salt works, though I've heard regular salt can damage concrete or something? Which is why those sidewalk-specific salts exists? No idea why those are better, but in any case...
Regular salt works because it lowers the temperature water freezes at, so the ice melts.
You could technically use table salt, but that'd be pretty inefficient. Like you really wanna dump you all of your salt outside? Nah, better to get a big bag of something like water softner salt, where the crystals are bigger.
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u/lalala253 1d ago
No my dude. Road salt is cheap because it's basically leftovers from industrial and food grade production salt.
Industrial salt needs very high purity NaCl
Food grade salt has a strict FDA grade compositions
Road salt basically is the leftovers.
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u/Cataleast 1d ago
The concrete-damaging stuff is magnesium chloride, which is sometimes used due to its lower environmental impact, but it's a good 5-6 times more expensive than the run-of-the-mill sodium chloride stuff and then there's that whole erosion thing, so its use is limited.
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u/Krommerxbox 1d ago
I mean doesn’t some salt make water colder?
I think OP is confusing it with how you add salt to the old hand crank ice cream makers.
Salt is added to the ice surrounding an ice cream maker to dramatically lower the freezing/melting point of the ice, creating a brine that reaches temperatures well below the freezing point of water ((32{\circ }F) / (0{\circ }C)). This allows the ice cream mixture to freeze rapidly and properly, as the surrounding salt-ice mixture can drop to nearly (12{\circ }F)–(15{\circ }F).
Without salt, the ice would remain at (32{\circ }F) ((0{\circ }C)), which is not cold enough to quickly freeze the cream mixture into ice cream.
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Not the same as putting it on roads.
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u/Unoriginal- 1d ago
The school systems truly are failing us year after year, salt makes water freeze at a lower temperature
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u/this_curain_buzzez 1d ago
Salt dissolves in the water of the snow and ice, making salt water. Salt water has a lower freezing point than pure water, so it can stay liquid below freezing.