Salt on icy roads vs salt to make homemade ice cream..


This winter has been filled with so much rain and snow. I started wondering what the difference is between the salt they place on icy roads and the salt you use to make homemade ice cream?

Is it the same kind of salt? What does the salt on the road do? Does it help with friction or melt the ice?

I thought the salt mixed with ice, to make ice cream, makes the freeze temperature lower?

I’m confused…

In: 0

They both do the same thing, and since the salt you use to make homemade ice cream isn’t going into the ice cream, you could use the same salt (although most people would probably use food-grade salt in case their ice cream ends up in contact with the salt-water).

Salt drops the freezing/melting point of water/ice. This means liquid water can be colder than 0 degrees C (32 degrees F). On the roads, this is good, because ice is more slippery than water. So if it’s 28 degrees F out, you can prevent ice by putting salt down.

With homemade ice cream… when you’re making the ice cream you want your cream/sugar mixture to be well below freezing. You can’t do this with regular liquid water, and solid ice doesn’t make good contact with things. Salt-water lets you have good contact with your ice cream bowl AND be below freezing temperature.

> what the difference is between the salt they place on icy roads and the salt you use to make homemade ice cream?

There isn’t necessarily a difference, but usually the salt you would make ice cream with is going to be cleaner. Much of the salt we use for roads is mined because it is cheaper to produce that way, but such salt is dirty because it is mixed with sand and rock dust. This is fine when you are just throwing it on the road since it is just more traction, but when you are churning ice cream may people don’t want a gray sludge in their bucket.

> What does the salt on the road do?

Salt mixes with the water and lowers the freezing temperature of water. This helps to melt the ice on the road, allowing it to just flow away.

> I thought the salt mixed with ice, to make ice cream, makes the freeze temperature lower?

This is also why it is used to make ice cream. By lowering the freezing temperature the brine is below the freezing temperature of fresh water and can freeze the ice cream you are making. You need to keep adding ice because as it transitions to a fluid it pulls heat out of the brine to keep it colder than the ice cream.

Its the same concept in both cases, it lowers the freezing point

You’re going to be more familiar with this from the other end. Boiling water is 100C right? If you have a pot of boiling water on your stove on low you get some vapor coming off. If you crank the stove up to high does the temperature of the water increase? Nope, just makes water boil off faster. All new energy added goes into the phase change (liquid-gas) and not into increasing the temperature

Same concept for freezing. A water-ice mix exists at the freezing point of the mix so a bag of ice water is pretty close to 0C. Add some salt and you lower the freezing point but the mix still needs to hang out at the freezing point so now your bag of ice-salt-water is -5C.

For a road, salt means that instead of needing the temperature to be above 0C to melt the ice on the road, it now just needs to be above -5C to melt the ice and not let it refreeze. The sun can add just enough energy to the mix to cause the mainly ice to transition to salty water with some ice still at -5C but the road is now clear

For ice cream, you need to get the temperature down below 0C to get good freezing of the sugary cream. By filling the outside of the maker with ice and salt and water you end up with a -5C cooler outside of the inner spinning liner which lets you freeze the mixture up a lot better than a 0C liner would.

Mixing in salt (of any kind) does indeed lower the temperature water freezes at. On roads, that’s helpful because it means that the road won’t freeze and get icy until it’s colder outside.

For ice cream, that’s helpful because of how the process of melting works. When something is melting (this is true for any solid, not just ice/water), it takes heat to break apart the solid structure into a liquid in addition to the heat it takes to warm it up. The solid can’t get any hotter than the melting point, because it can’t do that and stay solid, so it also keeps the liquid cool while it melts. This means that a partially-melted mixture will always stay at the freezing temperature. Once the ice all melts, it’ll warm up normally again. Making ice cream works better if you can keep it at a temperature that’s colder than the normal freezing temperature of water, and if you put in just ice, it’s going to become an ice/water mixture pretty fast. So, if you want to keep your soon-to-be ice cream at a temperature that is colder than the temperature water freezes at, you need to have ice that melts at lower than the usual temperature, which you achieve by adding salt.

You *can* use the exact same salt on roads and to make ice cream, but in practice they’re typically different. People use table salt for ice cream because if you’re cooking you probably already have it around. They use a variety of different salts on roads because they have different properties that give them different abilities to stick to the road, not harm the local environment, and have different prices.

They’re both pretty typically NaCl (sodium chloride) and yes, they’re doing the same thing – keeping things from freezing solid at a few degrees below water’s normal freezing temperature.

On the roads, they sprinkle salt to make ice melt and break apart so that the roads don’t become skating rinks. In ice cream, salt is used to keep the other ingredients in a nice thick slushy consistency which water can’t get cold enough to do without freezing solid.

When we want help with *traction* on roads rather than just plain melting the ice, we we use something insoluable alongside the salt, such as gravel or sand. The salt or sand doesn’t really help melt the ice at all, but it gives tires something to help grip onto on top of the ice.