A student asked me this today. I understand that salt doesn’t actually melt ice, but lowers its melting temp, but how do I explain this to a child?
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So what happens is that salt lowers the point where water freezes.
So when you’re making your ice cream usually you take like ice made from tap water and you put the ice cream mix in like a mixer or something above the ice. The ice has a lot of holes and gaps as it doesn’t totally fill the space evenly meaning there is a lot of surface area of the mixer not being touched by the ice.
So heat only transfers from the warmer object to the colder object but that doesn’t work very well when there is low contact surface to actually transfer that heat. So you melt the ice with the salt, the water still has the same temperature as the ice but now fills in all of the gaps from the ice which helps to make the heat transfer more conductive and therefore faster
Salt changes how ice melts, making it melt faster, by making it easier to pull heat out of the thing that the ice is touching. If that thing the ice is touching is a metal container of ice cream mix, then pulling the heat out of it makes it freeze solid. Mixing it during this process introduces air to the frozen mixture, making it super good.
You don’t put the salt in the ice cream, you put it on the ice. Ice melts by absorbing heat. The salt makes it absorb heat faster by altering the melting temperature thereby cooling the ice cream down faster.
Salt requires energy to dissolve and it is taken from the thermal energy of the water and ice mixture and it gets colder. When the meeting point drops some of the ice will melt. Turing ice to liquid water requires energy and reduces the temperature
The salt is not put in the ice cream but in the ice and water mixture around it. The mixture with the salt will not freeze because its melting temperature is lower. There is no extra salt in the ice cream so its melting temperature has not dropped.
When you add salt to a roadway its temperature will drop. There is a minimum temperature that is useful for a road that is above the minimum freezing temperature of the nine
This is one of those things that is only ever explained sorta right.
First, the part everyone misses: the ice you’re using to freeze the ice cream is well below freezing.
So far so good – you need it to be below freezing to freeze something, right?
Problem is it starts to melt. And when it does you’ve got cold ice swimming in warmer liquid water, which is now exactly at freezing — too warm to freeze the ice cream.
Toss some salt in and the melting water can now be liquid below the normal freezing point. That means that really cold ice is surrounded by nice cold water that is cold enough to freeze ice cream.
None of this would work if the ice wasn’t already colder than freezing.
Ice doesn’t transfer heat well, and air doesn’t transfer heat well, but water transfers heat much better than the other two. By adding salt to ice, you lower the melting point so you get colder liquid water, which freezes your ice cream faster.
At 0°C, both ice and salt are solid. But because of some insanely weird chemistry, salt water needs to be made much colder to freeze solid. This is due to how the sodium and chlorine ions interact with the polar water molecules and there’s really no child friendly explanation. Salt and water don’t like to sit still when they’re together so they stay liquid
It freezes ice cream because sugar, along with salt, decreases the freezing point of water. Therefore, to properly freeze a sugary water substance (ice cream), you need a temperature below the freezing point of pure water. Salt is used in ice cream machines because most homeowners don’t have a freezer large enough to fit an ice cream churn inside and that is the only way to effectively reach temperatures that would solidify a sugary water substance (ice cream). Salt melts ice ONLY when it is above the freezing point of salt water (also, most ice melting salt is not just normal sodium chloride). It just happens that many places on Earth do not get below the freezing point of saturated salt water for extended periods of time.
Edit: No one has pointed out that you actually need temperatures far below the freezing point of water to freeze ice cream. Commerical ice cream production doesn’t typically use salt water to freeze ice cream because they have specialized devices that can get colder then the freezing point of water to freeze the ice cream (typically glycol chillers).
Imagine water as Lego bricks.
When the bricks are clean, they snap together nicely and you get a big block of Lego. This is plain water freezing to make an ice cube.
Now imagine playing with Lego at the beach. The sand gets everywhere, between the individual Legos. This makes it really hard to make a nice big block of Lego. You have to clean all the sand away.
This is kind of what is happening with the salt in your water. It is getting between the water to prevent them from stacking up nicely to make an ice cube.
To make an ice cube from salt water, you have to do more work and make things colder to get the water clean in places to stack together nicely. Therefore, salt water freezes at a lower temperature than plain water.
Now that you have salt water, you have something that freezes colder than plain water, you can use it to freeze thing that normally don’t freeze with ice.
Particles in a fluid will increase its boiling point, and decrease its freezing point. The more particles, the more change from normal. You can actually use math to figure this out precisely.
Anyway salt dissolves easily in water and adds lots of particles. This makes the freezing point decrease, making it melt. In a traditional home ice cream maker the effect of melting ice (solid to liquid) absorbs a lot of energy (heat) from the ice cream mix making it cold. The ice cream has less particles compared to the salty mix, so it freezes while the salty mix is like a slushy.
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