and got intrested in wanting to know why salt was used in certain ice cream machines and here is what I found.
How does it do that?
For ice to melt, it has to get heat from something. In our ice cream project, it gets the heat from the ice cream mixture (and from your hands, which is why they get cold while holding the bag). When the ice is melting, it is at 0 degrees Celsius (32 degrees Fahrenheit).
When ice is melting, the surface of the ice is wet. At the surface, there is solid ice on one side, and liquid water on the other. The surface is exactly at the freezing point. This means that some water molecules are leaving the ice and moving into the water, but it also means that some liquid water is refreezing onto the ice. We say that the system is in equilibrium when the rate of melting is equal to the rate of freezing, and this happens at 0 degrees Celsius.
At equilibrium, the heat lost by the water as it freezes is equal to the heat gained by the ice as it melts.
Because plain ice can only barely cool something to the freezing point of water, we will need to do something to make it much colder than that, since our ice cream mixture freezes at a lower temperature than water.
The ice cream freezes because the salt and the ice mix to make a substance with a lower freezing point than ice alone. This means that the ice and salt mixture must get even more heat from somewhere in order to melt.
Salty water freezes at a lower temperature than plain water. But the ice is made of plain water, so it melts at 0 degrees Celsius. Since the ice keeps melting, but the water no longer freezes (because there is only salt water, which doesn't freeze at 0 degrees), the temperature goes down.
The heat gained by the ice as it melts is no longer offset by the heat given up by freezing water (since the water is no longer freezing back onto the ice). The heat gain has to come from somewhere else. It comes from the ice cream and your hands.
The sodium and chlorine in the salt split apart into charged ions, and these ions attract water molecules to form weak chemical bonds.
The resulting compound has a freezing point of -21.1 degrees Celsius (-5.98 degrees Fahrenheit). This is 21.1 degrees colder than ice (37.98 degrees Fahrenheit colder than ice).
When people put salt on the ice on a sidewalk or a road, the ice mixes with the salt, and the mixture of the two solids (ice and salt) produces a liquid, but the sidewalk actually gets colder than it was before.
If we add a different chemical to the ice, such as calcium chloride, we can get an even lower temperature (-29 degrees Celsius, or -20 degrees Fahrenheit).
“Now to him who is able to do immeasurably more than all we ask or imagine, according to his power that is at work within us, to him be glory in the church and in Christ Jesus throughout all generations, for ever and ever! Amen.” (Ephesians 3:20-21)
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