Under normal conditions, ordinary water freezes at 0°C, or 32°F. However, if you add salt to water, its freezing point becomes lower. Let's look at why a salt water solution has a freezing point below zero, and how you can use this fact to make ice cream!
At the right is a container of water with an ice cube in it. The water and ice are at 0°C, which is the melting point of ice and the freezing point of water.
Molecules of ice are constantly escaping into the water (melting), and molecules of water are being captured on the surface of the ice (freezing).
When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change. The ice and water are said to be in dynamic equilibrium with each other. The ice is melting, and the water is freezing, but both are occurring at the same rate, so there is no net change in either quantity.
This balance will be maintained as long as the water stays at 0°C, or unless something happens to favour one of the processes over the other.
Here is the same container, but where the water temperature is -10°C.
The molecules of water are moving more slowly, because they contain less heat. These slower-moving water molecules are more easily captured by the ice, and freezing occurs at a greater rate than melting.
Because there are more water molecules being captured by the ice (being frozen) than there are ice molecules turning to water, the net result is that the amount of water decreases, and the amount of ice increases.
Since more freezing is happening than melting, the water eventually all turns to ice.
This time we've warmed the water to 10°C.
Now the water molecules are moving more quickly, because they contain much more heat. These faster-moving molecules can't easily be captured by the surface of the ice, so not very many of them freeze. Freezing occurs at a slower rate than melting.
Because there are fewer water molecules being captured by the ice (being frozen) than there are ice molecules turning to water, the net result is that the amount of water increases, and the amount of ice decreases.
Since more melting is happening than freezing, the ice eventually all turns to water.
What we've learned so far is that when an ice cube is immersed in water, some of the ice is always melting, and some of the water is always freezing. If these are both happening at the same rate, the contents of the container are at the freezing/melting point. For pure water, this happens at 0°C.
If one of these processes is happening faster than the other, the water will completely freeze or the ice will completely melt.
So how does adding salt to the water affect the result? Let's look.
Here's the same container with the water at 0°C, only this time the water contains salt molecules. Adding salt disrupts the equilibrium.
The salt molecules dissolve in the water, but do not attach easily to the solid ice. There are fewer water molecules in the liquid because the some of the water has been replaced by salt. This means that the number of water molecules able to be captured by the ice (frozen) goes down, so the rate of freezing goes down.
The rate of melting of the ice is unchanged by the presence of the salt, so melting is now occurring faster than freezing. So the ice eventually melts.
In order to return the system to equilibrium, where the number of molecules of water that are freezing is equal to the number of ice molecules that are melting (this is the freezing/melting point), we must lower the temperature sufficiently to make the water molecules slow down enough so that more can attach themselves to the ice.
When the number of water molecules that are freezing equals the number of ice molecules that are melting, equilibrium will be reached.
In our example, this point is reached at -4°C, which would be the new freezing/melting point. The higher the concentration of salt, the lower the freezing point drops.
As ice begins to freeze out of the salt water, the fraction of water in the solution becomes even lower, and the freezing point drops further! However, this doesn't continue indefinitely. At some point the solution will become saturated with salt. This happens for salt in water at -21.1°C, which therefore is the coldest a saturated solution of salt and water can get.
At that temperature, the salt begins to crystallize out of solution, along with the ice, until the solution completely freezes. The frozen solution is a mixture of separate salt (NaCl·2H2O) crystals and ice crystals. This heterogeneous mixture is called a eutectic mixture.
Any foreign substance added to the water will cause a freezing point drop. For every mole of foreign particles dissolved in a kilogram of water, the freezing point goes down by roughly 1.8°C. Sugar, alcohol, or any chemical salt will also lower the freezing point and melt ice. Salt is used on roads and walkways because it is inexpensive and readily available.
You might suppose that larger molecules might inhibit the freezing of water molecules even more, and have a more dramatic effect on the freezing point. However, that isn't the case. Actual molecules are so tiny compared to the distance they move through the liquid that size is hardly a factor at all.
Now for the fun part! In order to make proper ice cream, which is smooth and creamy, you have to freeze it uniformly and quickly.
This is most easily done by using a mixture of ice, water, and salt, which will be at a temperature many degrees colder than 0°C.
Since you are using the salt/ice solution just for rapid cooling, and not eating it, you can use any kind of salt. Rock salt works well.
For your enjoyment, here are two recipes for home-made ice cream which use the principle of lowered freezing point.
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