Freezing Science: The Role of Salt in Making Ice Cream

We know salt gets mixed with the ice in hand-cranked ice cream machines. We see that it gets results, and yes, ice cream is made. But we’ve never quite been able to wrap our heads around it. Why the salt? What is it doing? Is it really necessary? Let’s see if we can get this straight.

Ok, the first concept to wrap our heads around is that the melting and freezing point of any liquid is just about the same. Water freezes at 32 degrees Fahrenheit, but it will also melt any smidgen of a degree above that. Make sense?

The next thing to understand is that ice cream freezes (and melts!) at a lower temperature than water. The sugar and fats in the mix interfere with the formation of ice crystals, and it takes a colder temperature to get the ice cream to really freeze. Therefore, we can’t use straight ice to chill the ice cream base, because the ice will melt before the base gets cold enough.

Salt provides the solution. Similar to sugar, salt affects how water freezes and effectively lowers the freezing/melting point of water. Creating a saltwater slush and packing this around our ice cream base allows us to cool the base enough so that it starts to thicken and freeze before the ice melts completely.

This whole process feels very counter-intuitive to us! We’re looking at a slushy, half-melting saltwater mix and thinking that it can’t possibly be colder than hard ice cubes. But amazingly, it is. And what’s more, it works to make ice cream and has done so for centuries!

(Image: Flickr member R. B. Boyer licensed under Creative Commons)

Comments (5)

Science!

Nothing better or more satisfying than hand cranked ice cream.

You skipped the most important part as to why it works.

You got that salt lowers the freezing/melting point of ice, but didn't indicate as to why this was significant. In order for ice to melt it needs heat, so it steals the heat from any source nearby, this has the effect of decreasing the temperature of everything around it. This seems counter-intuitive, but melting is in fact a cooling process. Thus what's really freezing the ice cream is the melting of the water, specifically to melt the ice is stealing the heat from the ice cream mixture causing it to freeze and create ice crystals and all that fun stuff that makes ice cream so gosh darn yummy.

@blpeders, oh, I'm so glad someone else jumped in. I can't resist going all science geek and thermodynamics is my favorite.

To make it extra nerdy, a phase change (phase change = melting or freezing or boiling)diagram would have been great!

I do have one quibble, though. Water doesn’t freeze slightly below 32 and ice doesn’t melt slightly above 32, water and ice exist at that temperature together but can’t exist together at any other temperature. If you've ever calibrated a thermometer with ice water, that's how you know a cup of water and ice is at 32F exactly.

It helps if you think of temperature as a measure of energy. Something warm has more energy than something cold. The ice/water mix wants to be at the same energy level as the outside air so the mix starts to pull energy from the surrounding air. Before the temperature can come up, though, the ice has to change phases into water since ice can’t exist above 32 degrees and that phase change takes energy. So the phase change from ice to water is using up all of the absorbed energy and the entire system is sitting at exactly 32 degrees until there isn’t any ice left. Then and only then can the energy from the air start changing the temperature of the water and moving it above 32 degrees.

The same thing happens at 212 degrees F which is why liquid water at a normal atmospheric pressure won’t ever go above 212 degrees.

Yea, it can be a little difficult to wrap your brain around without a little thermodynamics. There are two things going on in the ice cream freezer. One is the salt/water mixture has a lower freezing point than pure water, so a lower temperature can be achieved as a liquid. Another thing is the energy change that is going on when ice is involved and melts. The energy involved is quite large. To cool 1gram of water from 1 deg. C to 0 deg., one calorie of energy must be removed. Then to change that same gram of water at 0 deg. to one gram of ice, still at 0 deg. takes 80 calories of energy. This is the 'heat of fusion' of the water. This is a large energy exchange that is going into the mix just to accomplish freezing or melting. For each gram of ice that is melted in the salt water slurry 80 calories of heat must be exchanged, hopefully from the cream, sugar,... mix.

Yum! Isn't thermodynamics delicious.

Mythbusters did this one a while back. You can cool down cans of beer or soda more quickly if you add salt to the ice in your cooler.

This is why the sea doesn't really freeze.