Thermodynamics of Rock Salt and Ice Cream

Why do we salt the ice when making ice cream?

Recently I was making ice cream with my family and my 9 year old asked why we pour rock salt Tangentially, there’s nothing special about rock salt. It just happens to be the cheapest form of salt you can buy, as it’s not meant to be eaten. You could certainly make ice cream with table or kosher salt instead, it would just be more expensive. on the ice surrounding the canister of ice cream mix. I muttered something about melting ice, latent heat of fusion, and lower temperatures, hoping to misdirect my ignorance with jargon.

Unsatisfied with my explanation, I did some research. Here is what I’ve learned about melting point depression and its role in making ice cream.

Melting Points

The common understanding of why we put salt on ice is that it lowers the melting point. This makes sense when looking at icy roads in the winter: if it’s 22F (-5C) outside, the ice on all the roads won’t melt. When we add salt, the melting point is lowered past the ambient temperature and the ice melts away. Easy peasy.

But when I make ice cream, it’s hot outside! That’s why I’m making ice cream!! The ice in the bucket is already melting quite nicely, what good will it do to lower the melting point?

Equilibrium

Conveniently, all my confusion stemmed from a singular origin – a fundamental misunderstanding about the significance of the melting point of a substance.

I used to think of the melting point as “the temperature at which a solid will start to melt.” Get it up to that temperature, and it’ll begin to transition to a liquid. That’s what happens when I take ice out of the freezer, isn’t it?

This turns out to be a subtle, but critical, error. The melting point is more accurately described as “the temperature at which solid and liquid phases exist in equilibrium.” At this temperature, adding or removing energy (heat) from the system will change the ratio of solid::liquid, but the mixture will remain at the equilibrium temperature – the melting point – until all of it has melted (adding heat) or frozen (removing heat).

Digression – This equilibrium can exploited to calibrate a thermometer by measuring the temperature of a well mixed ice / water slurry. Any deviation from 32F (0C) and you know that your thermometer is bad. It doesn’t matter how much ice or water you use, as long as it’s been agitated To prevent any hot/cold spots from developing in the solution. and allowed enough time to come to a steady state.

Equilibrium Point Depression

So then, what does adding salt to the ice really do? It depresses the freezing point yes, but more accurately it lowers the equilibrium temperature. Salt scattered on an icy sidewalk causes it to melt by dropping the equilibrium point below the ambient temperature. Salt added to an ice / water slurry in an ice cream machine lowers the temperature of the mixture beyond the typical freezing point of water. This is usually around 15F (-10C). The lowest the equilibrium point of a salt + water solution can go is -5F (-21C), at which point salt and ice crystals will begin to precipitate from the solution.

This is important because ice cream – with all its added sugars – has a freezing point that has also been lowered below 32F (0C). Pure ice alone isn’t cold enough to freeze it, but an ice / water slurry whose equilibrium temperature has been lowered sure is!

Phase Changes are Expensive

This model of equilibrium depression carries a counterintuitive implication: you can take an ice / water slurry that is in equilibrium at 32F (0C), add some salt to it, and the temperature will go down.

I’m sorry, what? I can take a bucket of ice / water, put it outside on a hot summer day, add some salt, and it will get colder? That can’t be right, can it?

It turns out, yes! What happens is that when the salt is added some of the ice melts – pulling heat from the system – until the temperature has reached the new, lower equilibrium point. Melting ice is a thermodynamically expensive process, The amount of energy required to melt 1g of ice is enough to raise the temperature of 1g of liquid water 176 degrees Fahrenheit (80 degrees Celsius). so you only have to melt a small amount of ice before the system reaches the new equilibrium.

TLDR

Instead of lowering the melting point of ice, it’s more accurate to say that salt depresses the equilibrium point of the ice / water mixture, allowing it to become cold enough to freeze the ice cream. The ice / water solution will descend to and remain at the equilibrium point until it has entirely converted to solid or liquid.