Hello all! I think there is little else that is more enticing to me than ice cream (OK, I’ll admit it - crème brulee might top it, and realistically it is just a different version of sugary-creamy goodness). So what better scenario could I have this week then being able to make ice cream with my coworkers during a team building exercise?
And the more surprising part? I had 3 experienced, full grown adult, male engineers ask me why salt was needed to make the ice cream.
So, let’s talk science!
First of all, remember that this is when you are making ice cream by hand. Good old hand-crank, churn for long time, until the cream mixture freezes. The salt is required for the ice on the outside of the hand-crank machine.
OK, now back to the why…
Let’s talk about melting and freezing. They actually happen at the same temperature. For example, if I’m a solid and I want to become a liquid, I have to get enough energy (typically in heat form) to raise myself to my melting temperature. If I’m solid ice, I need to get up to about 0 °C. However, getting up to that temperature won’t automatically turn me into liquid (which is why you can still get ice, or snow, on days that are 0 °C). Liquid molecules have much more energy than solid molecules, which is why they are fluid and can’t hold their form. With all the energy required to go into the phase change, there is no temperature change occurring during the phase change.
So what about the salt?
Well, for ice cream, this melting / freezing point is actually at a lower temperature than 0 °C. The sugars and fats in the ice cream mixture require a colder temperature to freeze than pure water molecules would alone. This requires more than just ice to cool the ice cream mixture (otherwise the ice would melt before the ice cream is frozen).
Salt is the key! Salt itself has a much higher melting point than water, so making a salt-water solution will lower the point at which the solution melts!
But, in turn, it’s actually important for the salt-water mixture to melt. Why? Because the energy needed to change the salt-water mixture from solid to liquid needs energy (remember, the phase change we talked about?). That energy comes from the environment. And in what form? Heat. In other words, taking heat from the surroundings to melt the salt-water mixture ultimately makes everything colder, including the ice cream mixture – which needs to be colder than 0°C to freeze.
It’s not magic, it’s SCIENCE! And delicious science at that.