Q:
I'm doing a science project about freezing milk. Will the fat content affect the rate at which milk will freeze, for example whole milk, fat free milk and chocolate milk. We added vanilla and sugar to the first two. The answer to the experiment was that the fat free milk took longer to freeze. How can I explain this to a second graders. We did it as science fair project. Does the answer have to to with the fat? Please help me to explain in simple words for second graders.
- Carolina (age 29)
Richmond
A:

We love it when teachers ask questions like this, because that means that our answers get to reach the students too.

You may want to check some of our old answers on this same topic: listing.php?id=2721.

There are two broad reasons why the fat-free milk could have frozen sooner:

1. It might have a lower freezing point so it had to get colder before it froze.

2. It might have been able to supercool more before it froze. (You can find out a lot about supercooling by searching this site.  e.g. http://van.physics.illinois.edu/qa/listing.php?id=1618 )

Probably it was just that the freezing point was lower. You can check because if it was supercooling then you could get the fat-free to freeze as soon as the whole by shaking them.

Let's say it is that the freezing point is lowered. How can you explain that to the kids? Usually fat-free milk has some extra milk solids mixed in so that it doesn't seem too thin. In other words, to make up for the reduced fat some protein, milk sugar, and some salts are added. Then you added some more sugar to both the whole and fat-free milk, but not the chocolate milk. That might have given the whole milk about the same amount of dissolved sugar etc. as the chocolate milk, but it would make the fat-free milk have the most dissolved sugars etc.

Now you could draw a picture on the board with a couple of big globs of fat, far apart, to show what the fat looks like in the whole milk. In between you could put lots of little dots to show the molecules of sugar, protein, and the salt ions. The big globs don't do much of anything to the water freezing in between them. Why should that water care about a glob of fat somewhere else? Lots of little molecules and ions do make it harder for water to freeze. (That's why up north here we put salt on icy roads to melt the ice.) Even a small ice crystal has to push aside some of those molecules to form. That lowers the freezing point. We discuss a bit more about that here: https://van.physics.illinois.edu/qa/listing.php?id=1608.

Mike W.

(published on 04/14/2014)