Can someone clarify freezing point depression and boiling point elevation for me?

If freezing point is lowered, doesn't that mean that something would freeze more easily? My textbook gives an example of antifreeze, and it says that antifreeze lowers the freezing point. I'm confused. Can someone help? Thanks!

1 Answer
Nov 25, 2015

Answer:

Here's how you can think about this.

Explanation:

To answer your second question first, no, lowering a substance's freezing point does not mean that said substance will freeze more easily.

It's exactly the opposite.

In order for something to freeze, it must lose heat. The lower the boiling point of a substance, the more heat it must give off in order to freeze.

So when you lower the freezing point of a substance, you're actually increasing the amount of heat that must be given off by that substance in order to go from liquid, to liquid at its freezing point, to solid at its freezing point, to solid.

So, for example, let's say that you have a sample of pure water at room temperature, #20^@"C"#, and normal pressure.

The freezing point of pure water is #0^@"C"#. Under these conditions, water must lose enough energy to allow it to go from

  • liquid at #25^@"C"# to liquid at #0^@"C"#
  • liquid at #0^@"C"# to solid at #0^@"C" -># undergo a phase change
  • solid at #0^@"C"# to solid at whatever final temperature you want

Now let's say that you're using antifreeze on this sample of water. Antifreeze lowers the freezing point of water to, let's say #-20^@"C"#.

Now a sample of water + antifreeze at room temperature will need to give off more heat to reach its phase change step

  • liquid at #25^@"C"# to liquid at #-20^@"C" -># extra heat given off because of a bigger temperature interval
  • liquid at #-20^@"C"# to solid at #-20^@"C" -># undergo a phase change
  • solid at #-20^@"C"# to solid at whatever final temperature you want

So, looking at it from water's perspective, more heat needs to be taken away from the system (given off) in order to freeze at a lower freezing point than to freeze at the normal freezing point.

The exact same principle applies to boiling-point elevation, only this time it's the amount of heat that must be supplied to a water sample that increases with an increasing boiling point.

Now, to keep this answer from getting too long, I won't go into how what happens at a molecular level influences freezing-point depression and boiling-point elevation.

Check out this great video by Sal from Khan Academy on this issue