Eli5 – would it take more time/energy to boil a pint of frozen water or freeze a pint of boiling water?

Question

Eli5 – would it take more time/energy to boil a pint of frozen water or freeze a pint of boiling water?

In: 2

Answer 1

So this isn’t a great question. It takes energy to boil, it does not take energy to freeze

You remove energy to freeze, not add. The amount of energy to add and the amount of energy to remove are equal in your scenario, but going in opposite directions.

Heat transfer is generally proportional to temperature difference and surface area. Assuming the heat transfer is equal in both scenarios then time will be equal (ignoring the fact that one way may be slightly off depending on how if you count the phase change or not). However, it’s significantly easier to create a positive temperature differential to add heat than it is to create a negative temperature differential to remove. So, using conventional methods like a standard kettle & freezer the kettle will easily boil the frozen water before the freezer can freeze anything.

To illustrate, a freezer might run at -20C (and probably not even that cold). So 20 below freezing. A gas hob or electric kettle can easily reach several hundred C, so several hundred above boiling. A bigger temperature difference gives bigger heat transfer, and faster results.