Why is it incorrect to say that the boiling point of a substance is 100 C?

3 Answers
Nov 29, 2016

Perhaps because you haven't specified a pressure?


We usually quote the #"normal boiling point"#; i.e. the temperature at which the vapour pressure of the solution reaches #1# #"atmosphere"#, and bubbles of vapour form directly in the liquid. This is the #"normal boiling point"# by definition.

But we could perform a vacuum distillation, such that at LOWER temperatures than the #"normal boiling point"#, the vapour pressure of the liquid is such that it is equal to the ambient pressure, and it distills over.

I grant that the pressure here is likely one atmosphere (and that the substance is water), BUT we don't KNOW this for #"shure"#. Are you happy with this. I would certainly try to take this definition of #"boiling point"# on board, especially if you are an undergrad.

Nov 29, 2016

It changes with change in pressure.

Nov 29, 2016

Because you have no idea what the substance is.

A boiling point is intrinsic to what the substance even is. If it's table salt, then the boiling point is much higher than #100^@ "C"#. If it's ammonia, then the boiling point is negative!