Question

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

Answer 1

Perhaps because you haven't specified a pressure?

Explanation:

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.

Answer 2

It changes with change in pressure.

Answer 3

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!