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

Nov 29, 2016

Perhaps because you haven't specified a pressure?

#### Explanation:

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

But we could perform a vacuum distillation, such that at LOWER temperatures than the $\text{normal boiling point}$, 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 $\text{shure}$. Are you happy with this. I would certainly try to take this definition of $\text{boiling point}$ 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 ${100}^{\circ} \text{C}$. If it's ammonia, then the boiling point is negative!