Boiling point is the temperature at which a substance turns completely into vapour at atmospheric pressure. It sounds like a laboratory number, unrelated to smelling a strip of paper at room temperature.
But it is closely related, and understanding why is the foundation of this whole part.
Evaporation does not need boiling
At any temperature, some of the molecules at the surface of a liquid always have enough energy to escape as vapour. That tendency is called vapour pressure.
A substance with a low boiling point has a high vapour pressure at room temperature — many molecules escape every second. A substance with a high boiling point is the opposite.
So boiling point is an indirect indicator of how fast a substance leaves a smelling strip. It is not a direct measurement, but the correlation is close enough to use.
What decides boiling point
Two factors.
Molecular mass. Heavy molecules need more energy to escape. Fifteen-carbon beta-caryophyllene boils at 264 degrees; ten-carbon limonene boils at 176.
Polar functional groups. Hydroxyl, carbonyl and carboxyl groups form hydrogen bonds between molecules, holding them together. This factor is often overlooked but is no weaker than mass.
The clearest comparison: limonene and linalool both have ten carbons. Limonene has no functional group and boils at 176 degrees. Linalool has one hydroxyl group and boils at 198. The twenty-two degrees of difference come from exactly one functional group.
An extreme case
Acetic acid has only two carbons against limonene’s ten, and weighs less than half as much — yet it boils at 118 degrees, not below zero. The reason: the carboxyl group forms double hydrogen bonds, and acetic acid molecules usually travel in pairs.
Conversely, naphthalene, ten carbons with no functional group, sublimes at room temperature. The reason is not a loosely packed crystal — naphthalene crystals pack fairly tightly. The reason is that the only force holding the molecules together is weak dispersion, with no hydrogen bonds, so the vapour pressure of the solid at room temperature is still high enough for it to pass straight from solid to vapour.
How to use this number
On this site’s compound pages, each compound has a marker showing its position on the 150–290 degree axis. On dictionary entries and scent-family pages, the components are drawn as dots on the same axis.
One look at a band tells you whether a smell opens quickly or slowly, and how long it lasts. The citrus family crowds to the left; the woody and resinous family crowds to the right.
That is the whole meaning of the number: it is the clock of a smell.