In short
Vapor pressure measures how readily a substance enters the gas phase at a given temperature. It is the physics behind the volatility you timed in Level 1. Molecular weight alone misleads, since polarity and hydrogen bonding matter too. And it explains only half of what you perceive, because how strong a smell seems also depends on its odor threshold.
What you will learn
- Define vapor pressure and relate it to evaporation
- Use Raoult's law as a rough model, knowing its limits
- Calculate an odor value from a concentration and a threshold
Before this lesson: What Receptor Research Does and Does Not Explain
What sets vapor pressure
Molecules in a liquid hold on to one another. The stronger these attractions, the fewer escape. So:
- Larger molecules tend to have more attraction, and lower vapor pressure.
- Hydrogen bonding greatly lowers it, which is why alcohols are often less volatile than esters of similar size.
- Polarity generally lowers it too.
- Temperature raises it steeply. A warm day or warm skin can change what you smell.
Published values exist for many materials, but they vary between sources and methods, and they may be estimated rather than measured. Look up specific figures on PubChem and note their temperature, instead of relying on a number quoted without one.
Mixtures: Raoult's law as a rough model
In an ideal mixture, each component's partial pressure is its mole fraction times its pure vapor pressure:
p(i) = x(i) × p*(i)
Note the units. It uses mole fraction, not weight percent, so a line's share by weight must be converted using molecular weight. A worked example: a two-line blend has 10 g of a material with molecular weight 150 and 10 g of a material with molecular weight 300.
- Moles: 10 ÷ 150 = 0.0667, and 10 ÷ 300 = 0.0333.
- Mole fractions: 0.667 and 0.333.
So at equal weights, the lighter material is two-thirds of the molecules. Perfumes are not ideal mixtures, and real volatility shifts with composition and with whatever it is sitting on. So this is a model for thinking about direction, not a calculator for predicting a fragrance.
Why a mixture changes over time
As volatile components leave, the remaining mixture has a different composition, so the rest change their partial pressures too. On a strip or skin, absorption and evaporation also interact. That is why fragrance development is continuous rather than in stages, and it is the reason for the volatility work in Level 6.
Odor value: linking physics to perception
A very volatile material can be nearly odorless, while a barely volatile one can be strong. A common way to connect them is the odor value:
odor value = concentration in air ÷ odor threshold
If a material reaches a concentration of 10 units in the air above a strip, and its threshold is 0.1 of the same units, the odor value is 100. An odor value above 1 suggests that it should be detectable. The ratio has limits as a model of how strong something seems, since perceived strength does not simply scale with it, mixtures interact and thresholds vary between people and studies. It is still a good reason not to judge impact by share alone. The Lab's threshold guide takes it into dosing, and you saw it directly in your own threshold exercise.
What you can do with this
- Predict rough timing from a structure, then test it on a strip.
- Explain a gap in a formula's evaporation.
- Expect a formula to smell different in a hot room.
- Stop treating weight share as the same as smell share.
Practical exercise: Look up and rank five materials
- Choose five single molecules from your library that vary in type, such as limonene, benzyl acetate, linalool, phenylethyl alcohol and ethylene brassylate.
- Look up a vapor pressure for each. Note the source, the temperature and whether it is measured or estimated.
- Rank them, and compare with your own strip timings.
- Note where the two rankings disagree, and suggest why: strength, threshold, a difference in substrate or a poor figure.
- Calculate mole fractions for a 1:1 weight blend of two of them, using molecular weights from the same source.
You have done this well if every figure has a source and temperature, and your disagreements have a suggested explanation you could test.
Common mistakes
- Using molecular weight as volatility. Compare an ester and an alcohol, and it fails.
- Quoting vapor pressure without a temperature. It is not a fixed number.
- Treating odor value as perceived strength. It is a useful ratio, not a measure of intensity.
Key terms
- Vapor pressure
- The pressure of a substance's vapor in equilibrium with its liquid at a given temperature. Higher vapor pressure means faster evaporation, all else equal.
- Raoult's law
- An ideal-solution model: a component's partial vapor pressure equals its mole fraction times the vapor pressure of the pure material.
- Odor value
- A material's concentration in air divided by its odor threshold. Above 1, it is in principle detectable.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.