Odors are caused by small, volatile compounds
that trigger olfactory receptors in your nose.
The molecules that cause scents can usually be
detected in extremely small concentrations, down
to parts per billion (ppb).
When you smell perfume on a person or by
wafting perfume from a bottle, you are smelling
a vapor of the scent compounds that have become
volatilized (are in the gas phase) and diffuse
through the air and into your nose.
Perfume is a mixture of fragrant oils in an
ethanol/water solvent. When you spray perfume,
you create a fine mist of perfume droplets
suspended in the air. The droplets are usually
between 30 and 150 micrometers in diameter
depending on the sprayer used. The ethanol/water
mixture, which is volatile, evaporates from the
droplets within a few seconds, leaving behind a
droplet of the fragrant compounds in the
perfume. These compounds will also eventually
evaporate to form a vapor of the fragrant
molecules.* The vapor diffuses through the air,
spreading the scent.
Whether you are smelling the droplets or the
vapor probably depends on when you spray the
perfume. If you smell the perfume immediately
after spraying it, you are most likely smelling
the droplets. If you smell it from across the
room after someone sprays it, you are probably
smelling the vapor. How quickly the transition
occurs depends on the ethanol/water
concentration as well as which fragrance
molecules are present in the perfume and their
When you smell popcorn from the microwave,
you are smelling the vapor of popcorn scent
molecules (e.g. 6-Acetyl-2,3,4,5-
tetrahydropyridine from the popcorn itself and
diacetyl and acetoin from the butter). Heating
the popcorn volatilizes the scent molecules and
they diffuse into your nose.
*Most likely this process is not quite as
well-defined as I have described it and some of
the scent molecules will evaporate as the
ethanol/water mixture does.
James F. Davies, Allen E. Haddrell & Jonathan P.
Reid (2012): Time-Resolved Measurements of the
Evaporation of Volatile Components from Single
Aerosol Droplets, Aerosol Science and
Technology, 46:6, 666-677.
Rowe, David J. "Chapter 13." Chemistry and
Technology of Flavors and Fragrances. Oxford:
Blackwell, 2005. Print. Accessed at Blackwell
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