Chemistry of rain

The rain over the sea

Close your eyes… remember when you were young and outside was raining… remember the scent of rain on dry soil… that particular scent has a name: petrichor.

The term “petrichor” was first used in 1964 by scientists Isabel Bear and Dick Thomas and it derives from the Greek words “petra” (rock) and “ichor” (the fluid that flows in the veins of the gods in Greek mythology).

Petrichor is caused by a combination of different molecules, let’s have a closer look.

The first molecule responsible for petrichor is geosmin. Geosmin has an earthy arome and it is produced by actinomycetes, a group of bacteria living in the soil.

Earth atmosphere is composed of nitrogen (78%), oxygen (21%), argon (1%) and traces of other molecules.

Electric discharges from lightning can cause the disruption of the diatomic molecule of oxygen of air and induce the formation of ozone. Ozone molecules are carried down to the soil by raindrops and contribute to the pungent scent of petrichor. Do you know the feeling of smelling the air and say “It is going to rain”? Well, that sensation is due to ozone. This molecule is transported by the wind for miles and anticipates the rain.

Finally, the last two molecules are palmitic and stearic acid. These are oils produced by plants in dry periods. During rain, these molecules are released from the soil to the air adding the typical earthy smell to the petrichor.

Do you like the smell of rain?

References:

Bear and Thomas, “Genesis of petrichor”, (1966), https://doi.org/10.1016/0016-7037(66)90025-1

“The smell of rain”: https://www.compoundchem.com/2014/05/14/thesmellofrain/

“Petrichor”: https://pin.it/294e6no

“Che cosa causa l’odore della pioggia”: https://www.ilpost.it/2014/05/29/profumo-pioggia-odore-terra-bagnata/

Published by Martina Bodner

Biotechnologist, PhD Candidate in Food Chemistry, Science Teacher, STEM and autism advocate, mother.

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