# What is the speed of smell?

Curiosity Science Chemistry What is the speed of smell?

• If light is of photons and sound is of waves, what is a smell made of? What would be the speed of it? Thanks in advance for any answers.

• Sound is the collision of all particles in the air. Smell is caused by the chemicals in the air. So smell is a wave too. But its speed is dependent on different variables. Different scent chemicals traverse at different rates through air. Mostly depends on Mean Free Path of the molecules in action.

• The smell/scent/odour is due to the chemical nature of the compound that enters our nose. We perceive smell when these chemical compounds react with our olfactory receptors in our nose. I guess you already know this. As Farhan said, the speed depends on various factors. Elaborating on that, It’s basically the diffusion rate of that particular molecule among the gas around you. If the molecules have a high diffusion coefficient, then it will diffuse much faster (but not that fast enough to reach your nose). Again, a molecule can diffuse in many ways.

Diffusivity is not the only factor that governs the speed of smell. Turbulence in the air affects the speed of that particular molecule a lot. Hence, the convective currents play a dominant role compared to the molecular diffusion. This is the primary reason why you can smell stuffs around you. Convection brings you the compounds much faster than diffusion.

So practically, there is no definitive way to calculate the speed of the sound. Interestingly, if we could calculate some of the variables at STP and go by diffusivity, we can predict speeds of different smells relative to each other. As far as I know about this, that’s the closest thing to the speed of smell we can get to.

• Interestingly, if we could calculate some of the variables at STP and go by diffusivity, we can predict speeds of different smells relative to each other.

Could you tell me what are the variables?

• Even if you use Graham’s law, you will still be ending up with an approximate answer only. The system is too complex in an open environment.

• Eliminating any turbulent effects, you would need to calculate the molar mass of the two compounds that creates the smell. Then by simply applying Graham’s law,

$$\large \frac{Diffusion \ Rate \ 1}{Diffusion \ Rate \ 2} = \large \sqrt {\frac {Molar \ Mass \ 2}{Molar \ Mass \ 1}}$$

you can find which the faster smell is. Still, it won’t be that accurate.

Also, if you are so curious, you can analyze the diffusion co-efficient using the formula from the wiki I linked above.

• I think what you meant was ‘effusion rate’. Graham’s law clearly states that it is applicable only for effusion – “the rate of effusion of a gas is inversely proportional to the square root of the mass of its particles.”

• You are right. It’s effusion. Since I’ve already eliminated turbulence, I thought I made it clear about the gas having a similar flow state close to effusion (i.e. no collisions). I should’ve mentioned it. @24gata, I’m sorry that I didn’t mention it earlier. @yukuvt, thanks for pointing it out.

• Thank you all.