Re: Can you measure how fast an odour travells in air?

Robert:

That is an interesting question. The process you are describing is called diffusion, and if we accept a few postulates about gases moving and diffusing, we can actually make extremely accurate determinations of the rate and equilibrium of the diffusion. To answer your brief question, diffusion is extremely temperature dependent, as with increasing temperature, the individual molecules have a higher average kinetic energy, and they move faster, so the same amount of mixing will occur in a shorter period of time.

The definition of diffusion, as defined by the now ubiquitous Atkins Physical Chemistry is: The migration of matter down a concentration gradient.

What this means is that something will always move from an area of higher concentration, to lower concentration. When you squirt perfume in a corner, the concentration there is high, whereas the concnetration in the rest of the room is low, so the individual molecules diffuse to occupy the space equally.

The equation that roughly determines diffusion is given by:

				D=1/3 * Lambda * mean c   m-1 s-1

This means that diffusion decreases with increasing pressure and increases with increasing temperature. Diffusion is also greater for smaller molecules (e.g. with a smaller cross section.)

There are many derived equations, and integrals that physicists can use to determine many properties of diffusing gaseous molecules, particularly if we accept that the molecules are ideal, that is we ignore size, friction, kinetic losses and accept that they are in ceaseless random motion.

Ultimately, of general interest, is the limit of diffusion. That is, no matter how fast you move something or heat it up, there is a physical limit as to how fast the molecules can diffuse. Viscosity, temperature, pressure, molecular size and reactivity all affect the limit of diffusion. In a perfect scenario, the limit of diffusion is about 1.3*10^10 L/mol second.

If you would like derived equations for calculating the rate of diffusion of some substance at a particular temperature, consult a general chemistry or physical chemistry book.

Good luck

-Matt-