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OneTreePhil t1_j39rlya wrote

I remember calculating this in a ChemE class. If a bath tub of water no currents at all in it, any given molecule at one end would take something like six million years to diffuse to the other end. Any bulk current at all reduces the time to tens of seconds.

Please someone check me? It was an in-class problem in 1984 thanks for the memories Dr. Knickle & Coach

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purpleoctopuppy t1_j3a1c6u wrote

Diffusivity of water is on the order of 10⁻⁹ m² s⁻¹. Bath tub is on the order of a metre long, so it should take t ≈ L²/2D ≈ 10⁹ s. Since a year is e7.5 s, this means it would take on the order of 30 years.

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