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If I were to have two separate containers with solutions of different concentrations with a small opening, most of the molecules would flow down the concentration gradient. Does this occur just because more molecules can pass through the opening or is there a force pushing them out?

My second question is if the molecules could pass through the opening under normal conditions even against the gradient?

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All particles are constantly moving randomly due to their kinetic energy, and are thus constantly colliding into each other and the walls of their container (if any).

Particles in gases move rapidly and randomly and are spaced far apart, particles in liquids slide slowly over each other, and particles in solid vibrate around a fixed position.

Hence, diffusion can be defined as the overall movement of particles, from an area of higher concentration to an area of lower concentration, down a concentration gradient, due to their random motion.

Illustration of particles in a gas:

random motion of particles in a gas

Source: wikipedia.org/wiki/Kinetic_theory_of_gases#/media/File:Translational_motion.gif

This explanation relies on Kinetic theory and Brownian motion.

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  • $\begingroup$ So it’s just a random event when a particle goes down it’s concentration gradient? $\endgroup$ Oct 23 '20 at 20:22
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    $\begingroup$ @igorbernat For 1 particle it is random, for the huge set it is statistics. $\endgroup$
    – Poutnik
    Oct 23 '20 at 21:34
  • $\begingroup$ @IgorBernát Be aware molecules are by no means aware there is anything like concentration gradient. $\endgroup$
    – Poutnik
    Oct 24 '20 at 19:42
  • $\begingroup$ Yeah, this is the thing I’m confused about. How do molecules ”know” they are doing something that’s entropically positive? $\endgroup$ Oct 24 '20 at 20:15
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    $\begingroup$ @IgorBernát they don't. They eventually move from an area of high concentration to low concentration due to their random movement. This is why diffusion is such a slow process. $\endgroup$ Oct 25 '20 at 10:00

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