Diffusion states that a solute moves from an area of high concentration to an area of low concentration until the solute is evenly distributed through the solution.
So when I make hot tea and the sugar melts, why does the majority of sweetness stay at the bottom instead of diffusing evenly through my tea? I know for a fact that the sugar is completely melted because when I finally empty my mug there is no grit at the bottom.
It doesn’t make that big a difference in this setting - the diffusion coefficient is slightly temperature dependent, but the liquid will cool to ambient much faster than any effects of the high temperature will be seen.
Temperature does control convection, though. If the surface cools faster than the liquid below it, for example, the increase in density on the surface will cause the warmer water below it to rise, displacing the cooler water and mixing the mug. Convective mixing distributes the dissolved sugar by moving the liquid that contains the molecules rather than the molecules themselves, and is usually a much faster and stronger process than diffusion.
ETA: The other thing the temperature controls is the rate of dissolution. If you’re dumping sugar and letting it dissolve slowly, that will happen faster in a hot mug than a cooler one. But again, that’s a faster process than diffusion is.
I may be mixing up (heh) two different notions, but don’t weight and gravity produce a separating effect, partially countering diffusion? I thought that was one of Einstein’s biggies, where he figured out the actual energy (?) of Brownian Motion, by watching the two forces in competition. Heavier molecules want to sink to the bottom, but Brownian Motion stirs 'em up (and upwards) again.
Is this something that applies to objects suspended in water (like, say, pollen grains) but diffusion applies to chemicals actually dissolved in water, in which case I’m wrong and never mind?