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Name: Tahmil
Status: student
Age: 6-8
Location: NJ

Country: N/A
Date: 1/17/2005


Question:
What is diffusion in liquids and how does it happen?


Replies:
Hi Tahdmil!

Diffusion is mainly gases property. The kinetic theory predicts the ability of two or more gases to intermix or dissolve in each other in all proportions Diffusion is shown also by liquids but much less. Gases and liquids are fluids, that is they can flow easily, the gases more than the liquids. We can grossly imagine that fluids have as much of occupied space as empty space (or "holes") into their volume. Diffusion of a gas (or liquid) into a liquid corresponds to the occupation of these empty spaces. depending upon the liquid viscosity, temperature and other factors.

Thanks for asking NEWTON!

Mabel
(Dr, Mabel Rodrigues)


If you had a cup full of slippery rubber balls, and you were always shaking it enough so the balls were slightly bouncing around, occasionally the single red ball in the bunch would slip between a momentarily large gap between two other balls. Then it would have a new home. Eventually, this red-marked ball will get to every place in the cup.

The balls represent atoms, which are often both elastic (rubbery) and slippery. The bouncing is just about like heat. The "atoms slipping past each other" is diffusion.

This slipping happens rarely and slowly in solids. You can imagine that only one atom at a time is lucky enough to slip to a new position. So the solid mostly keeps holding its shape. Slipping happens very often and fairly fast in liquids. Significant percentages of the atoms in a liquid are slipping past each other at any given time, so the whole mass can deform smoothly in response to small pushes.

In this conceptualization, what holds these balls close together is gravity and the cup. With this kind of confinement, it is hard to imagine a sharp distinction between solid, liquid, and gas-like states.

To clearly imagine solid, liquid, and gas, you should imagine that each ball has a "magnet" in it, attracting all the neighboring balls with a modest pulling force that does not reach very far. So the liquid holds itself together in the presence of a certain amount of bouncing around. It can exist without gravity or the cup. If the bouncing is much weaker, the balls settle closer together and form a rigid stack of balls, which is much like solids do. If the bouncing is much stronger, the balls mostly escape far from each other's attraction and just bounce off walls and each other sporadically. That is like a gas.

If you viewed an exhibit having a big mass of such balls, if there was enough bouncing to be "liquid", the mass would look rather "squirmy". No wonder that things eventually move around in a liquid. If the bouncing was weak enough to simulate the solid state, the mass would look jittery but your eye could tell that it was mostly staying in place. Then it might be interesting to watch and wait for occasional "hops" and "trades". You would even notice a few "holes", spots with a ball missing, migrating around a little and not getting filled in. The scientific word for these holes is "vacancies". Vacancies are created and destroyed mostly at the surfaces of the mass. In the solid they slip around faster than atoms, so they help diffusion.

Most substances expand when they melt. You can think of it as a solid being invaded by about 10% "holes", enough to be lubricated to liquid-like motion.

You can see that diffusion will happen in all three states, at appropriately differing rates.

Jim Swenson


Tahmil,

Diffusion is a property by which gases or liquids mix spontaneously because of the random motion of their particles. I hope that this helps.

Sincerely,

Bob Trach



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