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Re: [Phys-L] Re: Sizes of atoms (was evidence for non-classical behavior)



On 07/10/05 14:17, Dan MacIsaac wrote:
Classic QUALITATIVE activities for elementary science teachers
providing supporting evidence of small particle nature of matter
include observing:

[1] drops of food colouring dissolve in hot and cold water in large
beakers or fish tanks
[2] opening a bottle of perfume near the door of a large room; have
students scatter throughout room and raise hands as they smell the
perfume
[3] watch sugar cubes and ice cubes dissolve in water (who says watching
stuff melt is boring :^)
[4] cold metal spoon held over the spout of a teakettle; also watch fog
form on cold mirror in bathroom while someone else showers (or in hall
after bathroom door is opened)
[5] ice melting in sealed baggie on digital balance; repeat same with
chunks of dry ice (solid CO2). Watch scale reading till baggie pops
seal for CO2.

This the sort of list I was hoping for.

I know these are billed as QUALITAIVE, but let's think about
what that means. For items [1], [2] and [3], advection will
lead to an overestimate of the mean free path, hence an
underestimate of the size of the particles. That's not terrible;
there's real value in having a nonzero lower bound.

Dye diffusing into water in a 1cm by 1m glass tube should
have much less trouble with advection. Such tubes should be
readily available in the chem stockroom. I like this idea
a lot.

I don't understand the teakettle method. What's the observable?
What's the connection to kinetic theory? Steam comes jetting
out of the teakettle ... don't "ballistic" advection effects
completely overwhelm any diffusion effects?

I don't understand the baggie-weighing method. How is the
observable connected to the size of atoms? What role does
condensation of atmospheric humidity play in the method?
(Is that the point, or is that a nuisance-effect?)

I just thought of the following variation: Glass tube,
1cm by 1m, with a smallish amount of water at the bottom.
Top is open, but surrounded by a box containing plenty
of dessiccant (anhydrous calcium chloride, calcium oxide,
silica gel, or whatever).
_______________
| |
| |
| dess | | dess |
|_____ | | _____|
| |
| |
| |
| |
| |
|W|
|W|

Observe how fast the water evaporates. Diffusion of water
vapor through air. Should take a rather long time. Repeat
with different-length tubes, to control for end-effects.
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