Diffusion Lab - Introduction.


In the scale of atoms and molecules, most of the world is made of space, through which these tiny particles fly, interacting through collisions or the weak attractions and repulsions that make up most “fly-bys.” If you add heat to a system, the energy makes that particles move faster, collide more often and spread themselves out more; cooling makes the particles slower and eventually more crowded - these translate to the expansion and contraction that go with heating and cooling. Whether they move fast enough to be completely independent of one another determines to a large extent whether the nature of the material is solid, liquid, or gas. Liquid or gas particles may bounce off the packed layer of a solid barrier. Liquids may interact with gases in ways that make the interface a bit of a barrier - for instance, the way that water molecules orient themselves and interact at a surface packs them more tightly together there and produces surface tension, a region denser and harder to get past than the rest of the water. To get free of attractions among the liquid and past the surface tension, molecules must be going very fast / be very warm - when water evaporates, it loses its hottest molecules, which is why the evaporation of sweat is a cooling process.

Particles in a solid are mostly locked together, although some evaporation may occasionally occur. Liquids’ particles move fairly freely but are fairly close together - evaporation is affected by the speed of the particles and how much they “hang on” to each other. Gas particles are freely moving and hardly interact at all beyond occasional collisions. Allowed to move freely, particles will spread out until they bounce off a barrier, and will spread out until evenly distributed within that barrier - this is the basis for how diffusion works.

Diffusion in gas can be observed by opening a bottle of bleach in a room - the chlorine atoms that have evaporated from the bleach are free to move into and around the room, and will do so - eventually, anyone who enters the room will smell the bleach. As long as the bottle acts as a source of the smell, the differences in chlorine concentration would let someone track the smell to the source; however, if the bottle was closed, the room would still smell like bleach, but the distribution of chlorine would balance out and you could no longer tell where the bottle was by tracking the smell - the chlorine atoms would have moved, in general, from the concentrated area around the bottle to the less concentrated area of the rest of the room. Diffusion is the net movement of particles from a higher-concentration to a lower-concentration area, until the concentrations become equal.