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Name: Phil
Status: other
Grade: other
Location: IL
Country: USA



Question:
Since matter is composed of chaotically, rapidly moving particles, why do solids keep their shape? Why do they not "evaporate"? A corollary question - If you clamp two pieces of wood together, why do they not join into one piece?


Replies:
Phil,

Solids do sublime (the equivalent of evaporation in solids). Think of dry ice and moth balls. The reason we do not observe most solids sublime is that the energy that has been transferred in order to get to room temperature is not high enough to allow most of the particles in the solid to escape from the bulk object.

Look up a graph called "Distribution of speeds" (it might show up as distribution of particle speeds or distribution of kinetic energy). The represents the distribution of particle motion at one particular temperature. Thus, evaporation (of liquids) means that a certain percentage of particles are fast enough (have enough kinetic energy) in order to escape the liquid phase and enter the gaseous phase. Draw a vertical line on the far right of this graph and call this the escape velocity - particles to the right of this line will be fast enough to escape (evaporate).

Now imagine that for a solid, this graph is skewed to the left (lower speeds), there will now be fewer particles which have the speed to escape. Thus, while it is true that particles are in motion, they do not have enough motion to break the solid-solid interactions in order to enter the gas phase.

The same reasoning can be used for why wood does not bond. While solid particles do move around, they move around in place. In other words, they vibrate. As such, there is no translational motion, just a vibration that essentially retains the same center of mass for each particle, despite the motion.

Greg (Roberto Gregorius)



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