Sublimation 1/12/22

Science with Richard Bleil

We’re all familiar with the three major forms of matter, solid, liquid and gas (although there are others, such as plasma and the glassy state).  Chemists, of course, are not only interested in the basic matters of state, but the processes in which one converts to another.  Some of these processes are also well-known, such as conversion from solid to liquid (melting) and from liquid to gas (evaporation).  If you think from liquid to gas as “boiling”, you get partial credit (what can I say; I was a professor).  See, boiling specifically occurs at the boiling point which varies from chemical to chemical and is dependent on pressure.  Boiling occurs at the maximum temperature for a liquid, and only at that temperature based on what you are boiling and the pressure. 

Actually, boiling is kind of interesting.  It’s based on the liquid’s vapor pressure.  Highly concentrated alcohol feels cold to us because its vapor pressure is so very high.  Water has a relatively low vapor pressure, unusually low, in fact, because of hydrogen bonding.  As we heat a liquid, it’s vapor pressure grows with the temperature, until it reaches a very specific temperature where the vapor pressure of the liquid equals the vapor pressure of its surroundings (atmospheric pressure).  At this point, the temperature of the liquid cannot increase further (at least not without some chemical tricks), and it begins to boil.  As we were applying heat below the boiling point, that heat was going to raising the temperature of the liquid, but at the boiling point the heat is used solely to convert from liquid to gas, creating vapor bubbles of the chemical inside of the liquid that we know as “boiling”.

But the thing is, liquids will evaporate even below the boiling point.  This is why puddles, for example, dry up, and why I have to keep adding water to a demonstration I’m working on.  On a molecular level, liquids evaporate when two things occur.  First, understand that the kinetic energy of the liquid is dependent on temperature.  The higher the temperature, the higher the mean (or average) kinetic energy.  But this is just the average; much like vehicles on a highway, some have randomly higher kinetic energy (speed), and some lower.  So in evaporation, a molecule has to have enough kinetic energy to leap from the liquid to the gaseous state, but if it’s deep inside of the liquid, it still won’t evaporate.  It must also be on the surface of the liquid.  So when a molecule is on the surface of the chemical and randomly has enough kinetic energy, it’ll leap into the gaseous state which leads to evaporation.  Obviously, the higher the temperature (or with additional energy sources such as the sun), and the larger the surface area of the liquid, the faster the evaporation.

Most of us are familiar with sublimation.  The same rules apply.  Sublimation is the process of going from the solid state directly to the gaseous.  For this to occur, the molecules at the surface of the solid must have randomly enough kinetic energy to make the transition from solid to liquid.  It’s all a game of chance, but add sunlight to the equation, and that can provide the energy for sublimation as well.  Perhaps you’ve noticed snow disappearing even if the temperature never gets above freezing, especially on sunny days.  This is sublimation.  I went for ice the other day, but my trays, once full, are now empty.  Again, this is sublimation.

Let’s talk about some of the transitions with which you may not be as familiar, although you might know some, or even all, of them.  For example, you probably know condensation.  You’ve seen this, especially on a cold glass on a hot and humid day.  Condensation is the process of going directly from the gaseous state to the liquid.  Freezing, of course, is going from liquid to solid below the freezing point, but there is the possibility of gas going directly to a solid.  This is called “deposition” and is the principal process behind the formation of geodes.  Cavities trapped deep inside of the earth eventually will become stone with certain minerals within them such as silicates.  These silicates undergo the process of sublimation and become a gas.  Eventually, they’ll find the wall of the cavity and recondense in the solid (or, more specifically, the crystalline) state in the process of deposition.  This takes millions of years until, eventually, the cavity is filled with beautiful crystals that have literally grown out of the walls of the cavity.

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