Wednesday, February 2, 2011
Can't Wait To Sublimate
These patterns of frost on my windshield this morning reminded me of a group of interesting words: sublimate, sublimation, sublime, subliminal. These words can take us into the realm of psychology, poetry, manners, etc., but I'm thinking of a scientific sense.
Have you ever seen frost form right before your eyes? Better yet, have you ever seen it disappear before your eyes? I don't mean melt, but rather disappear into "thin air." When a solid (such as ice) changes directly into a vapor without first melting (that is, passing through a liquid state), it's called sublimation. On a frosty morning, if the air is dry enough (low relative humidity), and the temperature stays below the freezing point of water, the frost will gradually disappear into the air. In other words, it will sublimate. This morning, I couldn't wait for that to happen. Had to get to work. Plus, it's possible the temperature would have soon risen above 32 degrees Fahrenheit and then the frost would have melted.
It is even more fascinating to watch frost form in the first place. We don't get many opportunities to see this because it usually happens at night and greets us in the morning. However, on more than one occasion I have been on a mountain top, such as Lassen Peak, when conditions were just right to watch frost form before my eyes in broad daylight. When a mass of relatively humid air flows into a mountain whose local temperature is lower than the air mass, the moisture in the air will condense directly into ice, especially on rocky surfaces. On one climb up Lassen in late fall, around the 9,500' mark, the rocks were already covered by huge ice crystals, some 6" long. With my gloved hand, I brushed a bunch of these off a rock and watched as they immediately re-formed - or, rather, new ones formed in their place. If memory serves, they'd grow an inch every couple of minutes until they were about as big as the originals. It was so fascinating, my kids and I repeated the experiment several times.
This same phenomenon is notorious for sinking ships in the Grand Banks fishing area off the East Coast. The ice can form on a ship's rigging so fast that literally tons can accumulate faster than the crew can knock it loose. It's called rime ice.
Iodine is an example of an element that doesn't exist in liquid form. If you heat the solid crystals with a Bunsen burner, they will sublimate - that is, change directly into a brownish-purple gas. If you think you've bought liquid iodine, it's actually an aqueous solution of an iodine compound.
I'm still trying for the ultimate frost photo. I'll be ready tomorrow morning!