I'm going to start a new monthly feature called Science Fun. It will deal with actual science and not math, but of course, since most things in science are explained with math, numbers will be involved. The first topic is the triple point, an interesting phenomenon I never heard about when I was in school. (Full disclosure: I took no physics or chemistry in college, which may be why I hadn't heard of this. I wrote to several friends and one American heard about it vaguely in a high school class, while an Australian friend remembered the topic being covered extensively.)
By the time we are in first grade, we are expected to know about solids, liquids and gases. We also know that while some things burn when exposed to a high enough heat, water can both be turned into a gas we call steam or water vapor when heated and turned into a solid called ice when cooled. These temperatures are 32 degrees and 212 degrees Fahrenheit, or 0 and 100 degrees Celsius respectively, and anyone with a refrigerator and a stove top has seen all forms of water before even attending school. Later, we learn that metals can melt at very high temperatures and gases can be made liquid at very low temperatures.
Your teacher may have told you that the boiling point for water changes depending on altitude. For example, in Denver, at over 5,000 above sea level, the boiling point is closer to 202 degrees F. or 94.5 degrees C. The real cause of the change is the change in pressure. As the pressure drops, the boiling point drops noticeably and the freezing point rises almost unperceptably. Since the high number is coming down and the low number is going up, do they ever meet?
As we see on this diagram nicked from robbwolf.com, they do meet at Point A, Which is called the triple point. The temperature ever so slightly above 0 degrees C., and the pressure is at .00603 atmospheres, or about 1/166 times less pressure than we experience at sea level. It's not a true vacuum, but a living organism would not survive long at this low a pressure. Beside not being able to breathe so little air, our bodies are used to much higher pressure and would begin to burst, like what happened to Arnold Schwarzenegger on Mars in Total Recall. To give you an idea, the air outside a plane at 35,000 is at about 1/5 normal atmosphere, and that is very thin air indeed.
It's a matter of semantics whether we can say that water at the triple point is solid, liquid and gas all at the same time or it is none of the above. The standard is to say all three at once. If we keep water at below 0 degrees C. and lower the pressure farther and farther, the ice will turn to vapor without ever being a liquid during the transition. This process used to remove water is known as freeze drying, which is used for coffee crystals and drying some foods eaten by hikers. I don't drink coffee, so I don't know if freeze-dried coffee is really any improvement over the taste of other instant coffees. I do recall that several diners at a fine restuarant loved Folger's Crystals, but it can be scientifically proven that some measurable percentage of people in any taste test are freaking idiots. Another example of this can be found in your freezer if you leave ice cubes in a tray for a long period of time. Any shrunken ice cube has had some of the solid ice turned into water vapor without ever being water. One of the physical properties at work in a freezer is lowering temperature by keeping the volume constant while lowering the pressure. It never gets near the pressure of the triple point, but even so, it can cause some phase shifting.
I think the triple point concept is pretty cool. (You don't have to pardon the pun if you don't want to.) It works for any simple chemical compound that can freeze or boil, but each compound has its own pair of temperature and pressure values at which it becomes solid, liquid and gas all at the same time.
As my father is fond of saying, you learn something new every day if you aren't careful.