Freezing by Boiling
Martian surface pressure is near 6 millibars, occasionally the temperature rises to 0 °C. This combination of temperature and pressure is called the triple point of water. At the triple point, a container can contain water as solid and liquid and gas forever. You can create these conditions in your classroom with a vacuum pump.
Cut the bottom out of the Styrofoam cup.
Put the cup upside down in the bell jar.
Cover the top with a loose layer of Saran wrap.
Put a tablespoon of water into the Saran wrap.
To Do and Notice
Turn on the vacuum pump.
Watch the water. At first the water will "boil."
This boiling is a combination of boiling and out gassing.
Then nothing will happen for a while.
Finally the water will bubble again slowly, it is boiling, then it will freeze.
Open the bell jar and remove ice from the cup.
What's Going On?
When the pressure on the water is reduced it can boil at lower temperatures. When water boils, the fastest, hottest water molecules leave first. Left behind are the slower cooler molecules. Boiling is a cooling process. At Martian pressures near 6 millibars water will boil at 0 °C. Thus, under Martian conditions water can boil and have ice cubes in it at the same time.
Boiling and evaporation are different processes. Evaporation can only occur at the surface of a liquid. It can occur at any temperature and pressure. Boiling can occur within a liquid. The transition from liquid to gas in boiling creates bubbles within a liquid. This transition from liquid to gas occurs at a precise temperature at any given pressure. This temperature is the boiling point. When there is less pressure squeezing down on the surface of a liquid, the liquid can boil at lower temperatures.
In very clean liquids the temperature of the liquid can exceed the boiling point without the formation of gas bubbles. This liquid is said to be superheated. It is extra difficult to start to form a small bubble, just as it is most difficult to start inflating a balloon. Once the balloon begins to inflate it becomes easier and easier to blow it up. The same is true with gas bubbles in a liquid. If there are gas pockets already present then boiling will occur at the boiling point. Otherwise the liquid will superheat without forming bubbles.
At pressures under 6 millibars water cannot exist as a liquid. It becomes "dry ice" just like carbon dioxide is on earth. Water goes directly from the solid state to the gas state. The surface pressure on Mars ranges from 3 to 10 millibars. The triple point is actually 6.11 mb and 273.16K.
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Scientific Explorations with Paul Doherty
10 October 2003