Carbon Dioxide (Or, conversations with your roommate's BF)

After dinner last night, my roommate, her boyfriend, and myself were sitting around the kitchen--drinking tea and procrastinating on the dishes. I'm uncertain how we reached the topic, but a question was posed to my roommate (also a scientist) and myself:

"Why do we call Dry Ice "Dry Ice?" It's not actually water."

Now, I'm sure many people have seen dry ice, either in Halloween fog machines, or in a special freezer at the grocery store. For those who haven't seen it, dry ice is the solid form of carbon dioxide (CO2). The same gas that you exhale, and that gives soda its fizzy taste, also makes an extremely useful coolant in the solid form. "Dry Ice"as it is so called, is dry because it does not melt and leave a puddle. Instead, it converts directly to carbon dioxide gas. To understand this, we need to talk about phase changes. 

A phase change is the process by which a state of matter is altered. We talk about solids melting, like ice cream on a hot day. Or vapor condensing to liquid, like dew onto grass in the evenings. To illustrate my point further, I'm going to use two examples: water and carbon dioxide. Let's start by looking at the different phases of water--which people are generally more familiar. 

Figure 1: Phase diagram of water (1).

For water, we are accustomed to seeing three different phases. The solid form (ice), liquid (water) and  gas (steam). At 1 atm (for non-scientists, this is approximately the pressure at sea level), water exists as a solid below 273K (0ºC) and as a gas above 373K (100ºC) (2). As we change pressures or temperatures, different phases are favored. One special point on the graph above is called the triple point--this is the temperature and pressure at which we observe all three phases at once. For water, the triple point is 273.2K and 0.61 atm. 

Now let's consider dry ice. 

Figure 2: Phase diagram of carbon dioxide (3). The scale on this y axis is in bar (1 bar ~ 1 atm). 

For carbon dioxide, at normal sea level pressure, we commonly observe two phases: the solid (dry ice) at temperatures below 195K (-78ºC) and the gas (carbon dioxide gas, CO2) at temperatures above that value. In other words--at normal, everyday pressures and temperatures, dry ice sublimes. It skips the liquid phase, instead transitioning directly from the solid phase to the gas phase. It is the lower temperature of the solid, combined with the fact that there is no residue to clean up, that makes dry ice an effective coolant. And the triple point? The place where we observe all three? This point exists at -56.4ºC, and 5.13 atm. We will never see liquid carbon dioxide at room temperature, unless it is under exceptional circumstances (4).

Phase changes are important in chemistry, where we often exploit them to drive reactions in a favorable direction. But they are also important in geology and biology. Though I've touched on it only briefly here, the unique phase diagram we see in water is perhaps one of the reasons life even exists on this planet. 

References:

1. http://www.kmacgill.com/documents/phase-diagram.gif

2. An over-simplification, true.

3. Wikipedia entry on "Dry Ice." Accessed April 8th, 2013. 

4. Hanson and Green. Introduction to Molecular Thermodynamics. Sausalito, University Science Books 2008. Chapter 12 has a experiment that will allow you to observe liquid carbon dioxide. Note: a blast shield and appropriate safety gear is required!