Would You Like Ice in Your Drink?

Q: “Would you like ice in your drink?
A: “Yes, but make it ice Ih, please.”

Many of us may recall our high school or college physics class, and learning about water and its physical characteristics. We learned from our instructors that it has three phases: liquid, ice, and vapor; and that it has a triple point: a single specific pressure and temperature at which all three phases of water can coexist, any of which can abruptly change from one state to the other.

They lied.

As vacuum engineers, many of our applications deal with water in one of its forms. It may be drying, distilling, or packaging, to name a few. Thus understanding its characteristics is important.

Water is a very complex compound, and we are still discovering new things about it. For example, we now have identified at least 14 phases of water, not just the three we were taught about in school. Other than the liquid and vapor phases, all other phases are of different forms of ice. These phases depend on pressure and temperature. Each phase of ice is called by its number (e.g., ice VI is ice six, ice XI is ice eleven). The ice you may be enjoying in your iced-tea glass is called ice Ih, as in “one h” (the “h” stands for hexagonal—the shape of the crystals; it’s why snow crystals have six sides). Crystals in some forms of ice are cubes (but that’s not why we have ice cubes), such as ice Ic; some are trigonal, some are tetragonal.

We were taught also that ice floats in water, with nine-tenths below the surface; that’s because it has a density of 0.92, which is lower than liquid water’s 1.0. But some ices would sink straight to the bottom, because their density is far greater than liquid water—up to 2.51. Higher density ice is created by higher pressures—the kind of pressures that might be found at the bottom of 800+ miles of ice on other planets.

Rather than the single triple point, we now know that there are at least 12 triple points for ice and water. Most of these triple points are for the various phases of ice itself. Alas, all these other triple points and phases of water will remain a mystery for most of us in our day-to-day engineering; we’ll still be dealing with the triple point and three states of water that we were taught about in school.

But, the next time you are asked if you want ice in your drink, you can now say that you want ice Ih.

Then, sit back and chill.