Curious Nature: The chemistry and poetry of ice formation (column)
If you live in the mountains, then you are familiar with ice and the many hazards that come along with it. Slippery roads and sidewalks pose a challenge throughout the winter, but ice also comes with its own ephemeral beauty. The feathery crystals of surface hoar on top of the snow in the morning or the lacy, delicate structures that form above icy-cold rivers all add to the exquisite splendor of winter in the high country.
But it turns out that not all ice is created equal. You might have recognized that already as you go from cursing out the clear, almost invisible ice covering your walkway to appreciating the delicate crystal frost that is growing on your car window. But if all ice is composed of water, why does it freeze in so many different ways?
Freezing seems like a very commonplace occurrence to us, but in much of the world, it is a foreign phenomenon. And it turns out that the physical and chemical properties of water, especially as it freezes, have a great impact on things that live in cold places. But for the purpose of this piece, we will focus on the way ice forms and the many different shapes it can take.
First, all atoms and molecules have forces that attract them to one another, and these forces vary depending on the particle’s attractive forces. Those with really strong attractive forces freeze at very high temperatures, while molecules with lower attractive forces, such as nitrogen, freeze at extremely low temperatures (which is why liquid nitrogen can be cooled to such a low temperature without freezing). Water molecules have a moderate attractive force, so they freeze at a moderate 32 degrees.
You probably recall that a water molecule, H2O, is made up of two hydrogen atoms bonded to one oxygen atom. But like all atoms and molecules, these molecules are also attracted to one another, and in water, the molecules are held loosely together by hydrogen bonds.
These bonds form primarily because of the innate polarity of a water molecule. (Where the hydrogen atoms have a slightly positive charge and the oxygen atom gets a slightly negative charge.) This is part of the reason why a drop of water on a table takes the shape of a circle, because the molecules are essentially holding on to one and other, attracted to the side of the nearest molecule with the opposite state.
In the liquid state, these molecules are constantly forming and breaking the bonds between one another, and any given molecule might be bonded with two or three other molecules at a given time. But as water cools and the particles have less energy, the forces of the hydrogen bonds cause the water molecules to align themselves such that they maximize attractive forces and minimize repulsive forces.
This results in the water molecules forming the crystalline shapes that we are familiar with in snowflakes and ice crystals. When the water cools too quickly, the molecules don’t have time to align in the lattice, and we get less defined crystalline shapes, like the more solid, clear ice we see in icicles and on sidewalks.
As a general rule, the more complex and beautiful ice crystals form the slowest, freezing at just the right rate and temperature to allow the molecules to line up like perfect soldiers in the ranks. It is amazing to me that the logical, chemical push and pull of atoms and molecules can create such poetic precision. What a wonderful life.
Jaymee Squires is the director of graduate programs at Walking Mountains Science Center in Avon. Squires is not a big fan of the cold, but she loves the beauty and serenity that winter brings to the valley.