365 Days of Climate Awareness 52 - Properties of Ice

Ice is the frozen state of water. Being inorganic, it is considered a mineral. Its properties vary based on a number of factors such as purity, temperature and pressure. It's generally clear, but will take on the color of suspended impurities. Some chemical forms appear bluish-white. (One theorized explanation for why the lookout on the Titanic couldn't see the iceberg until too late: it was old, denser, blue ice.) There are eighteen or more molecular formations, known as "packing geometries", for ice, mostly hexagonal in nature. As ice continues to cool and is subjected to greater pressure, the packing geometry can change.

The crystal structure of ice is held together by hydrogen bonds, just like the liquid-phase molecules. But being in a much lower energy state, ice molecules are fixed in position by those hydrogen bonds. Ice is significantly less dense than water. Its most common form is about 9% less dense than fresh water, though ice's density does increase slightly as continues to cool. Some crystalline forms are more dense than liquid water, but these occur only at very low temperatures and very high pressure.

When water freezes into ice, it releases 79.7 calories per gram (cal/g) of heat, known as the molecular heat of fusion. The water does not change temperature while this happens, but the lost heat energy means the water molecules now lack the kinetic energy to overcome their hydrogen bonds. The hydrogen bonds lock the molecules into a fixed structure: the ice crystal. Water rarely includes other molecules when it freezes into its crystal lattice. Suspended particulates might be included, which will discolor the ice. But chemically the water is pure, or nearly so. This is likely because as the water nears the freezing point, it expands and rises to the top of the water column, and heavier salt ions do not migrate with it.

Water vapor directly becoming ice is known as deposition, and the vapor must deposit onto a solid surface: for example the ground, glass, a particle of dust, or other ice. Vapor doing this takes on the beautiful hexagonal structure we see in snowflakes and frost. When vapor deposits directly to ice, it likewise does not change temperature--it is already at the freezing point--but it loses 619.7 cal/g of heat, which is the combined latent heat of vaporization and fusion (538 + 79.7). That's a lot of heat!

One aspect of ice which still defies scientific understanding is ice's slipperiness. Several theories involve localized, very thin melting of the ice at the boundary with an object rubbing against it, but this does not account for very low temperatures. Without this slipperiness, glaciers would not move as they do, and we couldn't do things like skate. It's strange when such basic aspects of our world escape our analysis! But this is why we continue to explore, experiment, theorize and learn. Science is nothing more than the attempt to rationally explain our world, including our own bodies and minds. We continue to pursue it because our understanding is very, very incomplete. Such as, on the question of why ice is slippery!

Tomorrow: intro to glaciers.

Be well!