What makes ice special in nature?

Simply put, from my perspective the pursuit of scientific curiosity is just awesome. Every now and then you really get blown away by how nature works. In this explainer, I want to share with everyone an implication of ice I learned that happened to be one of those moments for me.

We all likely have a good sense about how when water freezes the resultant ice would float if placed in liquid water. Familiar and real-world examples are pictures of that giant iceberg floating in the Artic Ocean or, perhaps closer to home, the ice cubes you put into a refreshing glass of tea. As a fun fact, the act of freezing makes the once liquid water about 10 percent less dense. This is why approximately 10 percent of an iceberg and an ice cube in a drink are floating above the water surface, while the rest of the 90 percent of the ice is found below the water line. Let’s dive into this a bit more!

Quickly, the mathematical relationship for density is: p = m/V or density (p) equals m (mass) divided by volume (V). So, the relationship shows that as mass (m) stays the same and volume (V) increases, density (p) becomes less. The key then to make a massive object float is to increase the volume that the total mass occupies until the density of that object is a smaller value than the density of the fluid you intend to float on. Good to know!

This physical density relationship has significant engineering implications since substances tend to occupy increasingly less volume to become denser as they are cooled. With a caveat, not water! Water does shrink in volume like most substances, but only when cooled to about 39 degrees Fahrenheit (°F) or 4 degrees Celsius (°C). Then the magic, so to speak, happens! Further cooling towards the freezing point (32°F or 0°C) starts a process when water molecules expand as crystalline structures (think of snowflakes). Consequently, the mass of water is the same, but total volume jumps and its density must drop. This fact alone is pretty cool, but there is more to the story applying it to the environment. Now to what blew me away and the implication of water’s unique density pivot when approaching the freezing point!

Winter in the higher latitudes of the planet can be harsh! How does wildlife in lakes survive the winter season if the lake surface turns to ice? Considering the above information, we can walk through an amazing sequence of events. As the seasons change and colder temperatures settle in, the exposed surface water to the winter air above loses energy to the surrounding environment. As sufficient cooling occurs, this top water layer eventually reaches that special 39°F threshold. Since density is now at a maximum, water at the upper reaches of the lake naturally begins to sink. Displaced water below now suddenly finds itself rising in response to eventually be exposed to the chillier air above. The process repeats until, finally, the colder air temperatures above the lake freeze the surface water. As this frozen surface is less dense than the relatively warmer water below, sinking motion ceases. Not only that, but the top ice layer now insulates and protects the lake wildlife below taking refuge from the much colder atmosphere!

The depth of ice can continue to deepen with time, though, so a lake does need to be deep enough to prevent all out freezing of the lake's profile in order to protect wildlife until the spring thaw. Speaking of the spring thaw, when the top layer ice begins to melt and warm back to 39°F, the sudden increase in density at that water layer promotes a new round of lake profile mixing! From a biology standpoint, seasonal “lake turnover” is also vital for distributing dissolved oxygen back to lower depths to benefit the lake ecosystem.

I find this 39°F (4°C) density nuance for water to allow for lakes to remain “alive” year-to-year incredible! I now challenge you to have fun discovering a new science fact that can blow you away!

Photo Credit: Jonny William Malloy