Field Notes

By Adrienne Funderburg, research coordinator at the Lilly Center

Have you ever been so cold you couldn’t think straight? Water is the same way – the colder it is, the denser it is!

Density – How compact a substance is; how much material is packed into a given volume of space

While silly, that analogy is an easy way to remember a fundamental characteristic of water: the colder it is, the denser – or heavier – it is. In a lake in the summer, that heavier water sinks to the bottom of the lake and forms a cold water layer, which we call the hypolimnion. The prefix “hypo” refers to the layer’s position under the warm water layer, which we call the epilimnion, or the upper layer. Sandwiched between the two is the metalimnion, a thin barrier in which the water quickly changes from warm to cold.

Like oil on water, the denser liquid underneath doesn’t mix with the lighter liquid above, so the lake’s contents in each layer, like dissolved oxygen, don’t mix, either. (That’s why the Lilly Center samples both the top and bottom layers for data on their temperature, dissolved oxygen, nutrients, and other characteristics in order to capture a full view of the lake.)

That analogy doesn’t give the whole story, however. Unlike many other substances, water is most dense in its liquid form, at 4°C, or about 39.2°F. Just a bit colder and the water approaches freezing temperatures. The molecules start to spread out again, and at 0°C (32°F), they form the solid crystals we know as ice. This spacing out of water molecules means that ice is less dense than water, so it floats. And it’s a good thing that it does – if ice was denser than liquid water, lakes would freeze from the bottom up, killing fish and other lake life, and the bottom of our deeper lakes might stay frozen all year!

Water isn’t the only substance that expands when it freezes (the metallic element gallium is another, for example), but it certainly is an unusual property that allows for life – including lake life – as we know it.