Why Does Ice Float?

One of our family’s favorite winter activities is ice skating on the lake where their grandparents have a little cottage. On good years, the lake freezes smoothly and we can skate across it, listening to the booms as it expands and contracts beneath our blades.

On not great years, the wind is blowing while the lake freezes, making ridges and ripples across the ice - not so great for skating or hockey, but good for holding snow.

One year, Marina asked why ice floats to the top, rather than sinking. (Must have been during our sink/float experiment times…) I assumed it was because ice traps little air bubbles in it, but didn’t really know.

Turns out, water is the most dense at 39°F (or 4°C). This means that the water molecules are the closest together at this temperature, which means gravity pulls on them the most.

But since it’s above 32°F (or 0°C) it stays as liquid water and sinks to the bottom.

Ice - being less dense when its frozen - has less gravity pulling on those molecules because they’re further apart. So, they float!

And it’s a good thing this is how the Earth works, otherwise the organisms that live in our lakes, ponds and rivers wouldn’t be able to stay alive through the winter.

So, the next time you see ice forming on a body of water in the winter, you can smile knowing the science that makes this happen and the animals that are swimming around beneath the ice.

Pair this learning with experiments on which objects sink or float, or read aloud the book Twelve Kinds of Ice by Ellen Bryan Obed and illustrated by Barbara McClintock. Journey with the characters in the book as they explore the different stages of ice through winter and how they enjoy each one.