Liquid Water

We have already seen water molecules freeze into ice on the surface of dust grains in space. We now meet an amazing thing about water. When they are as warm as the Earth, water molecules still stick to each other with hydrogen bonds, but they have enough movement energy to move around, sticking first to one partner and then to another.

When they are warm, water molecules can change partners

They form a liquid. As we will see, this movement is essential for life.

Ice floats on water

One odd thing about water is that liquid water takes up less space than ice, because as the molecules move past each other they are able to squash closer together. This is unusual. Most substances take up more space in liquid than solid form. The result of this is that water expands when it freezes, so that ice floats on top of liquid water. In most liquids the frozen part sinks to the bottom.

Without this very unusual property, ice would sink as it froze and during the winter as the air becomes cold the whole oceans would freeze. Life as we know it would be impossible. So life could never have prospered on Earth without this amazing property of ice to expand upon freezing.