The fact that ice floats on water is widely recognized and easily confirmed. It floats in a glass of ice water, on frozen lakes, in the ocean as polar ice and icebergs. You may think that because ice is a solid it should be heavier than liquid water and should sink, but fortunately this is not the case. When water freezes the molecular makeup actually makes it less dense than the water it may be floating in.
I’ll go further into the explanation as to why, but first here is a poem I penned to help explain.
Ice on the lakes the ponds or the river
May certainly give you reason to shiver
But the thing about ice that may make you think
Is when water is frozen, it doesn’t sink
Most things when frozen will certainly shrink
But not the world’s number one drink
No water when frozen will actually float
Much like you would in a raft or a boat
If all the ice did sink far below
In no time our lakes would be quite shallow
And besides the fish having nowhere to go
There’d be other problems when temps fell to zero
Like think of the beaver the duck and the otter
If it wasn’t for the unique character of water
So when I think of Ice, I think that it’s nice
That it carries its own flotation device
Water has the unique property of being the most dense at 40° F. That means that hot water and ice are both less dense than cool water. We know that substances that are of lower densities will float on top of more dense substances. A tennis ball will float and but rocks not so much.
Water will also expand when frozen which is also unique as most substances will shrink when cooled.
In cold climates, the potholes you try to avoid are created by the water freezing in cracks and gaps in the pavement expanding the imperfections and degrading the surface. Freezing a glass bottle of water is also not a good idea because the expanding properties of freezing water will most times break the container.
Back to the topic at hand, the reason ice floats on water. Water of course is made up of one oxygen and two hydrogen atoms. These atoms form strong covalent bonds and make up the molecules of water. Between water molecules there is also another weaker bond that forms known as a hydrogen bond. This hydrogen bond between water molecules becomes a major player to make ice less dense.
Hydrogen Bonds Not A Factor in Water Vapor
The hydrogen bond is weak and when water is warm the water molecules form and break bonds with other water molecules quickly.
Hydrogen Bonds Break and Reform often – Liquid Water
As water gets below 39° F They don’t have the energy to move and break and form bonds so easily.
Stable Hydrogen Bonds in Frozen Water
At this point the molecules will form more hydrogen bonds with other molecules in a lattice structure with more space between molecules. This extra space between molecules is what water floats and will expand when frozen.
The fact that ice floats is very important aquatic life on earth. If ice sank to the bottom of lakes and streams it could eventually freeze the entire lake completely from the bottom up which would be bad for life in the water.
Fortunately because of the properties of ice it will form on top of lakes and streams and not impact the creatures below.
Meteorologist Mike Morrison