Pop Quiz

Why does ice float?

Clue: 39.2 degrees has something to do with the answer.

substance floats if it is less dense, or has less mass per unit volume, than other components in a mixture. For example, if you toss a handful of rocks into a bucket of water, the rocks, which are dense compared to the water, will sink. The water, which is less dense than the rocks, will float. Basically, the rocks push the water out of the way, or displace it. For an object to be able to float, it has to displace a weight of fluid equal to its own weight.

Water reaches its maximum density at 4°C (40°F). As it cools further and freezes into ice, it actually becomes less dense. On the other hand, most substances are most dense in their solid (frozen) state than in their liquid state. Water is different because of hydrogen bonding.

A water molecule is made from one oxygen atom and two hydrogen atoms, strongly joined to each other with covalent bonds. Water molecules are also attracted to each other by weaker chemical bonds (hydrogen bonds) between the positively-charged hydrogen atoms and the negatively-charged oxygen atoms of neighboring water molecules. As water cools below 4°C, the hydrogen bonds adjust to hold the negatively charged oxygen atoms apart. This produces a crystal lattice, which is commonly known as ‘ice’.

Ice floats because it is about 9% less dense than liquid water. In other words, ice takes up about 9% more space than water, so a liter of ice weighs less than a liter water. The heavier water displaces the lighter ice, so ice floats to the top. One consequence of this is that lakes and rivers freeze from top to bottom, allowing fish to survive even when the surface of a lake has frozen over. If ice sank, the water would be displaced to the top and exposed to the colder temperature, forcing rivers and lakes to fill with ice and freeze solid.

IDO’s own Cliff Clavin

What a chemistry major??

You did that too fast Jesse, plus I’ll give you credit for cut and paste.

A couple years ago I was reading about ice fishing and the author explained it like this. Water is the densest at 39.2 degrees and so the water in any body of water is always going to have the water that is closest to 39.2 degrees on the bottom. For example if the surface is 72 degrees the water at the bottom of the lake may be 62 degrees. Since 62 degrees is closest to 39.2 degrees than 72 degrees the 62 degree water is denser.

The same holds true in the winter months when we walk on water. The surface temp is 32 degrees and the bottom might be 37 degrees. The 37 degrees is closest to the 39.2 degrees or the temperature that water is the densest.

The water is getting close to that temperature right now and there is another magic happening that occurs. You see when the surface temperature reaches 39.2 degrees the bottom of the lake is the same temperature and the oxygen content of the water is the same from the surface to the bottom. This is what is known as a lake turning over. Lakes turn over two times a year.

ah-hem…the question should be…why does water sink.

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Why does ice float?

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Cuz your glass is full… Drink up.

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A couple years ago I was reading about ice fishing and the author explained it like this. Water is the densest at 39.2 degrees and so the water in any body of water is always going to have the water that is closest to 39.2 degrees on the bottom. For example if the surface is 72 degrees the water at the bottom of the lake may be 62 degrees. Since 62 degrees is closest to 39.2 degrees than 72 degrees the 62 degree water is denser.

The same holds true in the winter months when we walk on water. The surface temp is 32 degrees and the bottom might be 37 degrees. The 37 degrees is closest to the 39.2 degrees or the temperature that water is the densest.

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If this is true, at what point will ice sink? If water becomes less dense as it warms, an ice becomes less dense as it cools, what would happen if you put an ice cube in 150 deg water?

It will melt.
DT

Simple, so we can walk on it to go ice fishing.

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You did that too fast Jesse, plus I’ll give you credit for cut and paste.

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I knew the answer but a quick copy and paste was much easier than typing it all out.