Why Is It That Water Freezes On The Surface Of A Lake But Not Below It?

This is a great question! To answer it, we first need to think about phases of matter: Gas, liquid, and solid. Matter is made up of particles called atoms and molecules. If the particles are moving around very fast and they are spread far apart the matter is in the gas phase. If the particles are closer together and moving slowly around each other the matter is in a liquid phase. Finally, if the particles are locked in position and not hardly moving at all the matter is in a solid phase.

Water is the name of the liquid phase of H2O molecules, and ice is the name of the solid phase.

The second thing we need to think about is density, which tells us how tightly matter particles are packed together. Tightly packed matter is more dense than loosely packed matter. Also, less dense matter floats on top of more dense matter. So when you place ice cubes in your soft drink (which is mostly water) and your ice cubes float, you know that ice is less dense than water. This is a very special property of water. Almost every other type of matter gets more dense when it changes phase from a solid to a liquid, but water gets less dense. Why? Because ice is a crystal, which means it has a regular pattern with spaces in between molecules. The spaces in the crystal are larger than the spaces between molecules in the liquid. More space = less dense, so ice is less dense than water.

So now we understand why ice floats on top of water, but how does this work on a lake? Imagine that it is the beginning of winter, and the temperature has just gotten below freezing. Air changes temperature faster than water -- this is why a swimming pool seems much warmer in the evening. The air cools down at night, but the pool water remains almost as hot as it was during the day. So although the air is freezing cold, the water isn't freezing yet. The water at the very top of the lake is in direct contact with the cold air, so it freezes first. And because ice is less dense than water, the sheet of ice doesn't sink. If the temperature stays very cold, the sheet of ice gets thicker, but the ice also acts as a barrier, or insulator, between the cold air and the warmer water underneath. The sheet of ice helps the rest of the water from becoming ice!

The amazing fact that water freezes on the surface of a lake but not below it allows plants and fish to live through the winter in northern lakes and ponds. Imagine a lake without fish! So it is a good thing that H20 has the very strange property that its solid phase is less dense than its liquid phase.

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