Fortunately for life on Earth, water is a unique compound: it actually gets less dense when it freezes. This is unusual in the physical universe: as a general rule, all chemicals are at their densest when they freeze, grow less dense as liquids, and finally are least dense as gases. That water should be different is convenient for life on Earth, because it means that frozen ice – like an ice cube, or like the layer of ice over a frozen pond – will actually float on top of liquid water, instead of sinking down to the bottom.
PHASES OF MATTER
In everyday life, matter comes in three states: solids, liquids, and gases. (There is another phase, plasma, which is much hotter than gas, but you’re unlikely to encounter plasma in natural form unless you’ve somehow managed to dive into the middle of the Sun.) Which state of matter a substance is in depends upon its thermal energy, or, in popular terms, how hot it is. The coldest form of a substance, and/or the one under the highest pressure, is a solid; when heated, this solid melts into a liquid, and eventually evaporates into a gas. (Or you can go the other way: gas can condense into a liquid, and then liquid can freeze into a solid.)
Exactly what temperatures these changes in state will occur at varies depending on what substance one is working with. In this particular case, water will change from a solid (which we call “ice”) to a liquid (which we call “water”) at 0 degrees Celsius, or 32 degrees Fahrenheit, and will boil into gas (which we call “steam”) at 100 degrees celsius, or 212 degrees Fahrenheit.
All matter has density, a measure of how far apart the molecules in a sample are spaced. In general, solids are the most dense: the molecules are very densely packed together (which is why they seem solid to us). Liquids are less dense, and gases are least dense.
Water, however, is an exception. As water freezes, it actually becomes less dense, not more dense. The reason for this lies in chemical bonding, the process by which atoms combine to form molecules. Water is made up of two hydrogen atoms and one oxygen atom (which is why we call it “H2O”), which bond together as positively charged hydrogen ions that are attracted to negatively charged oxygen ions. However, water is unusual; these bonds are just weak enough that the hydrogen ions which have already bonded to a single oxygen ion (and thus formed a water molecule) will still be slightly attracted to other oxygen ions.
As a result, as water cools and turns into ice, the water molecules begin to form a complicated crystal or lattice framework. (You can see much larger expressions of this lattice in such beautiful, visible structures as snowflakes.) These lattices hold atoms farther apart than they would normally be in a densely packed solid, making ice less dense than liquid water.
LESS DENSE OBJECTS FLOAT
This chemical property of water is important because of a second chemical process – how objects float. In general, a solid object will float in a liquid if it is less dense, and it will sink if it is more dense. This is because a denser liquid will hold up a less dense solid. In this case, because ice is less dense than liquid water, it will float.
If the reverse were true – if water were less dense than ice, as is common with solids and liquids in the natural universe – life on Earth would be very different indeed. Every year, winter ice would begin to form at the top of lakes and rivers, but would then sink to the bottom, in the process killing all life in the water.