With winter upon us, snow and ice illuminate the world in a winter wonderland. As children skate across that frozen pond, the question comes to mind of why ice floats. After all, it is just water in another form. The reason that ice floats can be answered by turning to science.
The easy answer would be that ice is less dense than water, causing it to rise to the top. However, when considering that the density of an object increases with the cooling process, it begs the question of why water is different. Fresh water is the exception to the rule.
This particular behavior can be explained by the makeup of water. A water molecule is made up of two hydrogen atoms and one oxygen atom. The covalent bond between the two hydrogen atoms and the oxygen atom is strong. When the water becomes colder than 4 degrees Celsius (or 40 degrees Fahrenheit), the hydrogen bonds that connect to different water molecules will adjust to keep all the negatively charged oxygen atoms apart. As this occurs, ice is formed.
Water changing over to ice crystals will move the network of water molecules apart. This causes the density of the ice to decrease by about 10%. Although the decrease in temperature usually causes density to increase, that bond between water molecules causes the density of ice to decrease. The ice covers more space than liquid water as well. If you were to freeze a jar of water, the jar would burst as the water froze. This is due to the water expanding as it freezes.
Anything that is less dense than the liquid it is placed in will float. When you look at a pond during the winter, the top layer is ice. It is floating on the water underneath. If water did not act in this manner, during the winter, any fish or other creatures living in ponds would cease to exist.
The next time you look at a lake in the winter, take a minute to think about the properties of water. The strange makeup of water is what makes ice float. Those hydrogen atoms and oxygen atoms bond together in such a way that when water freezes into ice, it is less 9% less dense than water. The density of water will decrease as it drops to zero degrees Celsius (32 degrees Fahrenheit). The increase in water density will not occur until the temperature reduces to 4 degrees Celsius (or 40 degrees Fahrenheit).