The ice cometh: The hard work of freezing

In freshman high school science class my teacher led us through a simple but enlightening experiment to show the difference between water and ice. It’s worth describing as we wait for our lakes to freeze over, likely in a month or so from now.

The teacher had us half-fill two glasses with water. Into one we dropped an ice cube, and into the other an equal volume of ice water. We then recorded the temperature of the water in both glasses for half an hour.

In the glass that received the ice water, the temperature dropped instantly, but then began rising and kept doing so. In the glass that received the ice cube, the temperature dropped more slowly, but then bottomed out and stayed down as the ice melted. The teacher then asked: Which would be the better way to cool a drink on a summer day?

The answer was obvious. What the experiment illustrated was a property of water called the heat of fusion, which will soon come into play on your lake and mine. Heat of fusion for water is the amount of heat energy that has to be removed to turn it from liquid to ice.

The definition of a calorie is the heat required to raise the temperature of one gram of water by one degree Celsius. It turns out that it takes a great deal more energy to turn water into ice than simply to change its temperature. In fact, 80 calories must be removed from that one gram of water in order to freeze it – and during the process the water’s temperature doesn’t change.

This works in reverse, too: It takes 80 calories to melt a gram of ice. In other words, it takes 80 times as much energy to melt ice as to warm water by one degree; or the same energy to melt ice as to warm water from zero degrees all the way to 80 degrees Celsius.

Now, let’s think about our lakes. As the days and nights get colder, the water temperature is dropping, rather quickly. Overnight low temperatures are now routinely below the freezing point of water, which is zero degrees Celsius (32 degrees Fahrenheit). So, why don’t our lakes freeze sooner? It’s because of that heat of fusion.

The water temperature will get down near the freezing point fairly quickly, but once it does, the water must give up a very large amount of heat energy before becoming ice. Finally, as the days and nights keep getting colder, we’ll see ice crystals forming along the shore, then a skin over the shallows, and finally, after a very cold, still night, a sheet of ice over most or all of the lake.

I saw the ice go off our lake last May. This year will be my first chance to watch day to day as a lake freezes. I am looking forward to it.