Water, as everyone knows, freezes at O°C. So a simple plan to make an ice rink would be to fill a space with water and cool it down. It may sound trivial but in practice it is not, owing to the huge amoun of energy involved. To understand why, let's first take a look at the physics of cool- ing. The coldest anything can get is a temperature known as abso-lute zero, which is a tad below -273°C. While the January chill may make it painfully cold to wait in line outside for a ticket to an Oilers' game in Edmonton, it's rare for outdoor temperatures to get below -50°C. According to The Guinness Book ofWorld Records, the coldest outdoor temperature ever recorded was -89°C, in Vostok, Antar-tica (which is still quite balmy compared to absolute zero). Anything above -273°C contains some heat, or thermal energy. The amount of thermal energy contained depends on the temperature, the mass, and the stuff of which the object is made. A gallon of water at 20°C has more thermal energy than a gallon of water at 10°C. And two gallons of water at 15°C have twice as much thermal energy as one gallon of water at the same temperature. When scientists and en- gineers developed the steam engine in the nineteenth century, they became fascinated with turning liquids into gases and solids. Devel-oping an understanding of these phase changes, as physicists now call them, was a necessary step in making better engines. A crucial problem was figuring out exactly how much energy it took to raise or lower the temperature of a given substance, say water, by one de-gree Celsius. This quantity is called the substance's heat capacity; for water this amount is around 4.2 joules/gram/oC. In other words, for every gram of water that we want to cool down by one degree, we have to take away 4.2 joules of energy. We can now imagine how much energy is needed to resurface the ice at Madison Square Gar-den for a Rangers game using warm water. Making ice from scratch is extremely costly. The size of a hockey rink is about 1,600 square meters, so filling it to a depth of 2 cm requires 32 million grams of water. If the water is initially at room temperature, the thermal energy that must be extracted to cool this much water down to O°C is some 2.7 billion joules, enough energy to power an average house for two weeks. It would take weeks of steady work by a typical household refrigerator just to cool the rink at Madison Square Garden down to the freezing point. And that's assuming the water is insulated from its environment, which isn't the case. Fortunately, as we will see later,there are systems better than regular appliances to cool hockey rinks.
Water has a rather large heat capacity compared to most liquids, and that's why it is a popular choice for cooling (or heating) systems, whether in thermonuclear reactors, car engines, or air conditioners. Water also has a huge influence on climate. Large bodies of water like oceans and lakes have a stabilizing effect on coastal climate because they act like massive heat reservoirs, which explains why Chicago has more drastic temperature fluctuations over the year than New York. Unfortunately for those maintaining ice rinks, water's large heat capacity means that creating ice involves a lot of energy.
So assume we have used 2.7 billion joules of energy to cool down our rink to O°C. But we don't have ice yet. Why? Because it takes additional energy just to change a liquid into a solid, even if the temperature stays the same. This amount of energy is called the latent heat of fusion, and for water it is 340 JIg. The mass of water we've flooded the Garden with now needs an extra 11 billion joules just to
freeze! For our poor household fridge, this would mean a lot of extra hard work.
Temperature is not the only parameter that changes during cool-ing: density also varies, and in a very peculiar way for water. Water has a density near 1 g/cm 3 (which was at one point used to define the gram), but, like most liquids, its density tends to increase at lower temperatures. However, at 4°C the process is reversed and water's density drops gradually to 0.99984 g/cm 3 at O°C. When freezing oc- curs, density drops further to 0.9167 g/cm 3 , which allows ice to float with 8 percent of its volume rising above the liquid water surface. Saltwater is heavier than pure water, so icebergs will float with about one-tenth their volume above the ocean-the proverbial tip of the iceberg. Water is one of the few substances that expand upon freezing; other liquids do the opposite. If water followed the same rule, icebergs would sink and the Titanic disaster would never have happened.
This peculiar density behavior of water plays a role in the forma-tion of ice on lakes and ponds, the surface on which so many grow up playing hockey. Going from your basement to the attic on a warm summer day, you'll notice the temperature will increase, because hot air rises. The same is true for a lake: as temperatures drop during the fall and winter, the cooler and denser water remains at the bottom and the warmer water rises to the top. But once temperatures fall below 4°C, the situation changes. The colder water rises to the top and stays there. This inversion process is called the "turning over" of a lake. Because of this, natural ice only forms once all the water in the lake or pond has reached 4°C or colder. Eventually the top layer becomes cold enough and turns to ice. This means lakes and ponds freeze from the top down, creating a layer that grows downward. This phenomenon is behind the warning that you may be "skating on thin ice"-and why anyone venturing onto a seeming frozen lake must be very careful.
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