Because water starts to expand when you chill it to within a certain temperature range. If you want to know why this strange fact is true, keep reading.
More detailed answer…
Through our early years, we all learned at one point or another that heat makes things expand and cooling things generally makes them contract or shrink. As we all grew up, most of our practical experiences verified and validated this seemingly unassailable law of nature. Whenever a body part became swollen as a result of an injury or a bug bite, applying a bag of ice to the affected area would consistently reduce the swelling… a form of contraction. If ever we placed an empty plastic bottle in a refrigerator or freezer, it would shrink and crumple in on itself… also a form of contraction. We have all also witnessed the opposite end of this assertion every time we’ve watched the evaporation that occurs off of a pot of boiling soup. As the soup is heated, some of it expands as it transforms into gas and seemingly disappears before our very eyes. This phenomenon of heat induced expansion as well as contraction under conditions of extreme cold generally holds true in the physical world. As with most rules of nature however, there is an exception to this general law. To put it plainly, not everything contracts when it is cooled. As a matter of fact, one of the most plentifully present fluids and primary source of life on our planet disobeys this law within a particular temperature range. That’s right… water actually expands when it is cooled to a certain temperature. This rather strange behavior of water is technically referred to as the “anomalous expansion of water” (anomalous is just a fancy word for abnormal or strange). To understand this phenomenon completely, we’ll have to start by dipping our “intellectual toes” into the vast labyrinths of physics and chemistry.
On a fundamental level, everything we see around us is made up of one or more chemical elements which have a certain fundamental energy level. Each molecule of water (H2O) is made up of the elements hydrogen (H) and oxygen (O), each molecule of common salt (NaCl) is made up of the elements sodium (Na) and chlorine (Cl), and each molecule of glass (SiO2) is made up of the elements silicon (Si) and oxygen (O). By the way, a molecule is simply the smallest possible unit of a chemical substance that possesses all the characteristics of that substance. A loose everyday metaphor to help you understand what a molecule is, is to envision a rectangular toy house built entirely from red lego bricks. Let us assume that our imaginary LEGO house has the following dimensions 120” in length, 40” in width, and 80” tall. Let us also assume that the red LEGO bricks that were exclusively used to build this toy house are each 3 inches long, 1 inch wide, and 2 inches tall. Each individual LEGO brick in this our imaginary house is analogous to one “molecule” of a substance because it is pretty much the smallest fundamental unit of our imaginary toy house that shares all its characteristics. Each individual LEGO brick is the same color as the house, the same shape as the house, and shares the same dimensional size proportions with the house. In an analogous way, a single molecule of water for instance shares all the chemical properties of water.
Each molecule of water has a certain fundamental energy which can be altered by the amount of heat or cold applied to it. An increase in the heat applied to a body of water will increase the kinetic energy of the molecules contained within it. Kinetic energy is the energy that a thing possesses by virtue of its motion. We’ve all seen a boiling pot of water on a stove and can attest to the the fact that water seems to move more when it is boiling as compared to when it is left still at room temperature. If you think through it, it makes sense that the kinetic energy of water would increase as it is heated because water always gets more “animated” when it is heated, with a symphony of bubbles forming and popping all over the place as it rises in temperature. On the other side of this coin, cooling water down will reduce the kinetic energy of its molecules. In plain english, this means that water molecules in a body of water will move much more slowly in cold conditions relative to warm conditions. You intuitively know this if you have ever lived in a city with bitterly cold winters like Pittsburgh, PA. It sometimes gets much harder to talk while waiting outdoors during the winter months for the next bus because the cold weather makes the muscles in and around the jaw actuate (or move) much more slowly than they otherwise would in warmer climates. This law of nature is also why empty plastic bottles shrink when you leave them in a refrigerator… the air molecules in the bottle lose kinetic energy which means that they collide less often and less forcefully with the walls of the bottle which in return, makes the bottle shrink. In general, the cooler the conditions, the less kinetic energy or movement the individual molecules exhibit.
As the movement of individual water molecules becomes slower due to a reduction in the temperature they are subjected to, these water molecules are now afforded the time to form hydrogen bonds with one another. A hydrogen bond is a relatively weak electrostatic link between two partially charged entities. It is a bond that occurs quite a bit in nature and is actually the exact same type of bond that holds both helical strands or your DNA together. The nature of the chemical covalent bonds that join the pair of hydrogen atoms and the oxygen atom in a water molecule together is such that both hydrogen atoms end up with a slight positive charge, while the oxygen molecule ends up with a slight negative charge as shown in the image below. We could go into the electrochemistry of why that is the case, but let’s leave that to a future article, or your own research.
When water is chilled to about 4oC, its molecules slow down enough for the slightly positive and negative charges that exist on each water molecule to begin to align in a very orderly way. Specifically, the slightly negative charges on the oxygen atom of one water molecule will form a hydrogen bond with the slightly positive charge on the hydrogen atom of another water molecule. In the same way, the slightly positive charges on each of the hydrogen atoms of one water molecule will each form a hydrogen bond with the slightly negative charge on the oxygen atom of another water molecule… and on and on. When water reaches its freezing point at 0oC, all water molecules would have arranged themselves relative to one another in a hexagonal crystalline structure that closely resembles the image in the figure below.
This ordered molecular structure of ice comes at the cost of space. The water molecules can no longer pack together as closely because of the distance imposed by the hydrogen bonds that are formed when liquid water turns to ice. As a result, more space is required to contain the same initial amount of water. This is why a plastic bottle will crack if you completely fill it with water and leave it in the freezer for too long. As the water freezes, it expands and needs more space than the water bottle you initially filled with it can sustain… so it breaks the water bottle. I swear… nature is the ultimate badass! You might be wondering how something as weak as hydrogen bonds between molecules can crack through plastic, and that line of thought is reasonable at first glance. Looking deeper though, it becomes evident that although the strength of just one hydrogen bond is woefully insufficient to burst through a plastic bottle, if we consider the combined strength of millions of hydrogen bonds combined together, it is a totally different story. It’s like hitting a punching bag with one straight jab versus somehow managing to accumulate the power of 100 million straight jabs in just one punch and hitting the same punching bag. I can’t tell you for sure if the “100 million jab punch” will break the bag (because I have neither designated the calibre of punching bag I am using for this analogy nor conducted the experiment to unequivocally confirm it) but you should understand the general idea. The “100 million jab punch” will certainly have a much more devastating effect on the punching bag than just one straight jab.
The other weird inference that comes from this anomalous expansion of water is that the density of water actually changes when you freeze it. The formula for the density of an object is its mass divided by its volume. Upon freezing water to ice, you effectively increase its volume which in plain english means it takes up more space even though its mass remains roughly the same. As a result, water is at its least dense when frozen or at 0oC and at its most dense when at 4oC. Weird… I know.
Since mother nature is often ruthlessly efficient, it is highly likely that this weird behavior of water at lower temperatures is no accident. As a matter of fact, the anomalous expansion of water serves a very important purpose for maintaining the important marine life on our planet. The fact that our lakes and rivers stay liquid underneath a thick top layer of ice even in places as cold as the arctic means that cold water fish and sea lions don’t all freeze to death and die off. Ever wonder why water stays liquid beneath sheets of ice even in places as cold as the arctic? Well, it is due to the anomalous expansion of water between zero and four degrees centigrade. Let’s take a closer look to understand this phenomenon. When the temperature drops due to a change in seasons or as a result of just generally being at one of the poles where it is always cold, the top layer of water starts to chill. Once the temperature of the top layer of water drops to 4oC (the temperature at which water is at its most dense), it falls to the bottom of the body of water displacing another layer of water to the top. This new top layer also gets chilled to 4oC and drops to the bottom of the body of water because it is more dense than other layers of water… and on and on. This continues until the temperature of the entire body of water drops to 4oC. At this point, the top layer finally freezes and stays afloat because remember that water is actually at its least dense when frozen. This is why even when the top layer of a body of water is frozen, there is often a thriving marine wildlife ecosystem in the still liquid body of water below it.
The anomalous expansion of water is one of nature’s many elegant tricks that are designed to maintain life on this planet. As an interesting aside, all of the life on this planet is thought to have actually started in the oceans and seas so it is an exceptionally good thing that mother nature figured this trick out. If she hadn’t, it is very likely that I wouldn’t be typing this article, and you wouldn’t be reading it because neither of us or the entire human race for that matter would have ever happened on this earth.
Oyolu B.C. Ph.D.
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