Chemistry

Why Water has a Boiling Point of 100 Degrees Celcius



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Water boils at one hundred degrees Celsius for the very good reason that we said that it does. (No joke!) Water's boiling and freezing points were used to define the Celsius temperature scale. It was an entirely arbitrary decision, of course, but setting boiling at one hundred and freezing at zero provides a very nice range to work with (and is much more intuitive than the antiquated Fahrenheit system).

A person still might wonder why water's boiling point is seemingly so high. After all, other common liquids like lighter fluid, nail polish remover, and alcohols boil at lower temperatures. For reference, here are a few such liquids, with their boiling points:

Ethanol (drinking alcohol) boils at 78.2 degrees Celsius.

Isopropanol (rubbing alcohol) boils at 82.3 degrees Celsius.

Butane (in cigarette lighters) boils at a mere -0.5 degrees Celsius (which is why they keep it under pressure).

Acetone (nail polish remover) boils at 56.05 degrees Celsius.

There's even diethyl ether and chloroform (generally used to knock people out) which have boiling points of 34.5 and 61.1 degrees Celsius, respectively.

Hopefully some of those substances are familiar to you, along with the fact that they evaporate much more quickly than water. If you find yourself doubting, pour out equal amounts of water and one of these chemicals on a countertop (far away from any ignition source!) and see which puddle evaporates faster. The water will lose every time. There is a very good reason that water (a very tiny molecule) has a higher boiling point than any of these chemicals. That reason bears the name "hydrogen bonding".

Hydrogen bonding is the strongest of the intermolecular forces (forces of attraction between separate molecules). There's actually a full Helium article on the topic ("Intermolecular Forces") so I won't go into too much detail here. The short of it is, water is ideally suited for hydrogen bonding, having not just one, but two hydrogen atoms that are ideally suited for grabbing onto the oxygen on a neighboring water molecule. The result is that your average glass of water is actually a network of molecules that are clinging onto one another fairly tightly, so it takes a lot more energy to make that water boil away into individual gas molecules.

Comparatively, the alcohols can only do a little bit of hydrogen bonding (notice that their boiling points are still fairly high). They each have but one available hydrogen, and are bulkier molecules. The other chemicals we have listed can't do any hydrogen bonding at all, so they don't cling to one another at all. When a bit of heat is applied, there's very little attraction at all with which to resist evaporation, and the molecules just go sailing away as a gas.

If you really wanted to, you could think of boiling as a game of tug-of-war. On one side, there are the molecules of whatever liquid you want to evaporate, and on the other side is a scale (Celsius) of how hard you have to pull to get a molecule to the gas state. In acetone, each molecule stands alone, so it doesn't take very much effort to win (low temperature). With alcohol, there are a couple "friends" standing by to help in the struggle, so a slightly higher temperature is needed to beat them. When water's turn comes up though, the whole team pitches in, and much more effort (higher temperature) is required.

There are, of course, chemicals which have higher boiling points than water. Octane (a major component of gasoline) boils at a slightly higher temperature (125.6 degrees Celsius) simply because it is so large. (Imagine playing tug-of-war with the five-hundred pound Sumo Champion.) Then there are substances like metals which are so interconnected that they don't even melt until you reach hundreds of degrees. They do boil eventually, generally at temperatures of thousands of degrees. We don't even think about such things in everyday life, since this pretty much only happens in labs or stars.

Going back to the original question though, water boils at one hundred degrees Celsius, 212 Fahrenheit, 273.15 Kelvin, and 8724 Bifferuminites because each temperature scale was defined in such a way that water had that boiling point. (Guess which scale I just made up.) You could just as easily define your own temperature scale, on which water can boil at 0, 42, or 1000 degrees. Just don't expect anyone else to use it. (Bifferuminites has yet to catch on with the general public, I must admit.)

[Thanks to the 82nd CRC Handbook of Chemistry and Physics for all those boiling points.]

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