Molecular Monday: When Water Boils…

I started writing Molecular Mondays with the goal of teaching myself a little chemistry. Chemistry was always my worst subject, but as a science fiction writer, I need to know this stuff. I’ve found that while researching and writing this series, I’ve had to unlearn much of what I learned in school.

Here’s a good example: my teachers told me that the boiling point of water is 100 ºC (212 ºF). That’s it. End of story.

Except it isn’t.

Sp05 Boiling Water

Water boils at 100 ºC if it’s under one bar of pressure (equal to normal atmospheric pressure at sea level). Also, water boils at 100 ºC if it’s pure water. Lower the pressure, and the boiling point drops. Raise the pressure, and the boiling point rises. Adding impurities like salt will also drive the boiling point up. These factors can also affect water’s freezing point.

As a science fiction writer, I spend most of my time (metaphorically speaking) on other planets, with atmospheric pressures often much higher or lower than one bar. And for those rare worlds that do possess liquid water, like Europa, Enceladus, or maybe Mars, that water undoubtedly contains loads of impurities.

As a result, water freezes and boils at very different temperatures throughout the Solar System. In many cases, due to the low or no pressure environments often found in space, water just skips its liquid phase entirely, transforming straight from ice to vapor and vice versa.

Since we live on Earth, I might be willing to forgive my old science teachers for failing to mention any of this space stuff… except even on Earth, the boiling point of water can vary greatly.

At sea level, the boiling point is 100 ºC. Go to Kansas, with a mean elevation of 2,000 feet above sea level, and the boiling point is roughly 98 ºC. That may not seem like a huge difference, but it’s a difference. Climb the Rocky Mountains, reaching a maximum elevation in excess of 14,000 feet above sea level, and the boiling point drops to 85 ºC. Climb Mt. Everest (over 29,000 feet above sea level), and water will boil at only 72 ºC. (Click here for a chart showing water’s boiling point at various altitudes.)

So while 100 ºC may be water’s boiling point under certain specific conditions, it’s hardly the universal standard I was led to believe it to be.

In the next Molecular Monday post, I’ll continue my research on water. I’m pretty sure there are a few more old science lessons I need to unlearn.

3 thoughts on “Molecular Monday: When Water Boils…

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