The "Watched Pot" of Planet Earth

Yesterday we looked at one of the characteristics that makes water so unusual and unique: its surface tension. Today we will take a look at another unusual characteristic of water: its heat of vaporization. And we’ll see how this characteristic profoundly affects our planet.
Looking back at yesterday’s analogy, we see in liquid water the “Conga line” of molecules all linked to one another with hydrogen bonds. Apparently, these molecules really enjoy this arrangement, because it takes a tremendous amount of energy to break up this party. And break it up we must, if liquid water is to be converted into a gas. This process is called evaporation or vaporization.
In the gaseous state, each molecule of water must be independent. If the molecules cling to the Conga line, they cannot leave the liquid state. The energy required to break the hydrogen bonds and make each molecule an independent entity – plus the energy required to give each molecule the velocity needed to remain in the gaseous state – is called the heat of vaporization.
How much energy is required? For one gram of water, the energy requirement is 540 calories. Is this a lot, or is it a little? Let’s compare it with a few other substances. For hydrogen sulfide, it’s 131 calories. For ethyl alcohol, it’s 204 calories. For the most common component of Earth’s atmosphere, nitrogen, it’s 47 calories. For carbon dioxide, it’s 87 calories. Water is the champ – and by a wide margin.
You’ve heard of the expression: “A watched pot never boils.” This is a testament to water’s incredibly high heat of vaporization (and its specific heat, which we’ll get to in a future blog.) Five seconds on the stove, and you would burn yourself on the empty cooking pot, because it changed temperature so readily. But fill that pot with water, and it takes quite a while – and quite a bit of energy – to change the temperature, and ultimately to convert the liquid water into a gas.
This is great news for our planet. Water doesn’t easily evaporate or change temperature. Since ours is the Water Planet, that means that the temperature of our planet remains pretty constant. Just look to our nearest celestial neighbor, the Moon. Average day time temperatures are above water’s boiling point, and average night time temperatures are over 450 degrees F less! The lack of water and other greenhouse gases in the lunar atmosphere allows this huge temperature change to take place – a change so radical that life is impossible on the Moon.
So once again, water is our friend here on Earth, providing great stability to our global temperatures thanks to its incredibly high heat of vaporization. Instead of becoming impatient while you’re waiting for the water to boil on the stove, be grateful for this unique property of water.

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