Water The Greenhouse Gas in a Glass
Structure of Water Molecules and Role in Climate Change
Sep 15, 2008
The atmosphere naturally holds billions of tonnes of H2O in the form of water vapour, which absorbs heat energy reflected from the surface of planet Earth and stops it escaping into space. Without this natural “greenhouse effect”, Earth would be much much colder than it actually is and most life forms would simply never have evolved.
What about the structure of water enables it to behave as a natural greenhouse gas and to play a pivotal role in controlling Earth's climate?
Structure of Water Molecules and Chemical Bonds
Every single water molecule is made up of two hydrogen atoms and one oxygen atom joined together with what are called“chemical bonds”. One can imagine the bonds holding the atoms together in the typical shape of a water molecule, like the one shown in the picture. The bonds between the atoms are not rigid like steel rods but are flexible and stretchy like rubber.
Imagine a person’s head and torso to be an oxygen atom, joined to their clenched fists (the hydrogen atoms) by their outstretched arms, which represent the bonds. This simple model indicates quite well how a water molecule can behave. For starters the arms can be waved up and down, imitating a flying motion. If the fists are moved in and out towards the head, by bending the arms in a horizontal plane, an alternative type of interaction between the atoms is illustrated.
Keeping the arms rigid, horizontal at head height and then moving the fists towards one another like a big nutcracker,demonstrates the third way in which the molecule can vibrate. It may sound ridiculous but it’s a good model for the way in which the atoms move, relative to one another, in a water molecule. Technically these movements are called “modes of vibration" but think of it as arm wiggling and most people get the general idea.
The Size of Water Molecules
Consider a glass of water. The glass has a volume of 250ml. It therefore contains about 250g of water. A quick calculation indicates that this mass of water is made up over 8 million, million, million, million molecules of water. A shorter way to express a huge number like this is 8.0exp 24, which means the number 8 followed by twenty four zeros.
That’s a truly staggering figure and very difficult, if not impossible, to visualise as an abstract number . If all the water molecules were set down like marbles, they would cover an area the size of approximately 630 million million square kilometres. This huge area would cover the entire surface of more than one million planets each the size of Earth.
We are led to the simple, but fascinating conclusion, that astronomical number of incredibly small molecules can be packed into a relatively small volume.
Vibrating Bonds and the Natural Greenhouse Effect
Water molecules are able to absorb heat energy, at specific frequencies, because of the ability of their chemical bonds to vibrate in the way that has been described. Other so called “greenhouse” gases, like carbon dioxide and methane, can also do this but water vapour absorbs far more heat energy than all the other greenhouse gases put together.The fact that the water molecules are so small means that a huge number of them are present in the atmosphere and thus their ability to act as energy absorbers is increased.
Made from trillions of tiny vibrating molecules, the most familiar and ubiquitous fluid of all, water is truly a greenhouse gas in a glass and an extremely important agent of climate control.
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