How Big Is a Molecule?
We do not as yet know all there is to be learned about molecules. We know through the wonders of chemistry that small as a molecule is, it is still made up of smaller particles called atoms. An atom is the smallest division of anything that can be imagined. We have found by chemistry that even a molecule is capable of being divided, i.e., it is made up of still smaller particles, but molecules are small enough. An eminent scientist, Sir William Thomson, has given us probably the nearest approach to a correct way of saying something of the size of a molecule. “If a drop of water were magnified to the size of the earth, the molecules would each occupy spaces greater than those filled by small shot and smaller than those occupied by cricket balls.”
To get at what water is made of we must separate it through chemistry into its parts or atoms. When we do this we find that a molecule of water is made of three atoms or parts. Two of these are exactly alike and consist of a gas called hydrogen, and the other part is another gas called oxygen, concerning which gases we have already learned much in the answers to other questions in this book. In other words, when we separate water, which is a liquid, into its parts, we change the relations of the molecules in the water which move in irregular lines, into parts which move in straight lines and, when the molecules of a substance, as we have already seen, move in straight lines, the substance becomes a gas. On the other hand, when you freeze water, it becomes a solid (ice), and in doing that you fix the molecules in the water so that they stick to each other.
Men thought for a long time that water was an element like oxygen and hydrogen, i. e., that its molecules could not be separated in its parts and was, therefore, considered one of the things which could not be divided up, but this was due to the fact that it requires a great amount of power to break up the molecules of water.