It becomes hard, in our information-saturated world, to find things to say that are novel and interesting. What do I have to add to the subject of water that someone else hasn't thought of before?
I suppose I have one good piece of information, and one that will seem ridiculous at first. The most basic fact of chemistry, known by everyone, is the molecular formula for water: H2O. But, I will demonstrate, this is not actually what water is.
In chemistry, a distinction is made between a substance, which is a collection of identical molecules, and a mixture, which is (as the name suggests), many different substances mixed together on a very fine level. Conventionally, water is a single substance, while (for example) air is a mixture of several different substances, molecular nitrogen and oxygen, carbon dioxide and elemental argon, amongst many others. Distinguishing between a mixture and a substance is one of the basic tasks of thinking about things in a chemical way.
So, conventionally, water is a substance, H2O. But let us look at some of the properties of water, as we observe them in every day life.
- Water conducts electricity: That water is electrically conductive is a scientific fact that was impressed on me by my parents in kindergarten, or before. Water's electrical conductivity is quite high, and quite dangerous.
- Water supports life: Of course, water is necessary to support our life, but things can actually live in water. Fresh and salt water support a bewildering host of plants and animals, in every phylum. Again, this is a kindergarten or preschool level scientific fact.
- Water corrodes and interacts with the earth around it: This is a little less apparent, and a little less important, than the other two mentioned above. But water's ability to erode through some substances, and to corrode and destroy the rock and earth it moves amongst, is responsible for shaping most of our planet.
What is important about these three properties of water is that, while being properties of water, they are not properties of H2O. H20 is a very poor conductor of electricity, its conductive power only comes from ions dissolved within it. H20 is not able to support life, since everything that lives within it must breathe oxygen. And H20 does not have the power to dissolve certain materials, since much of that comes from the acid or base added to H20 by dissolved chemicals, or by the cavitation effect of dissolved gases.
In other words, these are three (amongst others) common properties of water, that are not properties of H20. This being so, does it make sense to say that water is H20?
The obvious rejoinder is that water is H20, but that it has other substances dissolved in it, which alter its properties. Given the fact that most (fresh) water is, by weight, at least 99% H20, it seems reasonable to describe water as H20 with some impurities. However, there are two problems with this. First, following this line of reasoning, someone could also claim that air was nitrogen, and that the other constituents of air were just impurities. Most chemists would not posit such a thing. Secondly, some of the properties that substances other than H20 bring to water are essential to our understanding of water. The fact that water has enough dissolved oxygen gas in it to support life is not an incidental curiosity.
Thus, I would posit that water and H20 are not identical. Instead of saying that water is a substance that contains small numbers of impurities, it makes as much sense to think of water as a mixture, with the basic physical properties of it being formed by H20, but with many of its important properties coming from other substances.