Tuesday, April 30, 2013

Chemical Names for Water

I've entered 'grading hell' as Major Projects roll in and I prepare next week to start grading take-home tests.

I've been thinking about chemical names for water. People occasionally come to my blog searching for chemical names for water, and land on my (presumably not very helpful) posts about the longstanding reasons in philosophy of chemistry for not regarding water as H2O but as either a chemical process or active substance composed of a wide number of closely related and interacting molecules of which H2O is the most important -- H2O is still the water-constituting molecule, since if you take a bunch of H2O molecules, you get water as they decompose into other molecules (such as OH) and recompose as H2O; it is, so to speak, the index molecule for water.

In any case, the chemical names for the water-constituting molecule are interesting in their own right. There are different common systems for naming molecules and water is covered by several of them. Four seem to be most important, at least as far as this non-chemist can tell.

The first is just hydrogen oxide; this is a bare structural name. Sometimes people jokingly refer to it as dihydrogen monoxide, but while this doesn't seem to be absolutely incorrect, it's redundant and used only in jokes.

Another gives it the name oxidane. This is a basic structural name, given on the same model as sulfane (two hydrogen, one sulfur) and selane (two hydrogen, one selenium) and azane (three hydrogen, one nitrogen -- also known as ammonia). The -ane indicates that it is a hydride, formed with hydrogen; the rest of the name obviously indicates what is bonded to the hydrogen. It apparently indicates a parent molecule in a group of molecules with similar structures. (I'm not hugely familiar with it or contexts in which it might be used, but it is clearly established by IUPAC.)

Another way to approach naming is by way of considering acids and bases. Acids have their naming conventions and bases their naming conventions. Water is amphoteric, so it actually has both. With certain bases, like ammonia, water acts somewhat like an acid; with certain acids, like hydrogen chloride, it acts somewhat like a base. The acid-name for water is hydroxic acid, although one also sees hydroxylic acid; I'm not sure which one is most correct according to current naming conventions. The alkali name (alkalis are bases that dissolve in water, and obviously if water counts as a base, it's trivially true that it dissolves in water) is hydrogen hydroxide.

Of course, none of these are hugely common; in practice, chemists call H2O either 'water' or 'H2O', depending on the context, and only use these other names when they are doing very specific things. But as Whewell noted a long time, scientific names are not mere trivialities or practical conveniences: they summarize information, and sometimes a great deal of information. There's a lot to a molecule like H2O; we are capable of locking down some of its important features according to different naming conventions.


  1. Vishal Mehra1:45 AM

    Chemically, I believe, water is a dynamic hydrogen bonded network of H2O molecules and possibly other related ions. The hydrogen bonding is crucial for the bulk properties since in its absence even if you take a bunch of H2O molecules, you will NOT get water even if they
    decompose into other molecules (such as OH) and recompose as H2O.

    So water is a giant molecule that may be written as (H2O)_N where N is of order of Avogadro's number.

  2. branemrys7:24 AM

    Hydrogen bonding is a fairly weak bonding, so we'd have to be using 'molecule' a bit loosely, but certainly in a manner of speaking.

    While you're right that the hydrogen bonding is important, we also can't ignore the other related ions, since disassociation into them and association of them into H2O seems to be necessary for many of the properties of water (and given the structure of H2O and the properties of hydrogen and oxygen is inevitable, in any case). It's rather remarkable; water is such a common substance, and yet if you talk to chemists who study it, you find that to an immense degree we are still in the process of learning about how it works.


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