Water: Schizophrenic or Versatile?

I take a brief divergence from the recent topic of acids and bases to talk about something relevant but tangential: water. As we know, water is necessary for life on the planet. Its chemical formula is H2O, and, as freshman chemistry students are told until their ears bleed, when you draw this molecular structure, the bent shape of the molecule makes it polar.

Essentially, a molecule being polar means that the electrons in the bonds of the atoms making up the molecule are shared unevenly, resulting in partial charges on different parts of the molecule. In the case of water, you can think of the oxygen as an “electron-hog” (or as being more electronegative), pulling electrons more toward itself and making itself slightly more negative than the hydrogens.

Water’s polarity is very important, as it contributes to many of its unique properties. For example, because water is polar, it can hydrogen bond with itself. Hydrogen bonding entails the partially-positive hydrogens of one water molecule interacting with the partially-negative oxygens of other water molecules in a way that weakly “bonds” them together. Hydrogen bonding is the strongest of all intermolecular forces, and plays an important part in making water less dense when it freezes. Any child straight out of a general science class knows that, because water is less dense frozen, it floats on the surface of liquid water and insulates any body of water that freezes in the wintertime. This protects marine life and keeps our oceans from freezing solid. A pretty important function, I would say.

However, this isn’t by any means the only cool thing that water does. One of water’s party tricks is that it acts as both an acid and a base, and as such, participates in acid-base reactions with itself.

Let’s step back and think about this for a moment. From our overview of acids and bases, we know that an acid is a proton-donator, and a base is a proton-acceptor. That means that, if water were to react with itself as both an acid and a base, some water molecules would have to donate protons, and others would have to accept them.

Turns out, this is exactly what happens, and this nifty little trick is called water auto-ionization. Consider the following reaction:

H2O (l) + H2O (l) ⇌ H3O+ (aq) + OH (aq)

Here, we have two water molecules reacting with each other. One donates a proton to another, forming hydronium (H3O+) and hydroxide (OH). As you can see from the above reaction, when water dissociates or self-ionizes, it forms equal amounts of hydronium and hydroxide. The concentration of hydronium directly relates to the pH of a solution, whereas the hydroxide concentration directly relates to the pOH. Since pOH is simply 14 – pH, when pH = pOH, both equal 7. As we know, a pH of 7 indicates a neutral solution. Thus, pure water, although it undergoes an acid-base reaction with itself, remains neutral.

As seen by the above reaction, the acid-base reaction of water with itself goes to equilibrium instead of completion. There are other acids and bases that do this, as well. What makes water and those acids and bases different from other acids or bases that dissociate completely? The difference, turns out, is one of acid and base strength.

Questions? Comments? Come at me, bro!


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