That's an interesting question! In order to answer it, we need to first make the distinction between dissociation and impurity ions. Most water contains both. In fact, the complete elimination of ions from liquid water is next to impossible!
Dissociation ions are ones formed from water molecules themselves. Even at lower temperatures, these molecules are in constant motion. Now and then, this motion loosens the molecular bonds, causing H2
0 to break apart and its components to recombine. A water molecule that loses a hydrogen becomes a negatively charged ion, known as "hydroxide" (OH-
), while a water molecule that gains a hydrogen becomes a positively charged ion, known as "hydronium" (H3
). This process is commonly referred to as the "self-ionization" of water, and it is given by the reversible chemical reaction shown below.
Here's the catch... Water does
self-ionize, but not very much. At 25°C (room temperature) and standard atmospheric pressure, one liter of pure, neutral water contains only 0.0000001 moles of each dissociative ion. This is a very, very small amount compared to all of the H2
O -- nearly 56 moles! Thus, it is clear that dissociation ions make up only a tiny fraction of water under these conditions.
Impurity ions, on the other hand, are ions of entirely different elements (usually metals) that find their way into our water by simply leaching in from materials with which it comes into contact -- rocks, pipes, etc. Water loves
to interact with other substances in this way... It even leaches carbon dioxide (CO2
) from the air to form carbonic acid, which then dissociates into hydrogen (H+
) and hydrogen carbonate (HCO3-
) ions. There are plenty of methods of removing these impurities, but even so-called "deionized" water is still not truly free of ions. Instead, they're simply present to a lesser degree. In fact, commercial suppliers of deionized water must usually include expiration dates on their products, because they tend to leach impurities and -- over time -- ionize again.
Now, to answer your question...
There is not much of a correlation between temperature of water and impurity ion content because impurity content depends on so many other factors, but water's self-ionization increases with temperature, creating more hydronium and hydroxide ions. This is a relatively simple concept to grasp: As water is heated, its energy increases, and more energy means more motion and a greater tendency to dissociate.
To summarize, all water has ions, but hot water has more ions from dissociation.
Hope that helps!
(published on 10/11/2008)