About Acids:
Water: Acid and Base
We've been leaving out an important detail in acid chemistry up until now. When an acid is dissolved in water, and hydrogen ions dissociate from the acid molecules, the little hydrogen ions don't just float around by themselves. When a hydrogen ion leaves the acid molecule, it joins a water molecule. Together, the hydrogen ion and the water molecule form a new molecule called a hydronium ion, and its symbol is H3O+.
If you think about it, you'll realize that water is acting as a base when this happens, because it is stealing a hydrogen ion from the aspirin molecule.
Strangely enough, if a base comes near a water molecule, it will take a hydrogen ion from the water molecule. Let's look at a base called ammonia, or NH3. Ammonia dissolved in water makes a good window cleaner, but we're more worried about what ammonia molecules and water molecules do when they come together. When this happens, the ammonia molecule will steal a hydrogen ion from the water molecule.
In this case, the water molecule gave a hydrogen ion to the ammonia molecule. This means it was an acid!
Water is one of those strange molecules that can be both an acid and a base. Watch what can happen when two water molecules come together:
How about that! A water molecule just swiped a hydrogen ion from another water molecule. Here the one that did the swiping acted as a base, and the one that lost the hydrogen ion acted as an acid. But this doesn't happen very often. In any glass of pure water, only 2 in 10 million water molecules will actually do this.
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