Strong vs. Weak

By now you've experimented with a number of different acids, and while they have some pretty interesting reactions, they haven't been at all scary to handle. So why do acids have such a bad reputation as dangerous chemicals?

The simple answer is that there are two kinds of acids - strong acids and weak acids. You probably won't be surprised to learn that everyday acids, like vinegar, lemon juice, or cola, are weak acids. The really dangerous acids are strong, and you won't be doing any experiments with them! OK, strong vs. weak acids - isn't that just like strong vs. weak coffee? What if you just add more water to a strong acid - doesn't that just make it weak? No, the difference isn't that simple. Some acids are weak, and some are strong, and it doesn't matter how concentrated the acid solution is. You can't turn a weak acid into a strong one, or the other way around.

The important difference between strong and weak acids has to do with those hydrogen atoms that you noticed are present in all acids. The picture below shows this difference. When a molecule that makes a strong acid is dissolved in water, all of the hydrogens (gray balls) separate from the other part of the molecule (green balls), and it's those hydrogens that make the solution acidic. But when a molecule that makes a weak acid is dissolved, very few of the hydrogens (gray balls) separate from the rest of the molecule (purple balls). In a weak acid, most of the molecules stay together.
So the "dangerous" part of an acid is the unattached hydrogen atoms - the more of them, the stronger the acid. Even though strong acids have to be handled very carefully, they are still very important. In fact, you carry a strong acid around with you all the time! Your stomach contains hydrochloric acid, which helps to digest your food.