Cracking the AP Biology Exam
The Chemistry of Life
THE ACID TEST
We just said that water is important because most reactions occur in watery solutions. Well, there’s one more thing to remember: Reactions are also influenced by whether the solution in which they occur is acidic, basic, or neutral.
What makes a solution acidic or basic? A solution is acidic if it contains a lot of hydrogen ions (H+). That is, if you dissolve an acid in water, it will release a lot of hydrogen ions. When you think about acids, you usually think of substances with a sour taste, like lemons. For example, if you squeeze a little lemon juice into a glass of water, the solution will become acidic. That’s because lemons contain citric acid.
Bases, on the other hand, do not release hydrogen ions when added to water. They release a lot of hydroxide ions (OH–). These solutions are said to be alkaline. Bases usually have a slippery consistency. Common soap, for example, is composed largely of bases.
The acidity or alkalinity of a solution can be measured using a pH scale. The pH scale is numbered from 1 to 14. The midpoint, 7, is considered neutral pH. The concentration of hydrogen ions in a solution will indicate whether it is acidic, basic, or neutral. If a solution contains a lot of hydrogen ions, then it will be acidic and have a low pH. Here’s the trend:
An increase in H+ ions causes a decrease in the pH.
You’ll notice from the scale that stronger acids have lower pHs. If a solution has a low concentration of hydrogen ions, it will have a high pH.
One more thing to remember: The pH scale is not a linear scale—it’s logarithmic. That is, a change of one pH number actually represents a tenfold change in hydrogen ion concentration. For example, a pH of 3 is actually ten times more acidic than a pH of 4. This is also true in the reverse direction: A pH of 4 represents a tenfold decrease in acidity compared to a pH of 3.