Cracking the AP Biology Exam

2

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.