1. Introduction

In the previous tutorials in this unit, we’ve looked at how water’s polarity and the hydrogen bonding that results affects water’s properties. In this tutorial, we’re going to look at another water-related topic: acids, bases, and the pH scale.

If you’ve taken a previous chemistry course, most of what’s below will be review. And while none of what’s below will show up as questions on the AP Bio exam, it’s important background for understanding many key AP Bio topics.

2. Solutions, Acids, and Bases

When you dissolve sugar, salt, or lemonade in water, you’ve made a solution. In a solution, one substance, the solute, is dissolved in another substance, the solvent. In the solution shown in the image to your left, the solute is sugar, and the solvent is water.

In this tutorial, we’re going to focus on two special types of solutions. Acidic solutions have a sour taste. If you have a cut on your skin, and you put an acidic solution on it, it will sting. Strong acids can cause burns, and even dissolve metals. Almost all fruit juices and sodas are acidic. Sulfuric acid and hydrochloric acid are two strong acids that you might have heard of.

Basic solutions taste bitter. Strong bases have a soapy feel when you touch them. Unlike acids, bases don’t dissolve metals, so they can be used to unclog pipes by dissolving the hair and other debris that might be clogging them, without dissolving the pipe. Many bleaches and cleaning agents are bases. The word “alkaline” means the same as “basic.”

To see if you’ve mastered the concepts above, take the quiz below.

3. The Chemistry of Acids and Bases: What Biology Students Need to Know

What causes a solution to be acidic or basic? A water molecule, or H₂0, can dissociate, or fall apart, to form two charged particles or ions. One of these ions is called a “hydrogen ion,” written as H⁺ (and pronounced “H-plus”). An H⁺ is simply a proton, floating around in a solution. The other ion is written as OH^– (which is pronounced as “O” “H” minus). The OH^– ion is also called “hydroxide.” Here’s the equation for the dissociation of water.

H₂O → H⁺ + OH^–  

In a chemistry class, you’ll learn that the H⁺ immediately combines with a water molecule to form H₃O⁺, also known as the hydronium ion. For biology students, it’s very useful to continue to think of water as dissociating into H⁺ and OH^–

If dissolving something in water causes it to have more H⁺ than OH^–, then the solution is acidic. For example, if the compound HCl (hydrochloric acid) is dissolved in water, the hydrogen ions dissociate from the chlorine as shown below:

HCl→H⁺ + Cl^–  

The added H⁺ ions make the resulting solution an acid.

Conversely, if dissolving something in water causes it to have more OH^– than H⁺, then the resulting solution is a base. For example, when Sodium Hydroxide (lye) is dissolved in water, the sodium dissociates from the hydroxide ion:

NaOH → Na⁺ + OH^–  

If the amount H⁺ is equal to OH^–, then the solution is neutral. Pure water has a pH of 7, but what comes out of the tap is usually a weak acid.

Acidic and basic solutions are measured on the pH (pronounced “P” “H”) scale. On the pH scale, the acids with the most hydrogen ions are at 0 (or rarely, below 0). The most concentrated bases are at pH 14 (or above). A neutral liquid is 7.

The scale works by powers of 10, so a solution with pH 5 is 10 times more acidic than a solution that is pH 6. A solution that is pH 12 is 10 times more basic than a solution that is pH 11.

Here’s a graphical version of the pH scale.

4. Acids, Bases, and pH: Checking Understanding

5. Buffers

A buffer is a substance that when added to a solution, enables that solution to resist changes in pH when an acid or a base is added to it (source: slightly modified from the New Oxford American Dictionary). Blood, for example, resists changes in its pH much more than an equivalent quantity of water. That’s because blood contains buffers that are capable of soaking up hydrogen ions (H⁺) when acids enter the blood, and which release hydrogen ions when blood’s pH starts to rise.

One of the buffers which enables blood to do this is carbonic acid. The formula for carbonic acid is H₂CO_3. Carbonic acid can dissociate by losing a hydrogen ion, making it into the bicarbonate ion (HCO₃^–), which is a base. You can see this in the chemical reaction below.

H₂CO₃ → HCO₃^– + H⁺

Carbonic acid itself is a weak acid. When added to water, a small amount of carbonic acid will dissociate and release H⁺ into the solution (which is why it’s an acid). But if another acid is added to the same solution, then the H⁺ released by the second acid will combine with the bicarbonate ion, creating more carbonic acid. Because carbonic acid is a buffer, that decreases the pH of the solution (making it less acidic) by absorbing the H⁺. By contrast, when a base is added to the solution (causing the pH to rise), more carbonic acid will dissociate, releasing H+ into the solution, causing the pH to fall.

6. Conclusion: Acids, Bases, pH, and Life

Acids and bases are incredibly important to understanding how living things work. One of the most important aspects of homeostasis in any organism is keeping the pH in a cell’s cytoplasm, outside the cell, and or within a body compartment (the stomach, or the inside of a blood vessel) within a very specific range. Exceeding that range, even by a little bit, can be harmful or fatal. We’ve seen above how buffers help maintain the pH of the blood. We’ll continue to see, throughout this course, the variety of adaptations that organisms have for controlling pH.

7. Topic 1.1 Flashcards: Water and Hydrogen Bonding

What now?

• This is the last tutorial in this series about the Chemistry of Water and Hydrogen Bonding. The next AP Bio topic is Topic 1.2, Carbon and the Elements of Life.