We saw in Unit 4 that for reactions in which water is a reactant, the water molecules split. When a water molecule splits, it produces a hydrogen ion (H⁺) and a hydroxyl ion (OH^-). We saw that most of these reaction, including hydrolysis, hydration and dehydration, were catalyzed by one or the other of these two ions. It turns out, that even in pure water, there is also a small number of water molecules that split to form hydrogen and hydroxyl ions.

The concentrations of hydrogen ions and hydroxyls ions in pure water is quite low. It is around 0.0000001 moles/liter = 1 x 10^-7 moles/liter, which is much less than the molar concentration of pure water, which is 55.5 moles/liter. This means that only one in every half-billion water molecules spits to form a hydrogen ion and a hydroxyl ion. Even though these numbers are very small, these species are still very reactive and their concentrations are very important in biological systems. For example, if the hydrogen ion concentration in blood, as expressed by pH, drops from 7.4 to 6.8, coma and death can result.

The pH is an alternative may of expressing hydrogen ion concentrations in solutions. The pH of a solution is defined as:

where [H⁺] represents the concentration of hydrogen ions in moles/liter. The pH values are often used instead of molar concentrations because they compress a rather wide range of concentration values, which for H⁺ can vary from 1 mole/liter to 0.000000000000001 mole/liter, to a pH value range that varies from 0 to 15. When this is applied to the hydrogen ion concentration in pure water:

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The Operational Definition of Acids and Bases

By "operational" we mean you do some kind of operation to determine if something is an acid is an acid or a base. For example, you could after dissolving a substance in water, then taste it. Typically acids will taste sour, while bases taste bitter. This is a rather subjective way of distintinguishing acids from bases, and is also risky from a health stand point. Another way to test for acids and bases is to use chemicals called acid-base indicators. These chemicals turn different colors in the presence of acids or bases. For example, the indicator litmus turns pink when exposed to acid and blue when exposed to base. And still another way is to use an instrument, called a pH meter, which measures the pH of a solution. When an acid is added to pure water, it has been found that the pH of the solution drops below 7.0. On the other hand, if a base is added to pure water, the pH of the solution rises above 7.0. These operational definitions of acids and bases are summarized in the table below:

	Acid	Base
Operational Definition	When added to water pH < 7	When added to water pH > 7

Let's strap on our safety goggles and go into the lab and try this out. Below is a virtual lab simulator which we will use to investigate the chemical properties of acids and bases. You can either follow along by using the lab shown in Figure 1, or open the lab up in a separate window by clicking here.

This lab will allow us to mix up solutions and determine the pH of those solutions using a pH meter. The blue area in the center is the workbench. On the left is a stockroom where you can obtain chemicals. The lab also provides you with glassware, and here is the best part, you do not need to clean your glassware when you are finished using it. To get started, you might want to view this video, which highlight these various tools and resources and demonstrates how to use them.

I have created a stripped down stockroom that contains just the reagents that we will be using in this elaboration. You can load this setup by selecting "Load Homework..." from the "File" menu in the Virtual Lab:

Figure 2: Selecting a Lab. This is the dialogue box that appears when you select "Load Homework..." from the "File" menu. For this Elaboration, you should select "Elaboration - Definitions of Acids and Bases" in the "Unit 6" folder. You can do this either with the active lab window in Figure 1, or open the lab up in a separate window by clicking here.

After selecting the "Elaboration- Definitions of Acids and Bases" lab setup, the window should look like that shown below in Figure 3. The Stockroom should contain the limited number of reagents shown in this figure.

Figure 3: The Elaborations - Definitions of Acids and Bases Lab Setup. The Stockroom now shows only the reagents that you will need for working through this Elaboration. Also shown, is the popup menu where you can check out glasswear.

Let us use the lab to make 0.01 M solution of HCl, and measure it's pH value. The Stookroom does not contain a 0.01M solution of contains a 1.0 M solution of HCl. A 0.01 M solution can be made by diluting 1.0 mL of the 1.0 M HCl solution with 99.0 mL of water. Try this on you own. Figure 4 shows what it should look like just before the 1 mL of 1.0 M HCl is added to the 99 mL of water.

Figure 4: Using the Virtural Lab Simulator to make 100 mL of a 0.01 M solution of HCl.

When you have finished mixing up the 0.01 M solution of HCl, measure its pH by clicking on the beaker containing the solution (Figure 5).

Figure 5: Measuring the pH of the 0.01 M solution of HCl. Note that the pH has dropped from 7.0 for distilled water, to pH 2.0 for the 0.01 M HCl.

As shown in Figure 5, the pH has dropped to 2.0. By our operational definition, that is what makes HCl an acid; when HCl is dissolved in water is causes the pH to drop below 7.0