Lab Activity 14: Using Red Cabbage to Identify Acids and Bases

Background:

Acids ionize in aqueous solution to produce hydronium ions (H3O+). The strength of an acid depends on the degree to which it ionizes. Strong acids ionize almost completely, while weak acids ionize to a lesser degree. Bases dissociate in aqueous solution to produce hydroxide ions (OH-). The properties of acids depend on the presence of free H3O+ ions as the predominant ion in a solution; and the properties of bases depend on the presence of free OH- ions as the predominant ion in a solution.

Well, acids react with hydroxide ions (OH-), which are produced by bases in aqueous solutions, to form water (H2O) and a salt.

This reaction is called a neutralization reaction (neutralizing the acid and base effect by producing H2O and a salt). A salt is an ionic crystalline compound. (Figure 1).

There is a measure of the strength of Acids and Bases called the pH scale. This scale ranges from 0-14 were the relative strengths of the acid or base are measured. The zero end indicates a strong acid and the 14 end indicates a strong base. Precisely in the middle, 7, indicates that the solution is neither basic or acidic, it's neutral. Therefore orange juice (pH=3.5) is a weaker acid than battery acid (pH=1.0). Get it?

Materials:

Safety:

Always wear safety goggles and a lab apron or coat when working in the lab.

Procedures:

Preparing the Red Cabbage Indicator: Obtain several leaves of red cabbage and 1 L of distilled water. Bring the water to a boil, add the cut-up leaves and mix well. Allow the solution to cool.

Part A
· Obtain 15 ml of each solution from beakers 1-3. Using the computer and pH probe, determine the pH for the given solutions and record your data in Table 1.

Part B
· Add 5-8 drops of cabbage extract to each of the test tubes in Part A. In Table 1, record any changes in color that may have occurred following the addition of the cabbage extract.

Part C
· Select one of the household items from those available and add approximately 15 ml of the solution to a separate test tube. Add 5-8 drops of cabbage extract to the test tube. Record any color changes in Data Table 2. Estimate the pH of the item by comparing the color change observed with those that were produced in Part A and Part B. Repeat Part C for each household product.

Data Tables:

Table 1: pH Determination of Given Solutions

Table 2: pH Determination of Household Substances