) This unit has introduced you to some of the general properties of acids and bases. You’ve learned about the pH scale that helps tell you about the relative strength of acids (pH lower than 7) and bases (pH higher than 7). You’ve also learned about a few different compounds, called indicators, that can help you visually determine the pH of a liquid. Today, we will be making and using our own indicator to help us determine the pH of some common household liquids.

Learning Goals:

Students will follow a procedure to make their own indicator solution
Students will use their indicator to make observations of different household objects
Students will make predictions about the properties of the observed liquids
Students will explain the chemistry of a neutralization reaction

Vocabulary:

Acid: A substance that generally increases the concentration of H+ (H3O+) ions in a solution
Base: A substance that generally decreases the concentration of H+ (H3O+) ions, or increased the concentration of OH- ions in a solution
pH Scale: The scale used to quantify the strength of an acid or base relative to pure water (with a pH of 7)
Indicator: A compound used to visually determine the relative pH of a solution
Hydronium Ion: H3O+. Often created when an acid donates an H+ ion to a solution.
Hydroxide Ion: OH-. Often created when a base donates an OH- ion to a solution, or when a base removes an H+ ion from a solution.
Neutralization: A reaction involving an acid and a base where H+ ions combine with OH- ions to create neutral water (H2O)

Part 1: Making the indicator

Material Needed:

One head of red cabbage (green cabbage will NOT work)
Large kitchen knife and cutting board
Large stock pot
Water
Stove or other controlled heating surface
A pitcher or large bowl
A strainer or colander
Procedure:

Using the cutting board and kitchen knife, cut up your head of red cabbage into small pieces.
Place a few cups of your cut-up cabbage (2-4 cups will be enough) into your large stockpot
Pour water into your stock pot until the water level just covers the cabbage.
Turn the stove on to high heat and bring the water to a boil
Once the water starts boiling, turn of the stove and allow the water to cool (20-30 minutes)
Set out your storage container for your cabbage indicator solution, either a pitcher or a large cup or bowl.
Strain the cabbage mixture into your storage container, making sure to catch any large chunks of cabbage. Set aside your cabbage indicator solution for use later. You should have a few cups worth of indicator

Your cabbage indicator should have a deep purple color. Red cabbage contains the pigment anthocyanin that changes color when it is mixed with an acid or base. The pigment turns red when mixed with acids and turns blue or bluish-green when mixed with basic solutions. The amount of color change is related to the strength of the acid or base. A deeper red color would indicate a stronger acid than a light red or red-purple color.