Have you ever sipped a drink that tasted sour? The sour taste was caused by the acid in what you drank.
What is acid? Generally, it is an aqueous (water) solution that contains hydrogen ions. A hydrogen ion (H+) is a hydrogen atom with the electron removed. Because the electron is not present, the hydrogen ion has a positive charge.
The positive charge of the hydrogen ion makes it chemically active. One example would be the chemical action of the hydrogen ions on your taste buds, producing the sour taste.
Acids can be strong or weak, depending on the concentration of hydrogen ions. Vinegar is a weak acid, having a low concentration of hydrogen ions. The high concentration of H+ in a strong acid, such as sulfuric acid, can eat away metal!
Scientists have a numerical scale for the relative strength of acids called the pH scale. The H in pH refers to the concentration of hydrogen ions present in the solution. The p comes from “potenz,” a German word referring to power or potential.
The pH scale ranges from 0.0 (very high concentration of hydrogen ions) to 14.0 (very low concentration of hydrogen ions). Liquids with a low pH are highly acidic and have a sour taste. Liquids with a pH of 7.0 have a neutral taste. Pure water has a pH of 7.0. Liquids with a pH higher than 7.0 have a bitter taste. Such liquids are called bases.
The pH range of 0.0 to 7.0 is called acidic, and the range from 7.0 to 14.0 is called basic. For more information about the pH scale, go to Digging Deeper .
A pH indicator is a substance whose change of color indicates a change of pH in an aqueous solution.
In this activity, you will make a pH indicator from something you may have eaten before – namely red cabbage! As you add other substances to water drained from soaked red cabbage, you will see color changes indicating a change in the pH.
- Red cabbage
- Coffee filter or paper towel
- Container for water (at least 250 ml or 1 / 2 pint)
- Three transparent cups (about 100 ml or 3 ounces) or other similar containers
- Hot water (e.g., from a faucet, heated in a microwave oven, etc.)
- Thermometer
- Vinegar
- Baking soda
- Safety goggles
- Tongs or fork
- Eyedropper or drinking straw
- Craft stick or toothpick
- Extra sticks or drinking straws for stirring
Peel off several red cabbage leaves and tear them into pieces about 2.5 cm (1 inch) square. Place the red cabbage pieces into the container.
Some of the purple color from the cabbage may get on your fingers. Wash this away with soap and water as soon as you are finished with this step to avoid staining your clothes or other materials. |
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Cover the cabbage pieces with hot water (not boiling water) and soak for about 30 minutes. As a safety precaution, use the thermometer to make sure that the temperature of the water is not higher than 50 degrees Celsius (about 125 degrees Fahrenheit).
With tongs or a fork, remove and discard as many cabbage pieces from the container as you can. |
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Carefully pour the remaining cabbage water from the larger container through the coffee filter into the three smaller cups or other transparent containers. We recommend that you wear safety goggles whenever pouring liquids.
Note the purplish or dark blue color. If the cabbage water has a pH of about 7.0, which is typical for plain tap water, the color will be more purplish. If your water is treated by a water supply facility, it may have a pH around 8.0, resulting in a dark blue color. This water color is your baseline. You will be comparing other colors to this as you proceed. |
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Use a drinking straw or eyedropper to transfer vinegar from its container to one cup of cabbage water, five drops at a time. Stir the cabbage water after each vinegar transfer. Note any color change. Continue transferring vinegar five drops at a time until the color of the cabbage water has changed to a color different from that of the original cabbage water.
Use a craft stick or toothpick to transfer baking soda from its container to another cup of cabbage water, just a few grains at a time. Stir the cabbage water after each baking soda transfer. Note any color change. Continue transferring baking soda a few grains at a time until the color of the cabbage water has changed to a color different from that of the original cabbage water.
Investigate other household liquids and dissolvable solids to see how they change the color of red cabbage water.
Click here to see our results. |
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