BIOLOGY

DOUGHERTY

The pH of Common Substances

Directions:

In this experiment you will use a pH indicator to measure the pH of some common household items. Many foods and household cleaners are either acids or bases. Acids usually taste sour, and bases bitter. Household cleaners are poisons so you should never taste them.

Materials
  • pH paper and color chart (range pH 2 to12)
  • vinegar
  • distilled water
  • easy off oven cleaner
  • milk
  • lemon juice
  • ammonia
  • orange juice
  • 3 beverages (cola, carbonated non-cola, milk)
  • 10 mL graduated cylinder
  • 8 small beakers, clear cups or glasses
  • 1 clean stirring rod or spoon
Instructions
  1. Dip an unused strip of pH paper into the sample, compare with the color chart, and record the result in your data table. Repeat the same process for the remaining samples. Be sure to use a clean, unused strip of pH paper for each one.

Data Table

Sample Name / pH / Acid or Base?
Vinegar
Distilled Water
Easy -Off
Lemon juice
Ammonia
Orange juice
Cola
Biuret’s Solution
Milk
Questions and Conclusions:
  1. Locate each of the tested substances on the pH scale below. Place a vertical arrow to correspond to the substance tested. Label each arrow with the name of the substance tested.
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0 2 4 7 10 12 14
ACID NEUTRAL BASE
  1. Is the vinegar an acid or a base?
  2. Is ammonia an acid or a base?
  3. Did distilled water have a neutral pH?
  4. Were you surprised that distilled water did not have a neutral pH? Why do you think this happened?
  1. What is the pH of cola soda and orange juice?
  1. Explain why you think we are not hurt by drinking cola soda or orange juice even though they both have an acidic pH.
9.Was milk acidic or basic?
10.What variables were controlled during this experiment?

Questions and Conclusions - ANSWERS

  1. Locate each of the tested substances on the pH scale below. Place a vertical arrow to correspond to the substance tested. Label each arrow with the name of the substance tested.

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0 2 4 7 10 12 14

ACID NEUTRAL BASE

Accept reasonable results from laboratory experimentation

  1. Is the vinegar an acid or a base?

Vinegar is an acid, and in this experiment it will display a pH of about 4. Vinegar at pH 4 turns pH paper yellow and most other pH indicators red.

  1. Is ammonia an acid or a base?

Ammonia is a base and in this experiment it will display a pH of about 12. Bases turn most pH indicators blue.

  1. Did distilled water have a neutral pH?
No
  1. Were you surprised that distilled water did not have a neutral pH? Why do you think this happened?

PURE distilled water would have tested neutral, but pure distilled water is not easily obtained because carbon dioxide in the air around us mixes, or dissolves, in the water, making it somewhat acidic. The pH of distilled water is between 5.6 and 7. To neutralize distilled water, add about 1/8 teaspoon baking soda, or a drop of ammonia, stir well, and check the pH of the water with a pH indicator. If the water is still acidic, repeat the process until pH 7 is reached. Should you accidentally add too much baking soda or ammonia, either start over or add a drop or two of vinegar, stir, and recheck the pH.

It is NOT expected that students would have this detailed a response – reasonable answers or hypothesis should be accepted.

  1. What is the pH of cola sodas and orange juice?

The colas are acidic, primarily because they contain carbon dioxide to make them fizz, and carbon dioxide and water produce carbonic acid. The pH of these beverages varies with the amount of carbon dioxide and other ingredients in them, but it is usually below 4. The orange juice is also an acid with a pH around 5. This is due to the acetic acid and citric acid content of oranges.

  1. Explain why you think we are not hurt by drinking cola soda or orange juice even though they both have an acidic pH.
Accept reasonable answers
8.Was milk acidic or basic?

Milk can be slightly basic or slightly acidic depending on its age and how it was processed at the dairy.

  1. What variables were controlled during this experiment?

Volume of each sample, the pH indicator, same type of beakers or cups, temperature if all samples were at room temperature, accept reasonable responses.