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Using Acid-Base Indicators to Test Unknown Substances


Pre-Lab Discussion


The science laboratory is not the only place where acids and bases are found. Many items commonly found at home are acids or bases. For example, many of the foods you eat contain acids. Many commonly used cleaning products owe their effectiveness to the fact that they are alkaline, or contain bases.

Indicators are special chemicals that can show whether a given substance is an `acid, a base, or neither. Indicators usually react with an acid or a base to form a slightly different chemical with a different color. Two examples of indicators are litmus paper (blue and red) and pH paper. Blue litmus paper turns red in an acid and stays blue in a base. Red litmus paper turns blue in a base and stays red in an acid. One type of pH paper turns a different color at each of several pH values ranging from 2 to 10.

In this investigation, you will test a number of substances using litmus paper and pH paper.

Problem : How can you determine whether a substance is an acid or base?

Materials (per group)

Materials to be tested

1 spotplate

Ph paper

Ph meter


1)      Procedure Use the four corners of the spotplate for you samples.

2)      Only place the samples in those spots, thus only test 4 per trial.

3)      Fill spotplate half full with a sample. Be sure to keep track of what is in each spot.

4)      Test the pH with the pH paper then with the pH tester.

5)      Fill in the information on your table.

6)      Clean out the spotplate and dry with a towel.

7)      Fill spotplate with 4 more substances.

8)      Continue until you are done.

Be sure to get distilled water.







Sample number

PH paper reading

PH meter reading

Avg. pH



Substance guess



























































Answer these on a separate sheet of paper.

1. Which substances are acids?

2. Which substances are bases?

3. Which substances are neutral?

4. Which substances are the strongest acids? Explain

5. Which substances are the strongest bases? Explain

6. Is litmus paper useful to find the exact pH of a substance? Explain.

Critical thinking

1. Describe 3 situations in which acid-base indicators might be useful in everyday life.

 2. Suppose you are manufacturing a certain type of cosmetic. You know that it can be slightly acidic, but it should not be strongly acidic. Which of the indicators you have just studied will help you determine the degree of acidity? Explain your answer.

3. You may have found that the results you obtained were different from those of other groups. What variables might have affected your results?

4. Design an experiment in which you could investigate which of two antacids is more effective in       neutralizing stomach acid.

5. What was your unknown? Explain.