Explore Air

Acid Rain Lesson Plan

Activity 2 – Understanding the difference between an acid and a base

Time: 1-2 hours

Behavioral Objectives

At the end of this lesson the student will be able to:

  1. Explain the difference between an acid and a base.
  2. Understand that an acid can be made more neutral by adding something which is basic or interacting with a natural buffering agent in nature.
  3. Become familiar with the pH scale.


You will need:

  1. Vinegar
  2. Lemon juice
  3. Tomato juice (pure)
  4. Distilled water
  5. Salt water (3 tbs./1 cup distilled water)
  6. Ammonia
  7. Milk of Magnesia
  8. Alka-Seltzer
  9. Blank Chart (Figure 4) - one per student or group
  10. Enough wide-range (0-14 pH) litmus paper to give each group twenty-one 1-1/2 inch strips
  11. Eight 6-8 oz. cups (Because the students will need to use the comparison chart included with the litmus container, you may wish to obtain enough dispensers for each group to have one.)

Instructions to Teacher

  1. Refer to "Sources of Acid Pollution and the pH scale Figure 1.
  2. Distribute litmus paper and Figure 4 chart to students.
  3. Direct supervision is necessary when working with these materials.
  4. Put one of the seven samples in a cup and pass these among he students or groups for testing. The students should test each sample three times and arrive at an average pH using the following formula:
    Example: Test 1 = pH 2, Test 2 = pH 3, Test 3 = pH 2, Total (2+3+2) = 7
    Formula: Average pH = Total / 3 ( pH = 7/3 or pH = 2.3)

Instructions to Students

  1. Using the wide-range litmus paper, test each different sample three times.
  2. Using these three tests, calculate the average pH of the sample.
  3. Record your results on the data sheet, Figure 4. Include the three pH test figures, the sum of these, and the average pH.
  4. Repeat this for each of the seven samples.
  5. With the help of your teacher, add two Alka-Seltzer tablets to a cup of vinegar. Test this solution for pH.

Questions to Students

  1. How do your results of this test compare with the answers of the rest of the group?
  2. What is an acid? A base? Look for answers in reference books such as encyclopedias, science books, etc.
  3. Which samples are acidic? Neutral? Basic (alkaline)?
  4. Did the pH of the vinegar change when you added the Alka-Seltzer tablets? Why? Hint: Make up a cup of Alka-Seltzer and test it for pH. What does this tell you about the pH change in the vinegar - Alka-Seltzer solution?

updated on 04/20/2006  I   http://www.nature.nps.gov/air/edu/Lessons/Activity2.cfm   I  Email: Webmaster