A chemical change takes place when an acid reacts with a base. Depending on the pH strength of the acid or base, neutralization may occur. If a base neutralizes an acid, there is no longer an acid or a base in solution and the solution would, therefore, lose its acid or basic properties. A neutral solution has a pH value of 7.

PART 1

Materials:

graduated cylinder
6 test tubes with rack
3 droppers
3 50ml beakers
vinegar (pH 2)
ammonia (pH 10)
Universal indicator (pH 0-10)

Note to Student: Refer to this table to complete your data table.

(Note to the teacher: Prepare a set of test tubes ahead of time as students have a difficulty matching actual colors to expected colors.)

pH Color Acid/Neutral?

2 orange-red acid
3 red-orange acid
4 orange acid
5 yellow-orange acid
6 yellow-green acid
7 green neutral

Procedure:
1. Number the test tubes 1 - 6.
2. Add 5ml vinegar to each test tube.
3. Add 2 drops of universal indicator to each test tube.
4. Add ammonia to test tube #1 drop by drop until it reaches a pH of 2.
Count the drops as you add hem to the test tube.
Record the number of drops of ammonia in the data table.
Record the final color of the solution by comparing it to the teacher’s sample.
5. Add ammonia to test tube #2 until it reaches a pH of 3.
Record the number of drops of ammonia and the final color.
6. Repeat with test tube 3 to reach a pH of 4. Record results.
7. Repeat with test tube 4 to reach a pH of 5. Record results.
8. Repeat with test tube 5 to reach a pH of 6. Record results.
9. Repeat with test tube 6 to reach a pH of 7. Record results.
10. Answer the key questions before cleaning up.
11. Dispose of the chemicals and materials as directed by your teacher.

Data Table:

TEST TUBE #DROPS COLOR pH ACID, BASE, OR
NUMBER AMMONIA NEUTRAL

1
2
3
4
5
6


Key Questions:

1. Why did the pH range change as you added ammonia to the vinegar?
2. What happened to the acid as it reached pH 7?
3. How do you know that the vinegar was neutralized?
4. Why was ammonia used to change the pH of the acid?
5. What do you think would happen if you added vinegar to the solution in test tube 6?
6. What do you think would happen if you added ammonia to the solution in teat tube 6?
7. How much stronger was the solution in test tube 4 than in test tube 5?
8. Why is universal indicator a better indicator for this investigation that litmus paper?

PART 2

Materials

4 test tubes with a rack
3 droppers
3 50ml beakers
graduated cylinder
vinegar (pH 2)
ammonia (pH 10)
Universal Indicator (pH 0-10)

Note to the Student: Refer to this table to complete your data table.

pH Color Neutral or Base?

7 green neutral
8 blue-green base
9 green-blue base
10 blue base

Procedure:

1. Number the test tubes 1 - 4.
2. Add 5 ml of ammonia to each test tube.
3. Add 2 drops of Universal indicator to each test tube.
4. Add vinegar drop by drop to test tube 1 until it has a pH of 10.
Count the drops of vinegar as you add them to the ammonia.
Record the number of drops in the data table.
record the final color in the data table.
5. Repeat with test tube 2 until it has a pH of 9. Record your results.
6. Repeat with test tube 3 until it has a pH of 8. Record your results.
7. Repeat with test tube 4 until it has a pH of 7. Record your results.
8. Answer the Key Questions before cleaning up.
9. Dispose of the chemicals and materials as directed by your teacher.
10. Complete the Analysis Questions.

Data Table:

TEST TUBE #DROPS COLOR pH ACID, BASE OR
NUMBER VINEGAR NEUTRAL

1
2
3
4

Key Questions:

1. Explain the relationship of the pH range to the amount of vinegar added.
2. What was happening to the characteristics of the ammonia as the pH got closer to 7
3. When was the base no longer a base and how did you prove it?
4. Why was vinegar used to neutralize the ammonia?

Analysis Questions:

1. How is the role of Universal indicator different from that of other indicators like litmus paper or bromothymol blue and yellow?
2. Why would you not use phenolphthalein as the indicator in this investigation?
3. Explain why you think pH paper would be a better or not be a better indicator for this investigation.
4. Bleach feels slippery. Is it a base? How could you test your hypothesis?
5. Read the label on the Universal Indicator. Why did the investigation stop at pH 10?
6. Oh No! You just got Universal Indicator on your hands. Your hands are stained orange. What is the pH of the skin on your hands?
7. How many times more Hydrogen ions are dissolved in test tube 3 (Part 2) than in test tube 2?
8. During neutralization, the hydrogen ions that came from the acid combine with the OH ions that came from the base. What neutral compound do you think the ions form?
9. If there were fewer Hydrogen ions that OH ions, would the solution be acid, base or neutral?
10. If you had a strong, unknown acid in your basement, what are some things that could be done to dispose of it safely?