Comparing Acids and Bases

Name ______Date ______

Question: What are the identifying characteristics of acids and bases?

Introduction:

Acids and bases look the same. Scientists use indicators, substances that change color, to distinguish between acids and bases. Other tests can also be useful: What happens if you drop a piece of metal into a sample of acid and a sample of base? How do the conductivities of acids and bases compare? In this laboratory investigation, you will examine some acids and bases and compare them with respect to several characteristics. Finally, you will examine an unknown substance to determine if it is an acid or a base.

Materials: Acids: hydrochloric acid, acetic acid; Bases: sodium hydroxide, ammonium hydroxide; litmus paper; phenolphthalein; water; magnesium; conductivity tester, spot plate

Procedure:

1.  Add 5 drops of each substance to your spot plate, as shown in the diagram below.

1. / 3. / 5.
2. / 4. / 6.

2.  Dip a piece of red litmus into the unknown solution, then dip a piece of blue litmus into it. Record your observations in the data table. If neither color changes write NO CHANGE. If both pieces of litmus are blue after the test write BLUE. If both pieces of litmus are red after the test, write RED. Repeat this procedure using fresh paper in the remaining spots.

3.  Test the conductivity of the unknown solution by dipping the electrodes of the conductivity tester into the liquid. Look carefully to see if the LED bulb is lit at all. If it is that means that the substance conducts electricity; write YES in the data table. If the LED does not light up at all the substance does not conduct; write NO. Repeat for all substances.

4.  Put one drop of phenolphthalein into each of the spots. Note the color. Record your observations in the data table.

5.  Drop a piece of magnesium into each of the spots. Look for the presence of bubbles. Record your observations in the data table.

6.  To clean up carefully rinse off your spot plate and dry it. Put caps on all bottles.

Observations:

Spot / Name / Formula / Litmus Test / Phenolphthalein / Reaction
with Magnesium / Conductivity
1 / Unknown / ?
2 / Water / H2O
Acids
3 / Hydrochloric Acid / HCl
4 / Acetic Acid / HC2H3O2
Bases
5 / Sodium Hydroxide / NaOH
6 / Ammonium Hydroxide / NH4OH

Conclusion Questions:

1.  What do the formulas of all acids have in common? What do bases have in common?

2.  How do acids and bases compare with respect to the litmus test?

3.  How do acids and bases compare with respect to conductivity?

4.  What color is phenolphthalein in acid? In base?

5.  How do acids and bases compare with respect to their reaction with magnesium?

6.  Is the unknown an acid or a base? How do you know?