PURPOSE:  To observe the neutralization of acid snow by different minerals.

MATERIALS: 2 large glass or plastic water-tight containers, 2 plastic pails, limestone and granite samples; pH meter, pH color comparators or narrow range pH paper. 

INTRODUCTION:  Precipitation has a natural pH of approximately 5.6 which is slightly acidic.  However, recent studies have measured precipitation with pH values as low as 3 in some areas.  Oxides of sulfur (SO₂ and SO₃) and nitrogen (NO and NO₂) are emitted from power plants, factories, and cars.  Corrosive sulfuric and nitric acid are formed when these oxides combine with water vapor in the clouds.

Acid precipitation in the form of acid snow is especially hazardous. In spring, "acid shock" may result when melting snow runs into lakes and streams.  This surge of acid may be lethal to organisms. Even though the pH may slowly rise during the summer, the damage has been done.

The bedrock in the drainage basin of a lake can reduce the effects of acid on organisms.  The minerals in some bedrock neutralize the acid.  Lakes with this type of bedrock may receive a great deal of acid with little long term change in pH.  In other lakes, neutralization cannot occur because of different soils and bedrock.

In this activity you will test the ability of different minerals to neutralize the acid in snow.  Granite (feldspar and quartz, SiO₂) and limestone (calcium carbonate, CaCO₃), two common types of bedrock, will be used.

PREDICTION: Use what you know about granite, limestone, and the chemistry of acids to make a prediction.

1. Which rock will best neutralize the acid in snow?

PROCEDURE

A.  Line the bottom of one container with a layer of granite and the bottom of a second container with limestone.

B.  To simulate a lake, add distilled water to a depth of 5 cm above the stones.  Record the pH of the water in both containers.

C. Gather 2 buckets of snow from the same location.  Record the pH of a melted snow sample from each bucket.

D. Add half of each bucket of snow to each container.  The same amount of snow should be added to each.  Record the pH of the water in both containers.

E. Allow the containers to sit overnight to permit neutralization reactions to occur.

F.  The following day, record the pH of the water in each container.

DATA TABLE:

Snow pH: __________

CONCLUSIONS:

2. How did the results compare with your prediction?

3. Explain your results based on your knowledge of acids and minerals.

DISCUSSION:

4. The shells of fossil marine animals are often found in limestone deposits hundreds of miles from the nearest ocean.  Based on this fact, speculate on the history of an area rich in limestone deposits.

5. Based on your results, explain why lakes in the Adirondack Mountains of New York State have a serious acid problem while the Finger Lakes region, only about 150 miles away between Buffalo and Syracuse, do not.  (Both receive nearly the same amount of acid precipitation).

6. Define "acid anhydride".

7. What are the major acid anhydrides responsible for the acid precipitation?

8. Write a chemical reaction for the production of sulfuric acid from sulfur trioxide.

9. Write a chemical reaction for the neutralization which occurred in this activity.