TOPICS:  Minerals, acids and bases, ecology

LEVEL:  Middle and high school

TIME:  Lab procedure takes 15-30 minutes on first day, 10 minutes on subsequent day; additional time necessary to discuss predictions and analyze results.

ADVANCE PREPARATION:  This activity can be done as a class demonstration or, if enough containers are available, by groups of students .

MATERIALS: If available, pH meters will give the best results.  Inexpensive color comparators (similar to swimming pool test kits) are the next best choice and are available from science supply houses. You may wish to test the local bedrock or soil in a third container. Container size is unimportant as long as the containers are similar.  Possible containers are aquariums, plastic pails or plastic bottles.  Avoid metal containers as they may react with the acid .

PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: The geology lab test distinguishing limestone and its mineral constituent calcite is its reaction with dilute HCl; the fizzing that occurs is the carbon dioxide (CO₂) given off as the calcium carbonate neutralizes the acid. Limestone (CaCO₃) will best neutralize the sulfuric acid (H₂SO₄) in acid snow as follows:

SO_3(g) + H₂O_(l) ---------> HSO₄^-_(aq) + H⁺_(aq)

CaCO_3(s) + H⁺_(aq) -----> Ca⁺⁺_(aq) + HCO₃^-_(aq)

HSO₄^-_(aq) + Ca⁺⁺_(aq) + HCO₃^-_(aq) ----> CaSO_4(s) + H₂O_(l) + CO_2(g)

STEP C: The snow should be collected from the same spot; snow from different locations may have slightly different pH values .

STEP E: You may wish to allow more time for the reactions to occur.

QUESTIONS 4-9 require higher-order thinking skills.  We recommend that you discuss these questions in class prior to having students write out their answers.  You may wish to omit questions 6-9 for those students without chemistry background .

2. Answers will vary .

3. Calcium carbonate reacts easily with acids yielding water, carbon dioxide, and a salt.  Feldspar and quartz are relatively unreactive with acids .

4. Areas with limestone deposits were once covered with sea water.  Limestones are sedimentary rocks; some precipitate directly from water rich in calcium carbonate while others are composed predominantly of the calcium carbonate remains of marine life.  Often the remains of sea life are incorporated into limestone bedrock .

5. The Adirondacks have a granite base while the Finger Lakes lie on a predominantly limestone base .

6. An acid anhydride is a soluble, nonmetallic oxide which, when added to water, produces an acidic solution .

7. SO₂, SO₃, NO, NO₂

8. SO_3(g) + H₂O_(l) ------> HSO₄^-_(aq) + H⁺_(aq)

9. CaCO_3(s) + H⁺_(aq) --> Ca⁺⁺_(aq) + HCO₃^-_(aq)