TOPICS:  Weather, acids and bases, ecology

LEVEL:  High School

TIME:  One or more class periods to collect snow samples and measure pH, depending on location and number of sampling sites.

ADVANCE PREPARATION: The best time to do this activity is right after a snowstorm.  For several weeks ahead, collect weather maps from local newspapers. Daily wind direction data in the newspaper may be used as a general guide to the source of snowstorms .

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. Even narrow-range pH paper may not be sensitive enough to detect slight differences in snow pH.  .

PREDICTIONS: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: pH in various places should not be very different, since snow pH is primarily determined by atmospheric conditions rather than ground conditions.  pH in snow layers from different snowstorms are more likely to vary, depending on the origin of the storms .

PROCEDURE: You may wish to have different groups of students do the snow site procedure (steps A-C) and the snow layer procedure (steps D-H) .

STEP A: In class discussion, have the students generate numerous possibilities for sampling. Keep a copy of this list, including names of students analyzing each site.  Teams of 2-3 students may be appropriate .

STEP C and G: Store snow samples in a freezer or outdoors if pH measurements cannot be completed on the same day samples are collected .

STEP D: A good place to find undisturbed snow is in a woodlot where there is less wind .

RESULTS: The data table will probably reveal no systematic differences in snow pH among sites. Since conventional lab activities are usually designed to demonstrate differences, your students may need guidance in dealing with negative results.  "No difference" is a common conclusion in scientific research, and it offers a chance to discuss other explanations for the phenomenon under study. See answer to question 3. 

QUESTIONS 3-8 require higher order thinking skills.  We recommend that you discuss these questions in class before having students write out their answers .

3. Sites should not differ very much since snow pH is primarily determined by atmospheric conditions rather than ground conditions. See Results note above .

4. Differences can be due to experimental error, such as sampling snow from more than one snowfall, contaminating the sample, error in determining pH, and so on .

5. Answers will vary.  Air which has spent a long time over an industrial area should yield the lowest pH.  Slow moving high pressure systems can cause this to happen .

6. Answers will vary depending on your location and data from weather maps .

7. The acid which has collected in the snowpack over the winter can be released to lakes and streams in a short period of time without being buffered by the soil.  Many species of plankton, amphibians, and fish produce young in early spring which are more sensitive to variation in pH than adult organisms .

8. Most research has shown that summer rain is more acidic, but the accumulation of acid in the snow over the winter may have a greater impact on ecosystems (see question 7).