Investigating soil acidity: Comparing pH and buffering capacity of various soils
As part of acid rain study, (environmental chemistry lab) students investigate the differences in pH of soil samples. They will collect and dry samples from the local prairie including grassland, transition, forest and riverbank and compare with teacher collected samples from a mixed forest and lake. Students determine pH levels using Universal Indicator Solution and use Tums to simulate buffering. Students write a lab report describing their results. Students also develop a new, testable question relating to acid rain.
Students practice lab safety.
Students collect, analyze and present data.
- Acidity of soil samples varies because of soil type and vegetation.
- The effect of acid rains varies with the natural acidity of soils and the buffering capacity of some soils.
- Lab results vary accuracy in collection, measurement, observation.
Vocabulary: pH, buffer
Context for Use
Resource Type: Activities:Lab Activity
Grade Level: Middle (6-8)
Description and Teaching Materials
At least 4 dried soil samples with origin labeled (under spruce tree, bog, prairie etc.)
1 small squirt bottle of Universal Indicator solution
1 Universal Indicator Color Chart
1 small squeeze bottles (150-200 ml) w/ distilled water (labeled)
1 small squeeze bottles (150-200 ml) w/"Acid Rain" (1 molar solution of Sulfuric Acid)
1 graduated cylinder from measuring 20 ml quantities
1 tall clear flexible plastic cup (9 oz.)
1 coffee filter paper-basket type
1 rubber band
1 Acid Rain Data Sheet
Collect soil samples labeling each soil type by location found. Our students collect samples at MSUM Regional Science Center by removing the sod and slicing a sample 6-8 inches down and placing in a labeled locking bag. Sample labels should be descriptive, such as 'prairie', 'forest' etc. Soils need be dried. Because I do not have a fume hood, I placed mine outside on aluminum foil, making sure to protect it from the rain.
As a class, determine the pH of 'normal rain' and 'acid rain' using 4 drops of Universal Indicator solution and 20 ml of each 'rain'. Use this pH level as a standard.
Add 4 drops of Universal Indicator solution directly in the plastic cup.
Suspend the coffee filter over the top of the cup, holding it in place with the rubber band. Put about a tablespoon of dried soil on the filter. Pour 20 ml of normal rain (distilled water) on the soil. Compare the color of the 'rain water' or 'lake' that passed through the soil with Universal Indicator Chart to determine pH of soil. (White paper under cup makes color more visible.) overhead using "R" for rain.
Have the class discuss the differences in pH of the various soils. Students discuss and journal possibilities for differences. Discuss pH tolerance levels of lake organisms (chart) and discuss which soils would support life with rain.
Students repeat the procedure with 'acid rain' using dry soil from the same sample. Record results on group chart with "A" for acid rain. Discuss and journal the results.
Discuss buffers and introduce Tums as a buffer for stomach acid. Buffer is a substance that brings an acidified solution to a less acidic pH. Give students a Tums (mostly CaCO3) calcium carbonate. Crumble the Tums in each cake and chart results (R-T, A-T). Discuss buffers and natural soil buffers.
Set aside several of the samples for at least 15 minutes or until the next class period to allow the buffer time to work
Teaching Notes and Tips
"Acid Rain solution" - Prepare by adding 1mL sulfuric acid to two liters of distilled water. (Use goggles and extreme care, remember, "Do as you oughter, add acid to water!") You could use full strength white distilled vinegar but sulfuric acid is preferable.
Soil samples: Have students collect from homes or trips making sure to label the location on bag. I collected from the lake cabin, choosing some from the cattail bog, under a spruce tree, the ravine and lake bottom.
Acid Rain by LHS GEMS in Behind the Scenes contains great background information on acids, bases, buffers, acid rain, the problems it causes, and proposed solutions. It also includes information on soil collection and preparing 1M sulfuric acid solution.
This activity presents acids and bases in a real-world context, that of soils and acid rain. We extend the environmental fieldtrip into a lab learning experience.
9-12.I.B.4-sources of error