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Acid Rain Socratic Questions

Created by Dorothy Merritts, Franklin & Marshall College (dorothy.merritts@fandm.edu)
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


Questions regarding acids in the atmosphere (and acid deposition) are arranged in a suggested sequence in which the teacher can direct a session of Socratic questioning. General answers are provided in italics within brackets. Main concepts addressed are causes of acid deposition and the residence time of anthropogenically-derived acids in the atmosphere.

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Learning Goals

To consider what happens to sulfur and nitrogen oxides when they react with water and oxygen in the atmosphere. To examine why acid rain deposition occurs within hundreds of km of the source of the pollutant (i.e., concept of residence time).

Context for Use

This example is suitable for in-class use during a lecture period. No equipment is required unless the instructor wishes to use supplemental images. In that case, the images can be shown either with an overhead or computer projector.

Teaching Materials

Sample questions (with general answers in italics ) are arranged in sequence for Socratic questioning regarding acid rain:

Background to Questions:Acid deposition (i.e, acid "rain") caused by pollutants (particularly sulfur and nitrogen oxide gases) in the atmosphere has only a local or regional effect. Areas impacted typically are just hundreds of km downwind from the source. Acid rain with sources in the Pennsylvania-Ohio steel belt, for example, generally is deposited throughout New England and, more rarely, into Canada. To explore why acid deposition occurs so close to its source, let's first examine what happens to emissions of sulfur and nitrogen oxide gases from a smokestack, for example, as the oxides move into the lower atmosphere (troposphere).

What substances are in the atmosphere that might react with the sulfur and nitrogen oxides to form other substances? [ Water and oxygen; sulfur and nitrogen oxides oxidize to become sulfates and nitrates; sulfates and nitrates react with water to form sulfuric and nitric acids ]

Questions (and General Answers):Once pollutants react with water to form acids, they are easily mobilized. Why? [ The acids can be transported back to Earth in rainfall, as acid "rain". ] Because acids fall back to Earth in rain (or also as dry deposition), they have short residence times in the atmosphere. How is residence time related to the distance over which the acids can fall with respect to their sources? Hint: On average, sulfates and nitrates remain in the atmosphere on the order of days (typically 5-10 days. [ If the substances are traveling with air masses at velocities of several kms to tens of km per hour, then the travel distances are limited to hundreds to a few thousand km at most. ]

Teaching Notes and Tips

Tips: As in all Socratic questioning, give students time to reflect before answering questions, and make an effort to call on different students throughout the class period. Let students know at the beginning of class whether or not you will call on students randomly, or ask for hands to be raised, or both.


Many simple details can cause problems when using Socratic questioning. For example, students might feel that they never are given quite enough time to reflect on the answer before called upon. They might not be able to hear some of the other students' answers, especially in a large classroom. They might find it very challenging to take notes during the questioning and response session, and at least will find it more difficult to take notes than during a traditional lecture/chalk class.

The best way to determine what problems are occurring is to give students a questionnaire after each of the first few classes in which the approach is tried. Ask students directly if they think that you are allowing sufficient time for reflection. Ask if they are concerned about hearing other students' responses, and so forth. Add one question that asks students to make note of any problems not referred to in the questionnaire.

Each student should be able to answer any of the questions that was posed during the Socratic questioning session. A good way to assess what the students have learned from a Socratic questioning class is to give a short quiz in which several of the questions from the previous class are listed. This quiz can be given at the beginning of the next class period. If the class is large and grading frequent quizzes is too burdensome, the questions can be designed with multiple-choice answers.

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