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Effect of Pollution on the pH of Rainwater

based on 4 ratings
Author: Cy Ashley Webb

Grade Level: 5th - 8th; Type: Chemistry

Objective:

The goal of this experiment is to learn how acid rain forms, and explore how acid rain damages the environment.

By measuring the acidity of rain water in their environment, students will learn just how polluted their environment may be. By comparing the acidity of runoff in different environments, students will get a rough sense about how much of the acid rain may be attributed to car exhaust and how much is due to power plants and other industrial sources in their particular locale.

Research Questions:

  • What is pH? What is acidity?
  • What is wet deposition? What is dry deposition?
  • What are the causes of acid rain?
  • What is the effect of acid rain in plants and animals?
  • How does the acidity of runoff after rainstorms differ in areas with intense traffic
  • compare to less heavily trafficked regions?

Gases containing sulfur and nitrogen compounds from power plants and car exhaust reacts with water and returns to earth when it rains. This process is called acid rain – or more accurately, wet deposition. Wet deposition does not explain the entire story because dry deposition occurs as well. Dry deposition occurs when particulate matter lands on the roadway and other surfaces and forms acids when it rains. Regardless whether the process involves wet or dry deposition, the process of forming acidic compounds from nitrogen and sulfur compounds is the same.

Nitrogen dioxide gas from power plants and other sources dissolves in rain and forms nitrous and nitric acid.

2 NO2 (g) + H2O (l) Ü HNO2 (aq) + HNO3

Sulfur dioxide undergoes similar reactions with oxygen and water, ultimately forming sulfur trioxide. The sulfur trioxide reacts further with water, forming sulfuric acid. SO2 (g) + O2 (g) Ü H2SO3 (aq) SO2 (g) + H2O (l) Ü H2SO3 (aq) SO3 (g) + H2O (l) Ü H2SO4 (g) Ultimately, these acidic compounds damage the health of plants and animals, degrade buildings and damage the finish of cars. The acidity of rain can be measured after rainstorms using pH paper.

Materials:

  • pH paper
    • pH paper can be found in lab supply houses or wherever you buy pool and garden supplies. pH paper is frequently sold for a limited range of pHs. Make sure you get full range paper, or paper that measures pH in the range from 7 to 3.
  • Notebook
  • Transportation to heavily trafficked city streets, as well as to suburban or rural environments

Experimental Procedure

  1. Identify at least four different areas that have widely different traffic patterns. Ideally, the areas will include a heavily trafficked urban center, a region near a busy freeway (not on the freeway itself!), a suburban street and a rural area.
  2. After a rainstorm, go to the areas that were identified in step one and look for puddles of runoff. Take the pH of this run-off and mark it down in your notebook. Take several samples in the same environment. Take pictures of your areas.
  3. Create a spreadsheet of your values. Is there a difference in the acidity of the runoff from different areas? What is driving this difference?

References:

U.S. Environmental Protection Agency: What is Acid Rain? http://www.epa.gov/acidrain/what/ Almanac of Policy Issues: What is Acid Rain and What Causes it?

http://www.policyalmanac.org/environment/archive/acid_rain.shtml U.S. Environmental Protection Agency: What is pH? http://www.epa.gov/acidrain/measure/ph.html

U.S. Environmental Protection Agency: Measuring Acid Rain http://www.epa.gov/acidrain/measure/index.html

Books:

Iacopi, Robert L and Fred W. Fisher. Earthquake Country. Sunset Books, 1996

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