Acid Neutralization in Nature

Introduction

Why do different bodies of water in the same geographic region have different pH ranges? Perform this visual demonstration to explain how the chemical makeup of the soil around a body of water can affect the pH of the water.

Concepts

• Acid rain
• pH
• Soil types

Materials

Safety Precautions

Hydrochloric acid solutions are toxic by ingestion and are corrosive to body tissues. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Reacted samples may be disposed of according to Flinn Suggested Disposal Methods 26a and 26b.

Prelab Preparation

1. Place 10 g of granite chips in a 600-mL beaker. Label this beaker Lake A.
2. To remove any contamination in the granite chips, add 50 mL of 0.1 M hydrochloric acid to the beaker. Decant the hydrochloric acid into a sink with running water. Rinse the granite chips several times with distilled water.
3. Place 10 g of marble chips in a second 600-mL beaker. Label this beaker Lake B.
4. Prepare the simulated acid rain solution by placing 5 mL of the 0.1 M hydrochloric acid in a clean 600-mL beaker. Dilute to 500 mL with distilled or deionized water.
5. Using a pipet, add 1 mL of bromcresol green indicator solution to the simulated acid rain solution and stir the mixture. If the solution is not yellow add a few drops of 0.1 M hydrochloric acid.

Procedure

1. Pour approximately 100 mL of the simulated acid rain solution into the beaker labeled Lake A (granite chips). Stir with a stirring rod and note the results. Clean the stirring rod with distilled or deionized water.
2. Pour approximately 100 mL of simulated acid rain solution into the beaker labeled Lake B (marble chips). Stir with a stirring rod and note the results.
3. The solution in the granite chip beaker (Lake A) will remain yellow, which means it is still acidic. The solution in the marble chip beaker (Lake B) will turn bluish-green, indicating that the acidic water is being neutralized.

Student Worksheet PDF

13841_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration seven times: 70 g of granite chips, 70 g of marble chips, 500 mL of 0.1 M hydrochloric acid and 20 mL of bromcresol green. The marble and granite chips may be rinsed and reused for future demonstrations.
  
• Bromcresol green is an acid–base indicator that changes color from yellow to blue/green when the pH rises above 5.4. Bromcresol green is a valuable indicator for this demonstration because the color change occurs right at the minimum pH value of acid rain.
  
• After students have seen this demonstration, have them take the pH of bodies of water in your area and compare the readings to the type of soil surrounding the bodies of water.
  
• This demonstration may also be repeated using rocks or soils in your area.
  
• For a different effect to show water percolating through soil, place the granite and marble chips in columns (glass tubes) and pour the acid rain solution through the column. Observe the color of the water as it drains from the simulated soil column.

Answers to Questions

1. Draw the set-up of this demonstration. Label the contents and color of each beaker and the solution that was used to simulate acid rain.
   {13841_Answers_Figure_1}

   The acid rain solution was composed of 0.1 M hydrochloric acid, which had been diluted with deionized water, and bromcresol green indicator.

2. Which “lake” was less affected by acid rain, Lake A containing granite or Lake B containing marble?

   Lake B was less affected by acid rain because granite has neutralizing capabilities as displayed by the color change of bromcresol green which changes to a blue/green color as the pH rises above 5.4. Lake A was more affected by acid rain because the granite does not have neutralizing capabilities as demonstrated by the fact that the solution remained yellow.

3. What is acid rain? Name one negative effect acid rain might have on a body of water.

   Acid rain is precipitation that has reacted with compounds in the atmosphere and has a pH of about 5.4 or lower. If a body of water cannot effectively neutralize the acid rain, the water may become so acidic that organisms can no longer function in such an environment.

Discussion

The acidity of different bodies of water in a specific area can vary greatly. An increase in acidity is generally attributed to pollution in the form of acid rain. Acid rain is precipitation that has absorbed and reacted with compounds (mainly sulfur oxides and nitrogen oxides) in the atmosphere. Generally, acid rain is termed so when the pH reaches 5.4 or lower.

Given different bodies of water in the same relative area that are exposed to the same amount of acid rain, why do some bodies of water become more acidic than others? Waters that are able to maintain a generally neutral pH do so largely because the chemical makeup of the surrounding soil. Soils that are composed of carbonates, such as the marble chips (limestone) used in this demonstration, are able to neutralize acidic solutions. In contrast, soils that are composed mainly of silicates (i.e., granite in this demonstration), have little or no acid neutralizing capabilities. When acidic rainwater flows over soils high in carbonates, bicarbonate ions are produced and the rainwater runoff becomes more basic before entering a body of water.

The majority of the lakes, rivers, and streams in the United States have pH values in the range of 6.5 to 8.2. As the pH of water drops below this range, several negative events may occur. The physiological processes within aquatic organisms can be disrupted or even disabled. Toxic metals are also chemically released readily in waters that have a low pH. The toxicity of the water may even reach a level where fish and other organisms can no longer survive.

References

Shakashiri, B. Z., Chemical Demonstrations: A Sourcebook for Teachers; University of Wisconsin: Madison, 1983; Vol. 3, pp 125–127.