Materials Included In Kit

Additional Materials Required

Safety Precautions

Have students wash hands thoroughly upon completion of the laboratory work. Follow all normal classroom guidelines.

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. The solutions in this activity may be disposed of according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students.
• Encourage students to take the water sample tubes home to gather the water samples. Complete all testing in the classroom.
• Additional water samples may be gathered, if desired.
• Students should share the 5 pH Color Comparison Cards.
• A vial of 1–12 pH test paper (100 strips) has been included in case student water samples do not fall between the pH of 3–6.
• Acidic buffer solution may be used as simulated acid rain solutions, if desired. Buffer solutions and buffer capsules are available from Flinn Scientific.

Sample Data

Answers to Questions

1. What was the pH of your water sample(s)? Is your water sample considered acidic?

   Answers will vary.

2. How is acid rain formed?

   Acid rain is formed when rain is reacted with compounds, mainly SO_x and NO_x, in the atmosphere to form acids.

3. How does acid rain affect water, soil, and plants?

   Acid rain causes damage in many ways. Lakes may become so acidic that organisms may cease to exist. Acid rain dissolves and mobilizes trace metals in soil such as aluminum and lead and can cause their concentration in the soil to increase to toxic levels. Acid rain may harm plants directly by damaging leaves and preventing seeds from germinating.

4. What can be done to help stop the formation of acid rain in your area?

   Answers will vary. Reduce amount of NO_x and SO_x in the atmosphere, etc.

Introduction

What exactly is acid rain and how does it affect our surroundings? In this activity, the pH of freshwater sources will be tested.

Concepts

• Acid rain
• pH
• Pollution

Background

Acid rain is precipitation that has absorbed and reacted with compounds in the atmosphere to form acids. In more precise terms, acid rain is precipitation with a pH less than 5.6. A pH of 5.6 is generally considered to be the pH of “normal” rainwater. The term “acid rain” dates back to mid-19th century England. Following a long period of deforestation, homes and businesses gradually converted to burning coal as a primary source of fuel. It was noted by scientists and others that over this period the pH of rain falling in England and nations to the east was becoming more and more acidic. Eventually the connection between the increasing acidity and the combustion of coal was made.

The pH of pure water is theoretically 7.0, a value considered to be neutral on the pH scale of 0 to 14. Carbon dioxide gas (CO₂), naturally present in the atmosphere, dissolves in and reacts with water by the following equation:

The free hydrogen ions on the right side of the equation above are the cause of the moderate acidification and lower pH of 5.6. With other minor contributors, the pH of normal precipitation may on occasion range as low as 4.0. Although rare cases have been reported where the pH of rain has dropped to a pH of 2.

The chemicals primarily responsible for acid rain fall into two basic classes: sulfur oxides (SO_x) and nitrogen oxides (NO_x). It is important to understand that acid rain is created both naturally and by manmade sources. The primary natural sources for SO_x are volcanoes, fires, wetlands and other systems with significant concentrations of anaerobic bacteria. Manmade sources for SO_x are the burning of coal, oil and gas (fossil fuels), ore smelting and other industrial processes. Natural sources for NO_x include fires (high temperature combustion) and lightning. The most significant manmade source of NO_x is automobile emissions. In industrial regions, human generated sources of both SO_x and NO_x greatly outweigh contributions from natural sources.

Sulfur is present as a contaminant in fossil fuels. Most notably in coal and oil, and to a much lesser extent in natural gas. The combustion of these fuels results in the production of sulfur dioxide (SO₂). Compounds naturally present in the atmosphere are capable of further oxidizing the SO₂ to form sulfuric acid (H₂SO₄). These oxidants include hydroxyl radicals, hydrogen peroxides, dissolved oxygen and ozone.

Nitrogen oxides are formed by the combination of nitrogen and oxygen. Since our atmosphere is approximately 78% nitrogen (N₂) and 21% oxygen (O₂) the reactants are certainly abundant. However, the reaction to create acid rain will only take place when these reactants are involved in a high temperature combustion. Truck and automobile engines are ideal environments for this reaction and are by far the greatest sources of NO_x emissions. Oxygen and nitrogen combine to form nitric oxide (NO) which further reacts with oxygen to form nitrogen dioxide (NO₂). Ultimately nitrogen dioxide reacts with hydroxyl radicals and ozone to form nitric acid.

Acid precipitation causes damage in many ways. Most people have heard how acid rain can damage lake ecosystems. Aquatic organisms (ranging from microorganisms to fish) can be very sensitive to the pH of their liquid environment. Many lakes in sections of the United States, Canada and Europe have become so acidic that the organisms that used to flourish there have disappeared. It is not just lakes that are damaged in this way, soils can become too acidic or their essential minerals can be depleted as acid precipitation washes them away. The acid can also harm plants directly by damaging leaves and preventing seeds from germinating. Acid precipitation also dissolves and mobilizes trace metals in the soil such as aluminum and lead and can cause their concentration in the soil and in nearby lakes and streams to increase to toxic levels. On a more visible, though less life-threatening level, acids released into our atmosphere damage statues and erode artwork and buildings.

Materials

Safety Precautions

Follow all normal classroom guidelines.

Procedure

1. Obtain a water sample in the water sample tube. Cap if sample is to be transported to another location. Record the location where the water was collected in the data table.
2. Obtain a piece of acid rain pH paper.
3. Dip the pH paper into the water sample in the water sampling tube. Immerse for two seconds.
4. After two seconds, remove the paper and shake off excess water.
5. Obtain a Color Comparison Chart.
6. Compare the color of the acid rain pH paper to the Color Comparison Card.
7. Record the pH of the water sample in the data table.
8. Obtain other water samples as instructed by the teacher.
9. Answer the Post-Lab Questions.
10. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

13550_Student1.pdf