Water Planet Challenge

The Effect of Acid Rain - Middle School

ESSENTIAL QUESTIONS

• How much acid is needed to change the pH of a given volume of water?
• How does acid rain impact aquatic organisms?

OBJECTIVES

• Use titration to establish the proportion of acid that is needed to change the pH of distilled water.
• Discuss how acid rain impacts aquatic organisms.

TEACHER NOTES

It's a common misconception that pollution is caused only by direct discharge of substances into water bodies. Acid rain is a byproduct of sulfur pollution primarily from fossil fuel electrical power plants. The pollutants involved include sulfur dioxide, anthropogenic carbon dioxide and various nitrous oxides. Unpolluted rain has a pH of 5.6, primarily due to natural sources of carbon dioxide. Acids form when these gases combine with water in the earth’s atmosphere. Rain is called “acid rain” when the pH is decreased to between 4.4 and 4.2. Lakes are considered acidic once their pH decreases to 4.5. However, some aquatic life forms are affected at a slightly higher pH. Acid rain is an indirect (or non-point source) pollutant. These gases can also increase the acidity of bodies of waters directly by wind action.

In this activity, students learn how a small input of acid rain can change the acidity balance in a body of water. Distilled water simulates a pristine lake.

The aim of the activity is to use titration to establish how much acid (vinegar) needs to be added to detect incremental changes in acidity. The activity demonstrates that a tiny ratio of added liquid (acid) to the distilled water can cause a chemical change.

Students will conclude that acid rain can change the acidity of lake water. Such changes in water chemistry can profoundly affect wildlife.

TIPS

• Only white vinegar should be used. Do not substitute other types of vinegar.
• Although vinegar is safe, it is acidic, so students should use appropriate safety equipment.
• Red-green color-blind students may have difficulty estimating pH from litmus paper. Pair such students with normal vision students.
• The activity is simplified if you use a standard titration setup. However, the instructions provided will be enough to accomplish the lesson objectives.
• Results will be more accurate if you use a pH meter (less than $30 from most scientific supply stores).

Engage

• Review some of the water Fun Facts (see Teacher Background).
• Introduce students to key concepts (see Teacher Background) to explain why we should care about how acid rain changes lake water chemistry.
• Show the students pictures of habitats affected by acid rain.
• Add a pinch of baking soda to a small amount of vinegar. T he fizzing demonstrates an acid-base reaction.
• Review the EPA information on effects of acid rain on fish and other aquatic organisms (see link to EPA website).

Explain (Sample Answers to Analysis Questions)

1. Answers will vary. Only a few drops of vinegar should be enough to change pH.
2. Answers will vary. If 1 mL of vinegar changes the pH of 1 L of water, the proportion is 1:1000.
3. Sulfuric acid
4. Perch and frogs.
5. Bass, clams, crayfish, snails and mayfly.
6. Since many aquatic animals do not tolerate acid water, the wildlife of the lake would either die or have to move away. If food animals such as snails and mayfly die, animals they eat them would also die or have to move elsewhere.

USEFUL LINKS

http://www.shodor.org/UNChem/basic/ab/
http://www.chem.ubc.ca/courseware/pH/section14/index.html
http://www.epa.gov/acidrain/effects/surface_water.html
http://www.bigstockphoto.com/image-811938/stock-photo-aerial-of-power-station

STUDENT RESOURCE PAGE

The Effect of Acid Rain – How Much Acid Can Change Water Chemistry

In this lab, you will determine how much acid is needed to cause a detectable change the pH of water. You will then investigate the effects of acid rain on aquatic organisms.

OBJECTIVES

• Use titration to establish the proportion of acid that is needed to change the pH of distilled water.
• Determine how acid rain can impact the chemistry of lake water.

MATERIALS

Distilled or spirit vinegar (about 5% solution acetic acid), 100 mL
Distilled water, 1 L
Measuring cylinder, 1 L
pH test paper (blue litmus paper, or alkacid paper)*
Pipette (graduated in 0.1mL increments)
* If available, use a pH test meter for more accurate results

SAFETY PROCEDURES

Vinegar is acidic. Wear gloves, safety goggles and lab coats while conducting your experiments.

PROCEDURE

1. Measure out 1 liter of the distilled water.
2. Use the pipette to measure out 0.5 ml of vinegar.
3. Add the measured vinegar to the distilled water.
4. Measure the pH using the test paper. Note the result in the table provided on your worksheet.
5. Repeat steps 2 to 4, adding the measured vinegar to the distilled water.
6. Continue until the water pH is less than 5.0.

ANALYSIS

1. How much vinegar did you need to add to the liter of water to see a change in pH?
2. What is the proportion of the volume of water to the volume of vinegar needed to change the pH?
3. What is the name of the acid that forms when water reacts with sulfur dioxide to create acid rain?
4. Review the diagram below, which indicates tolerance of different aquatic organisms to acidity. If the lake water changed to pH 4.5, which animals would survive?
5. Which animals would not survive in a lake with water of pH 5.0?
6. What can you conclude about the impact of acid rain on the wildlife of a lake?

From http://www.epa.gov/acidrain/images/waterspecies.gif