Ozone in Our Neighborhood

Students will experiment to understand variations in the amount of ground-level ozone between different places in their neighborhood, town, or city.

Learning Objectives

Students will experiment using Schoenbein paper to understand variations in the amount of ozone between different places.
Students will be able to draw conclusions about ozone levels based on test results.

Materials

For the class:

Potassium iodide
1/4 teaspoon measuring spoon
Hot plate
Corn starch
Glass stirring rod
250 ml beaker
Oven mitt
Distilled water

For each group of four:

Distilled water in a spray bottle
Four sheets of 8 1/2 x 11” paper
Glass pie plate
Four pieces of filter paper
Small paintbrush
Zipper storage bag
Scissors
String or wire
Access to Google Maps

Directions

Introduction

Introduce students to ozone as an air pollutant. You may wish to have students read and discuss Ozone in the Troposphere.
Introduce students to the experiment. They will measure relative ozone concentrations, comparing where ozone is higher and lower in their neighborhood.
To measure relative ozone concentration, students will need Schoenbein paper. Prepare the Schoenbein paper solution as a presentation for the class. With more advanced students that have background in chemistry, describe the reaction that causes this solution to change color when in contact with ozone (see Background Information).

Place 100 ml of distilled water in a 250 ml beaker.
Add 1 1/4 teaspoon of cornstarch.
Heat and stir mixture until it gels. The mixture is gelled when it thickens and becomes somewhat translucent.
Remove the beaker from the heat source and add 1/4 teaspoon of potassium iodide and stir well. Cool the solution before applying to the filter paper.

Students Make the Ozone Test Strips

Have student groups collect the supplies they need.
Instruct students to brush both sides of the filter paper with the paste with the paper on the glass pie plate, applying the paste as uniformly as possible. Wash hands after applying the potassium iodide mixture. (Although potassium iodide is not toxic, it can cause mild skin irritation.)
Set the paper out of direct sunlight and allow it to dry. A low-temperature drying oven works well if available.
Have student groups cut their filter paper into 1-inch wide strips and place strips in a zipper storage bag out of direct sunlight.

Students Design the Experiment

Ask students to list places where they think ozone levels would be higher. (For example, students may choose a location near a road or where vehicles idle at stoplights.)
Ask students to list places where they think ozone levels would be lower. (For example, students may choose a park or backyard.)
Have students use Google Maps or Google Earth to plan the locations of their testing strips. Guide this process to ensure student safety. You may wish to have students place their test strips on their way home from school.
Have student groups choose at least two locations that they think will have higher ozone and two locations that they think will have lower ozone. (If one of the locations does not follow the expected pattern, there is an opportunity for more research.)
Have students decide how many test strips they will place at each site and where they will be placed (in trees, near the ground, on a lamppost, or a combination of high and low, for example.)
Have each group (or each student) write up the design for their experiment including a hypothesis about where they think ozone will be higher and lower and the plan for how they will collect the data.

Students Collect the Data

Have students spray a strip of test paper with distilled water and hang it at a data collection site out of direct sunlight. Make sure the strip can hang freely. Use wire or string to secure the test strips.
Expose the paper for approximately eight hours. When you collect test strips, identify their location with a pen.
After eight hours of exposure, collect test strips and seal the test strips from each location in a zipper storage bag if the results will not be recorded immediately. Make sure the location and group members are identified on the outside of the bag.

Results, Analysis, and Conclusions

To observe and record test results, spray the paper that has been exposed for eight hours with distilled water. Observe the color.
Have students take photos of all their test strips for their report, indicating the location where each was placed in the neighborhood.
Have students compare the color of the test strips from one environment with the color of test strips from the other environment, noting the following:
- What change in the test paper, if any, did you observe? (The paper will vary in color depending on the amount of oxidation. The color of the paper may not be uniform.)
- Compare your test paper from the different environments. Do all the test papers appear the same? (The individual test papers will vary depending on the amount of oxidants at various sites. For example, sites near highways will show greater color change due to oxidants from car exhaust and nitrous oxides in heavy traffic areas.)
- Why do you think the various test papers did not all appear the same? (Student answers will vary. Tropospheric ozone levels vary widely due to the type and number of sources of ozone. Students measuring ozone in their home may report little color change of the paper, but if they live on a busy street, a measurement near the curb will register greater color change. Humidity and oxidants are present in varying levels depending on the time of day, the weather, the season, etc.)
Ask students to develop new questions about ozone levels in their neighborhood based on the results of this experiment. (If time permits, have students develop new hypotheses that are informed by their initial data.)
Have each student group (or each student) write a results section for their report, including a photo of their test strips, a conclusion, and at least one question that they have for more research about ozone in their neighborhood.

Discuss as a Class

Was there any variation in ozone levels on the map?
Where on the map were the concentrations the highest? The lowest?
Looking at the area of highest concentration, does there appear to be any obvious explanation for the variation?
Why do you think there were ozone level variations?

Notes:

The results of individual test papers will vary depending on the specific relative humidity of the site. See the reaction in the Background Information section. Notice that water is a reactant, so humidity will affect the reaction. Sites near lakes or streams may show greater change.
This activity works best in areas of low humidity and high ambient ozone concentrations. In some parts of the country, this activity may not be very conclusive.
You may wish to extend this activity with ozone testing equipment that is quantitative instead of qualitative. In this experiment, students can compare their test strips to understand relative differences in ozone. Check your school’s science supplier for ozone test equipment that is quantitative if you’d like students to partake in both quantitative and qualitative analysis.
For younger students, consider doing the experiment as a class instead of in groups to guide the inquiry.

Background

Ozone: the Air Pollutant

In the stratosphere, ozone molecules play an important role - absorbing ultraviolet radiation from the Sun and shielding Earth from dangerous rays. But in the troposphere, near ground-level, ozone molecules are both air pollutants, threatening the health of living things, and greenhouse gases, trapping heat and contributing to climate change.

A small amount of ozone does occur naturally at ground level. Plants and soil release some. Some migrate down from the stratosphere. But neither of these sources contributes enough ozone to be considered a threat to the health of humans or the environment. Most of the ozone that is found near the ground comes from vehicle exhaust and emissions from factories, power plants, and refineries. Since 1900, the amount of ozone near the Earth's surface has more than doubled due to more automobiles and industry.

Unlike most other air pollutants, ozone is not directly emitted into the air. Tropospheric ozone is formed by the interaction of sunlight, particularly ultraviolet light, with hydrocarbons and nitrogen oxides, which are emitted by automobile tailpipes and smokestacks. In urban areas, high ozone levels usually occur during warm summer months. Typically, ozone levels reach their peak in mid to late afternoon, after exhaust fumes from morning rush hour have had time to react in sunlight. A hot, sunny, still day is the perfect environment for the production of ozone pollution. At the end of the day, as the Sun starts to set, the production of ozone begins to subside. To form, ozone needs sunshine to fuel the chemical reaction.

About Schoenbein Test Strips

Christian Friedrich Schoenbein discovered ozone in 1839 during his tenure as a professor at the University of Basel, Switzerland. He used the reactivity of ozone to measure its presence and demonstrate that it is a naturally occurring component of the atmosphere. He developed a way to measure ozone in the troposphere using a mixture of starch, potassium iodide, and water spread on filter paper. The paper, called Schoenbein paper, changes color when ozone is present. Ozone causes iodide to oxidize into iodine.

Ozone in the air will oxidize the potassium iodide on the test paper to produce iodide. Specifically, potassium iodide (KI), ozone (O3), and water (H2O) react to form potassium hydroxide (KOH), oxygen (O2), and iodide (I2). Here's the equation:

2KI + O₃ + H₂O ---> 2KOH + O₂ + I₂

The iodide reacts with starch, staining the paper a shade of purple. The intensity of the purple color depends on the amount of ozone present in the air. The darker the color, the more ozone is present.

Credits

This activity was adapted from UCAR’s Project LEARN by Lisa Gardiner of the UCAR Center for Science Education.