Air Pollution and Properties of Gases - Investigation 3

Investigation #3: Air Pollution and Properties of Gases

Asking Questions

  • What is an independent variable and how is it changed during experimentation?
  • What is the difference between a chemical and a physical change?
  • What gases are inhaled and exhaled?
  • Which locations nearby likely have the highest/lowest ozone and PM concentrations?

Preparing to Investigate

This investigation examines the properties of atmospheric gases and tests for pollutants, focusing on ozone and particulate matter (PM).

  • Ozone: Blocks harmful ultraviolet (UV) light.
  • Particulate Matter (PM): Inhalable particles, categorized by size:
    • PM_{10}: Average diameter of 10 \mu m.
    • PM_{2.5}: Average diameter of 2.5 \mu m.

While ozone and PM occur naturally (e.g., from forest fires), human activities like driving, electricity generation, and manufacturing increase their concentrations through combustion reactions. Refer to Chapter 2 of Chemistry in Context, 10e, for further details.

This investigation involves creating devices to measure air pollutant levels in different locations.

Preparing Gases

  • Oxygen Gas (O₂): Prepared by using potassium iodide (KI) as a catalyst to decompose hydrogen peroxide (H2O2) into water (H2O) and oxygen (O2):
    2 H2O2(l) \longrightarrow 2 H2O(l) + O2(g)
  • Carbon Dioxide (CO₂): Prepared by mixing baking soda (sodium bicarbonate, NaHCO3) and vinegar (acetic acid, HC2H3O2). This reaction produces sodium acetate (NaC2H3O2), water (H2O), and carbon dioxide (CO2): NaHCO3(s) + HC2H3O2(aq) \longrightarrow NaC2H3O2(aq) + H2O(l) + CO2(g)

These prepared gases, along with room air and exhaled air, will be used in tests to determine their chemical properties.

Chemical Tests

  • Limewater Test: Reacts in the presence of specific gases.
  • Bromothymol Blue: Indicator that is blue when acid is absent, yellow when acid is present.
  • Wood-Splint Test: Detects the flammability of a gas.

Making Predictions

Select various outdoor locations to test for air pollutants, including a control location with minimal expected pollution. The ozone and PM investigation locations do not need to be the same. Locations for ozone testing should not be exposed to direct sunlight. Identify the variables in this investigation. Form a hypothesis predicting ozone and PM levels at these locations.

Create a table outlining predictions for each chemical test, including the test name and anticipated results. Researching the expected results of these tests may be helpful.

Gathering Evidence

Part I. Detecting PM

  1. Obtain four index cards, cut a 3 cm \times 3 cm square in the center, and label each with your name and location.
  2. Cover the square hole with clear packaging tape, sticky side outward, avoiding contact with the adhesive.
  3. Place three cards in different locations to test particulate levels and the fourth card in a controlled location, ensuring the square hole is unobstructed. Prop them up suitably.
  4. Leave the cards in place for at least 24 hours, protecting them from precipitation by temporarily storing them in a container, avoiding tape contact.
  5. After a day, seal the particles by applying a fresh piece of tape over the sticky surface.
  6. Observe the particles under a magnifying glass and a microscope, counting them and describing their size and properties. Note the differences and similarities between the two observation methods; record observations.

Part II. Detecting Ozone

  1. Mix 50 mL of distilled water and 4 g of corn starch in a 150-mL beaker. Heat gradually while stirring with a glass rod or magnetic stirrer until a thick, translucent gel forms.
  2. Remove the beaker from the heat and add 1.5 g of potassium iodide (KI). Stir until mixed, then let it cool for 2 minutes.
  3. Obtain four filter paper pieces (at least 50 mm in diameter) and place each on a large watch glass. Apply the gel solution uniformly to both sides using a small paintbrush.
  4. Label and mount the test strips at the chosen test sites and the control site, ensuring they hang freely. Spray each strip with distilled water and leave them in place for at least 30 minutes.
  5. To analyze a test card, spray it with distilled water and compare it to the control, noting observations and recording them. If immediate analysis isn't possible, store strips in a sealed plastic bag or glass jar in a dark place.

Part III. Gathering Gases

Gases are stored in sealed plastic zipper bags. Ensure the bags are completely sealed by pressing on them to check for leaks.

A. Exhaled Air

  1. Insert a straw into a zipper bag. Exhale into the bag through the straw, crimping the straw to prevent backflow, until the bag is inflated. Seal the bag.

B. Room Air

  1. Squeeze room air into a clean pipet.

C. Carbon Dioxide

  1. Measure about 2 g of sodium bicarbonate into the corner of a zipper bag.
  2. Fill two 4-mL plastic transfer pipets with acetic acid.
  3. Place the pipets containing vinegar into the bag with the sodium bicarbonate. Minimize air in the bag and seal it, being careful not to spill any vinegar.
  4. Hold the bag sealed and slowly squeeze one pipet to release vinegar onto the NaHCO_3. Observe and record what happens. When the first pipet is empty, squeeze the second. Keep the bag sealed during this process.
  5. The sealed bag will partially fill with carbon dioxide (CO_2) gas. Leave the bag standing upright, leaning against something on the lab bench.

D. Oxygen

  1. Use a spatula to measure about 0.5 g of potassium iodide into the corner of a zipper bag.
  2. Fill two 4-mL pipets with 10% hydrogen peroxide (H2O2). Caution: Hydrogen peroxide is corrosive; handle it with care.
  3. Place the pipets containing H2O2 into the bag with the potassium iodide. Smooth out the bag to remove most of the air; seal the bag, making sure not to spill any H2O2.
  4. Hold the sealed bag and slowly squeeze one pipet to release H2O2 onto the potassium iodide. Observe and record your observations. When the first pipet is empty, gently squeeze the second pipet so the potassium iodide reacts with the remaining hydrogen peroxide. Keep the bag sealed during this process.
  5. You now have a sealed plastic bag filled partially with oxygen (O_2) gas. Leave the bag standing upright, leaning against something on the lab bench.

Part IV. Running Tests

Use a clean plastic transfer pipet for each gas sample in each test. Record observations.

How to fill a pipet with gas samples:

Squeeze the bulb of the pipet to expel air. With the bulb squeezed, push the tip against the zippered seal at the corner of the bag containing the gas. Open the bag just enough to insert the pipet while keeping the rest of the seal intact. Release the bulb to draw in the gas, ensuring the pipet doesn't touch liquids or solids in the bag. Have a partner quickly remove the pipet and seal the bag.

A. Limewater Tests

  1. Add 10 drops of limewater to four wells in a well plate. Place the plate on a dark surface to clearly see results.
  2. Fill a clean pipet with exhaled air. Place the pipet tip in the first well and gently bubble the gas through the limewater. Observe and record results.
  3. Repeat the test with air, carbon dioxide, and oxygen in the remaining wells.

B. Tests with Indicator

  1. Add 10 drops of water and one drop of bromothymol blue indicator into four wells. Place the plate on white paper to see results clearly.
  2. Fill a new, clean pipet with exhaled air. Place the pipet tip in the first well and gently bubble the gas through the indicator solution. Record observations.
  3. Repeat the tests with air, carbon dioxide, and oxygen in the next three wells. Record observations.

C. Wood-Splint Test

  1. Use a clean pipet to obtain a sample of exhaled air.
  2. Ignite the end of a wood splint or toothpick with a match.
  3. Have your lab partner hold the pipet with exhaled air so the tip is close to the glowing ember. Gently squeeze a puff of exhaled air directly at the glowing portion. Observe and record results.
  4. Repeat with air, carbon dioxide, and oxygen on a new wood splint.

Clean Up

Dispose of waste as instructed. Do not rinse materials down the sink unless instructed. Ensure matches or toothpicks are completely extinguished before disposal, preferably by dipping them in cold water.

Analyzing Evidence

  1. Rank test sites by particulate concentrations and identify potential sources based on appearance. Note any differences in particle numbers, sizes, or types between sites.
  2. Describe physical differences in ozone test cards and rank sites by ozone levels, noting the range of differences.
  3. Describe how gases were generated in this investigation.
  4. Identify which gases reacted with limewater.
  5. Record the colors observed for each gas when mixed with the indicator solution.
  6. Describe how the burning wood splint reacted when exposed to each gas.

Interpreting Evidence

  1. Compare investigation results to initial hypotheses. Evaluate the accuracy of predictions and suggest improvements for future investigations.
  2. Identify the main source of particulate pollution at test locations and how particulate numbers and types changed based on the independent variable.
  3. Identify the main source of ozone pollution at test locations and how ozone concentrations changed based on the independent variable.
  4. Rank gas samples from most to least carbon dioxide and oxygen present, specifying which tests aided in this ranking.
  5. Describe the purpose of the limewater test, identifying gases with the most and least dramatic results and explaining why.
  6. Based on indicator solution results, determine which gas leads to more acidic solutions and discuss the environmental impact.
  7. Determine whether exhaled air or regular air contains more CO_2 and explain how to tell.

Making Claims

Make claims about sources of ozone or particulate pollution in chosen locations, using evidence from the investigation. Make claims about inhaled and exhaled air based on obtained results. Identify which gas is more prevalent in the air based on findings.

Reflecting on the Investigation

  1. Identify which tests involved chemical changes and which involved physical changes, providing explanations.
  2. Discuss factors other than pollution that may have affected results.
  3. Describe any unusual results observed during the investigation.
  4. Use the Smog City 2 simulator (www.smogcity2.org) to identify variables most important for minimizing ozone and particulate concentration.
  5. Visit the AirNow website (www.airnow.gov):
    a. Look up ozone and particulate pollution for your zip code, recording the AQI and describing the corresponding health category.
    b. Compare your area to other regions in the United States.
  6. Suggest actions to improve air quality in your community and globally.
  7. Discuss the impact of oceans absorbing anthropogenic CO_2 emissions. Predict the effects on the oceans' pH and the consequences.
  8. Explain how CO_2 fire extinguishers work based on observations of carbon dioxide and the glowing splint. Then, explain why liquid oxygen is extremely hazardous based on observations of oxygen and the glowing splint.
  9. Describe what might happen if your blood becomes too acidic. This causes your blood O2 levels to increase and your CO2 levels to decrease.
    a. Write the equation for the reaction of CO2 with water. b. Explain why reducing the amount of CO2 in the blood will reduce the acidity of the blood.
  10. Determine whether carbonated beverages are acidic or basic and what happens to the pH of a carbonated soft drink as it "flattens" over time.
  11. Explain the purpose of using regular air in these investigations.