Part 1: Engage (Anchoring Phenomenon)

Phenomenon: In parts of the Appalachian region, historic coal mining has exposed certain rocks to air and water, creating streams that run bright orange and are highly acidic. In these “acid mine drainage” streams, almost no fish or aquatic life can survive. Environmental engineers often build treatment plants alongside these rivers that slowly release substances into the water to “fix” it before it flows downstream.

Questions:

  1. Why do you think the water turns so acidic and changes color when it contacts these exposed rocks?
  2. What kind of substance do you think the environmental engineers are adding to the water to “fix” it, and how might that affect the pH?
  3. What might happen to the river ecosystem if the engineers add too much of this substance too quickly?

Part 2: Explore (Simulation Investigation)

Access the simulation: Appalachian Acid Mine Drainage: Titrations & Buffers

Investigation Instructions:

  1. Start with the simulation reset. Observe the initial state of the river. What is the starting pH and what is the “River Status”?
  2. Select Sodium Hydroxide (0.10M NaOH) - Strong Base from the dropdown menu.
  3. Slowly increase the Doser Flow Rate using the slider.
  4. Carefully monitor the pH graph. Record the Doser Flow Rate required to bring the river pH to exactly 7.0 (neutral).
  5. Continue increasing the flow rate slightly past this point. What happens to the pH? Does it change gradually or suddenly? Record the flow rate where the “Ecological Fine” warning appears.
  6. Press Reset. Now select Crushed Limestone (0.05M CaCO₃) - Weak Base.
  7. Again, slowly increase the Doser Flow Rate. Try to reach a pH of 7.0.
  8. Continue increasing the flow rate to its maximum. Describe the shape of the pH curve compared to the NaOH curve. Did you trigger an ecological fine?

Part 3: Explain (Sensemaking)

Using your data and observations from the simulation, answer the following questions:

  1. Comparing Reactions: Both NaOH and CaCO₃ are bases that neutralize the acid in the river. However, the pH graphs looked very different. Explain the difference between adding a strong base (NaOH) and a weak base/buffer (CaCO₃) to the acidic river.
  2. Atomic Rearrangement: When the base (e.g., NaOH) reacts with the acid (e.g., HCl or H₂SO₄) in the river, a neutralization reaction occurs. Describe in general terms how the atoms rearrange during this chemical reaction to form products that are less harmful to the environment.
  3. Designing a Solution: If you were the lead engineer for this treatment facility, which chemical (NaOH or CaCO₃) would you choose to use on a daily basis? Justify your choice using evidence from the simulation, specifically considering safety, the risk of overdosing (ecological fines), and effectiveness.

Part 4: Elaborate/Evaluate (Argumentation & Modeling)

Task: Construct a scientific explanation for why crushed limestone (CaCO₃) provides a safer, more stable solution for treating acid mine drainage than a strong base like NaOH.