In a petri dish, you will simulate a little environment with local sources of industrial pollutants that cause acid rain. You will create the equivalent of little power plants and cars that emit primary pollutant gases (SO₂ and NO) that can be transformed in air, on dust, and in water to strong acids like H₂SO4 (sulfuric acid) and HNO₃ (nitric acid). You will also prepare some little lakes to see how these gases are transported in air and how the lake pH responds to nearby industrialization.

You will generate SO₂ and NO from chemical reactions, instead of using combustion reactions to burn coal and gasoline. However, the pollutant gas transport in air and how they dissolve in lakes should mimic some aspects of the real environmental conditions.

Supplies:

• 3 petri dishes (plastic works better than glass)
• 1 white ceramic spot plate
• NO source (made by cars): 0.5 M NaNO₂ (sodium nitrite)
• SO₂ source (made by power plants): 0.5 M Na₂SO₃ (sodium sulfite), prepare fresh
• Initiator: 2 M H₂SO₄ (sulfuric acid)
• Acid-base probe: bromothymol blue green (blue/green in base, yellow in acid)
• "Imported water": 0.01 M NaHCO₃ (sodium bicarbonate, baking soda)
• Disposable pipettes and bulbs (for mixing NaHCO₃ lakes)
• pH paper (range 0-14)

Experiment 1: Will SO₂ make lakes acidic?

Prepare a petri dish with several lakes of bromothymol blue at various distances from the SO₂ source. For the SO₂ source, put 1 drop of Na₂SO₃ in the dish. When you are ready to start, initiate SO₂ production by adding 1 drop of 2M H₂SO₄ to the Na₂SO₃ puddle. Put the petri lid on quickly and watch what happens.

Experiment 2: Will NO make lakes acidic?

Similar to Experiment 1, but now the source is 1 drop of NaNO₂, followed by 1 drop of sulfuric acid to initiate NO production. Decided where to place your bromothymol blue lakes relative to the source. Initiate with H₂SO₄, put the lid on, and watch what happens.

Experiment 3: How will lakes with bicarbonate (from limestone) respond?

Your lakes will now contain both bromothymol blue (the indicator) and varying amounts of NaHCO3. First prepare 4 different lake solutions in a spot plate as follows:

Lake	NaHCO3 solution	Bromothymol blue solution
#1	1 drop	9 drops
#2	2 drops	8 drops
#3	3 drops	7 drops
#4	4 drops	6 drops

 Now take drops (USING DIFFERENT DISPOSABLE PIPETTES to prevent cross contamination) from 4 different mixtures to make petri dish lakes, which now represent lakes in different geological regions with varying amounts of limestone (CaCO₃) in the lakebed and/or soil. Limestone dissolves in water to make bicarbonate (HCO₃^-).

Use either the NaNO₂ or Na₂SO₃ solution as the source and H₂SO₄ as the initiator, as above, to make NO or SO₂. How does the time vary to observe acid effects with lakes made with different amounts of bicarbonate?

Disposing of petri dish solutions:

Wash contents down the sink with lots of water.

Some Chemical Reaction to help explain our results:

pH = - log₁₀[H⁺] pH 7 is neutral. Less than 7 is acidic, more than 7 is basic.

• SO₂ (g) + oxidant (source of O) --> SO₃ (g)
• SO₃ (g) + H₂O (in rain or lake) --> H₂SO₄ (in water)
• H₂SO₄ (in water) --> H⁺ (aq) + HSO₄^- (aq)

• NO (g) + O₂(g) --> NO₂ (g)
• NO₂ (g) + H₂O (in rain or lake) --> HNO₂ (aq) + HNO₃ (aq)
• HNO₃ (in water) --> H⁺ (aq) + NO₃^- (aq)

• CaCO₃ (limestone) + H₂O --> Ca²⁺ (aq) + CO₃^2- (aq)
• CO₃^2-(aq) + H₂O --> HCO₃^- (aq)+ OH^- (aq)

• HCO₃^- (aq, in lake) + H⁺ (in rain) --> H₂CO₃

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