Background:

Before falling to Earth as precipitation, water vapor in the atmosphere normally reacts with carbon dioxide gas to form a weak acid, carbonic acid (H₂CO₃). As a result, the pH of rainwater is approximately 5.6, making typical rainwater slightly acidic.

However, water vapor in the atmosphere will also react in a similar fashion with other gases, including sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and nitrogen trioxide (NO₃). These gases are pollutants that result from the burning of fossil fuels: for example, in car exhaust and factory emissions. These reactions will produce acids stronger than carbonic acid, including sulfuric acid (H₂SO₄), nitric acid (HNO₃), and sulfurous acid (H₂SO₃). If the concentration of these acids in the atmosphere is high enough, the pH of rainwater can be lowered considerably.

Rain with a pH lower than 5.6 is considered "acid rain" and can have significant environmental effects on natural ecosystems and on buildings, statues, and other structures.

In this experiment, you will create samples of "acid rain" in the laboratory, using the pH Sensor to quantify the change in acidity of the "rain water". You will also complete several simple laboratory tests to determine what effects acid rain has on common substances.

The Questions:

Can acid rain be simulated?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: Wear protective equipment while handling chemicals - this lab uses a strong acid (HCl) - take all the necessary precautions (gloves, apron, eye protection).

Materials:

Procedure:

Step 2:

Connect the pH sensor to your computer - if you are using a USB sensor, your apparatus will look something like this:

Clamp the pH Sensor to a support rod, as in Photo A below:

Step 3:

Follow these steps to generate "Acid Rain":

• Obtain a 1-gram sample of sodium sulfite (Na₂SO₃) in a test tube. Place the test tube inside the plastic zip-seal bag, holding the test tube upright from outside the bag. See photo "B" (the person in the photo should have been wearing protective gloves - do you have yours on?):

• Carefully fill a disposable pipette with 6 M hydrochloric acid (HCl). CAUTION: Hydrochloric acid (HCl) is corrosive. Avoid spilling, and avoid breathing fumes. Use a wash bottle to rinse off any acid on the outside of the pipette.
• WITHOUT squeezing the bulb of the pipette, so that no acid is released from it, place the pipette filled with acid inside the test tube which is inside the plastic bag. Be sure the tip of the pipette does not contact the sodium sulfite -- do not start the reaction yet!
• Add 15 ml distilled water to the plastic bag, making sure that the water does not come into contact with either the acid or the sodium sulfite.
• With the assistance of your lab partner(s), move the plastic bag assembly to the support rod where the pH Sensor is clamped. Remove the sensor from the clamp, and position the plastic bag assembly so that the sensor is inserted into the distilled water at the bottom of the bag. (The water level should just cover the tip of the sensor.)
• While one lab partner holds the test tube / plastic bag assembly in place, a second person should seal the bag as tightly as possible around the sensor wire. The plastic bag "environment" should now be closed, with the sensor ready to record the pH of the water. See photo "C":

Step 4:

Click the Start button ( ) to begin collecting data (pH of "rain" water). Keeping the sensor in place, slowly and carefully squeeze the pipette bulb so that all of the acid drips onto the solid sodium sulfite in the test tube. Keep the test tube upright so that the contents do not spill, and keep the bag SEALED.

Step 5:

Allow the reaction in the test tube to proceed for 1-2 minutes, tapping gently on the test tube as needed. (You may need to continue to hold the pipette above the test tube while the reaction proceeds so that fumes can escape the test tube.) See photo "D":

After the reaction appears to stop, gently swirl the water in the bottom of the bag for 1-2 additional minutes. Be careful the water does not come into contact with the contents of the test tube, but that the sensor tip stays submerged.

Click the Stop button ( ) to end data collection after the pH level stabilizes.

Step 6:

Carefully open the top corner of the plastic bag and remove the assembly from the pH Sensor. Rinse the tip of the sensor with distilled water from the wash bottle.

Use the 2nd (clean) disposable pipette to transfer the liquid at the bottom of the bag into the 3 remaining test tubes. Transfer 5 ml to each test tube and label them A, B, or C.

Seal the plastic bag with the test tube still inside, and set it aside for disposal.

Step 7:

Into each of the 3 test tubes A, B, and C, add one of the following: small piece of magnesium ribbon, 2 marble chips (calcium carbonate, CaCO3), and a small piece of fresh apple skin. Observe the results and record them in a table such as the following:

Analyzing and Interpreting:

1.	Perform the following data analysis:

Scale the axes of the graph to fit the data using the Scale to Fit button () in the Graph toolbar.

Use DataStudio's built-in analysis tools to find the maximum and minimum pH values, and the change in pH during the reaction. Hint: Click the Statistics button () drop-down arrow; "minimum" and "maximum" should already be selected. Use the Smart Tool () to pinpoint exact coordinate points on the graph and find the change in pH.

Label the graph to describe "typical rainwater" and "acid rain". Click the Annotate button (), and then click the graph. The 'Annotation Properties' dialog box will appear where you can enter your text. After entering text and clicking "OK", use the cursor to position the text box and the pointer ("tail") on the graph in the appropriate location.

Forming Conclusions:

Based on the data you have collected, answer the following questions:

2.	Can acid rain be simulated? In what ways is this simulation like the process that occurs in the atmosphere to produce acid rain? In what ways is it different?
3.	What effect did the SO2 gas have on the acidity of the "rain" water in the bottom of the plastic bag? Can this water now be classified as "acid rain", according to the pH data?
4.	Judging by the effect of the acid rain samples on the contents of test tubes A, B, and C, what prediction can you make about the effect of similar rain on statues or buildings?

Extending:

Once you have learned how to balance chemical equations, consider returning to this lab to answer the following questions:

• The gas formed by the reaction inside the test tube was sulfur dioxide (SO₂). Sodium chloride (NaCl) and water were the other products. Write a complete balanced equation for this reaction.
• The acid that was produced as the gas mixed with water inside the sealed bag was sulfurous acid, H₂SO₃. Write a complete balanced equation for this reaction.
• Write a correctly balanced equation for the reaction of "acid rain" (H₂SO₃) that occurred in test tube "A" (Mg ribbon), and a second correctly balanced equation for the reaction that occurred in test tube "B" (marble chips).