Visitors Create Acid Rain in a Glass Dish by Reacting Two Chemicals. the Reaction Creates Sulfur Dioxide Gas

EXPERIMENT Acid Rain

Visitors create acid rain in a glass dish by reacting two chemicals. The reaction creates sulfur dioxide gas. The gas reacts with a drop of water, causing it to become acidic.

OBJECTIVES: Visitors learn that certain gases in the atmosphere react with rainwater to produce an acid rain that is harmful to plants and animals.

SCIENCE TOPICS PROCESS SKILLS VOCABULARY

Chemical Reactions Observing Acid pH/lndicators Measuring Chemical Reaction Properties of Gases

Experiment: Acid Rain Operating Guide

See Materials Prep (with amounts to have on hand) for more details • Plastic petri dish with lid (keep two on hand) • Four 30-ml dropper bottles

• Na2SO3 (sodium sulfite) (keep 200 g on hand) • 1M HCl (hydrochloric acid) (keep 100 ml on hand) —OR— 12.1M (concentrated) HCl (keep 100 ml on hand) • Two plastic 500-ml storage bottles • One plastic 125-ml storage bottle • Bromthymol blue sodium salt (keep 5 g on hand) • One white plastic mat • cloth dish towel • Large plastic bowl

Setup/Takedown Procedures

Label Petri dish with round label with numbers 1-5 on it. Label the three 30-ml dropper bottles “Hydrochloric Acid (HCl),” “Sodium Sulfite (Na2SO3) and “Bromthymol Blue.” Label the plastic bowl “towel”.

Remake stock solutions of 1M Na2SO3 (sodium sulfite), 1M HCl (hydrochloric acid), and bromthymol blue indicator as necessary. (Keep 50 ml of each on hand.) (See Materials Prep.) Refill all three labeled dropper bottles from the stock solutions.

Set out the visitor instructions in a Plexiglas holder. On a tray lined with a white mat, set out the following: • Labeled plastic petri dish (with three sections) • Plastic lid for petri dish • Labeled dropper bottle of bromthymol blue • Labeled dropper bottle of HCl (hydrochloric acid)

Experiment: Acid Rain Operating Guide DO NOT SET OUT the dropper bottle of NaOH (sodium hydroxide).

Remake the stock solutions of 0.5M Na2SO3 (sodium sulfite), 0.5M NaOH (sodium hydroxide), 1M HCl (hydrochloric acid), and bromthymol blue indicator as necessary. (Keep 50 ml of each on hand.) (See Materials Prep.) Refill all four labeled dropper bottles from the stock solutions. (HCl is in the acid cabinet, NaOH is in the hydroxide cabinet, and bromthymol blue solution is in the indicator cabinet.) Bromthymol blue should be green to start. If it is blue, add one drop of 1M HCl (hydrochloric acid). If it is yellow, add one drop of 0.5M NaOH (sodium hydroxide). add these to the STOCK BOTTLE, not the dropper bottle.

Wash the plastic petri dish and return it to the tub. Tightly cap all bottles and store them in an upright position in the tub. Drape the towel across the counter or public copy holder to dry out overnight. If towel is stained and yucky looking, switch it out for a clean one.

Clean and leave the tray at the station. Clean the glassware and return it to the tub. Clean and return the mat to general lab storage.

◊ Once visitors have added the Na2SO3 (sodium sulfite) to the HCl (hydrochloric acid), there is a tendency for the visitors to watch this drop (in area “5”) for a reaction, The reaction is not a visible one, Advise visitors to watch the bromthymol blue drops in areas “3” and “4.”

◊ If the bromthymol is too acidic (yellow) or too basic(blue), pour the contents of the dropper bottle back into the stock bottle, mix, and then refill the dropper bottle. Do not try to adjust the pH in the little dropper bottle itself.

NO2 (nitrogen dioxide) is another common origin of acid rain. It can be found in car exhaust, among other places. It will react with water to

Experiment: Acid Rain Operating Guide

form nitric acid in a manner identical to that of the SO2 (sulfur dioxide) in this experiment.

Na2SO3 (sodium sulfite) and HCl (hydrochloric acid) are hazardous substances; follow the handling and disposal instructions in Materials Prep.

Consult the Material Safety Data Sheets (MSDS) for additional information.

To prepare 1M HCl (hydrochloric acid): CAUTION: concentrated HCl (hydrochloric acid) is extremely corrosive. Handle it with care. Wear protective gloves, an apron, and eyewear. Use it only in the fume hood. Avoid contact with skin or clothing. Neutralize any spills with baking soda and clean up with large amounts of water. CAUTION: A large amount of heat will be evolved after you mix the HCl with water. Do not handle the beaker until it has cooled. Turn on the fume hood. Wear protective eyewear, chemical safety gloves, and an apron or a lab jacket. In the fume hood, measure 83 ml of concentrated (12.1M) HCl.

Add the acid to 917 ml of dH2O (deionized water) to make one liter. Store the extra solution in the labeled and dated bottles in the acid cabinet. Turn off the fume hood.

To prepare 0.5M Na2SO3 (sodium sulfite): Wear protective eyewear, chemical safety gloves, and a lab jacket.

Weigh 62.5 g Na2SO3.

Add 500 ml dH2O (deionized water).

Mix to dissolve the Na2SO3. Store the solution in a labeled/dated bottle.

To prepare bromthymol blue solution: Weigh 0.1 g bromthymol blue sodium salt.

Add 100 ml dH2O (deionized water) and mix to dissolve the bromthymol blue. Store the solution in a labeled and dated 125-ml plastic storage bottle in the indicator cabinet.