Authors: Rachel Casiday and Regina Frey Department of Chemisattempt, Washington College St. Louis, MO 63130

Natural Acidity of Rainwater

Pure water has a pH of 7.0 (neutral); yet, natural,unpolluted rainwater actually has actually a pH of about 5.6(acidic).

You are watching: Sulfur dioxide gas is bubbled through water

The acidity ofrainwater comes from the herbal existence of 3 substances (CO2,NO, and SO2) found in the tropospright here (the lowestlayer of the atmosphere). As is viewed in Table I, carbon dioxide(CO2) is current in the greatest concentration andtherefore contributes the many to the natural acidity ofrainwater.


Natural Sources


Carbon dioxide CO2 Decomposition
355 ppm
Nitric oxide NO Electric discharge
0.01 ppm
Sulhair dioxide SO2 Volcanic gases
0-0.01 ppm

Table 1

Carbon dioxide, developed in the decomposition of organic product, is the major resource of acidity in unpolluted rainwater.

NOTE: Parts per million (ppm) is a prevalent concentration meacertain supplied in environmental chemisattempt. The formula for ppm is offered by:


Carbon dioxide reacts through water to develop carbonic acid(Equation 1). Carbonic acid then dissociates to offer the hydrogenion (H+) and the hydrogen carbonate ion (HCO3-)(Equation 2). The ability of H2CO3 tosupply H+ is what classifies this molecule as anacid, hence lowering the pH of a solution.





Nitric oxide (NO), which additionally contributes to the naturalacidity of rainwater, is created throughout lightning storms by thereactivity of nitrogen and oxygen, 2 prevalent atmospheric gases(Equation 3). In air, NO is oxidized to nitrogen dioxide (NO2)(Equation 4), which in turn reacts via water to give nitric acid(HNO3) (Equation 5). This acid dissociates in water toyield hydrogen ions and nitprice ions (NO3-)in a reactivity analagous to the dissociation of carbonic acidshown in Equation 2, aget lowering the pH of the solution.








Acidity of Polluted Rainwater

Unfortunately, human commercial task produces additionalacid-creating compounds in far higher amounts than the naturalresources of acidity described over. In some areas of the UnitedStates, the pH of rainwater deserve to be 3.0 or lower, approximately1000 times even more acidic than normal rainwater. In 1982, the pH ofa fog on the West Coast of the USA was measured at 1.8!When rainwater is also acidic, it can reason difficulties varying fromkilling freshwater fish and damaging crops, to eroding buildingsand monuments.

Questions on Acidity of Rainwater

1. List 2 or even more methods that you could test the acidity of asample of rainwater.

2. Write a well balanced chemical equation for the dissociation ofnitric acid in water. (HINT: Draw an analogy through Equation 2.)

3. The gaseous oxides found in the setting, including CO2and also NO are nonmetal oxides. What would happen to the pH ofrainwater if the atmosphere contained metal oxides instead?(HINT: Think ago to Experiment 1.) Briefly, define your answer.

Sources of Excess Acidity in Rainwater

What reasons such a dramatic boost in the acidity of rainfamily member to pure water? The answer lies within the concentrationsof nitric oxide and sulhair dioxide in polluted air. As displayed inTable II and also Figure 1, the concentrations of these oxides aremuch greater than in clean air.


Non-Natural Sources


Nitric oxide NO Internal Combustion
0.2 ppm
Sulhair dioxide SO2 Fossil-fuel Combustion
0.1 - 2.0 ppm

Table II

Humans cause many type of combustion processes that substantially increase the concentrations of acid-creating oxides in the setting. Although CO2 is current in a much better concentration than NO and SO2, CO2 does not develop acid to the very same level as the other two gases. Hence, a huge boost in the concentration of NO and SO2 considerably affects the pH of rainwater, also though both gases are present at much lower concentration than CO2.


Figure 1

Comparichild of the concentrations of NO and also SO2 in clean and also polluted air.

About one-fourth of the acidity of rain is accounted for bynitric acid (HNO3). In addition to the naturalprocesses that develop tiny amounts of nitric acid in rainwater,high-temperature air combustion, such as occurs in automobile enginesand power plants, produces large quantities of NO gas. This gas thencreates nitric acid via Equations 4 and also 5. Therefore, a procedure thatoccurs normally at levels tolerable by the atmosphere have the right to harmthe environment once human task causes the procedure (e.g.,formation of nitric acid) to occur to a much higher degree.

What about the various other 75% of the acidity of rain? Most isaccounted for by the visibility of sulfuric acid (H2SO4)in rainwater. Although sulfuric acid may be produced normally intiny quantities from organic decay and also volcanic activity(Figure 1), it is produced virtually completely by huguy activity,particularly the combustion of sulfur-containing fossil fuels inpower plants. When these fossil fuels are melted, the sulfurincluded in them reacts with oxygen from the air to form sulfurdioxide (SO2). Combustion of fossil fuels accounts foraround 80% of the total atmospheric SO2 in theUnited States. The results of burning fossil fuels deserve to bedramatic: in contrast to the unpolluted atmospheric SO2concentration of 0 to 0.01 ppm, polluted city air deserve to contain0.1 to 2 ppm SO2, or approximately 200 times more SO2!Sulhair dioxide, prefer the oxides of carbon and nitrogen, reactsvia water to develop sulfuric acid (Equation 6).



Sulfuric acid is a solid acid, so it readilydissociates in water, to provide an H+ ion and an HSO4-ion (Equation 7). The HSO4- ion might furtherdissociate to provide H+ and SO42-(Equation 8). Therefore, the visibility of H2SO4reasons the concentration of H+ ions to increasesignificantly, and so the pH of the rainwater drops to harmfullevels.





Questions on Sources of Acidity in Rainwater

4. At sea level and also 25oC,one mole of air fills a volume of 24.5 liters, and also the density ofair is 1.22x10-6 g/ml. Compute the mole fraction (i.e.,moles of component /complete moles) and also molarity of SO2 when the atmosphericconcentration of SO2 is 2.0 ppm (view note in Table I).

5. One strategy for limiting theamount of acid pollution in the environment is scrubbing.In certain, calcium oxide (CaO) is injected right into thecombustion chamber of a power plant, wbelow it reacts via thesulhair dioxide created, to yield solid calcium sulfite.

a. Write a well balanced chemical equation for this reaction. (HINT: Consult the table of prevalent ions in the tutorial assignment for Experiment 1 to watch the structure and formula for sulfite; also, usage your understanding of the routine table to deduce the charge of the calcium ion. Using these facts, you deserve to deduce the formula for calcium sulfite.)

b. Approximately one ton, or 9.0x102 kg, of calcium sulfite is produced each year for eexceptionally person served by a power plant. How a lot sulfur dioxide (in moles) is prevented from entering the atmosphere when this much calcium sulfite is generated? Show your calculation.

c. The final phase in the scrubbing process is to treat the burning gases with a slurry of solid CaO in water, in order to trap any kind of remaining SO2 and also convert it to calcium sulfite. A slurry is a thick suspension of an insoluble precipitate in water. Using the solubility guidelines gave in the lab hands-on for this experiment, predict whether this phase of the scrubbing process will produce a slurry (i.e., precipitate) or a solution (i.e., no precipitate) of calcium sulfite .

d. If MgO, quite than CaO, were offered for scrubbing, would the product of the last phase be a slurry or a solution of magnesium sulfite? (Assume that a really huge amount of magnesium sulfite, relative to the amount of water, is produced.)


Environmental Effects of Acid Rain

Acid rain triggers a variety of not natural and biochemicalreactions with deleterious environmental effects, making this aflourishing eco-friendly problem worldwide.

Many lakes have actually become so acidic that fish cannot live in them anyeven more. Degradation of many kind of soil minerals produces steel ions that are then washed away in the runoff, resulting in a number of effects: The release of toxic ions, such as Al3+, right into the water supply. The loss of essential minerals, such as Ca2+, from the soil, killing trees and also damaging plants. Atmospheric pollutants are easily relocated by wind currents, so acid-rain results are felt far from where pollutants are produced.

Stone Buildings and Monuments in Acid Rain

Marble and limestone have lengthy been preferred products forbuilding long lasting buildings and also monuments. The Saint Louis ArtMuseum, the Parthenon in Greece, the Chicearlier Field Museum, andthe United States Capitol structure are all made of theseproducts. Marble and limerock both consist of calcium carbonate(CaCO3), and also differ just in their crystallineframework. Limestone is composed of smaller sized crystals and is moreporous than marble; it is offered more extensively in structures.Marble, via its larger crystals and smaller sized pores, have the right to obtain ahigh polish and also is thus desired for monuments and also statues.Although these are well-known as highly resilient materials,structures and also outdoor monuments made of marble and limestone arenow being gradually eroded away by acid rain.

How does this happen? A chemical reactivity (Equation 9) betweencalcium carbonate and also sulfuric acid (the main acid componentof acid rain) outcomes in the dissolution of CaCO3 toprovide aqueous ions, which subsequently are waburned ameans in the watercirculation.



This procedure occurs at the surconfront of thestructures or monuments; thus acid rain deserve to conveniently ruin thedetails on relief work (e.g., the deals with on a statue),yet mostly does not affect the structural integrity of thebuilding. The level of damages is identified not just by theacidity of the rainwater, yet additionally by the amount of water flowthat a region of the surface receives. Regions exposed to directdownpour of acid rain are extremely at risk to erosion, butregions that are even more sheltered from water flow (such as undereaves and also overhangs of limestone buildings) are a lot bettermaintained. The marble columns of the monarchs Marcus Aurelius andTrajan, in Rome, provide a striking example: big quantities ofrainwater flow directly over certain parts of the columns, whichhave been badly eroded; other parts are protected by wind effectsfrom this flow, and are in extremely great condition even afteralmost 2000 years!

Even those parts of marble and limestonestructures that are not themselves eroded can be damaged by thisprocess (Equation 9). When the water dries, it leaves behind theions that were dissolved in it. When a solution containingcalcium and sulfate ions dries, the ions crystallize as CaSO4l 2H2O,which is gypsum. Gypamount is soluble in water, so it is washed awayfrom areas that receive a heavy circulation of rain. However, gypsumaccumulates in the same sheltered locations that are defended fromerosion, and also attracts dust, carbon pwrite-ups, dry-ash, and also otherdark pollutants. This outcomes in blackening of the surfaces wheregypsum accumulates.

An also more significant situationarises once water containing calcium and sulfate ions penetratesthe stone"s pores. When the water dries, the ions develop saltcrystals within the pore mechanism. These crystals can disrupt thecrystalline arrangement of the atoms in the stone, causing thefundamental framework of the rock to be disturbed. If thecrystalline structure is disrupted sufficiently, the stone mayactually crack. Therefore, porosity is a vital aspect indetermining a stone"s durability.

Questions on Effects of Acid Rain

6. Based on the information explained over aboutthe calcium ion, and the formula of calcium carbonate (CaCO3),deduce the charge of the carbonate ion. Also, in the framework ofthe carbonate ion, are any type of of the oxygens bonded to one another,or all the oxygens bonded to the carbon atom? (HINT: Consult theframework of the prevalent ions offered in the tutorial for Experiment1).

7. In water, H2SO4 candissociate to yield 2 H+ ions and one SO42-ion. Write the net ionic equation for the reactivity of calciumcarbonate and also sulfuric acid. (See the introduction to Experiment2 in the lab manual for a discussion of net ionic equations.)

8. Which is a more long lasting structure product,limestone or marble? Briefly, define your thinking.


Further Links:


Brown, Lemay, and also Buster. mslsec.com: the Central Science,7th ed. Upper Saddle River, NJ: Prentice Hall, 1997. p. 673-5.

Charola, A. "Acid Rain Effects on Stone Monuments," J.Chem. Ed. 64 (1987), p. 436-7.

Petrucci and also Harhardwood. General mslsec.com: Principles andModern Applications, 7th ed. Upper Saddle River, NJ:Prtempt Hall, 1997. p. 614-5.

Walk, M. F. and P.J. Godfrey. "Effects of Acid Depositionon Surface Waters," J. New England Water Works Assn.Dec. 1990, p. 248-251.

Zumdahl, S.. Chem. Principles, 3rd ed. Boston:Houghton Mifflin, 1998. p. 174-6.

Stryer, L. Biomslsec.com, fourth ed., W.H. Freemale andCo., New York, 1995, p. 332-339.


The authors thank Dewey Holten (Washington University) formany kind of advantageous suggestions in the writing of this tutorial.

See more: How To Divide A Circle Into 6 Equal Parts ? How To Divide A Circle Into 6 Equal Parts

The development of this tutorial was sustained by a approve fromthe Howard Hughes Medical Institute, through the UndergraduateBiological Sciences Education regime, Grant HHMI# 71192-502004to Washington University.