Rainwater naturally acidic

As rainwater falls, it absorbs atmospheric carbon dioxide. Once in the rainwater, the carbon dioxide reacts with water to form an acid known as carbonic acid, H2C03, which, as we discuss in this chapter, makes rainwater naturally acidic. As the rainwater passes through the ground, the carbonic acid reacts with various basic minerals, such as limestone, to form products that are water soluble and thus carried away by the underground flow of water. This washing-away action over the course of millions of years creates caves.The world s most extensive cave system is in western Kentucky in Mammoth Cave National Park, where more than 300 miles of networked caves have been mapped. 

Why is rainwater naturally acidic, even in the absence of polluting gases such as SO2 ... 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Charlson, R. J. and Rodhe, H. (1982). Factors controlling the acidity of natural rainwater. Nature 295, 683-685. 

Rainwater is naturally acidic, containing about 48 micrograms of acidity per liter. Express this concentration in units of ppm, ppb, and ppt. 

Carbonic acid, as its name implies, behaves as an acid and lowers the pH of water. The C02 in the atmosphere brings the pH of rainwater to about 5.6— noticeably below the neutral pH value of 7. Because of local fluctuations, the normal pH of rainwater varies between 5 and 7. This natural acidity of rainwater may accelerate the erosion of land and, under the right circumstances, can lead to the formation of underground caves, as was discussed in this chapter s introduction. 

Rainwater is naturally acidic since it dissolves carbon dioxide gas from the atmosphere as it falls. Natural rainwater has a pH of about 5.7. In recent years, especially in central Europe, the pH of rainwater has fallen to between pH 3 and pH 4.8. This increase... 

Butterfield DA, Massoth GJ, McDuff RE, Lupton JE, Lilley MD (1990) Geochemistry of hydrothermal fluids from Ashes Vent Field, Axial Seamoimt, Juan de Fuca Ridge Sub-seafloor boihng and subsequent fluid-rock interaction. J Geophys Res 95(B8) 12895-12921 CharlsonRJ, Rodhe H (1982) Factors controlling the acidity of natural rainwater. Nature 295 683-685 Charlson RJ, Lovelock JE, Andreae MO, Warren SG (1987) Oceanic phytoplankton, atmospheric sulfur, cloud albedo and chmate. Nature 326 655-661... 

Rainwater is naturally acidic (with a pH less than 7) as a result of the dissolving of carbon dioxide in the moisture of the atmosphere and the forming of carbonic acid. (See Chapter 12 for more about carbonic acid as well as the pH scale.) This interaction results in rainwater having a pH of around 5.6. The term acid rain, or acid deposition, is used to describe a situation in which rainfall has a much lower (more acidic) pH than can be explained by the simple dissolving of carbon dioxide. Specifically acid rain is formed when certain pollutants in the atmosphere, primarily oxides of nitrogen and sulfur, dissolve in the moisture of the atmosphere and fall to earth as rain with a low pH value. 

R. Charlson H. Rodhe, Factors Controlling the Acidity of Natural Rainwater, Nature, 295, 1982. 

Rainwater is naturally acidic, with a pH range of 5 to 6, and can become more acidic when it comes into contact with decaying plant matter. Because carbonate ion is the conjugate base of the weak add, hydrogen carbonate ion (HCOs, it readily combines with hydrogen ion. 

The presence of SO2 in the atmosphere and the sulfuric acid that it produces result in the phenomenon of acid rain. (Nitrogen oxides, which form nitric acid, are also major contributors to acid rain.) Uncontaminated rainwater is naturally acidic and generally has a pH value of about 5.6. The primary source of this natural acidity is CO2, which reacts with water to form carbonic acid, H2CO3. Add rain, however, is more addic than normal rainwater and typically has a pH value of about 4. This addity has affected many lakes in northern Europe, northern United States, and Canada, reducing fish populations and affecting other parts of the ecological network within flie lakes and surrounding forests. 

Limestone (calcium carbonate, CaCOj) is a common sedimentary rock that was laid down eons ago on ocean floors, mostly from the remains of decomposed marine organisms. Subsequent uplift of the land brought limestone above sea level. Carbon dioxide dissolved in rainwater undergoes the reactions shown above. The natural acidity of rainwater lends itself to slightly dissolving limestone, releasing calcium ions (Ca ) into the environment, as shown ... 

Pure water is pH neutral, but rainwater is acidic. What causes rainwater to be acidic One contributing factor is that carbon dioxide from the atmosphere reacts with water to form carbonic acid, H2CO3, a diprotic acid. For a discussion of the natural sources of acidity in rainwater, and how human activities also contribute, go to the Focus On feature for Chapter 16, Acid Rain, on the MasteringChemistry site. 

The pH of rainwater in equUibrium with atmospheric CO2 is 5.6, a value frequendy cited as the natural background pH. However, in the presence of other naturaUy occurring species such as SO2,, NH, organic acids, sea salt, and alkaline cmstal dust, the natural values of unpoUuted rainwater vary... 

Sometimes we need to know how the concentrations of the ions present in a solution of a polyprotic acid vary with pH. This information is particularly important in the study of natural waters, such as rivers and lakes (Box 10.1). For example, if we were examining carbonic acid in rainwater, then, at low pH (when hydronium ions are abundant), we would expect the fully protonated species (H2C03) to be dominant at high pH (when hydroxide ions are abundant), we expect the fully deprotonated species (C032 ) to be dominant at intermediate pH, we expect the intermediate species (HC03, in this case) to be dominant (Fig. 10.20). We can verify these expectations quantitatively. 

Rainwater is naturally slightly acidic due to the dissolved carbon dioxide. Acid rain results when acidic sulfur and nitrogen oxides produced during the combustion of coal and oil react with rainwater (see Box 10.1). 

Chemical interactions also occur in the condensed phases. Some of these are expected to be quite complex, e.g., the reactions of free radicals on the surfaces of or within aerosol particles. Simpler sorts of interactions also exist. Perhaps the best understood is the acid-base relationship of NH3 with strong acids in aerosol particles and in liquid water (see Chapter 16). Often, the main strong acid in the atmosphere is H2SO4, and one may consider the nature of the system consisting of H2O (liquid), NH3, H2SO4, and CO2 under realistic atmospheric conditions. Carbon dioxide is not usually important to the acidity of atmospheric liquid water (Charlson and Rodhe, 1982) the dominant effects are due to NH3 and H2SO4. The sensitivity the pH of cloud (or rainwater produced from it) to NH3 and... 

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