Acid+Deposition+(HL)

= __ACID DEPOSITION (HL ONLY) __ =

Yao Xiong Assessment statements: E.11.1 -- Describe the mechanism of acid deposition caused by the oxides of nitrogen and oxides of sulfur E.11.2 -- Explain the role of ammonia in acid deposition

Recall: Acid deposition -- Process which acidic particles leave the atmosphere.

This sub topic looks at the formation of sulfuric acid, nitric and nitrous acid in our atmosphere. These acid generally results in acid deposition, such as acid rain. This will then be followed by looking at the role of ammonia in acid deposition.

Before starting the first section, the formation of hydroxyl radicals will need to be understood, as they play an important role in the formation of the acids mentioned above. Hence, they are also the basis of this section.

__Hydroxyl radicals __

A hydroxyl is a compound which contains an oxygen atom covalently bonded to a hydrogen atom. A hydroxyl radical HO .  is neutral compound of OH - , with the oxygen atom having a lone electron, as denoted by the <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;">beside the oxygen atom. <span style="font-family: 'Comic Sans MS',cursive;">

<span style="font-family: 'Comic Sans MS',cursive;">Hydroxyl radicals are very reactive as they are unstable, due to the absence of an electron which prevents them from having an octet structure OH <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">- <span style="font-family: 'Comic Sans MS',cursive;">.

<span style="font-family: 'Comic Sans MS',cursive;">These radicals are formed from reaction of water vapor with ozone or oxygen radicals in our atmosphere, as shown from the equation : <span style="font-family: 'Comic Sans MS',cursive;"> H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (g) + O <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (g)--> 2HO <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;"> (g) + O <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (aq), or H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (g) + O <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;">(g) --> 2HO <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;">(g)

__<span style="font-family: 'Comic Sans MS',cursive;">Formation of sulfuric and sulfurous acid __

<span style="font-family: 'Comic Sans MS',cursive;">Sulfuric acid is formed by sulfur dioxide (SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">), one of the primary pollutants which are in turn formed from the smelting of metal sulfate ores during production of iron, nickel and steel. <span style="font-family: 'Comic Sans MS',cursive;">The following equation shows its formation from the production of nickel: <span style="font-family: 'Comic Sans MS',cursive;">NiS(s)+ O <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g)--> SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + Ni (s)

<span style="font-family: 'Comic Sans MS',cursive;">As mentioned above, HO <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;"> is very reactive, hence SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> will then be oxidized rapidly by hydroxyl radicals to form HOSO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;"> radicals, and is then followed by the reaction with oxygen to produce sulfur trioxide. <span style="font-family: 'Comic Sans MS',cursive;">SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + HO <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;"> (g) --> HOSO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;">(g) <span style="font-family: 'Comic Sans MS',cursive;">HOSO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;">(g) + O <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) --> HO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;"> (g) + SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (g)

<span style="font-family: 'Comic Sans MS',cursive;">In the presence of water vapor in the clouds, SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> will dissolve in the rain to form sulfuric acid. <span style="font-family: 'Comic Sans MS',cursive;">SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (l) --> __H__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">4 <span style="font-family: 'Comic Sans MS',cursive;">(aq) __

<span style="font-family: 'Comic Sans MS',cursive;">Some of the unreacted SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> will also be converted to sulfurous acid in the way mentioned above. <span style="font-family: 'Comic Sans MS',cursive;">SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (l) --> __H__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (aq) __

<span style="font-family: 'Comic Sans MS',cursive;">__Formation of nitric and nitrous acid__

<span style="font-family: 'Comic Sans MS',cursive;">The formation of nitric acid and nitrous acid are similar to that of sulfuric acid. <span style="font-family: 'Comic Sans MS',cursive;">Nitrogen oxides NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">x <span style="font-family: 'Comic Sans MS',cursive;"> (refers to the NO and NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> oxides) will react with hydroxyl radicals to form nitric and nitrous acid, depending on the number of oxygen atoms in the nitrogen oxides. <span style="font-family: 'Comic Sans MS',cursive;">NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">x <span style="font-family: 'Comic Sans MS',cursive;"> is produced from the combustion of oxygen and nitrogen in high temperatures, such as car engines and forest fires.

<span style="font-family: 'Comic Sans MS',cursive;">Nitric oxide is able to form nitrous acid upon reacting with hydroxyl radicals: HO <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;"> + NO (g) --> __HNO__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (aq) __ <span style="font-family: 'Comic Sans MS',cursive;">Nitrogen dioxide, on the other hand, produces nitric acid in the reaction with hydroxyl radicals: HO <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">. <span style="font-family: 'Comic Sans MS',cursive;"> + NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) --> __HNO__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (aq) __

<span style="font-family: 'Comic Sans MS',cursive;">Alternatively, nitrogen dioxide can react with nitrogen oxide in the clouds to form nitrous acid: NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + NO (g) + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (g) --> __2HNO__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (aq) __ <span style="font-family: 'Comic Sans MS',cursive;">Nitrogen dioxide can also react with water vapor in the clouds, producing nitric acid in the process: 3NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (g) --> __2HNO__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 __<span style="font-family: 'Comic Sans MS',cursive;"> __(aq)__ + NO (g)

<span style="font-family: 'Comic Sans MS',cursive;">Nitrous acid can also be produced through the oxidation of nitrogen oxide in the presence of water: 4NO (g) + O <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (g) --> __4HNO__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">(aq) __

<span style="font-family: 'Comic Sans MS',cursive;">**Both the oxides of sulfur and that of nitrogen are able to result in the formation of sulfurous and sulfuric acids, as well as nitrous and nitric acids respectively, which are released from the atmosphere through acid rain.**

<span style="font-family: 'Comic Sans MS',cursive;">**The process of the formation of such acids usually involves in the reaction of sulfur and nitrogen oxides with moisture and oxygen in the clouds, where the oxides will be able to take in the hydrogen and oxygen atoms****. (See Acid Deposition SL)** <span style="font-family: 'Comic Sans MS',cursive;">**The other process of converting the oxides into acids is through the radical mechanism. These oxides will react with hydroxyl radicals in the clouds, and then oxygen and water vapor (for sulfur oxides) to form acids.**

<span style="font-family: 'Comic Sans MS',cursive;">__What about ammonia?__

<span style="font-family: 'Comic Sans MS',cursive;">Ammonia (NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;">) is a weak base which is present in our atmosphere to prevent acid deposition. <span style="font-family: 'Comic Sans MS',cursive;">This is done through the neutralization of the acid, resulting in the formation of ammonium salts. <span style="font-family: 'Comic Sans MS',cursive;">The following equation shows the neutralization involving ammonia and sulfuric acid, producing ammonium sulfate, a salt which is often used as soil fertilizer to reduce the soil's pH value. <span style="font-family: 'Comic Sans MS',cursive;">2NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">4 <span style="font-family: 'Comic Sans MS',cursive;"> (aq) --> (NH 4 <span style="font-family: 'Comic Sans MS',cursive;">) <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">SO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">4 <span style="font-family: 'Comic Sans MS',cursive;">(aq)

<span style="font-family: 'Comic Sans MS',cursive;">The next equation shows the acid-base reaction involving ammonia and nitric acid. NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">4 <span style="font-family: 'Comic Sans MS',cursive;">NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> is used as a fertilizer. <span style="font-family: 'Comic Sans MS',cursive;">HNO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;">(aq) + NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;">(g) → NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">4 <span style="font-family: 'Comic Sans MS',cursive;">NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;">(aq)

<span style="font-family: 'Comic Sans MS',cursive;">However, how on earth did ammonia get up to our atmosphere in the first place? <span style="font-family: 'Comic Sans MS',cursive;">They find their way up through the fertilization of agricultural areas and also nitrogen cycle, which occurs naturally.


 * <span style="font-family: 'Comic Sans MS',cursive;">Fertilizers contain ammonia, and in fact, 90% of ammonia consumption was for fertilizers in the U.S. alone.


 * <span style="font-family: 'Comic Sans MS',cursive;">Nitrogen cycle is shown by the picture on the right. Recall that nitrogen cycle is the process in which nitrogen is converted into many chemical forms in our environment. Ammonium (NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">4 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">+ <span style="font-family: 'Comic Sans MS',cursive;">) will be converted to NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> gas in alkaline soil, which will be lost to the atmosphere.


 * <span style="font-family: 'Comic Sans MS',cursive;">Another way which ammonia can be released to the atmosphere is through a process known as Biological Nitrogen Fixation (BNF). This involves in the conversion of N <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> to NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;">through the reaction with H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> obtained from the burning of fossil fuels. It also involves in the presence of an enzyme known as nitrogenase, as N <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> consist of strong triple bonds which are hard to break. BNF can be shown from: N <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + 3 H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) + energy --> __2NH__ __<span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive;"> (g) __

<span style="font-family: 'Comic Sans MS',cursive;">Back to ammonium salts, they will be deposited in the ground through rain, which migrates them into soil. <span style="font-family: 'Comic Sans MS',cursive;">In the soil, acidification and nitrification will then occur.

<span style="font-family: 'Comic Sans MS',cursive;">Soil acidification refers to the production of H <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">+ <span style="font-family: 'Comic Sans MS',cursive;"> ions in the soil, which is capable to reducing the pH level of the soil. <span style="font-family: 'Comic Sans MS',cursive;">Nitrification is the oxidation of ammonia to nitrate via nitrite in the soil, which is part of the nitrogen cycle. <span style="font-family: 'Comic Sans MS',cursive;">Both processes can be shown by the equation: <span style="font-family: 'Comic Sans MS',cursive;">NH <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">4 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">+ <span style="font-family: 'Comic Sans MS',cursive;"> + 2O <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;"> (g) --> 2H <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">+ <span style="font-family: 'Comic Sans MS',cursive;"> + NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">- <span style="font-family: 'Comic Sans MS',cursive;"> + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (l) <span style="font-family: 'Comic Sans MS',cursive;">The oxidation of nitrites to nitrites is part of the nitrogen cycle, shown by: NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">- <span style="font-family: 'Comic Sans MS',cursive;"> + H <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">2 <span style="font-family: 'Comic Sans MS',cursive;">O (l) --> NO <span style="font-family: 'Comic Sans MS',cursive; font-size: 80%; vertical-align: sub;">3 <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">- <span style="font-family: 'Comic Sans MS',cursive;"> + 2H <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">+ <span style="font-family: 'Comic Sans MS',cursive;"> + 2e <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%; vertical-align: super;">-

<span style="font-family: 'Comic Sans MS',cursive;">**The role of ammonia in the atmosphere is to neutralize acids which result in acid deposition, such as the ones mentioned in the above section. The salts produced from process will then sink into the ground in rain.** <span style="font-family: 'Comic Sans MS',cursive;">**With ammonia, the chances of acid deposition occurring can be reduced.**

__<span style="font-family: 'Comic Sans MS',cursive;">Works referred: __

<span style="font-family: 'Comic Sans MS',cursive;">Deacon, J. (n.d.). The Microbial World: The Nitrogen cycle and Nitrogen fixation. Retrieved October 14, 2010, from Nitrogen fixation: http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/nitrogen.htm

<span style="font-family: 'Comic Sans MS',cursive;">Hubbell, D., & Kidder, G. (n.d.). Biological Nitrogen Fixation. Retrieved October 14, 2010, from SL16/SS180: Biological Nitrogen Fixation: http://edis.ifas.ufl.edu/ss180

<span style="font-family: 'Comic Sans MS',cursive;">Neuss, G. (2007). Chemistry Course Companion. In G. Neuss, Chemistry Course Companion (p. 351). Oxford: Oxford University Press.

<span style="font-family: 'Comic Sans MS',cursive;">Nitrogen Cycle. (n.d.). Retrieved October 9, 2010, from Tech Notes Nitrogen Cycle | Environmental Monitoring Products: http://www.fondriest.com/tech_notes_nitrogen_cycle.htm

<span style="font-family: 'Comic Sans MS',cursive;">The Definition of Free Radicals. (n.d.). Retrieved October 2, 2010, from The Definition of Free Radicals: http://www.antioxidantsdetective.com/definition-of-free-radicals.html

<span style="font-family: 'Comic Sans MS',cursive;">U.S. Geological Survey. (2005, January). NITROGEN (FIXED)—AMMONIA. Retrieved October 14, 2010, from nitromcs05: http://minerals.usgs.gov/minerals/pubs/commodity/nitrogen/nitromcs05.pdf

__<span style="font-family: 'Comic Sans MS',cursive;">Pictures referred: __

<span style="font-family: 'Comic Sans MS',cursive;">Face Forward Nickel. (n.d.). Retrieved October 15, 2010, from The Cost of Money: http://images.businessweek.com/ss/06/07/money/source/3.htm

<span style="font-family: 'Comic Sans MS',cursive;">Ritter, M. (n.d.). Biogeochemical cycles. Retrieved October 1, 2010, from Biogeochemical cycles: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/earth_system/biogeochemical_cycles.html

<span style="font-family: 'Comic Sans MS',cursive;">Shellie. (2009, April 17). Amazing Ammonia. Retrieved October 15, 2010, from Amazing Ammonia: http://www.savingwithshellie.com/2009/04/17/amazing-ammonia/