Pure rain

What is the pH of rain formed in and falling through the Earth s atmosphere if the atmosphere were free of anthropogenic pollutants (we might call this pure 

Strategy. The answer is not 7.00 as some might guess, but rather it is somewhat lower due to the presence of C02 in the atmosphere. The C02 dissolves into the rainwater, creates some carbonic acid (H2C03), and lowers the pH of rain. Let us look at the reactions step by step. First, the C02 dissolves in the water. This is controlled by the KH value, which is known experimentally  

In some textbooks, the C02 dissolved in water is represented by H2C03 this notation is chemically incorrect - H2C03 is fully protonated carbonic acid. Another notation that you might encounter is H2C03, which is the analytical sum of true H2C03 and dissolved C02 at 25°C, dissolved C02 is 99.85% of this sum, so we will just use [C02]. Notice in this expression that the concentration of dissolved C02 is given in moles per liter (M) and the partial pressure is in atmospheres. We know that the atmospheric partial pressure of C02 is 380 ppm, which is 380 x 10-6 atm. Hence, 

we must consider the reactions of C02 with water  

HCO is called bicarbonate. The above reaction has an equilibrium constant of 

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I therefore took some of this flour and mix it either with pure rain water wherein were no animals or else with some of my spittle (having no Air bubbles to cause a motion in it) and then to my great surprise perceived that the aforesaid matter contained very small living animals, which moved themselves very extravagantly. The... 

If the pH of natural rain were measured, you might expect a pH of around 7.0. Because a pH of 7.0 indicates neutral conditions, many people assume this to be the pH of rain. The theoretical pH of pure rainwater is actually about 5.6. Pure rain is acidic due to the equilibrium established between water and carbon dioxide in the atmosphere. Carbon dioxide and water combine to give carbonic acid ... 

Theoretical pure rain, pH 5.6 Most fish species die, pH 4.5-5 0... 

Even pure rain water is not really pure. It has undissolved small particles, dissolved gases, dissolved chemical salts. Water, therefore, has to be purified before it can be used by us. 

Hence, the pH of pure rain is 5.62, which agrees well enough with our guess.2... 

This is actually quite remarkable—by adding just a little bit of a very water-soluble base (NH3 in this case), the pH of the rain goes almost back to that of pure rain. 

The appearance of acid solutions in the weathered layer cannot be substantiated physicochemically even if a high content of carbon dioxide in the atmosphere is assumed with =1 bar the pH of the saturated aqueous solution (pure water with no impurities of other anions and cations) is only 3.9—a value clearly insufficient for intensive removal of ferric iron in the form of stable Fe FeOH, or Fe(OH)2. Moreover, when pure (rain) carbonic acid waters react with rocks the pH value increases as the saturation with dissolved cations (mainly Mg, Na" ", K, and Ca ) increases. The constant presence of other volatiles of the acid fume group in the Precambrian atmosphere is not very likely, due to their high solubility... 

Portland cement concrete is even susceptible to pure rain water which can hydrolize the alkali silicates and aluminosilicates pres-... 

A question of interest to environmental chemists is the solubility of statuary marble in acid rain. This was considered in Example 6.7. How soluble is CaC03 in "pure" rain water is a related question in which the pH is determined by the 03 of the dissolved CaC03. 

Pure rain water is slightly acidic and has a pH of about 5.6. This acidity is caused by carbon dioxide in the atmosphere reacting with rain droplets to form carbonic acid. Rain water with a pH of less than 5.6 is termed acid rain. The main acids present in acid rain are sulfuric acid (H2SO4) and nitric acid (HNO3). 

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