 So normal rain is typically a little bit acidic because of carbon dioxide in the atmosphere. So the carbon dioxide dissolves in the water in the atmosphere producing carbonic acid. Carbonic acid can then ionize to produce hydronium ions and carbonate ions. So the increase in concentration of hydronium ions reduces the pH and makes it slightly acidic. But this isn't what we're really talking about when we're talking about acid rain. So natural rain is normally between 6.5 and 5.6 pH and that's because of that carbonic acid. Below 5.6 you can't really add any more carbon dioxide into the water, it just doesn't go in. It's already saturated. So when we're talking about acid rain, we're talking about rain with a pH of less than 5.6 and that's produced mainly from the result of human activities releasing oxides of nitrogen and sulfur into the atmosphere. So we'll start by talking about nitrogen oxides. So they're produced wherever you have really high temperatures. You need really high temperature to break the triple bond between nitrogen and acid. So between the two nitrogen atoms is a very strong triple covalent bond. You need to break that in order to get the nitrogen to react with anything. So in a combustion engine you have the nitrogen reacting with oxygen to produce nitric oxide which is NO. That nitric oxide then further reacts with oxygen to produce nitrogen dioxide. Nitrogen dioxide is an acidic oxide and it reacts with water in the atmosphere to produce nitric acid and nitrous acid. So here we have nitrogen dioxide reacting with water producing nitric and nitrous acid. Because we've got these a weak acid and a strong acid dissolved in the rainwater that lowers the pH and makes it acidic. Very important if you're using these equations you should probably show the ionization and the production of hydronium ions. Oxides and sulfur are produced whenever you burn anything that contains sulfur in oxygen rich environments. Often coal is a good example. Coal contains a lot of sulfur. So when you heat up the sulfur it reacts with oxygen to produce sulfur dioxide and again sulfur dioxide reacts in the atmosphere with oxygen to produce sulfur trioxide. Both of those sulfur dioxide and sulfur trioxide will dissolve in atmospheric water and produce weak sulfurous acid for sulfur dioxide and strong sulfuric acid for sulfur trioxide. So you get these weak and strong acids dissolved in rainwater and that again reduces the pH. Again if you're going to use these equations it's a good idea to show that it's acid by showing the ionization and producing of hydronium ions. So you can get acid rain occurring naturally. Usually after volcanoes go off you get a lot of sulfur going to the atmosphere. So you get sulfur dioxide being produced and sulfur trioxide. You can also get it from aerobic decay of dead plants and animals. When they ionize in water you get the hydronium ion being produced and that reduces the pH like I said. The main places that we notice acid rain in North America and Europe not so much in Australia. Also in Asia too.