Acid Rain Demonstration

Some common acids are listed below.  In solution the acid molecules dissociate (split).  The presence of H+ ions is what makes these materials acids.



Actually for a solution to be acidic it must have an H+ ion concentration that is greater than the H+ ion concentration found in distilled water.  The H+ ion concentration in pure water is 10-7 moles of H+ ions per liter of water.  A mole is just a number, a very large number (6 x 1023).  It's the same idea as dozen.  A dozen means you've got twelve of something.

We often use the pH scale to measure acid concentration.  An H+ ion concentration of 10-7 moles/liter corresponds to pH 7 (the pH value is computed by taking the -log10 of the H+ ion concentration, not a detail that you need to remember or worry about). 
 
A basic solution will have an H+ ion concentration that is lower than found in pure water. 



Pouring some acid into water would increase the H+ ion concentration (from 10-7moles/liter to 10-3moles/liter, perhaps).  Adding a base to water will decrease the H+ ion concentration (from 10-7moles/liter to 10-10moles/liter, perhaps).

Now we can proceed to the demonstration.  We will start with three 1000 mL beaker each filled with distilled water.  Some vinegar (contains acetic acid) was added to the left beaker. Some ammonia (a base) was added to the right beaker.

Then we added some bromothymol blue, a color indicator solution, to all three beakers.  Bromothymol blue has the amazing property of changing color depending on whether it is mixed with an acid or a base.



We add some Tucson tap water to a large 2000 mL beaker.  Tucson tap water is slightly basic.  So it turns blue when we add bromothymol blue to it.  A few small pieces of dry ice are put into a flask.  We close the flask with a stopper.  The end of a piece of tubing connected to the flask is immersed in the tap water.

Dry ice sublimates.  It turns directly from solid to ice (ordinary ice melts and turns from solid to liquid).  The gaseous CO2 is invisible but you can tell it is there because of the bubbles in the tap water.  Some of the CO2 dissolves as it bubbles through the water and slowly turns the water acidic.  You can tell that this is occurring because the bromothymol blue indicator turns from deep blue to green and eventually to yellow.

We haven't really produced acid rain.  We would need something different than carbon dioxide, something that would lower the pH below 5.6.

While we didn't actually produce acid rain, there is concern that increasing atmospheric concentrations of carbon dioxide will dissolve and acidify the world's oceans.  This is discussed in the following article from The Christian Science Monitor.  You can download a copy of the article here.



The main concern over increasing atmospheric carbon dioxide concentrations is global warming from enhancement of the greenhouse effect.  We will discuss this topic at some point during the semester.