Acid rain demonstration

Acid Rain Demonstration

Here is a short description of a colorful demonstration done in the lecture version of this course.

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. We often use the pH scale to measure acid concentration. An H⁺ ion concentration of 10^-7 moles/liter corresponds to pH 7. You're no doubt familiar with the term dozen. A mole is really just the same idea and is a unit of amount. A dozen eggs means 12 eggs. A mole is approximately 6 x 10²³ atoms or molecules of a particular substance.

A basic solution will have an H⁺ ion concentration that is lower than found in pure water.

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

At this point the three beakers are indistinquishable, they are all filled with a clear liquid. We'll add some bromothymol blue color indicator solution to each of the 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 larger 2000 mL beaker. Tucson tap water is slightly basic (pH 7.8 to pH 8). So it turns blue when we add some 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.

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 a little more detail here.