I think this is a great question to ask, especially because many people are concerned about the environment.

There are several different ways to test the acidity of rain but a very common one is to test its pH. First, as you may have guessed acid rain contains acidic materials and acids have a higher pH than pure water. pH is simply a measure of acidic or basic a substance is. If a substance has a pH of 7 it is neutral, if it is less than 7 it is acidic and if it is greater than 7 it is basic.

An example of a simple acidic substance is vinegar,and an example of a base we use everyday is soap. There are several ways to test the pH of something, the simplest is by using litmus paper. Litmus paper is a special kind of paper that will change color depending on what kind of substance you put it in.

With all of this said, one easy way to test the acidity of rain is to collect some in a clean plastic container. This could be done by leaving a container outside next time it rains. You have to be careful to place the bucket at least 2 meters above the ground to avoid contamination from dust which could alter the pH of the collect rain. After the rain has stopped, you could dip some litmus paper in the glass of rain and by comparing the color change in the paper to a corresponding chart, it would tell you the pH of the rain! Now if you find all of this as interesting as I do, you could get more accurate results by using a pH meter, although these cost more money.

To know if a sample of rain is "acid rain" we need to measure the concentration of hydrogen ions in the sample. Hydrogen ions are produced when any acidic substance is dissolved in water. The solutions of some substances, like carbon dioxide in the air, are weakly acidic. When they are dissolved in water there is not a large increase in the concentration of hydrogen ions. Other substances give strong acids when dissolved and they cause a large increase the concentration of hydrogen ions. Examples of these substances are sulfur dioxide and various types of nitrogen oxide. These are the main generators of acid rain. They come mostly from the emissions of energy plants that burn fossil fuels to produce electricity.

Back to your question, how do we measure the concentration of hydrogen ions? In a quick way, by using one of the many substances that change color depending on the concentration of hydrogen ions. They are called acid-base indicators. Some of them are natural products (mostly from plants). For example, if you take a cup of black tea and add lemon juice (acid!), the liquid in the cup goes from dark to light brown indicating a high concentration of hydrogen ions in the juice. More accurate measurements come from the use of a pH-meter. This instrument has a scale that goes from 0 to 14. This scale is the known as pH . The smaller the pH the more acidic, that is, the higher the concentration of hydrogen ions. Pure water is neutral, and therefore has a pH of 7. Unpolluted rain has a pH around 6 to 5.5. It is slightly acidic because some carbon dioxide is always dissolved on it.

Some samples of rain in the Ohio Valley have a pH of around 4, and this is very acidic! That rain damages the soil, as well as many living creatures.

If you want to know more about this subject, here is a website full of information: here

By collecting the rain water carefully without allowing for any contamination one can then determine by an instrument known as a pH meter the acidity of the solution.

The acidity tells us how much H₃O⁺ (or hydrogen ion) is present. An acid solution has a low pH . Tap water has a pH of about +7 and strong acid has a pH of +1 or 2.

The ability of a solution to conduct electricity is related to the hydrogen ion concentration and that is how some pH meters work. The conductivity of the solution is measured.

We can take a sample of the rain and test if it's acid. There are a few different ways of measuring acidity. One way is with indicator chemicals. They are dyes that change color depending on how acidic or basic the water is. Different indicators change at different pH values (a measure of how acidic the water is). So, by adding different indicators to parts of the sample of rain and seeing which indicators are which color we can say how acidic that rain sample is.

It's not too tough to measure how acidic the rain is. We can measure the acidity of something using electronic probes, little strips of treated paper, or other tools. Pet stores often sell kits to test the pH of aquarium water. Pool supply stores also sell them. If you can talk an adult into helping you, you can make your own acid test paper (called litmus paper). The directions are on: here

Acidity is ranked based on the pH scale, with neutral being 7. Acids have numbers less than 7, alkalines (bases) have numbers higher than 7.

Notice that each step down on the scale multiplies the acidity by 10. This is a very important feature of the scale. You would normally think of a rating of 4 being twice as high as a rating of 2. On the pH scale, something with a pH of 2 is 100 times more acidic than something with a pH of 4.

Normal rain has a pH of about 5.6, but this is already acidic. Generally, any rain with a pH lower than 5.6 is considered acid rain. Some rain in the US has had a pH of 4.3 making it about 20 times more acidic than normal rain. The EPA has a nice web site on acid rain at: see here