Students will be able to define pH and explain/use the pH scale
Students will be able to describe the formation of acid rain
Begin by asking students if they or anyone they know has ever had the unfortunate experience of acid indigestion or ‘heartburn’. Why does this happen? Explain to them that our stomach contains acid (HCl) that is used in digestion, but sometimes our body produces too much of it. How do we combat this? We usually take
. Have these packages available to the students and read out the ingredients. What do they all contain and how does it help reduce stomach acid symptoms?
Assorted antacid packages, transparency of Figure 1, various food, drink and water samples (vinegar, soda, ammonia, milk, rainwater etc.), pH paper
Make a transparency of (or draw on the board) a line that will represent the pH scale. Explain to the students that this is like a ladder that has 14 rungs. The acids are found with a pH lower than 7 and the bases have a pH higher than 7. The pH is a logarithmic scale with 7 being neutral. Have the students draw the line and pH 7. Explain to the students that pure water has a pH of 7 and explain the concept of neutrality and the process of neutralization.
Demonstrate to the students how to use pH indicator paper by dipping a strip in some pure water. Show the students on the pH indicator chart that the pH is 7 or neutral.
Divide the students into groups of two and supply them with about seven various samples, pH paper and an indicator chart. Have the students test their samples and write in the pH they find for each on their pH scale. Have the students share their findings with the class.
Ask students why the rainwater had a pH of less than 7. Explain to them that rainwater is naturally only slightly acidic because of the dissolved carbon dioxide forming carbonic acid. But, there are other gases that can dissolve in the rainwater that could lower the pH. Introduce the students to SOx and NOx and draw the equations of sulfuric and nitric acid formation on the board or on an overhead.
Ask the following questions:
What is an acid? Give an example.
What is a base? Give an example?
What is the pH of pure water?
Why does rainwater have a low pH?
How is acid rain formed?