It's all well and good to tell students that rainwater contains acids, mostly as a result of humans burning fossil fuels. But it is much better if you can show them the process in action.
It works like this. Pour a small amount of water into a flat pan or tray and cover it with a clear bowl turned upside down. This models a body of water and the atmosphere. Place a reading lamp that has a standard light bulb just above the bowl. This represents the sun.
Wait about 10 minutes until some water has condensed on the inside of the bowl. This is your rainwater. Get students to test and record the pH of the water by touching the inside of the bowl with a piece of litmus paper.
Then take a pinch of black powder (equal parts charcoal and sulphur) and place it on a metal spoon. Ask a student to lift the bowl as you heat the spoon with a lighter until the powder catches fire.
As you can imagine, this prompts a great reaction from students. But don't be distracted by the oohs and ahs - you need to catch the smoke in the bowl and place the bowl back over the tray of water.
Now repeat the heating effect of the sun to create another batch of rain. Once you have a decent amount, ask a student to lift the bowl, let the smoke out of an open window and again test the pH of the rain with a fresh piece of litmus paper.
The students will observe that the pH level has changed from about 7 (neutral) to about 1 (highly acidic), demonstrating that the chemicals in the burned powder - similar to those in fossil fuels - mix with water to form acids.
And, as any student with a fish tank knows, if the pH of the water becomes too low, living things will die. The class suddenly realises that this is not just a science lesson but a lesson for life, too.
Seth Robey is a science teacher in Chicago, US
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