Simulate acid rain by adding an acid to water. You can use pH paper to test whether the acidity is changed after water runs through different types of soil or through plant root systems.
Is the acidity of apples (malic acid) affected by their ripeness?
See if you can make your own pH indicator from common plants or chemicals.
Measure the pH of common acidic beverages (soft drinks, lemonade, orange juice, tomato juice, milk, etc.) and examine how easily they corrode metal (such as iron). Another idea: which is more corrosive? A salt solution or an acidic liquid?
Do all brands of orange juice contain the same amount of ascorbic acid?
Compare the effects of different acidic fruit juices and liquids (e.g., vinegar) at preventing apple browning.
Which animal's saliva has the lowest pH? You could test humans, dogs, cats, possibly other species.
What is the effect of pH on growth or survivability of daphnia (an aquatic crustacean)? You could test other factor, too, such as salinity or the presence of detergent in the water.
How does the pH of the water affect tadpole development?
Does acid rain (real or simulated) affect the number and type of organelles seen in algae under the microscope?
Which is a better conductor of electricity, an acid or a base?
Does the pH of water affect the growth or surivability of mosquito larvae?