You need a control so you have something to compare the experimental group on-- If you don't have a control group, how will you know what's changed?
The answer is each indicator has a narrow range. We need many different indicators to span the entire ph spectrum because each indicator has a narrow range.
This would decrease by 25 hope this helps !!
Answer:
reactions between air, water and minerals, chemicals in rain water
For apex the answer is no solution