I’ve got some beautiful hydrangeas blooming in my garden right now. They are one of my favorite flowers, especially because they can be blue, my favorite color.
They are also a fascinating application of acid-base chemistry. Hydrangeas get their blue color from aluminum, and in acidic soil is it freely available as Al³+ and taken up by the plant; but in basic soil, it is bound up in compounds and therefore not as available for uptake.
Here’s the chemistry for aluminum binding under basic conditions:
Therefore, hydrangeas in acidic soil have blue flowers, and those in basic soil have pink flowers. So hydrangeas work kind of like a litmus test for you soil, but interestingly the colors are switched from a standard litmus test, where acidic=red and basic=blue.
You can add fertilizers and compounds to your garden soil to manipulate the color of your flowers. I did not add anything to my hydrangeas, so I got an interesting mix of pink and blue and purplish flowers (sometimes even in the same bloom!)