Hydrangeas

Color Changing

All hydrangeas expereience some color alteration as their flowers age. The hydragnea's bloom color reveals the pH of the soil. Blue flowers indicate acidic soil, where as pink indicates neutral to basic soil. Because of this facinating trait, gardeners can chemically manipulate the bloom's color using soil additives. 

Chemistry 

Ultimately, the color depends on the availability of aluminum ions within the soil. For neutral to basic soil, the ions combine with hydroxide ions, which is needed for the blooms to change their color to blue. 

FUN FACT: 

There are around 49 species of hydrangeas, four being native to North America. Notoriously water-needy, the "hydra" refers to the seed capsules' resemblance to ancient Greek water-carrrying vessels.