Violet becomes red

If you ever come across an anthill in the woods, you can there and then do a small chemical experiment. Hold a violet flower, e.g. a bluebell, firmly over the ants. The insects feel threatened and spray a sharp smelling liquid over the flower. The places hit turn red.
The ants make a corrosive protective liquid in their hindquarters. You notice it if an ant nips you, though it is generally quite harmless. Since the flower turns red where the drops fall, you know that they are acid. The acid is called formic acid.