Answer:
Two in a whole and four in a pair. Six in a trio you see. Eight's a quartet but what you must get is the name that fits just one of me.
Tear me off and scratch my head, what once was red is black instead. What am I?
You can see me in water, but I never get wet. What am I?
Drop me and the world will shatter. You can always find me where you empty your bladder. What am I?
Better touch me before you proceed to second.
It flows out of the soil, It burns you if it boils, And holds us in its coils, More valuable than gold, As black as it is old.