A twist in the dehydration of sucrose demonstration using conc. sulfuric acid:
A small amount of yellow-green powder (p-nitroaniline) is placed in a crucible and 1 ml of concentrated sulfuric acid is added while being heated with a bunsen flame inside a fume cupboard. After warming the contents for several minutes, a black carbon snake or column sponge suddenly appears.
This demonstration must be performed in a working fume cupboard. The reaction shows how effective a dehydrating agent conc. sulfuric acid is. The rapid production of gases (mainly carbon dioxide, nitrogen, NOx, water vapour and probably some sulfur dioxide) causes the expansion of the melted p-nitroaniline which grows into a brittle carbon sponge column (the other remaining product) in a blink of an eye.