Concentrated sulfuric acid is extremely dangerous. Choking sulfuric acid vapor will rise from the container. Do this demonstration in a fume hood. The reaction takes a little while to get started, so don't be tempted to dump more acid on the sugar if nothing seems to be happening. If you have a hood, use it.
For the procedure, refer to Bassam Shakhashiri's Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1, pp 77-78 (1983).
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