There is a time delay of almost two to three minutes before the reaction proceeds because of the following processes:
1) The acid starts to go yellow as the dehydration begins.
2) The rate of dehydration then accelerates as the acid heats up because the reaction is exothermic/chemical (releases energy by light or heat). As the sugar molecules are stripped of water, the heat generated turns the water into steam which then expands the remaining carbon into a porous, smoking, black column.
3) This expands out of the reaction vessel, producing a choking acrid vapor and the smell of burned sugar.
At this stage, sulfuric acid is highly corrosive and will burn skin so one must avoid contact with it.
