Concentrated sulfuric acid can perform a dehydration reaction with table sugar. After mixing, the color changes from white to brownish and eventually to black. The expansion of the mixture is the result of vaporization of water and CO inside the container.  The gases inflate the mixture to form a snake-like shape, and give off a burned sugar smell. The granularity of the sugar can greatly affect the reaction: powdered sugar reacts very quickly but sugar cubes take longer to react.
When sucrose is dehydrated, heat is given out to the surroundings in an exothermic reaction, while graphite and liquid water are produced by the decomposition of the sugar:
:CHO  + HSO  + 1/2 O  → 11 C  + CO  + 12 HO  + SO 
As the acid dehydrates the sucrose, the water produced will dilute the sulfuric acid, giving out energy in the form of heat.
:CHO   → 12 C  + 11 HO