science from high school

[atron]

I remember doing an experiment in high school which involved adding sugar to hydrochloric acid or sulphric acid (one of the two, my memory isn't that good). When they reacted it turned black and expanded to about 2 feet. It was hard like rock but porous like sponge. Can anyone tell me what happened and why?

[Peter Pedant]

The sugar carbonises

[gen2]

Concentrated sulphuric acid reacts very strongly with water and will even remove it from compounds containing water.  This reaction generates a lot of heat (exothermic).
Sugar - sucrose - is a carbohydrate, a compound of carbon, hydrogen and oxygen.  C12H22O11
This can be thought of as 12 carbon atoms joined onto 11 water molecules.

The acid removes the water, leaving behind the carbon.  The heat generated produces steam which froths up the carbon into the sponge texture you describe.