Concentrated sulphuric acid produces messy results. Not only is it an acid, but it is also a strong oxidising agent. It oxidises some of the alcohol to carbon dioxide and at the same time is reduced itself to sulphur dioxide. Both of these gases have to be removed from the alkene.
It also reacts with the alcohol to produce a mass of carbon. There are other side reactions as well, but these aren't required by any current UK A level (or equivalent) syllabus.