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.
The dehydration of ethanol to give ethene
Ethanol is heated with an excess of concentrated sulphuric acid at a temperature of 170°C. The gases produced are passed through sodium hydroxide solution to remove the carbon dioxide and sulphur dioxide produced from side reactions.
The ethene is collected over water