It was a drain cleaning fluid containing hydrochloric acid. A drain cleaning fluid containing sulfuric acid would work equally well. The green fluid into which he passed the hydrogen was washing up liquid.
These can be dangerous chemicals, and as Stephen said on the show do not try this at home.
I left the uk 3 years ago so the episode was before then. Stephen asked that when people say 'that's gay' is it rude to gay people. The answer was no and that it was language evolution from 'that's queer' which means unique and novel. I have been looking for a clip everywhere because the other day a US charity had an advert in a national US newspaper advising people not to say it because it was hurtful to gay people. People please help me find what episode this was from and if there is a clip available?