Thanks SDA - we don't do the mean free path, so I have to bow to your argument - I can't see an "easy" way to do this without the maths, but it's a question from an example paper, and it a worded answer, like explain two difference before and after (2 marks) - so I'd have though this would be an obvious answer, but to me it isn;t "obvious". The first difference is easy though - increase in KE, the second not so obvious for me, although the mark scheme agreed with your answer.