Hmm this could be one of two things, a problem with your controller or a glitch in your game.
I suggest you try another controller for chemestry and see how that works out or quickly start a new game and wait for chemestry to come round and try it then and see if your still having problems.
I cant see any reason for them to blow up as long as you're pressing the correct button when its inside the 'box'.
Hope this helps
