The Best Chemistry Demo
The best demo I have come across so far is the Hydrogen Chlorine Photochemical Explosion or Hydrogen Chlorine Cannon.
Hydrogen and chlorine are lit by red, yellow and blue light, the reaction mixture explodes on blue. Theoretically any light shorter than 500 nm would work, so green may or may not work. I stayed away from it in case it used some reactants up. I used a 500 ml drink bottle with a bung in instead of the cap.
The Demo
Making Hydrogen and Chlorine
Be careful, Chlorine is toxic and corrosive, hydrogen chloride too, but less so. Use safety equipment, only for those with some knowledge of chemistry.
For the Demonstrations Itself