Interaction between two 3d models in unity 3D

I am working on a project using Unity 3D and vuforia.I intend to achieve something similar to this link https://www.youtube.com/watch?v=IpNrWKQFq6Q

I have already designed all my 3D models using Maya and imported them to unity 3D, I have few questions

1) I want me to be able to achieve collision using vuforia for instance, when i bring 3D model of oxygen and hydrogen closer to each other, it should bring up the 3D model of water and fade out oxygen and hydrogen model.

2) I have not been able to establish an interaction between my marker and my 3D models, for instance my camera should be able to detect oxygen marker and display the oxygen model.