Nitrogen triiodide explosion. Footage of a laboratory demonstrator making contact with a sample of nitrogen triiodide (NI3) to trigger a loud explosion. The initial explosion triggers secondary explosions on the other samples of NI3. The purple cloud is iodine vapour. Nitrogen gas is also produced. The instability of NI3, an example of a very sensitive contact explosive, is due to the steric strain of three relatively large iodine atoms bound to the relatively small nitrogen atom.