Explanation
Class 3 unstable reactive gas is defined as a material that in itself is capable of detonation or explosive decomposition or reaction at normal temperatures and pressures with a strong initiating source or if heated under confinement. This class of gas readily undergoes violent chemical change at elevated temperatures and pressures.
References:NFPA 55: Compressed Gases and Cryogenic Fluids Code, 2016 Edition, Section
3.3.51.14.3;International Fire Code, 2015 Edition, Appendix E, Section E102.1.1