Question

Is Hydrogen gas flammable?

Answer 1

Let us first clear the fact that diatomic hydrogen represented by ${H_2}$ (diatomic hydrogen gas) is highly flammable and we are not talking about elemental hydrogen. Hydrogen gas contains a covalent bond between the two Hydrogen atoms. When the conditions are just right hydrogen can become highly flammable. These conditions can be due to presence of oxygen in air or other substances to which hydrogen can react such as chlorine, oxygen, or other oxidizers

Complete answer:
Let us look at the flammability scale for reference,
Chemical substances are rated from 0-4 on the basis of their flammability and reactivity by the NATIONAL FIRE PROTECTION ASSOCIATION (NFPA) RATING SYSTEM.
Hydrogen gas is rated 4 for the flammability in this scale meaning that it is a material that completely vaporizes at normal pressure and temperature and burns readily.
Even at low concentrations such as 4 percent up to 75 percent hydrogen gas can catch on fire.
We know that Hydrogen gas exists as a diatomic molecular gas ( ${H_2}$ ). But when Hydrogen is mixed with oxygen (an oxidizer) and a small spark provides the energy, Hydrogen will get sufficient energy to break the covalent bond and react with the oxygen. Due to this highly reactive rate of hydrogen it will suddenly catch on fire even if a small amount of energy is given to it.

Note:
There is a difference between Combustion and flammability.
A combustible material is something that can combust (burn) in air. Flammable materials are combustible materials that ignite easily at ambient temperatures.
Flammable materials will catch on fire even at low temperatures (below ${37.8^o}C$ ) whereas combustible materials will ignite at a much higher temperature.