The TUKI dictionary says:
hewa (nm) = air, atmosphere:
Hali ya ~ = weather;
~ kanieneo = air pressure;
Punga ~ = get some fresh air.
I also found:
hewa (noun 9/10)
2. weather, climate
So it doesn't seem to be as specific as "gas" in the dictionary, but they seem to be using it that way here.
I don't know if this use of "hewa" represents a decision made by some language standardization committee, or it is just some individual's choice. I would have thought gesi would be a better word to use for "gas", even if its primary meaning is "natural gas / propane".
In any case, "hewa" cannot be simply back-translated as "air", because it leads to these FALSE statements that "Nitrogen is air", "Hydrogen is air", "Oxygen is air". Of course it is fine when translated as "air" in a sentence like "Hewa chafu ni mbaya kwa afya."
I am irritated by this translation....as earlier on in the exercise Haidrojeni ni hewa is translated "Hydrogen is air" and then in this sentence, "Nitrogen ni hewa" is translated "Nitrogen is a gas" If you say "Nitrogen is gas" it is marked wrong because you have to say "Nitrogen is a gas"