fossil fuel vs gas

fossil fuel and gas both are nouns.

Word	Noun	Adjective	Verb	Adverb
fossil fuel	Yes	No	No	No
gas	Yes	No	Yes	No

As nouns, gas is a hyponym of fossil fuel; that is, gas is a word with a more specific, narrower meaning than fossil fuel:

fossil fuel: fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content
gas: a fossil fuel in the gaseous state; used for cooking and heating homes

Other hyponyms of fossil fuel include coal, natural gas, crude, crude oil, fossil oil, oil, petroleum, rock oil.

fossil fuel (noun)	gas (noun)
fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content	a pedal that controls the throttle valve
	the state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency to become distributed uniformly throughout any container
	a state of excessive gas in the alimentary canal
	a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely
	a volatile flammable mixture of hydrocarbons (hexane and heptane and octane etc.) derived from petroleum; used mainly as a fuel in internal-combustion engines
	a fossil fuel in the gaseous state; used for cooking and heating homes

fossil fuel (verb)	gas (verb)
	show off
	attack with gas; subject to gas fumes