bubble vs soap bubble

bubble and soap bubble both are nouns.

Word	Noun	Adjective	Verb	Adverb
bubble	Yes	No	Yes	No
soap bubble	Yes	No	No	No

As nouns, soap bubble is a hyponym of bubble; that is, soap bubble is a word with a more specific, narrower meaning than bubble:

Other hyponyms of bubble include air bubble, foam, froth.

bubble (noun)	soap bubble (noun)
a dome-shaped covering made of transparent glass or plastic	a bubble formed by a thin soap film
an impracticable and illusory idea
a speculative scheme that depends on unstable factors that the planner cannot control
a hollow globule of gas (e.g., air or carbon dioxide)

bubble (verb)	soap bubble (verb)
expel gas from the stomach
form, produce, or emit bubbles
cause to form bubbles
rise in bubbles or as if in bubbles
flow in an irregular current with a bubbling noise