This is the 6157th most frequent Japanese word.
カパッ
"カパッ" represents the sound or action of something opening or popping open, often suddenly or cleanly.
カパッ here is an onomatopoeic word representing the sudden popping open of the bottle cap.
ビール瓶の蓋がカパッと外れた。
The beer bottle's cap popped open.
カパッ describes the sound or visual impression of the door opening suddenly in this context.
扉がカパッと開いた音がした。
The sound of the door quickly opening was heard.
カパッ expresses the swiftness and completeness of the action of opening in this sentence.