godan verb (-ru)
dilute with soda; mix with soda water
Used for mixing a drink, typically alcohol, with carbonated water. The phrase is a fixed expression.
ウイスキーを炭酸で割るのが好きです。
I like to mix whiskey with soda water.
焼酎を炭酸で割って飲む。
I drink shochu diluted with soda.
Compound of 炭酸 (carbonated water) + で (instrumental particle) + 割る (to dilute).