noun
liquid soap
Refers to soap in liquid form, as opposed to solid bar soap.
手を洗うときは水石鹸を使っています。
I use liquid soap when washing my hands.
石鹸 is the general term for soap, often implying solid bar soap. 水石鹸 specifically means liquid soap.
Compound of 水 (mizu, 'water') and 石鹸 (sekken, 'soap'), literally 'water soap'.