Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Dissolved mineral salts wash into the ocean. As ocean water evaporates, it leaves the salts behind. This makes the water saltier. Ocean water is about 3.5 percent salts. The main salt is sodium chloride. Answer: salts
A:
Concentration of what, the substance left behind when ocean water evaporates, is about 3.5 percent?