Lemon slices turn black, mainly because lemon is rich in vitamin C, which is a substance that is easy to be oxidized. If it is not stored properly, it is easy to be oxidized and discolored, and the lemon slices turn black, which is also a manifestation of deterioration. It is not recommended to continue soaking in water, and the nutrients of lemon slices will also be lost after oxidation and blackening. Don't buy too much at one time, so pay attention to keeping it. If you put it in a cool and ventilated place, you can't drink too much lemon juice at one time, which will stimulate excessive gastric acid secretion.