Tears leave a white mark after the eyes flow, because the main component of tears is not only water, but also other ionic components, including sodium and chloride. After the water in tears evaporates, inorganic components similar to sodium or potassium salts will be left behind, and the usual salty taste of tears is also due to this reason. After the tears evaporate dry, the final white component left behind is similar to the salt component, usually observed after a lot of sweating, in clothes will also remain white marks is the same reason, also because the sweat glands secrete sweat inorganic ions in the composition, and eventually after the water evaporates dry, will be left out of the sodium or potassium salts inorganic minerals.