I saw someone asking how to remove a yellow turmeric stain from a blender cup & lots of folks recommended leaving it out in the sun to break down the pigment. Why does this work when water & scrubbing doesn’t?
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Water can only remove water-soluble substances. Soap can help remove some fat-soluble substances also. But the ultraviolet part of sunlight can break down substances that can’t be removed by either. Color, especially in plants and animals, is often a property of complex molecules. When they break, they lose their color. Bleach works the same way.
Fun fact: rust stains are unaffected by sunlight, and insoluble in water or fat. We use acids on those, because they convert the rust into other compounds that are water soluble.
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