Why are lemons sour?
Lemons are sour because they each contain a chemical called citric acid.
Citric acid is a weak organic acid found in citrus fruits like grapefruit, oranges, limes and lemons.
In its pure form citric acid is a white crystalline solid and is sour to taste like all food acids. It is sold as a food addictive for cooking and can be found in most supermarkets.