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.

The IUPAC or systematic name for citric acid is 2-hydroxypropane-1,2,3-tricarboxylic acid. The molecular formula of citric acid is C₆H₈O₇

The chemical formula for citric acid is best shown by a diagram of its complete structural formula. This shows how the atoms are arranged in the molecule

Citric acid has three carboxyl or - COOH groups and is tripotic in nature. It weakly dissociates in water producing a weak acidic solution.

Citric acid(aq)   ==> citrate ions + H⁺(aq)
  92.1%                      7.9%            7.9%

The pH of a 0.1 M solution of citric acid is 2.1