The basic constituent of lemon are citric acid (around 5-6%) which makes the pH of lemon acidic (pH=2.2). When lemon juice is added to water which is neutral (pH=7) in nature, there is an increase in the [tex][H^+][/tex] ion concentration which means that the solution has turned acidic.
Increase in the [tex][H^+][/tex] ion concentration relates to the decrease in the pH of water.
[tex]pH=-log[H^+][/tex]
