Why is it that Lemon-Lime soda not sour but alone they are?

The sensation of sweetness is much stronger than sourness, and sweet tastes block sour tastes. The citric acid in lemon juice is weak and does not have a strong sour flavor on its own, but when mixed with sugar, the sweetness overrides the sourness.

On the other hand, lime juice is more sour than lemon juice and still tastes sour when mixed with sugar. This is because lime juice has a higher concentration of citric acid. Sweetness does not completely block the sour taste in lime juice, so the sour flavor can still be detected.

When combining lemon and lime juice, their acidity becomes milder and the sweetness enhances, creating the signature flavor of lemon-lime soda.