A whole and a cut lemon. Lemons are used primarily for their juice, though the pulp and rind (zest) are also used, primarily in cooking or mixing. Lemon juice is about 5% citric acid, which gives lemons a sour taste and a pH of 2 to 3. This acidity makes lemon juice a cheap, readily available acid for use in educational chemistry experiments.Photo credit: André Karwath