Here's why:

* Citric acid: Oranges are high in citric acid, which gives them their tart flavor.

* pH scale: The pH scale measures acidity and alkalinity. Values below 7 are acidic, 7 is neutral, and values above 7 are alkaline (or basic). Orange juice has a pH of around 3.5, which is well within the acidic range.

While orange juice may be acidic, it's important to note that it doesn't necessarily mean it's bad for you. Many fruits are acidic, and they can be part of a healthy diet.