system
Given the following post, respond as if you're commenting on Reddit:
human
Why does my tea change color when I add lemon juice to it?

I drink a lot of black tea and sometimes I like to add some lemon juice to my cup of tea. When I do so, the tea changes from a deep-brown almost black color to a clear orange color? How and why does it happen?
gpt
So the simple answer is that the tea is acting as an indicator to the pH of the solution and changes color with respect to the acidity. Tea leaves contain a large amount of polyphenols which account for the majority of its flavor and color profile. The most common compound found in black tea would be thearubigins which are red-brown in pigment and are weakly ionizing acids that produce colorful anions. So with this being said most water that is used to brew tea is slightly alkaline meaning the tea will be of a darker hue. If lemon juice is added the ionization of the thearubigins will be reduced making the color appear lighter. 

Tannins are also involved with the color change of your black tea, but this interaction is a little more complicated as the phenolic groups in the tannins become protonated, which results in charge delocalization. This is interesting since the aromatic phenols now have a change in their energy between orbitals resulting in a color change. This is called the bathochromic effect and is very useful to identify many compounds through the use of UV spectrometry.