Question

Why pH change as the cause of tooth decay.

Answer 1

Certain foods increase the acidity of the mouth – drop the pH level. ... In that time, the lower pH of the mouth creates an optimal environment for bad bacteria, which eventually causes tooth decay. Fortunately (or unfortunately), the pH of your mouth can change drastically depending on what foods you are eating

Answer 2

Tooth decay occurs if pH of the mouth lowers than 5.5. As we know tooth enamel is made up of calcium phosphate [Ca3(PO4)2] and it decays when mouth pH goes below to 5.5.

Because bacteria present in mouth produce acid by degration of sugar and food particles which remains in the mouth after eating. That's why in order to prevent tooth decaying we use toothpaste which are basic in nature.