What is the primary cause of the decrease in ph when lemon juice is added to water?

The basic constituent of lemon are citric acid (around 5-6%) which makes the pH of lemon acidic (pH=2.2). When lemon juice is added to water which is neutral (pH=7) in nature, there is an increase in the ion concentration which means that the solution has turned acidic.

Increase in the ion concentration relates to the decrease in the pH of water.

beta decay is the answer

no, the red giant is the hottest with 2.00.000.000 degrees celsius. while the white dwarf only 25,000 degrees celsius.