why the cloudy colour disappears when the HCl is added to the following equation after adding Ba?

Question

2CrO4^2-(aq) + 2H+(aq) --> Cr^2O7^2-(aq) + H2O(l)

Answer 1

Simple - barium chromate is insoluble, but barium dichromate is soluble. This reaction is an excellent demonstration of Le Chatelier's Principle. Adding H+ ions forces the reaction you have quoted to the right hand side, which forces the precipitate of barium chromate to dissolve.