Two test tubes without labels contain a solution of an acid and its salt. How to distinguish them chemically?

To determine in which test tube the salt is, it is necessary to carry out a reaction for the cation (when the cations of some Me are added to the salt, the burner flame paints it in a certain color)
To determine which test tube contains the salt solution, you need to measure the pH of the medium using litmus paper. If the piece of paper turns red, then the medium is acidic, therefore, there is a solution of acid in this test tube.