What happens when sulfur trioxide reacts with water?

An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?

$(a)$ Give the chemical names of acids present in:
$(i)$ ants
$(ii)$ lemon
$(iii)$ milk
$(iv)$ tomato

$A$ student prepared solutions of $(i)$ an acid and $(ii)$ a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since both the solutions are colourless,how will she distinguish between the two?

How is an aqueous solution with $pH$ $12$ compared to an aqueous solution with $pH$ $9$?

According to the $Brønsted-Lowry$ theory, what kind of substance is a base?