What is formed by the reaction of carbon dioxide with water?

$A$ 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?

Which of the following is a weak acid?

Which enzyme becomes active in the stomach during a low $pH$ value?

What is correct for an acidic aqueous solution?

$pH + pOH = ?$