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

Which of the following is(are) true when $HCl(g)$ is passed through water?
$(i)$ It does not ionise in the solution as it is a covalent compound.
$(ii)$ It ionises in the solution.
$(iii)$ It gives both hydrogen and hydroxyl ion in the solution.
$(iv)$ It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule.

$A$ 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?

When a $pH$ paper is dipped in an aqueous solution,it turns blue. What is the approximate $pH$ value of that solution?

Give reasons for the following:
$(i)$ Why is it recommended that acid should be added to water and not water to acid?
$(ii)$ Why do compounds like alcohol and glucose not show acidic behavior even though they contain hydrogen?

What are strong and weak acids? In the following list of acids,separate strong acids from weak acids: Hydrochloric acid,citric acid,acetic acid,nitric acid,formic acid,sulphuric acid.