Solution A turns universal indicator blue to purple, whereas solution B turns universal indicator orange to red.
Name any two substances which can give solutions like B.

Answer the following questions:

(a) Explain why the pH in a person's mouth becomes lower after each meal.
(b) What damage could be caused while the pH is low?
(c) How could the person change his eating habits to lessen the chances of suffering from tooth decay?

A group of students measured the pH of some substances they found in their homes. Their results are given in the following table:

(a) What would the students have used to measure the pH?
(b) Which solution is the most acidic?
(c) Which solution is the most alkaline?
(d) Which solutions are neutral?
(e) Which solution can be used to treat wasp stings?
(f) Which solution can be used to treat bee stings?

Hydrochloric acid reacts with a metal $\mathrm{X}$ to form a gas $\mathrm{Y}$ which burns with a 'pop' sound. Sodium hydroxide solution also reacts with the same metal $\mathrm{X}$ (on heating) to form the same gas $\mathrm{Y}$.
(a) Name X and Y.
(b) Write the chemical equation of the reaction of metal X with (i) hydrochloric acid, and (ii) sodium hydroxide.