According to the Bronsted-Lowry acid-base theory,which ion is responsible for the acidity of an aqueous solution of an acid?

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

Common salt, besides being used in the kitchen, can also be used as the raw material for making:
$(i)$ Washing soda
$(ii)$ Bleaching powder
$(iii)$ Baking soda
$(iv)$ Slaked lime

Which gas is evolved when dilute hydrochloric acid reacts with zinc? Write the molecular formula of this gas.

$A$ compound $X$ is a constituent of baking powder. It is used as an antacid. When $X$ is heated, it releases a gas $Y$ which, when passed through lime water, turns it milky, and a salt $Z$ is formed, which is the main constituent of washing powder. Identify $X, Y$, and $Z$. Write balanced chemical equations for the reactions involved.

Which of the following salts has a $pH$ less than $7$? Why?