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

Mention the range of $pH$ value for identification of a base.

Write true or false for the following statement:
Acids that give rise to more $H^+$ ions are said to be strong acids,and acids that give less $H^+$ ions are said to be weak acids.

Give the chemical name and formula of:
$(i)$ Soda ash
$(ii)$ Blue vitriol
$(iii)$ Baking soda

When $Zn$ metal is treated with a dilute solution of a strong acid,a gas is evolved,which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed.

If the $pH$ of the aqueous solutions $A, B, C$ and $D$ are $1.9, 2.5, 2.1$ and $3.0$ respectively,then what will be the order of acidity?