The correct order of $pH$ for $0.1 \ M$ solutions is:

Blue litmus turns red in the following mixture of acid and base:

$100 \ cm^3$ of $0.1 \ N \ HCl$ solution is mixed with $100 \ cm^3$ of $0.2 \ N \ NaOH$ solution. The resulting solution is

The degree of ionization of a solution depends upon:

Given below are two statements :
Statement $I$ : Aqueous solution of ammonium carbonate is basic.
Statement $II$ : Acidic/basic nature of salt solution of a salt of weak acid and weak base depends on $K_a$ and $K_b$ value of acid and the base forming it.
In the light of the above statements,choose the most appropriate answer from the options given below :

Which of the following,when added to $20 \ mL$ of a $0.01 \ M$ solution of $HCl$,would decrease its $pH$?