Which scientist introduced the $pH$ scale?

Which of the following are present in a dilute aqueous solution of hydrochloric acid?

The metal salt $A$ is blue in colour. When salt $A$ is heated strongly over a burner,a substance $B$ present in it is eliminated and a white powder $C$ is left behind. When a few drops of a liquid $D$ are added to powder $C$,it becomes blue again.
$(a)$ Identify $A, B, C, D$.
$(b)$ Write the chemical equations involved.
$(c)$ Give an example of a salt which also shows the above property.

$A$ metal compound $'A'$ reacts with dilute sulphuric acid to produce a gas which extinguishes a burning candle. Identify the compound $'A'$ and the gas produced. Write a balanced chemical equation for the reaction if one of the compounds formed in the reaction is sodium sulphate.

$(i)$ Give an example of a strong base and a weak base.

Which of the following salts is formed by the neutralization of a weak acid and a strong base?