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

$A$ sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the $pH$ paper yellowish-orange. Which of the following would change the colour of this $pH$ paper to greenish-blue?

Write true or false for the following statement:
Mixing an acid or base with water results in an increase in the concentration of ions $(H_{3}O^{+} / OH^{-})$ per unit volume.

$(i)$ State the purpose of developing $pH$ scale.
$(ii)$ Mention the $pH$ range for acids.
$(iii)$ What is the $pH$ of acid rain and how it affects the aquatic life?

Write true or false for the following statements:
Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The water must always be added slowly to the acid with constant stirring.

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.