Which of the following is used for the dissolution of gold?

Write true or false for the following statement:
Higher is the hydronium ion concentration,higher is the $pH$ value.

$A$ sulphate salt of a Group $2$ element of the Periodic Table is a white,soft substance,which can be moulded into different shapes by making its dough. When this compound is left in the open for some time,it becomes a hard solid mass and cannot be used for moulding purposes. Identify the sulphate salt and explain why it shows such behavior. Provide the chemical reaction involved.

The colour of $pH$ paper strip at the $pH$ values of $1, 7,$ and $14$ will be:

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

$(a)$ Identify the acid and the base whose combination forms the common salt that you use in your food. Write its formula and chemical name of this salt. Name the source from where it is obtained.
$(b)$ What is rock salt? Mention its colour and the reason due to which it has this colour.
$(c)$ What happens when electricity is passed through brine? Write the chemical equation for it.