What type of substance is $NaOH$?

Sodium carbonate is a basic salt because it is a salt of :

When a $pH$ paper is dipped in an aqueous solution,it turns blue. What is the approximate $pH$ value of that solution?

Explain why:
$(a)$ the colour of copper sulphate crystals changes on heating?
$(b)$ baking soda acts as an antacid?
$(c)$ an acid should be added to water while diluting?

At which value of $pH$ of the inner side of the mouth does the decay of teeth take place?

$15 \, mL$ of water and $10 \, mL$ of sulphuric acid are to be mixed in a beaker.
$(i)$ State the method that should be followed with reason.
$(ii)$ What is this process called?