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

What happens to the $pH$ of a basic aqueous solution if the concentration of $[OH^-]$ ions increases?

Which gas is evolved when dilute hydrochloric acid reacts with zinc? Write the molecular formula of this gas.

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?

Calcium phosphate is present in tooth enamel. Its nature is

Crystals of a substance changed their colour on heating in a closed test tube but regained it after sometime when they were allowed to cool down. Name the substance and write its formula and explain the phenomenon involved.