What happens on heating a substance containing sodium carbonate in the presence of water?Will it give carbonic acid or caustic soda or carbonic acid?

The correct answer to your question is both carbonic acid
and caustic soda will be produced in the course of the reaction. This reaction proceeds
as a two step process. In the first step we have this
occurring:

NaCO3 + H2O -> H2CO3 +
NaOH

Here, a double replacement reaction occurs in which
hydrogen replaces sodium, resulting in the formation of both NaOH, also know as caustic
soda or lye, and H2CO3, which is carbonic acid.

However
carbonic acid is a weak acid, and will dissociate in the presence of the base into water
and carbon dioxide. The full equation looks like
this:

NaCO3 + H2O -> H2CO3 + NaOH -> H2O +
CO2 + NaOH

So when the reaction is completed, you will be
left with three products: Water, carbon dioxide, and sodium hydroxide. If the reaction
is done in an open container, the carbon dioxide will bubble off, and the result will be
a caustic soda solution.