Question

Why is sodium carbonate more soluble than sodium bicarbonate in water. Can we give any general statement that carbonates are more soluble than bicarbonates

Answer 1

Actually, sodium bicarbonate more soluble than sodium
carbonate in water

If you think about their structures, the only thing that is different is that Na2CO3 has two atoms of NaNa, while NaHCO3 has one atom of NaNa and one atom of HH instead of a second NaNa atom. Sodium and hydrogen have different electronegativity values, giving NaHCO3 polar character.

Likely, this is due to hydrogen bonding. NaHCO3NaHCOX3 has one hydrogen bond donor site, but Na2CO3has none.
For an organic compound, one hydrogen bond for each four-five carbons may be enough to make it water soluble.