What is the precipitate in Na2CO3(aq) +HCl (aq)-->

Unless the solutions are very concentrated, there is no precipitate. There is however gas evolution.

Na2CO3(aq) +2 HCl (aq)--> H2O(l) + CO2(g) + 2 NaCl(aq)

If this solution is concentrated, salt crystals will form.