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when you titrate the base solution to the phenolphthalein end point you have caried out the following reaction: Na2CO3 + HCl = NaHCO3 + NaCl
You have reacted exactly half the Na2CO3. The remaining solution then contains the unreacted NaHCO3 from this reaction plus the unreacted NaHCO3 originally in the solution. Solution mixture of reaction (1) at the equivalence point is alkaline the answer is in the way Na2CO3 reacts with acids. its a two step equation.

Na2CO3 + HCl --> NaHCO3 + NaCl
NaHCO3 +HCl --> CO2 + H2O + NaCl

according to this mechanism, very little carbon dioxide is formed if you added HCl dropwise into a flash of Na2CO3. this is because the small amount of HCl would only take the carbonate to the end of the first reaction. it would take additional HCl to start producing the gas from the second reaction, which is why you only start seeing bubbles towards the end of the titration. the second reaction starts when there is no Na2CO3 left.

however if you titrated the carbonate into acid, a few drops of carbonate is going to meet alot of acid HCl. the carbonate completes the first reaction quickly and starts to produce gas in the second reaction almost instantly. so you see bubbles right from the start and throughout the whole titration. Neutralization reaction between Na2CO3 and HCl acid takes place into two steps- Na2CO3+ 2HCl => NaHCO3+ 2NaCl

NaHCO3+HCl =>NaCl + H2CO3
The ultimate reaction,
Na2CO3+ 2HCl =>2NaCl + H2CO3
In the first step, the solution is basic due to the formation of a salt where the basic part is stronger than the acidic part (NaHCO3).So, in order to determine the equivalent point of this reaction Phenolphthalein is used. As the salt that forms due to the neutralization reaction, produces more OH-, so the solution becomes a basic one and...
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