Question

What is the gas evolved when dilute hcl reacts with sodium hydrogen carbonate and how is that gas recognized?

Answer 1

Carbon dioxide gas evolved when dilute hcl reacts with sodium hydrogen carbonate .
We recognize the presence of carbon dioxide by passing it through lime water which turns milky or a white precipitate of calcium carbonate . 

Answer 2

HCl + NaHCO3 gives NaCl + CO2 + H2O


The gas can be passed over a flame to check whether the flame is extinguished or not, as CO2 does the same.

There u go....
Hope it helped....