Sodium Hydrogen Carbonate and Dilute Hydrochloric Acid Reaction
When sodium hydrogen carbonate (NaHCO3) reacts with dilute hydrochloric acid (HCl), the gas evolved is carbon dioxide (CO2). This reaction is a type of acid-base reaction known as a neutralization reaction.

Chemical Reaction
The chemical equation for the reaction between sodium hydrogen carbonate and dilute hydrochloric acid is as follows:
NaHCO3 + HCl → NaCl + H2O + CO2
In this reaction, sodium hydrogen carbonate reacts with hydrochloric acid to form sodium chloride, water, and carbon dioxide gas. The carbon dioxide gas is the effervescence observed during the reaction.

Explanation
1. When sodium hydrogen carbonate reacts with dilute hydrochloric acid, it undergoes a double displacement reaction where the hydrogen ions from the acid replace the sodium ions in the sodium hydrogen carbonate, forming sodium chloride and water.
2. The carbon dioxide gas is evolved as a byproduct of the reaction. This gas can be identified by the effervescence or bubbling observed during the reaction.
3. The production of carbon dioxide gas in this reaction is due to the decomposition of sodium hydrogen carbonate into sodium carbonate, water, and carbon dioxide when it reacts with an acid.
4. Carbon dioxide is a colorless, odorless gas that is heavier than air and can be tested using a flame, which will extinguish in its presence, or by bubbling it through limewater, causing the solution to turn cloudy due to the formation of calcium carbonate.
In conclusion, the reaction between sodium hydrogen carbonate and dilute hydrochloric acid leads to the evolution of carbon dioxide gas, which can be identified by its effervescence and characteristic properties.