When a mixture of citric acid and sodium hydrogen carbonate is added to water, it undergoes a chemical reaction to produce carbon dioxide gas. The reaction can be represented as follows:

3NaHCO3 + H3C6H5O7 → 3CO2 + 3H2O + Na3C6H5O7

In this reaction, sodium hydrogen carbonate (NaHCO3) reacts with citric acid (H3C6H5O7) in the presence of water (H2O) to produce carbon dioxide gas (CO2), water (H2O), and sodium citrate (Na3C6H5O7). The carbon dioxide gas bubbles out of the solution, creating a fizzy or effervescent reaction. This reaction is commonly used to make homemade effervescent drinks, such as lemon-lime soda, as it produces a refreshing and bubbly effect when mixed with water.