When lemon juice and sodium hydroxide are mixed, they undergo a chemical reaction called a neutralization reaction. This reaction results in the formation of sodium citrate, water, and carbon dioxide gas. The balanced chemical equation for this reaction is:

3NaOH + C6H8O7 → Na3C6H5O7 + 3H2O + CO2

In this reaction, sodium hydroxide (NaOH), a strong base, reacts with lemon juice, which contains citric acid (C6H8O7), a weak acid. The sodium ions (Na+) from the sodium hydroxide combine with the citrate ions (C6H5O73-) from the citric acid to form sodium citrate (Na3C6H5O7). The hydrogen ions (H+) from the citric acid combine with the hydroxide ions (OH-) from the sodium hydroxide to form water (H2O). The carbon dioxide gas (CO2) is released as a byproduct of the reaction.

The overall effect of this reaction is that the acid and base neutralize each other, resulting in the formation of a salt (sodium citrate), water, and carbon dioxide gas. This type of reaction is commonly used in chemistry to neutralize acids and bases.