When conducting a chemical reaction that produces gas, you should use a closed system. Here's why:

* Safety: A closed system prevents the escape of potentially harmful gases into the surrounding environment.

* Accuracy: A closed system allows you to better measure the volume of gas produced, which is often crucial for calculating reaction yields and understanding the reaction's stoichiometry.

* Control: A closed system allows you to control the pressure within the reaction vessel, which can be important for certain reactions.

Examples of closed systems:

* Erlenmeyer flask with a stopper: A common setup for small-scale gas-producing reactions.

* Burette: Used to collect and measure the volume of gas produced.

* Gas collection tube: Used to capture the gas produced over water.

Important Note: Never perform a chemical reaction in a closed system without proper safety precautions. Always ensure adequate ventilation and wear appropriate safety gear like gloves and goggles.