Which of the following indicates that a gas has formed during a chemical reaction?

The formation of gas during a chemical reaction is typically indicated by observable signs such as fizzling or bubbling. This is due to the release of gas bubbles that can be seen escaping from the liquid during the reaction. When a chemical reaction produces a gas, it often results in effervescence, which is the rapid escape of gas from the liquid. This is a clear and direct indication that a gas is being generated as a product of the reaction.

Other indicators, while they may signify a reaction is occurring, do not specifically confirm the formation of gas. For instance, a color change might suggest that a chemical transformation has taken place, indicating the formation of new substances, but it does not provide direct evidence of gas formation. Similarly, a temperature change, such as a drop or rise, can occur without the associated production of gas, making it a less definitive sign. Lastly, a scenario where no visible change occurs would not indicate any reaction, including the formation of gas, since evidence of change is essential to suggest that a reaction has taken place.