Which of the following is a sign that a chemical reaction has produced a gas?

• A. Bubbles/fizzing
• B. A change in volume or pressure
• C. A color change only
• D. A precipitate forms

Answer: (B)

Gas formation changes how the reaction system behaves because adding gas increases the amount of gas particles in the space available. In a sealed or rigid container, producing gas raises the pressure; in a flexible container, it can cause the volume to expand. This makes a change in volume or pressure a reliable sign that a gas has been produced, since it reflects the physical presence of new gas. Bubbles or fizzing are common visible signs, but they depend on gas escaping to the surroundings and aren’t as universal in all setups. Color change and precipitate formation, by contrast, indicate other types of changes and don’t confirm gas production.