Which of the following is an observable sign of an acid-metal reaction that is commonly expected in experiments?

• A. Fizzing and gas evolution.
• B. A color change of the metal alone.
• C. The solution turning clear without reaction.
• D. No energy change detected.

Answer: (A)

Fizzing and gas evolution is the clear sign you’d expect when a metal reacts with an acid. In these reactions, the metal is oxidized and hydrogen gas is produced, which forms bubbles that rise through the solution. This visible bubbling is a reliable indicator that a reaction is occurring, and you’ll commonly see it in simple experiments like reacting zinc or magnesium with hydrochloric acid.

Other signs shown in the options don’t fit as consistently. A color change of the metal alone isn’t typical for most metals in acid, since many metals keep their color even as they corrode. A color change of the solution to clear without any reaction would mean nothing is happening, which contradicts the idea of a reaction taking place. Finally, claiming no energy change is detected overlooks the fact that chemical reactions often release or absorb energy (often as heat) even if you don’t measure it directly.