International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What are two observations made when a metal reacts with an acid?

1. Color change and heat emission

2. Bubbles and metal disappears

3. Odor release and precipitation formation

4. Temperature increase and solid formation

The correct answer is: Bubbles and metal disappears

When a metal reacts with an acid, one of the key observations is the production of bubbles, indicating the release of hydrogen gas as a byproduct of the reaction. This effervescence is a clear, visual sign of the chemical change occurring. Additionally, the metal will typically start to dissolve, effectively "disappearing" as it reacts with the acid to form a salt and hydrogen gas. This combination of bubbling and the disappearing metal is a classic indication of a metal-acid reaction; it directly correlates to the formation of new substances. In contrast, other observations like color change, heat emission, or the formation of a solid (precipitate) may occur in different contexts or reactions but are not universally applicable to all metal-acid reactions. Thus, the observation of bubbles and the disappearance of the metal effectively capture the essence of the reaction taking place.