Question

which is evidence that a chemical reaction has occurred? a a phase change occurs b a new gas is observed in the form of bubbles c red and yellow paint mixing to make orange paint d the temperature of the substance remains constant

Explanation:

• Option A: A phase change (like melting, freezing) is a physical change, not a chemical reaction (no new substance formed).
• Option B: A new gas (bubbles) indicates a chemical reaction, as a new substance (gas) is produced (e.g., in a reaction like acid + carbonate → gas).
• Option C: Mixing paints is a physical mixing (color change due to pigment mixing), no new substance, so physical change.
• Option D: Constant temperature doesn't indicate a chemical reaction; reactions often involve temperature changes, but even if not, no new substance is implied here.

Answer:

B. a new gas is observed in the form of bubbles