Which of the following is a sign that combustion is occurring?

• A. Temperature increases and light is produced
• B. Temperature decreases
• C. Reactants form gas only
• D. Color changes from blue to red

Answer: (A)

Combustion is an exothermic reaction that releases energy, usually seen as heat and a flame. When a flame is present, you feel warmth and can see light, which are classic signs that a fuel is reacting with oxygen. So, temperature rising and light being produced are the telltale indicators of combustion, making that option the best choice.

The other possibilities don’t reliably signal combustion: a temperature drop would mean the reaction absorbs heat, not releases it; forming gas alone can happen in many non-combustion reactions; and a color change from blue to red isn’t a definitive sign of combustion and can depend on other factors like the specific fuel and flame conditions.