When Magnesium reacts with oxygen, heat is

Step 1: Analyze the question The question asks whether heat is taken in or given out when magnesium burns in oxygen, and what type of reaction this makes it.

Step 2: Explain the concept

• Exothermic reaction: heat is evolved (given out) to the surroundings. ΔH is negative.
• Endothermic reaction: heat is absorbed (taken in) from the surroundings. ΔH is positive.

The words must match: evolved goes with exothermic, and absorbed goes with endothermic.

Step 3: Apply it to magnesium Magnesium burns in oxygen with a bright, dazzling white flame and gives off a lot of heat:

2Mg(s) + O₂(g) → 2MgO(s) ΔH = –1204 kJ (approximately)

The negative ΔH shows that heat is released. The reaction is therefore exothermic.

Step 4: Check the options

• A. Absorbed and endothermic: the pairing is consistent, but magnesium burning does not absorb heat. Wrong.
• B. Evolved and endothermic: the terms contradict each other. Wrong.
• C. Evolved and exothermic: correct.
• D. Absorbed and exothermic: the terms contradict each other. Wrong.

**Answer: C.

Exam tip: Remember "EXO = EXIT". In an exothermic reaction, heat exits the system. Combustion reactions, such as burning magnesium, charcoal, or cooking gas, are always exothermic. Also, if two options contradict themselves, like B and D, you can eliminate them immediately.