When heat is absorbed during a chemical reaction, the reaction is said to be

1. Analyze the question The question asks for the term used when a reaction absorbs heat from its surroundings.

2. Explain the concept Reactions are classified by how they exchange heat with the surroundings:

• Endothermic: heat is absorbed from the surroundings (endo = "into"). ΔH is positive, and the surroundings feel cooler.
• Exothermic: heat is released to the surroundings (exo = "out of"). ΔH is negative, and the surroundings feel warmer.
• Adiabatic: no heat is exchanged with the surroundings at all. It describes a condition of the system, not the direction of heat flow.
• Isothermal: the temperature stays constant throughout the process.

3. Step-by-step solution

• The reaction absorbs heat, so heat flows into the system.
• Heat flowing in means ΔH > 0, which is the definition of an endothermic reaction.
• Option A (adiabatic) is wrong because no heat transfer takes place in an adiabatic process.
• Option C (exothermic) is wrong because it describes heat being given out.
• Option D (isothermal) is wrong because it only says the temperature is constant, not that heat is absorbed.

4. Answer The correct option is B. endothermic.

Examples you may meet in exams include the dissolving of ammonium chloride or potassium nitrate in water (the beaker feels cold), photosynthesis, and the thermal decomposition of calcium carbonate.

5. Exam tip Remember "EN-dothermic = ENters": heat enters the system. And "EX-othermic = EXits": heat exits. If a question mentions the container getting cold, think endothermic.