Which description defines an endothermic reaction?

• A. A reaction that releases energy in the form of heat.
• B. A reaction that absorbs energy in the form of heat and requires more energy to continue.
• C. A reaction that occurs only at high pressure.
• D. A reaction that produces a phase change from solid to liquid.

Answer: (B)

Endothermic reactions pull heat from their surroundings. The energy of the system increases as heat flows into it, so these reactions often need continual input of energy to keep proceeding. The description that a reaction absorbs energy in the form of heat and requires more energy to continue captures that behavior perfectly.

Releasing energy as heat would describe an exothermic process, not endothermic. A condition like “only at high pressure” doesn’t define whether a reaction is endothermic. And while melting (solid to liquid) involves heat absorption, describing a phase change isn’t the general definition of an endothermic reaction.