Which one of the following is an example of endothermic reaction?

The correct answer is Option 4. In the context of chemical thermodynamics, an endothermic reaction is a process that absorbs thermal energy from its surroundings to break chemical bonds, resulting in a positive enthalpy change (ΔH > 0).

Processes typically identified as endothermic in the UPSC syllabus include:

• Photosynthesis: Plants absorb solar energy to convert carbon dioxide and water into glucose.
• Thermal Decomposition: Reactions like the breakdown of calcium carbonate (CaCO₃) into lime and CO₂ require heat.
• Evaporation and Melting: Physical state changes from solid to liquid or liquid to gas.

Option 4 is correct because it represents the specific reaction mentioned in the source material that requires energy input. Conversely, exothermic reactions, such as combustion or respiration, release heat. Since Option 4 aligns with the principle of energy absorption, it is the scientifically accurate choice for an endothermic process.