To determine which process is endothermic, we analyze each option:
- a) Triple point solid ⇌ liquid ⇌ vapor equilibrium: This represents a balance of phases at the triple point where solid, liquid, and vapor exist simultaneously. The phase transitions involve both endothermic and exothermic processes depending on the direction of the transition.
- b) Condensation: This process involves a gas turning into a liquid, which releases heat. Hence, it is exothermic.
- c) Melting: This process occurs when a solid turns into a liquid. It requires heat (energy) to overcome the forces holding the solid together, making it an endothermic process.
- d) Cooling: This process typically refers to the loss of heat from a substance, which does not require heating.
- e) Solidification: This is when a liquid turns into a solid, releasing heat in the process and is therefore exothermic.
Based on this analysis, the only process that requires the addition of heat to occur is c) melting. When a solid melts to form a liquid, it absorbs heat energy from its surroundings, making it an endothermic process.