Which one of the following processes is endothermic? A) Boiling a liquid. B) Freezing a solid. C) Condensation of a gas into a liquid. D) Condensation of a gas into a solid.
The Correct Answer and Explanation is :
The correct answer is A) Boiling a liquid.
Explanation:
In thermodynamics, processes are classified as endothermic or exothermic based on whether they absorb or release heat.
- Endothermic Processes: These require the absorption of energy (usually in the form of heat) to proceed. In an endothermic reaction, energy flows from the surroundings into the system, causing the system’s temperature to increase while the surroundings cool down. For example, boiling a liquid is endothermic because heat energy must be absorbed by the liquid to overcome intermolecular forces and change from the liquid phase to the gas phase.
- Exothermic Processes: These release energy, causing the surroundings to warm up. When a substance condenses (gas to liquid), freezes (liquid to solid), or undergoes deposition (gas to solid), it releases energy. For example:
- Freezing a solid: Releases energy, causing the liquid to solidify.
- Condensation (gas to liquid or gas to solid): Releases energy as molecules slow down and move closer together.
In boiling (option A), the liquid absorbs heat, allowing particles to gain enough energy to escape into the gas phase, overcoming attractive intermolecular forces. The absorbed heat increases kinetic energy, promoting this phase change. This heat absorption makes boiling an endothermic process, as it requires a continuous input of energy.
In contrast:
- Freezing (option B), condensation to a liquid (option C), and condensation to a solid (option D) are all exothermic processes since they release energy as molecules transition to a more ordered state.
Therefore, boiling is the only endothermic process among the options given, as it requires heat to proceed, unlike the other options which release heat.