Final answer:
As a solid changes into a liquid or a gas, atoms gain energy, which is required to overcome the forces holding them in the solid state. This energy gain facilitates the transition to a less ordered state, such as liquid or gas, classifying it as an endothermic process. (option b)
Step-by-step explanation:
Energy transfer is indeed involved in a phase change. When matter changes from one state to another, energy is either absorbed or released. For a substance to transition from a solid to a liquid (melting) or from a solid to a gas (sublimation), it must absorb energy.
This absorption of energy increases the kinetic energy of the atoms, allowing them to overcome the intermolecular forces that keep them in a fixed position in the solid state.
Therefore, when a solid changes to a liquid, atoms gain energy, and similarly, when a solid changes to a gas, atoms also gain energy. This process is known as endothermic because it requires an input of heat. On the contrary, when a gas changes to a liquid or a liquid to a solid, atoms release energy, and these processes are exothermic. As such, the correct answers to the student's question are A. Atoms gain energy as a solid changes to a liquid and B. Atoms gain energy as a solid changes to a gas.
Hence, the answer is option b