Which sequence of phase changes represents endothermic processes?

Which sequence of phase changes represents endothermic processes?

• A. Solid -> Liquid -> Gas
• B. Gas -> Liquid -> Solid
• C. Liquid -> Solid -> Gas
• D. Gas -> Solid -> Liquid

Answer: (A)

Endothermic processes absorb energy. Melting a solid to a liquid requires energy to overcome the solid’s lattice forces, and vaporizing a liquid to a gas requires even more energy to overcome intermolecular attractions in the liquid. So a sequence that goes from solid to liquid to gas involves two energy-absorbing steps, making it endothermic overall. Other sequences include steps that release heat: gas to liquid (condensation) and/or liquid to solid (freezing) release energy, so they are not endothermic throughout.

Endothermic processes absorb energy. Melting a solid to a liquid requires energy to overcome the solid’s lattice forces, and vaporizing a liquid to a gas requires even more energy to overcome intermolecular attractions in the liquid. So a sequence that goes from solid to liquid to gas involves two energy-absorbing steps, making it endothermic overall.

Other sequences include steps that release heat: gas to liquid (condensation) and/or liquid to solid (freezing) release energy, so they are not endothermic throughout.