Heat energy is absorbed in which phase change?

• A. Br2(g) --> Br2(l)
• B. Br2(l) -> Br2(s)
• C. Br2(l) -> Br2(g)
• D. Br2(g) -> Br2(s)

Answer: (C)

Endothermic phase changes absorb heat. When a liquid becomes a gas, energy must be put in to overcome intermolecular forces and create more space between particles, so heat is absorbed. The temperature stays the same during this transition while the energy goes into latent heat of vaporization. The other listed changes release heat as the substance becomes more ordered (gas to liquid, liquid to solid, gas to solid). Therefore, the liquid-to-gas change is the one that absorbs heat.