Which statement describes the forward direction of H2O(l) ⇌ H2O(g) in terms of heat?

• A. It is endothermic
• B. It is exothermic
• C. It has no heat change
• D. It depends on pressure

Answer: (A)

The key idea is that changing from liquid water to steam requires energy input. To break the strong intermolecular forces in liquid water and allow molecules to move freely as a gas, heat must be absorbed from the surroundings. So the forward direction (liquid to gas) is endothermic. During a phase change, the added heat goes into changing the phase rather than raising temperature, which is why the temperature stays at the boiling point while boiling occurs. The reverse direction (gas to liquid) releases heat, so it’s exothermic. Saying there is no heat change is incorrect because latent heat is involved in vaporization. Pressure can shift the conditions for vaporization (boiling point) but does not remove the need for heat input for the forward process.