vaporization

Meaning: Vaporization is the conversion of a liquid to a gas. It occurs in two ways. Evaporation happens at the surface below the boiling point, as the fastest molecules escape and leave the liquid cooler. Boiling happens throughout the liquid once its vapor pressure equals the external pressure. The heat absorbed goes into separating molecules (potential energy), not into raising average kinetic energy, so temperature holds steady during the change. Stronger intermolecular forces, such as hydrogen bonding, give a larger ΔHvap and a higher boiling point.

Equation: q = nΔHvap, where q is the heat absorbed (J), n is the amount of liquid (mol), and ΔHvap is the molar enthalpy of vaporization (J/mol). On a mass basis, q = mLv, where m is mass (kg) and Lv is the latent heat of vaporization (J/kg). ΔHvap is positive; condensation releases the same amount of heat.

On the MCAT: Heating-curve passages show a flat segment at the boiling point that is longer than the melting plateau, because ΔHvap is larger than ΔHfus. Calculations add q = mcΔT for warming the liquid to q = mLv for vaporizing it. Other questions ask you to predict that lower external pressure lowers the boiling point, or to rank liquids by intermolecular forces.

Common confusion: Students think temperature keeps rising while a liquid boils. It does not, because the added heat is breaking attractions. Vaporization is also not the same as boiling, since boiling is one form and evaporation is the other.