Why can a burn from steam at 100 °C be more severe than a burn from liquid water at 100 °C?

Core Concept

At 100 °C both are at the same temperature, but the steam carries extra stored energy from having been vaporized. When steam touches cooler skin, it condenses first, releasing about 40.7 kJ per mole as intermolecular attractions reform. That additional delivery of energy comes before any ordinary cooling of the liquid. Temperature alone cannot capture the phase-change energy a substance can surrender.