Boiling water at standard atmospheric pressure remains at a constant temperature (100°C) during phase change, and adding additional heat increases the rate of vaporization rather than the water's temperature. This is a matter of basic thermodynamics and everyday observation, requiring no external citation.
The claim describes a fundamental thermodynamic property of boiling water and phase transitions: once boiling begins at saturation temperature, added heat goes into latent heat of vaporization rather than raising the sensible temperature. Because this is a trivial thermodynamic fact directly observable in daily life, it falls under STEP 0 as common knowledge, requiring no citations.