This added energy pulls the liquid molecules apart from each other. It completely breaks the attractive forces holding the liquid drop together. The heat does not make the liquid hotter during the phase change.
Instead, all the energy goes directly into separating the crowded particles. Liquids with strong attractive forces always need more heat to boil. Water has very strong hydrogen bonds between all its separate molecules.
This means water requires a massive amount of energy to vaporize. Chemists usually measure this energy requirement in kilojoules per mole (kJ/mol). They also use joules per gram (J/g) for practical everyday measurements.
The required heat gets smaller as the starting liquid gets hotter. The value finally drops to exactly zero at the substance’s critical point. At this extreme temperature, liquid and gas look exactly the same.
Students often mistakenly think that boiling breaks the actual internal chemical bonds. Boiling only pulls the separate whole molecules away from each other. The atoms inside each individual molecule always stay safely connected together.