# In the heating curve for water shown below, why is line WX longer than line YZ?

The $Y Z$ line is at a constant temperature. Specifically, at ${0}^{\circ} \text{C}$. If you recognize that as the melting point of ice, we are evidently performing a melting phase transition by heating the ice over time along the $Y Z$ line.
The enthalpy of melting the ice, $\Delta {H}_{f u s}$, is about $\text{6.02 kJ/mol}$ at ordinary pressures. The enthalpy of vaporizing the water, $\Delta {H}_{v a p}$, is about $\text{40.7 kJ/mol}$, which is larger.
Since more heat was required to vaporize the water along the $W X$ line, more time is thus required at some constant heating rate to vaporize the water along the $W X$ line.