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

Apr 2, 2017

Because trying to get water to boil takes much longer than melting ice.

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.

The larger enthalpy indicates a larger amount of heat applied at constant pressure in order to accomplish the phase transition.

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.