Question

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

Answer 1

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

The `YZ` line is at a constant temperature. Specifically, at `0^@ "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 `YZ` line.

The enthalpy of melting the ice, `DeltaH_(fus)`, is about `"6.02 kJ/mol"` at ordinary pressures. The enthalpy of vaporizing the water, `DeltaH_(vap)`, is about `"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 `WX` line, more time is thus required at some constant heating rate to vaporize the water along the `WX` line.