During the melting of a slab of ice at 273 K under atmospheric pressure, the following occurs:

As the ice melts, its volume decreases. Because of this volume reduction, the atmosphere exerts a positive work on the ice-water system. When heat is absorbed by the ice-water system, we denote the absorbed heat as , which is positive. The work done by the ice-water system is negative because it is being compressed by the external pressure.

According to the first law of thermodynamics:

In this equation, represents the change in internal energy, is the heat absorbed by the system, and is the work done by the system. Since the work done is negative and the absorbed heat is positive, the overall internal energy ( ) increases.