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 $ \Delta \mathrm{Q} $, 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:

$ \Delta \mathrm{U} = \Delta \mathrm{Q} + \Delta \mathrm{W} $

In this equation, $ \Delta \mathrm{U} $ represents the change in internal energy, $ \Delta \mathrm{Q} $ is the heat absorbed by the system, and $ \Delta \mathrm{W} $ is the work done by the system. Since the work done is negative and the absorbed heat is positive, the overall internal energy ($ \Delta \mathrm{U} $) increases.