Final answer:
Gases have two specific heats, Cp and Cv, because they can do work when expanding at constant pressure, affecting how they heat up. Solids and liquids, being incompressible, have negligible volume change when heated, leading to no significant work done, and thus only one specific heat value is provided.
Step-by-step explanation:
The reason why there are two specific heats for gases, Cp and Cv, yet only one given for solids is rooted in how gases and solids respond to heat. The specific heat at constant pressure (Cp) and at constant volume (Cv) differ for gases because adding heat at constant volume does not allow the gas to do work, leading to a straightforward temperature increase.
In contrast, at constant pressure, the gas expands, thereby doing work on its surroundings, which means not all the added heat goes into raising the gas's temperature. Solids and liquids, due to their incompressibility, have little to no volume change under normal conditions when heat is added.
Therefore, the distinction between Cp and Cv is negligible, and usually, a single value of specific heat is sufficient to describe their behavior under heat transfer. As the volume change is minimal, there's hardly any work done on or by the substance, thereby not necessitating the separate consideration that gases require.