Advanced Placement (AP) Chemistry Practice Exam

The standard state of an element is defined as the most stable physical form of that element at a pressure of 1 atmosphere and a specified temperature, which is conventionally set to 25 degrees Celsius (298 K). This definition ensures consistency when looking at thermodynamic data, allowing for comparisons of standard enthalpies, standard entropies, and other thermodynamic functions.

For example, in the standard state, elements can exist in different forms depending on their temperature and pressure conditions. Most metals are solid at this standard state, while nonmetals can exist as gases (like nitrogen) or solids (like sulfur). Therefore, B correctly represents this widely accepted definition.

The other choices do not accurately represent the standard state. For instance, 0 degrees Celsius is not the agreed-upon reference temperature; hence it does not fulfill the definition. Referring to gaseous states only or the least stable form does not align with the criteria for standard states, which center around stability and specific conditions. Thus, B is the appropriate choice.