What describes an isotherm?

An isotherm is defined as a line or curve that represents the relationship between pressure and volume for a substance at a constant temperature. This relationship is typically illustrated in phase diagrams or P-V diagrams.

When referring specifically to option B, a plot of pressure (P) as a function of molar volume at constant temperature accurately captures the essence of an isotherm. In this context, it shows how the pressure of a gas changes with its molar volume while maintaining a fixed temperature. The ideal gas law can be used to relate these variables: ( PV = nRT ), where you can rearrange the equation to express pressure as a function of volume. This is significant because it allows physicists and chemists to analyze behaviors of gases without changing the temperature, thus focusing solely on the effects of varying volume on pressure.

The other choices either describe relationships that do not maintain constant temperature or present plots that do not align with the definition of an isotherm. For example, a plot of P versus temperature at constant pressure relates to isobaric processes, not isothermal; a plot of volume against temperature at constant pressure reflects isochoric conditions; and a plot of density versus volume does not maintain the essential aspects of pressure-volume behavior