What does a phase diagram represent?

A phase diagram visually represents the different states of matter (solid, liquid, gas) of a substance as a function of temperature and pressure. It highlights the conditions under which distinct phases are stable and the transitions that occur between these phases, such as melting, boiling, and sublimation.

In a phase diagram, various lines demarcate the boundaries between phases; for instance, the line between solid and liquid represents the melting point, whereas the line separating liquid and gas indicates the boiling point. The areas separating these lines indicate the conditions where each phase is stable. Therefore, option B accurately captures the essence of what a phase diagram conveys regarding the interplay between pressure, temperature, and phase stability.

The other options, while they touch on aspects of thermodynamics or physical properties, do not encapsulate the role of phase diagrams in representing the stability of phases under varying conditions. For example, temperature changes during chemical reactions pertain more specifically to reaction kinetics rather than phase transitions, and thermal conductivity relates to how heat moves through different states rather than the states of matter themselves.