In the context of phase diagrams, when do phase transitions occur in a substance?

• A. Only when the pressure is zero
• B. Only when the temperature is at its maximum value
• C. Whenever the substance is exposed to light
• D. When the temperature and/or pressure reach the boundaries between different phases

Answer: (D)

Verified step by step guidance

1. Understand that a phase diagram maps the state of a substance (solid, liquid, gas) as a function of temperature and pressure.
2. Recognize that phase transitions occur at specific conditions where two phases coexist in equilibrium, which are represented by the boundaries (lines) on the phase diagram.
3. Identify that these boundaries correspond to combinations of temperature and pressure where the substance changes phase, such as melting, boiling, or sublimation points.
4. Note that phase transitions do not depend on pressure being zero, temperature being maximum, or exposure to light, but rather on crossing these boundary lines in the phase diagram.
5. Conclude that phase transitions occur whenever the temperature and/or pressure reach the boundaries between different phases on the phase diagram.