Which of the following statements best describes thermal equilibrium at constant pressure?

• A. At constant pressure, thermal equilibrium is reached when the internal energy of both systems is the same.
• B. Two systems in contact at constant pressure have no net heat flow between them when they reach the same temperature.
• C. Thermal equilibrium at constant pressure means the pressure of both systems is equal, regardless of temperature.
• D. At constant pressure, thermal equilibrium is achieved when both systems have the same volume.

Answer: (B)

Verified step by step guidance

1. Understand the concept of thermal equilibrium: Thermal equilibrium occurs when two systems in contact no longer exchange heat, meaning their temperatures are equal.
2. Recognize that temperature is the key factor in thermal equilibrium, not internal energy, pressure, or volume alone.
3. At constant pressure, the condition for thermal equilibrium remains that there is no net heat flow between the systems, which happens when their temperatures are equal.
4. Analyze the given statements and identify that the correct description involves no net heat flow and equal temperatures, not necessarily equal internal energy, pressure, or volume.
5. Conclude that the best description of thermal equilibrium at constant pressure is: 'Two systems in contact at constant pressure have no net heat flow between them when they reach the same temperature.'