What unit is used to measure the amount of energy required to change a substance's phase?

• A. Calorie
• B. Watts
• C. Joule
• D. Newton

Answer: (C)

The unit used to measure the amount of energy required to change a substance's phase is the Joule. Phase changes, such as melting, boiling, or sublimating, involve energy transformations that are quantitative, and Joules provide a standard measure for this energy.

Phase changes require specific amounts of energy to overcome intermolecular forces—energy is absorbed during melting and boiling (endothermic processes) and released during freezing and condensation (exothermic processes). A Joule is a derived unit in the International System of Units (SI) that quantifies energy, work, or heat, making it perfectly suited for such measurements.

While calories are also units of energy, primarily used in nutrition and in specific contexts like heat transfer, the Joule is the standard SI unit and is more universally applicable in scientific calculations, including those involving phase changes. Watts, on the other hand, measure power (the rate of energy transfer) and are not directly used for quantifying phase change energy. Newtons measure force and do not pertain to energy measurements. Thus, the Joule is the most appropriate and scientifically accepted unit for this context.