What happens to water when heat is applied, according to molecular principles?

When heat is applied to water, the energy transfer causes the water molecules to move more vigorously. This increased molecular motion leads to a greater average distance between the molecules, which in turn results in an increase in the volume of water. The application of heat prompts the liquid water's molecules to transition into a less dense state, especially as it approaches the boiling point and begins to turn into steam. This phenomenon, where volume increases with the addition of heat, is a fundamental principle of thermodynamics and is observed in many substances, including water. Therefore, the correct answer highlights this important relationship between heat and the physical properties of water at the molecular level.