Final answer:
Water undergoes two major phase changes: melting (solid to liquid), requiring 335 J/g, and vaporization (liquid to gas), requiring 2260 J/g. The energy required for vaporization is much more than for melting because vaporization involves overcoming all hydrogen bonds between molecules, while melting only requires breaking some of these bonds, allowing for freedom of movement without a temperature change.
Step-by-step explanation:
Phase Changes in Water
The physical change from solid to liquid and from liquid to gas in water involves significant energy changes. For a solid turning into a liquid, known as melting or fusion, water requires an energy input of 335 J/g, which is the enthalpy of fusion (ΔHfus). For ice to melt, energy is absorbed to break down the orderly arrangement of water molecules in the ice lattice without increasing the temperature of the substance. This enables the molecules to move more freely, resulting in a liquid phase.
Conversely, the transition from liquid to gas (vaporization) takes much more energy, specifically 2260 J/g for water, known as the enthalpy of vaporization (ΔHvap). This higher energy requirement is due to the need to overcome the strong hydrogen bonds between water molecules in the liquid state to allow them to move apart and enter the gaseous phase. Whereas melting only requires breaking some hydrogen bonds, vaporization requires breaking all intermolecular forces to release individual water molecules into the air as a gas.
Illustrating these processes at the particulate level, you would see an orderly grid of solid ice molecules gaining energy and spacing out slightly to form liquid water. For vaporization, you would see the liquid water molecules gaining energy, moving much more rapidly, and spreading out to become water vapor.