Final answer:
Water changes phase from ice to liquid through heat absorption without a change in temperature, remaining 11.19% hydrogen and 88.81% oxygen by mass, reflecting a physical change. The melting occurs at a fixed temperature, the melting point, which is the same as the freezing point, a signature physical property of water.
Step-by-step explanation:
Understanding Phase Changes in Water
Water changes phase due to the transfer of heat, such as when ice absorbs heat and undergoes a physical change from solid to liquid at its melting point. In this process, the molecular composition remains constant with 11.19% hydrogen and 88.81% oxygen by mass, indicating it is a physical change, not a chemical one. During a phase change like melting, the temperature of the substance does not change despite the heat transfer, because all the energy goes into changing the state rather than increasing the temperature.
When ice is exposed to warmth, for instance, from the Sun's rays on a rooftop, the ice melts into water. This is because the water molecules receive enough energy to break free from the rigid structure of the ice. Conversely, when water is cooled below its freezing point in an environment like an ice tray in a freezer, it releases heat and freezes into ice. The melting and freezing points of water are the same temperature, which is characteristic of a substance's physical properties.
Interestingly, water behaves uniquely in that its solid state, ice, is less dense than its liquid state, which is why ice floats on water. Should ice be near its melting point, the application of pressure can also induce melting, opposite to most other substances, due to the unusual positioning of water molecules in its solid and liquid forms.