Final answer:
Phase changes as a substance is heated are best represented in a phase diagram or heating curve with the x-axis representing time or heat added, and the y-axis depicting temperature. Horizontal lines indicate constant temperature during phase changes, highlighting the isothermal process of phase transition such as melting and boiling. Therefore, the correct option is D.
Step-by-step explanation:
The correct answer to the question of how phase changes can be diagrammed as a substance is heated is option D) The temperature is on the y-axis and the time is on the x-axis. In a phase diagram or a heating curve, the temperature of the substance is typically plotted on the y-axis, while the x-axis represents the amount of heat added or time. As the substance is heated, phase changes like melting or boiling occur at constant temperatures represented by horizontal lines within the phase diagram or the heating curve.
At these points, the substance is undergoing a change in phase, which is an isothermal process, meaning it occurs at constant temperature. Examples include the melting of solid ice into liquid water at 0°C, and the boiling of liquid water into steam at 100°C (at standard atmospheric pressure). These phases coexist along the lines known as phase boundaries, and the transitions are represented by flat sections of the curve indicating that energy is being used to change the state rather than the temperature.
An important concept to note is that phase diagrams also show pressure-dependency, but in the case of a phase change occurring at constant pressure (such as 1 atm), the diagram simplifies to a heating curve.