Final answer:
In a heating curve, the section showing the liquid phase of a substance gaining kinetic energy is the upward-sloping line between the melting point and boiling point plateaus, which indicates rising temperature due to increased molecular motion.
Step-by-step explanation:
The liquid phase of a substance gaining kinetic energy in a heating curve is represented by the upward-sloping line between the two plateaus. These plateaus indicate phase changes where the temperature remains constant. As the substance absorbs heat in its liquid state, the kinetic energy of the molecules increases, causing the temperature to rise until it reaches the boiling point.
This is visually represented by the section of the curve that slopes upward after the end of the melting point plateau and before the beginning of the boiling point plateau.
The slope of this line depends on the specific heat capacity of the liquid, which is the amount of energy required to raise the temperature of 1 g of the substance by 1°C. During this phase, although there is a consistency in the rate of energy input (heat added), there will be no phase change until the boiling point is reached, indicating that all the energy is going into increasing molecular motion, which is manifested as an increase in temperature.