Final answer:
The temperature remains constant during the melting point of a substance because the added energy is used solely for transitioning from solid to liquid phase, which is an isothermal process. This equilibrium is maintained until the complete melting of the solid phase has occurred.
Step-by-step explanation:
The temperature stays constant during the melting point of a substance because the energy being supplied to the system is used entirely to change the phase of the substance from solid to liquid without causing a temperature increase. This phase change is an isothermal process, meaning it occurs at a constant temperature. In the case of water (H₂O), ice melts at 0°C and during this phase transition, the temperature does not increase until all the solid has become liquid.
For example, in a perfectly insulated container, such as a high-quality thermos bottle, a mixture of ice and water can maintain this equilibrium for hours without a temperature change. This is because melting and freezing occur at equal rates, thus maintaining a balance. The temperature at which this balance occurs is known as the melting point or freezing point of the substance.
Therefore, the melting point at which the solid and liquid phases of a substance are in equilibrium is a significant physical property, which remains consistent until the phase change has been completed. Only after that will additional heat change the temperature of the now fully liquid substance.