Final answer:
The heating curve for an unknown substance with a melting point of 15°C and a boiling point of 95°C consists of increasing temperatures for the solid and liquid phases with plateaus at the melting and boiling points where the phase transitions occur.
Step-by-step explanation:
The heating curve of an unknown substance A with a melting point of 15°C and boiling point of 95°C will have three distinct parts: the increase in temperature of the solid until it reaches the melting point, a plateau during which the solid melts into liquid at constant temperature, and another increase in temperature once all the substance has melted, up until the boiling point. At the boiling point of 95°C, there will be another plateau where the substance transitions from liquid to gas, and after all the liquid has evaporated the temperature will rise again.
Two key points are worth emphasizing. First, at a substance's melting point or boiling point, two phases can exist simultaneously. For example, substance A in the solid state can start to turn into liquid at 15°C and continue until all the solid has melted. No temperature change occurs during this phase change, hence the plateau on the curve. A similar situation occurs at the boiling point where liquid turns to gas.
Due to pressure dependencies, the melting and boiling points can vary. However, under standard atmospheric pressure (1 atm), substance A conforms to these specific points of 15°C and 95°C, respectively.