Final answer:
A horizontal line on a heating curve indicates a change of state where temperature remains constant during a phase transition.
Explanation:
A heating curve displays temperature changes over time as heat is added to a substance, showing transitions between solid, liquid, and gas states. In a heating curve, a horizontal line indicates a change of state where the temperature remains constant during a phase transition, such as melting or boiling.
For example, at the melting point, a substance shifts from solid to liquid state, and the temperature stays constant until the phase transition is complete. This feature of a horizontal line on a heating curve signifies a change of state rather than a change in temperature.
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