On a temperature–time graph for a pure substance being heated at a constant rate, what does a horizontal (flat) section of the graph represent?

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Question

On a temperature–time graph for a pure substance being heated at a constant rate, what does a horizontal (flat) section of the graph represent?

Accepted Answer

The substance is undergoing a change of state — A horizontal section on a heating curve indicates a change of state (e.g. melting or boiling), during which energy input goes into breaking intermolecular bonds rather than raising temperature. Option B implies an external event not shown by the heating curve itself. Option C is incorrect; there is no physical maximum temperature imposed here. Option D is a misconception — specific heat capacity does not increase in a way that produces a perfectly flat section.

Answer

The substance has reached its maximum possible temperature

Answer

The heater has been switched off temporarily

Answer

The rate of heating has decreased due to increased specific heat capacity

Answer

The substance is undergoing a change of state

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