In a typical heating/cooling curve, the slope of the line when a change of state is occurring is zero.
A heating curve is a curve that shows the temperature changes that occur when a substance is heated.
A cooling curve is a curve that shows the temperature changes that occur when a substance is heated.
Heating and cooling curves for a pure substance have the same shape but are oppositely directed.
When a heated or cooled substance is undergoing a change of state, there is no change in temperature, hence the slope is zero.
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