Concept:A cooling curve shows how temperature changes as a substance loses heat.
A horizontal or flat portion means the temperature remains constant during a phase change.
Explanation:The molten substance is initially hot and cools down gradually.
When its temperature reaches
60∘C, the curve becomes horizontal.
This indicates that heat is being lost, but the temperature does not fall.
The substance is changing state from liquid to solid at this constant temperature.
This constant temperature is called the freezing point.
For a pure substance, the freezing point is equal to the melting point.
Therefore, the temperature
60∘C is the melting point of the substance.
Answer:D. melting point
