The figure given above shows the temperature (T) - time (t) plot when we start heating a piece of naphthalene. The temperature (T) at the plateau of the curve signifies

When heating a solid substance like naphthalene, the temperature-time (T-t) plot exhibits a plateau where the temperature remains constant despite the continuous addition of heat [1]. This plateau occurs during a phase change, specifically the transition from a solid to a liquid state, known as melting [2]. During this period, the heat energy supplied is absorbed as latent heat of fusion, which is used to overcome the interparticle forces of attraction rather than increasing the kinetic energy of the molecules [2]. For naphthalene, this constant temperature plateau typically occurs at approximately 78.2°C to 80°C, signifying its melting point. Once the entire solid has converted into a liquid, the temperature begins to rise again. Therefore, the plateau in a heating curve represents the melting point of the substance.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Explanation > p. 295
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Latent Heat > p. 294