The melting point is the temperature that a solid will become a liquid. At different pressures, different temperatures are required to melt a substance. Each pure element on the periodic table has a normal melting point, the temperature that the element will become liquid when the pressure is 1 atmosphere(2)

Melting Point of a Substance

The melting point of a substance refers to the temperature at which a solid substance changes into a liquid state at atmospheric pressure. It is an important physical property that helps in identifying and characterizing different substances. The correct answer to the given question is option 'D', which states that both the solid and liquid phases of the substance exist together at the melting point.

Explanation

To understand why option 'D' is the correct answer, let's discuss the other options and their implications:

a) The solid starts melting: This option suggests that the melting point is the temperature at which the solid substance begins to melt. However, it does not consider the fact that during the process of melting, both solid and liquid phases coexist. The melting point is not reached until the entire solid has transformed into a liquid.

b) Only liquid is present: This option implies that the melting point is the temperature at which only the liquid phase of the substance is present. However, this contradicts the definition of melting point, which specifically refers to the transition from solid to liquid. The presence of only the liquid phase would indicate a different temperature, such as the boiling point.

c) The solid is completely changed into liquid: This option correctly emphasizes that the solid substance is entirely transformed into the liquid state during the melting process. However, it does not consider the coexistence of both phases, which is a defining characteristic of the melting point.

d) Both the solid and liquid exist together: This is the correct answer. At the melting point, the solid and liquid phases of the substance coexist in equilibrium. The temperature remains constant until the entire solid has melted into liquid. This is because the energy supplied to the substance is being utilized to overcome the intermolecular forces holding the solid together, resulting in the transition to the liquid state.

Therefore, option 'D' is the correct answer as it accurately describes the melting point of a substance, where both the solid and liquid phases exist together at atmospheric pressure.