Difference Between Melting Point and Freezing Point

The key difference between melting point and freezing point is that melting point is the temperature at which a solid will go to the liquid state whereas freezing point is the point at which any liquid will change its state to a solid.

Phase changes are processes that involve energy release or energy gain. Melting point and freezing point are points at which phase changes occur. With these, many other properties of the material can also change. Theoretically, the freezing point and the melting point of a material are the same, but sometimes for particular substances, the two values differ from each other.

CONTENTS

1. Overview and Key Difference
2. What is Melting Point 
3. What is Freezing Point
4. Side by Side Comparison – Melting Point vs Freezing Point in Tabular Form
5. Summary

What is Melting Point?

Melting point is the temperature at which a solid will change into its liquid state. It is a physical property we can use to identify a compound. When a solid converts into a liquid stage, a phase change occurs. It occurs at a characteristic temperature for a given pressure. For this, we need to supply the required energy.

The phase change absorbs energy/ heat (endothermic) when going from solid to liquid. Most of the times, this energy is supplied in the form of heat. Heat is required to increase the temperature of the solid to its melting point state. Furthermore, the substance needs the energy to melt itself. We call this energy heat of fusion; this is a type of latent heat.

Latent heat is the heat that is being absorbed or released from a substance during a phase change. These heat changes do not cause temperature changes as they are absorbed or released. Therefore, at the melting point, the substance absorbs heat, but the temperature will not change accordingly. Thermodynamically, at the melting point, change of Gibbs free energy is zero. Following equation is valid for a material at melting point. It shows that the temperature does not change, but the enthalpy and entropy of the material change.

ΔS = ΔH/T

Since the substance absorbs energy, enthalpy increases at melting point. In the solid state, particles are well ordered and have less movement. But at the liquid state, their random nature increases. Therefore, at the melting point, entropy increases. According to the pressure, there is a specific melting point for a given material. We can only determine the melting points for solids.

In the laboratory, we can use many techniques to determine the melting point. Using a melting point apparatus is very easy. We can put some finely powdered solid into a capillary in which one end is sealed. Then we should put the sealed end that contains the solid into the apparatus. Here, the end should touch the metal inside. Thereafter, we can observe the solid through the magnifying glass window of the apparatus. There is also a thermometer to record the temperature. When temperature gradually increases, the metal will heat and, the solid in the capillary will also heat. We can observe the point at which the melting starts and finishes. And, this range corresponds to the melting point. The melting point of water is 0 °C.  Tungsten has the highest melting point, which is 3410 °C.

What is Freezing Point?

Freezing point is the temperature at which any liquid will change its state to a solid. The temperature of melting point and freezing point for a material is more or less the same value. For example, the water turns into ice at 0°C and its melting point is also 0°C. More precisely, the freezing point describes the change of phase of matter.

Moreover, the process of freezing a substance decreases entropy. There is a term in chemistry as “freezing point depression” which explains how the presence of a non-volatile solute in a solution decreases the freezing temperature of the solvent.

What is the Difference Between Melting Point and Freezing Point?

Melting point and freezing point are opposite directions of the same phase change. The key difference between melting point and freezing point is that melting point is the temperature at which a solid will go to the liquid state whereas freezing point is the point where any liquid will change its state to a solid. Moreover, there is also a difference between melting point and freezing point in terms of their entropy change; while melting a solid, the entropy increases, but in freezing, the entropy decreases. Besides, though theoretically, melting point and freezing point temperatures are similar for a given material, practically they vary slightly.

Summary – Melting Point vs Freezing Point

Melting point and freezing point are two physical properties of substances. The key difference between melting point and freezing point is that melting point is the temperature at which a solid will go to the liquid state whereas freezing point is the point where any liquid will change its state to a solid.

Reference:

1. Helmenstine, Anne Marie, Ph.D. “What Is the Freezing Point of Water?” ThoughtCo, Oct. 5, 2018, Available here.

Image Courtesy:

1. “Melting point of the elements (K)” By Albris – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “1550739” (CC0) via pxhere