Melting Point vs Freezing Point
Phase changes are processes where energy is released or required. Melting point and freezing point are points at which phase changes occur. With this, many other properties of the material also change.
Melting point is the temperature at which a solid will go to the liquid state. This is a physical property, which we are using to identify a compound. When a solid converts into a liquid stage, we say a phase change has occurred. It is a spontaneous conversion and occurs at a characteristic temperature for a given pressure. For this, energy has to be supplied. The phase change absorbs energy/ heat (endothermic) when going from solid to liquid. Most of the time, this energy is supplied in the form of heat. Heat is required to increase the temperature of the solid to the melting point state. Further energy is needed for the melting itself. This energy is known as heat of fusion, which 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 heat will be absorbed, but the temperature won’t change according to that. 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 is not changing, but the enthalpy and entropy of the material are changing.
ΔS = ΔH/T
Since energy is absorbed, enthalpy is increased at melting point. In solid state, particles are well ordered and have less movement. But at the liquid state, their random nature increases. Therefore, at melting point, entropy increases. According to the pressure, there is a specific melting point for a given material. Melting point can be determined only for a solid. 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. The sealed end that contains the solid is put inside the apparatus. The end should touch the metal inside. We can observe the solid through the magnifying glass window of the apparatus. There is a thermometer to record the temperature. When temperature is gradually increasing, the metal will heat and, therefore, solid in the capillary will be heated. We can observe the point at which the melting starts and finished. 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.
This is the point where any liquid will change its state to a solid. Most of the time temperature of melting point and freezing point for a material is more or less the same value. For example, water turns into ice at 0°C and its melting point is also 0°C.
What is the difference between Melting and Freezing Points?
• Melting point is the temperature at which a solid will go to the liquid state. Freezing point is the point where any liquid will change its state to a solid.
• At the melting point, the entropy of the material is increasing whereas, at the freezing point, it is decreasing.
• Though theoretically, melting point and freezing point temperatures are similar for a given material, practically they vary slightly.