Melting Point

Claimed by Clayton Maike Edited by Jennifer Vo

The melting point of a material is an intensive physical property that indicates the temperature at which the substance transforms from a solid to a liquid or vice versa.

Properties of Matter

On the most basic level, there are two types of properties of matter: chemical and physical properties. Chemical properties are classified as those that change when the substance or material undergoes a chemical reaction involving a fundamental change in the identity of the material. On the other hand, a physical property involves only a change in appearance of the material.

For example, consider the boiling a pot of water to cook a bowl of spaghetti. After sometime, the water begins to boil and form a vapor (i.e. steam). The water that boils off as steam has undergone a physical change in appearance, thus we can consider the boiling point of a substance as a physical property.

Now consider, a loaf of bread accidentally left out after a trip to the grocery store. Over the course of a week or two, mold will form on the bread indicating a chemical change in the identity of this bread molecules. The specific property that lead to this reaction would then be considered a chemical property.

Physical Properties

Physical properties are distinguished into two subgroups: intensive and extensive physical properties. Intensive properties are independent of the quantity of the material present where as extensive properties are not. For example, the density of a material is an intensive property while the mass of a material is an extensive property.

Melting Point

The melting point of a material is the characteristic temperature in which the solid transitions to a liquid under a fixed pressure. In theory, the melting point of a solid should be the same temperature as the freezing point of a liquid. It is difficult to heat the solid above its particular melting point, as the heat energy is used to convert the solid to a liquid. Because of this property, melting points are often used to identify chemical compounds and determine the purity of a sample.

Enthalpy of Fusion

The enthalpy of fusion (the heat of fusion) is the change in enthalpy due to change in heat energy. Enthalpy of fusion is the energy required to transition the solid to the liquid state without changing its temperature. This is because enthalpy of fusion is a latent heat, therefore the temperature does not change in this process. Therefore, the energy is not used to heat the substance, but to break the solid bonds. At the melting point, the change in Gibbs free energy of a substance is zero, but the enthalpy and the entropy of the material are increasing. Melting occurs when the Gibbs free energy of a liquid becomes lower than the energy for the solid of that material.

Clausius-Clapeyron Equation

The melting and boiling point of a substance is dependent upon the pressure. This relationship can be modeled by the Clausius-Clapeyron equation.

It is written as:

[math]\displaystyle{ \frac{dP}{dT} = \frac{PL}{T^2R} }[/math]

Using separation of variables and integrating from [math]\displaystyle{ P_1 }[/math] to [math]\displaystyle{ P_2 }[/math] and [math]\displaystyle{ T_1 }[/math] to [math]\displaystyle{ T_2 }[/math] this equation becomes:

[math]\displaystyle{ \ln\frac{P_1}{P_2} = -\frac{H_F}{R}(\frac{1}{T_1}-\frac{1}{T_2}) }[/math]

where [math]\displaystyle{ H_F }[/math] is equal the enthalpy of fusion, which is equal to the amount of energy that must be taken out or put into the system per mole of material for the phase transformation to occur. R is the gas constant.

Note: This equation should be used only in idealized situations as it does not include the temperature dependence of the heat of fusion. For this reason, there is some inaccuracies involved in this calculation.

Melting Point Depression

It is likely that you or someone you know has, at one point, spread salt over his or her driveway before the onset of a cold front. Somehow, this greatly reduces the amount of ice that forms on the covered surfaces. This helpful winter trick occurs due to a phenomenon know as melting point depression. While a much more in-depth explanation could be given, here is a easy way to think about. When a liquid freezes, the molecules are attempting to orient and pack themselves in a way so as to form a solid. If foreign particles are in the liquid, they will partially block the liquid particles from forming into a solid thereby lowering the freezing point of the liquid. To summarize, the melting point of a material will decrease when a foreign solute is added to the solution.

See [Freezing-point Depression] for a closer look on how this process occurs and the math behind calculating the change in melting point!

Example

Consider the hypothetical element Greconium, which is a liquid at room temperature and has a standard boiling point of 273.15K. If the pressure is increased to 5 atmospheres, what will be the boiling point of Greconium at this elevated pressure? Assume the heat of fusion is constant and equal to [math]\displaystyle{ 45000\frac{J}{mol} }[/math] for this problem.

Using the simplified version of the Clausius-Clapeyron equation derived above, we must solve for [math]\displaystyle{ T_2 }[/math]:

[math]\displaystyle{ \ln\frac{P_1}{P_2} = -\frac{H_F}{R}(\frac{1}{T_1}-\frac{1}{T_2}) }[/math]

Rearranging and solving for [math]\displaystyle{ T_2 }[/math]:

[math]\displaystyle{ T_2 = \frac{1}{\frac{1}{T_1}+\ln(\frac{P_1}{P_2})\frac{R}{H_F}} }[/math]

Plugging in for known variables:

[math]\displaystyle{ T_2 = \frac{1}{\frac{1}{273.15}+\ln(\frac{1}{5})\frac{8.314}{45000}} }[/math]

We find:

[math]\displaystyle{ T_2 = 297.3 K }[/math]

In this case, the increase in pressure causes the boiling point of Greconium to rise to 297.3K.

Note: This problem was meant only to demonstrate how to use the Clausius-Clapeyron equation.

Connectedness

While the concept of a substance's melting point is relatively simple, it has far-reaching implications on many industries. These include the automotive, chemical manufacturing, and chemical storage industries. For example, consider a cold winter night during which ice forms. In order to prevent the fluids inside of your car's engine from freezing, automotive manufacturers created antifreeze, which is a direct application of freezing point depression. In many chemical manufacturing roles, it is extremely important to know both the chemical and physical properties of the materials being handled. The melting point of substance is one of these properties that must be taken into account to maintain a safe working environment.

With respect to the field of chemical engineering, a substance's melting point is deeply routed in the field of thermodynamics especially in relation to the concepts of Gibbs free energy, enthalpy, and entropy. Further investigation into these topics will offer a more scientific explanation of what is occur during phase transformation. Knowledge of the melting point of a substance also allows chemical engineers to create process and reactions used in the separations and manufacture of other chemicals. Even more commonly, the boiling point of a substance will be used in distillation and condensation, which involve the separation of compounds by boiling point.

History

The concept of melting points has been used for thousands of years, although only in the past few centuries have scientists developed an accurate way to pinpoint the temperature at which the phase transformation occurs.

See also

For information on the transition of a substance from a liquid to a gas, see:

Boiling Point

Further reading

See this article from Cal Tech for more information on the:

[Thermodynamic Explanation of Phase Transformations]

References

[UCDavis ChemWiki Properties of Matter]

[UCDavis ChemWiki Clausuis-Clapeyron]

[Galen Lew the Science Dude!]