Melting Point

Claimed by Clayton Maike Edited by Jennifer Vo

The melting point of a material is the characteristic temperature in which the solid transitions to a liquid under a fixed pressure.

Properties of Matter

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. A physical property can be observed or measured without altering the composition 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 dependent. 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, as such, 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

Melting-point depression is the lowering of a material’s melting point by adding solutes. A mixture containing solutes melts at a lower temperature and a larger temperature range than a pure compound. Added solutes disrupt the crystalline structure of a solid, so that a smaller amount of energy is required to break the bonds of a solid.

An example of the phenomenon is salting one's driveway in the winter.

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.The earliest recorded discovery of a melting-point apparatus is attributed to a work published in 1823 by French chemist, Michel Chevreul, who used melting points in organic chemistry to characterize various fatty acids, waxes, and oils. However, this technique was not well known until 1832, when a famous paper was published by German scientists, Justus von Liebig and Friedrich Wöhler. The paper detailed the chemistry of the benzoyl radical and reported the melting point of benzamide. A year following the paper, Liebig reported the boiling point of liquid acetal and began the practice of reporting the physical properties until the procedure of reporting the compounds physical properties became standard. Lieberg’s collaboration with Wöhler regarding organic compound melting points later led to the formulation of the concept of isomerism by Berzelius in 1831.

By the 1890s, new apparatuses were built to increase the accuracy and speed of recording melting/boiling points. Most of these devices were composed of a melting point capillary attached to the stem of a thermometer which was suspended in a long-necked, round-bottom flask filled a liquid with a high boiling point. These apparatuses were heated at the bottom of the flask using a Bunsen burner.

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!]