The melting point is the temperature at which a solid substance transforms into its liquid phase. While many factors can influence the melting point of a substance, pressure is one of the most significant ones, therefore, understanding how pressure affects melting point can help us better comprehend the behavior of the substance under different environments.
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Effect of Pressure on Melting Point
The melting point of a substance can be affected by several factors, such as its molecular and crystalline structure, molecular weight, purity, and pressure of the surrounding environment.
When a solid substance melts, its molecules or atoms need to overcome the intermolecular forces that keep them in a solid lattice structure. Thus, the pressure can influence the melting point of a substance by changing the spacing between molecules or atoms in the lattice.
According to Le Chatelier’s principle, if a system is disturbed by a change in pressure, temperature, or concentration, the system will adjust itself to counteract that change and restore equilibrium. Therefore, the effect of pressure on the melting point depends on the nature of the substance and its behavior during the melting process.
When a solid substance turns into a liquid, it can either contract and become denser, or expand and become less dense. In most substances, the latter case occurs.
As a substance melts, its particles gain energy and move more freely, leading to volume expansion. Hence, the spacing between molecules or atoms in the lattice increases, making it easier for them to overcome the intermolecular forces holding them in place. This is common in most materials such as metals, glass, and plastics.
Consequently, higher pressure increases the melting point of these substances. In accordance to Le Chatelier’s principle, an increase in pressure counteracts the volume expansion caused by melting, thus, raising the melting point.
On the other hand, some substances, like ice, bismuth, and antimony, contract when melted. This is because of the unique properties of their atomic and molecular structure.
For example, in the case of water, the molecules form a hexagonal crystal structure in the solid state, or ice, which creates more space between the molecules than when it is in its liquid form. As the water molecules gain energy and move more freely, they become more tightly packed, resulting in the contraction of solid water when it melts into a liquid.
In this case, higher pressure reduces the melting point. This is because increasing the pressure encourages the contraction of the substance due to melting, thus, reducing the melting point.
Melting Point Behavior of Most Solid Substances
The relationship between the pressure and melting point of a substance is called the melting curve and is shown on the substance’s pressure-temperature phase diagram.
To give a sample illustration of how the melting point of most solid substances behaves with respect to pressure, the phase diagrams of titanium and carbon dioxide are shown below.
As shown in the phase diagrams of titanium and carbon dioxide, the typical slope of the melting curve is generally positive. This means that as pressure increases, so does the melting point. However, it is important to note that this boundary isn’t always a straight line; the melting point usually behaves nonlinearly with respect to pressure.
Melting Point Behavior of Water
Water and some other substances, such as antimony and bismuth, behave differently. The phase diagram of water illustrates that the boundary between its solid and liquid phases slopes downwards towards the triple point. This means that for water and some other rare substances, as pressure increases, the melting point decreases.
It is important to note that the melting point of a substance is the temperature at which its solid and liquid phases coexist in equilibrium. Hence, strictly speaking, the melting point is not an exact temperature value, but rather a range. This is because the melting process gradually occurs over a range of temperatures as the substance transitions from a solid to a liquid state.
This is particularly true for impure substances, as their melting point range is typically lower and wider than that of pure substances. The presence of impurities disrupts the orderly arrangement of molecules in the solid phase, making it easier for the substance to melt at a lower temperature and over a wider temperature range.
Melting Point Calculation
Derived from the Clapeyron equation, the relationship between the change in pressure and melting point can be equated as:
- ΔT = change in melting point temperature [K]
- Tm = standard melting point temperature [K]
- ΔP = change in pressure [Pa]
- ΔfusVi = change in molar volume [m3/mol]
- ΔfusHi = enthalpy of fusion [J/mol]
Note that the equation above assumes a constant change in molar volume and transition enthalpy. This is because, in the case of solid-liquid equilibrium, the change in molar volume for melting is not significantly impacted by pressure.
Similarly, even if very large pressures are applied, the change in melting point is relatively small. Hence, to a great extent, the transition enthalpy remains constant, except for extreme geological situations.
Also, note that the change in molar volume during melting for most substances is positive— that is, the molar volume of the liquid phase is larger than the solid phase. For select substances, like water, bismuth, and antimony, this becomes negative, resulting in a negatively sloped melting curve.
Importance of Considering Pressure in Calculating Melting Point
The impact of pressure on the melting point temperature of a substance is significant. It is essential to take its influence into account when calculating the melting point, especially in applications that require varying pressures.
For instance, in many industrial processes that involve melting materials, it is crucial to consider the impact of pressure to accurately design and optimize the operation of these processes. In geology, the minerals’ melting points in the Earth’s mantle and crust are influenced by the high pressures to which they are subjected. Understanding the correlation between pressure and melting points is vital for studying the behavior of these materials in geologic processes such as magma formation and volcanic eruptions.
In food science, pressure conditions during cooking and food processing can affect the melting point of fats and oils. Knowing how pressure affects melting points can help in developing new cooking and food processing methods that yield the desired properties in foods.
Moreover, because of the fact that it is virtually impossible to heat solids above their melting points and pure solids tend to melt over a narrow temperature range, melting points are commonly used to identify compounds. For example, by determining the melting point of a small sample, we can differentiate between glucose, fructose, and sucrose which have different melting point temperatures of 150°C, 103-105°C, and 185-186°C, respectively.
Lastly, measuring the melting point of a solid provides information about the substance’s purity. Pure, crystalline solids have a distinct melting point. The melting point of pure substances is so precise that they can be measured to 0.1°C.