Heating a solid will increase the energy of its atoms or ions until the energy levels become high enough to partially disrupt the forces holding the atoms or ions in their fixed position. At this point, the solid start to transition into a liquid state called melting. The melting point is the temperature at which the solid and liquid phases are in equilibrium. The melting point depends on the attractive forces between the atoms or ions in the solid - the higher the attractive forces, the higher the melting point. Ionic crystals typically have a higher melting point than molecular compounds due to their desirable forces within the three-dimensional crystal lattice.
Image from Labster's Melting Point Analysis: Pure or impure? Virtual Lab,
Basic knowledge is important to understand every topic. To understand melting point analysis, the students must know the melting points of the studying compounds. The 2nd question is how we determine the purity of the compound by melting point analysis. The answer is if there is a single compound, it will melt at a specific temperature. Simply if the compound under observation melts completely at a single point, it is pure.
If the compound melts at a single point, it means that it is pure. When there is more than one compound in a mixture, they definitely have different melting points. If the mixture does not completely melt at a single melting point, another compound is definitely in the mixture. But students face more difficulties when the melting point of the compounds present have nearly the same melting points. So this requires close attention and experience; they have to focus on the mixture and note whether there is a continuity in melting. If the mixture is melting in parts means that it contains an impurity.
Theoretical understanding of melting point analysis is tricky because it involves phase changes. If the compound is in powder form and, after heating, it melts into liquid, the students have to monitor this phase change, which is impossible to understand theoretically. Students readily understand the melting point analysis process practically or by video graphics. If you cannot perform the practical, use video graphics so that students understand it easily.
Before going to the melting points must review the following basic terms:
The melting point range is the span of temperature from the point when the solid starts melting to the point at which the entire solid is in a liquid state.
The heating curve is a graph of temperature versus the amount of heat added. The heating curve shows the changes in temperature that result as the substance absorbs an increasing amount of heat. We can also observe that the temperature remains constant while the change of state is in progress. The temperature plateaus because all the heat energy is used to break the intermolecular forces.
Intermolecular forces are the attractions between molecules that determine many of the physical properties of a substance. The intermolecular forces are generally a lot weaker than molecular bonding. For example, it requires 17 kJ/mol to overcome the intermolecular forces of liquid HCl to gaseous HCl. However, it requires much more energy, 431 kJ/mol, to break the covalent bonds between the hydrogen and chlorine atoms.
Specific heat capacity (J/g oC) is the amount of heat required to raise the temperature of one gram of a substance by 1°C . Specific heat capacity only depends on the kind of substance. For example, 100 g of gold will have the same specific heat capacity as 1 kg of gold. Water has a relatively high specific heat (~4.2 J/g °C for liquid, 2.09 J/g °C for solid ) compare to metals (mostly less than 1 J/g °C). Therefore, it's easier to heat metal than water.
Latent heat (kJ/mol) is the change of total heat energy involved in phase changes. Three types of latent heat are involved: latent heat of fusion, latent heat of evaporation, and latent heat of sublimation.
Latent heat of fusion or enthalpy of fusion (Δ_H_fus) is the amount of heat required to change one mole of a substance from the solid state to the liquid state.
The latent heat of evaporation or enthalpy of evaporation (Δ_H_vap) is the amount of heat required to change one mole of a substance from the liquid state to the gas state.
The enthalpy of sublimation can be estimated as the sum of its enthalpy of fusion and vaporization.
It has become common lab practice to measure the melting point of the solid product after completing organic syntheses. But how can it help to assess the purity of your product?
It is simple. In preparation for your organic synthesis, you will need to research melting point data recorded for the pure sample of the compound you are trying to synthesize. This is available in many textbooks, published scientific articles, and online. By researching this, you can simply compare this melting point range with that measured for your solid sample. This method works since different solids have their own characteristic properties. If the molecular structure of your sample differs from that of the pure sample, your melting point data will be affected. To find out more, you can access this theory page about the melting points of solids.
It is important to understand how the melting point machine actually works. This can be broken apart to identify the machine's key functional parts that allow it to simultaneously hold and heat the sample steadily while informing the user of the current temperature within the machine. The four key parts of the machine are the heating chamber, the capillary tube, the magnifying glass, and the temperature display.
The tube is inserted in a heating chamber or an oil bath, where the temperature can be raised through a control pane. Old instruments also present a mercury thermometer, while more modern ones show the temperature on display. These modern machines also allow setting a heat gradient, which is how much the temperature should rise in a determined time period (units/minute).
The capillary tube acts as the container of the sample. The capillaries are made of glass, as it’s vital that the operator can see through them and the changes of phase in the sample. Furthermore, glass has good resistance to heat.
It is imperative to have a way to measure the temperature and monitor it. In the machines, a useful part is a screen displaying the current temperature inside the heating chamber at any given time, so we can determine the exact temperature range at which the solid melts.
Lastly, another important part of the machine is the magnifying lens. This acts as either a visible window or an eyepiece to look through so that you get a clear zoomed-in view of the melting process. This is important as the tube is very thin, and there will only be a small amount of sample inside.
This technique can be used to determine the purity of a solid. If the melting point range of a pure solid is known, we can estimate the purity of our impure solid as the difference in the melting point compared to the pure solid. An impure solid melts over a wide range of temperatures and at a temperature lower than that of a pure solid. The closer the impure solid's melting point range gets to the pure solid's melting point, the fewer impurities it will contain.
To perform a melting point range in a solid, we should follow the following steps:
Grind your crystals until you have a fine powder
Tap a capillary tube on top of your sample to load it, and then tap it against the workbench to make it go to the other end
Insert the melting capillary tube into the melting point apparatus and turn it on
Determine the temperature at which the solid starts melting
Determine the temperature at which the solid is completely melted
If, at the end of the protocol, the melting point range of the impure solid is below the melting point of the pure solid, it means that we still have impurities in our solid.
Figure 1. Melting Point Range protocol (Image source Labster theory).
Moistening Point: The moistening point is the initial stage of melting a solid. There is no liquid present at this stage.
Sintering Point: At this stage, the loose solid becomes more compact and begins to melt but not to the point of complete liquefaction.
Collapse Point: The point where the melting starts. The temperature at this point should be the beginning of your melting point range.
Meniscus Point: The lowest temperature at which enough liquid is present to form a meniscus (a curve where the liquid adheres to the walls of the tube).
Clear Point: The lowest temperature at which all the solid has melted, and the only liquid remains in the tube.
Figure 2: Stages of melting (Image source Labster theory).
There are also many synthesis-related factors, such as temperatures used, solvents, proper drying, and filtration, that will affect the purity of your final product. Even though preparing a sample for a melting point analysis is not hard, there are some considerations to keep in mind to avoid having biased measurements:
The sample has to be dry, as the presence of water will create condensation and influence the reading of the melting point.
The sample has to be well-packed inside the tube. This is usually achieved by tapping the tube with the sample on the working bench. Remember to do it lightly, as the tube is made of glass, be gentle.
Only 1-3 mm of the sample is required in the tube, and if more samples are analyzed, it’s good practice to always use the same amount. Taller samples take extra heat to completely melt and usually display larger melting ranges than their shorter counterparts (and thus are less precise in their readout)
The open ends of a sample capillary tube must be sealed for crystals that are hygroscopic (to avoid moisture uptake) or that sublimate at high temperatures (to avoid sample loss during testing.
Ensure that the apparatus is switched on and is pre-heated to near the anticipated starting temp of the sample.
With technological advances, it is much easier to explain complex and challenging processes with the help of simulations. Now, you can simulate experiments without the need for any valuable equipment. In this regard, you can take help from Labster’s virtual lab simulations. These simulations engage students through interactive learning scenarios. Students dive into a 3D world, where they visually learn and apply their concepts to solving real-life problems.
In Labster’s interactive Melting Point Analysis: Pure or impure? Virtual Lab, you will be introduced to the melting point analysis technique and apply it to organic syntheses you would carry out in the lab. Uncover one of the simplest and most informative techniques for indicating substance purity and learn how to determine whether you synthesized a pure substance using the recorded data and graphs.
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