Melting Point Definition
The melting point of any given substance is the temperature at which the substance experiences melting, or the moment that the substance changes from a solid into a liquid. Some materials, like metals, glass, and other substances which are solid at room temperature have a high melting point. In other words, they require a lot of energy between the atoms in order for the substance to become a liquid.
By contrast, most substance that we experience normally as gases, such as oxygen and carbon dioxide, have a relatively low melting point. Carbon dioxide, in its solid form, is commonly known as dry ice. This material has a melting point of -56 degrees Celsius (-69 degrees Fahrenheit). When exposed to room temperature air, the drastic change that much past the melting point causes sublimation, or direct conversion of carbon dioxide into a gaseous vapor.
The exact melting point of a substance is determined by many factors, which determine how the atoms within a molecule interact and bond with each other. Because of the differences in atoms and how they interact, one substance can have a different melting point compared to a similar molecule with atoms in a different arrangement.
Interactions that Determine Melting Point
There are several interactions studied in chemistry which determine a substance’s melting point. The first of those is temperature, which directly influence the amount of free energy available to all atoms and molecules within a system. Temperature, as we experience it, seems to be heat sources such as the sun or a fire. While heat does come from these sources, overall temperature is really a measurement of how quickly molecules are moving and the energy they have to repel each other.
Consider a thermometer, the device used for measuring the temperature. In side of the thermometer is mercury, as seen in the image below. This chamber of mercury is measured to a specific size, and filled with a certain volume of mercury. The small chamber extends up the thermometer as a narrow tube. As the temperature rises, the mercury gets warmer and has more energy between individual molecules of mercury. As the molecules bounce off of each other, they expand the liquid and it travels up the thermometer.
Mercury makes a perfect metal for thermometers because it has an unusually low melting point for a metal. This means that at most temperatures experienced, mercury will remain a liquid and can accurately measure the temperature. However, mercury has a melting point of -38 degrees Celsius, or around -38 degrees Fahrenheit.
Melting point is not only affected by temperature because temperature is not the only thing that affects the movement and energy of molecules and atoms. Melting point is also heavily affected by pressure, which pushes the molecules together. Thus, if you increase the pressure the melting point also increases. Using this method of increasing the pressure, scientists have been able to solidify typically gaseous substances.
A third thing that influences melting point is the structure of the substances themselves. When you consider substances like water, the melting point is largely determined by the interactions between the molecules themselves. Water is a polar molecule, meaning that different areas of an individual molecule have different electrical attractions. In a solution of water, these molecules interact with each other to create hydrogen bonds, or weak bonds between the hydrogen of one water molecule and the oxygen of the next. This series of interactions stabilizes the entire solution, and lowers the melting point of the solution in general.
Scientists have found many other molecular interactions which influence melting point, such as van der Waals forces, molecular symmetry, and atomic forces with determine the interaction and stability of different substances. All of these small, seemingly minute interactions are important for determining how much energy it takes to get molecules to move from a solid to liquid form. Differences in melting point is the basic reason different substances are found in different phases (solid, liquid, gas) at the same temperature.
Chart of Common Melting Points
|Substance||Melting Point (In Degrees Celsius)|
How is Melting Point Determined?
A simple experimental setup for determining melting point can be set up with a beaker or crucible, a thermometer, and heat source such as a Bunsen burner. A sample of the material being studied is place in the beaker or crucible, heat is applied, and the temperature is observed. At the point when the substance begins to liquefy, the temperature is noted. This procedure should be repeated several times for accuracy.
However, there are much more advanced methods of determining melting point, such as the device below. This device works on the same basic principle, but can more accurately measure the temperature and energy input into the system. Further, scientists can now use computer applications to simulate the melting point of unknown substances, though they are not always correct.