Knowing the melting point of a substance is a requirement when working in the laboratory. The melting point is the temperature and pressure condition in which a substance is in equilibrium between a solid and a liquid state. This condition is also called thermodynamic equilibrium. Because the pressure considered is atmospheric pressure and is constant, the value to be considered remains that of temperature.
When a substance melts, it absorbs a certain amount of heat. This heat loosens, to the point of breaking, the bonds between molecules. The material, once liquefied, continues to increase in heat. In reality, however, most materials, such as glass, do not have an exact liquefaction point. They begin to liquefy at a certain temperature and gradually become softer and softer until they liquefy completely. Only crystalline solids have this characteristic.
How to determine the melting point
In order to determine the melting temperature of a certain material, a special apparatus must be used. In chemical laboratories, to calculate the melting point temperature of a substance, a sample is heated and the temperature at which melting is observed is determined. The sample is inserted into a capillary tube so that the tube is filled to 5 mm. The tube is heated until the temperature is reached. The temperature must be observed empirically by the operator. The temperature can be detected automatically when the melt is transparent, i.e. when the transmittance exceeds 90% as a conventionally established value.
Sophisticated instruments are used to measure the temperature.
Sophisticated and manual instruments
Today, there are very sophisticated instruments to accurately determine the melting point automatically. With the possibility of working on two samples, it is possible to make comparisons and average values between the two. There are still glass instruments for manual calculation of the melting point, however, for those who trust their experience more than technology.
At this point, we could address the tma of melting point determination from an educational point of view.
Chemistry lab: the melting point
In every chemistry laboratory, the melting point is one of the most important operational concepts. In particular, by means of the melting temperature it is possible to identify substances with absolute certainty and subsequently determine their purity. Almost all solid substances, when heated, melt. This means that they go from the solid state to the liquid state.
Melting of pure substances
Only for a pure substance, however, can the relative melting point be determined. With reference to pure substances, the melting temperature can be defined as the temperature at which the substance is in both the solid and liquid states at a pressure of 101.325 KPa. To define a substance as 'pure', the melting range must be between 0.5 and 1°C, remembering that the melting range is the difference between the temperature at which the substance begins to melt and the temperature at which, in its practical state, it is fully melted. The higher the melting interval, the lower the melting temperature, meaning that, in this case, you are dealing with an impure substance.
Equipment for determining the melting point
In the laboratory, there is no single apparatus for melting point determination. In fact, two different types of apparatus can be used.
analogue instrument or Thiele tube
The first is of the analogue type and is known as Thiele's Tube, named after the German chemist who invented it. It is a relatively simple instrument that is characterised by indirect heating carried out manually with a bunsen burner and a thermometer that is also analogue. The Thiele tube consists of a test tube with a V-shaped side channel in which oil is contained. Using a manual flame, this side channel is heated. The oil is heated very evenly due to the special shape of the Thiele Tube. At this point, to determine the melting point, a capillary is attached to the thermometer and immersed in the oil. In this way, the substance heats up gradually and the precise melting point can be determined.
There are also digital instruments, which are much more modern and have a much lower level of uncertainty. These are devices that use electric heating and are equipped with LCD displays that act as measuring instruments.
Known and unknown compounds
Determining the melting point of a known compound
How do you determine the melting point of a known compound using the Thiele Tube? You must start by adjusting the flame of the bunsen burner to medium heights and then placing it under the V-shaped section of the Thiele Tube. Since it is a known compound, its melting temperature is known, so it is necessary to heat the oil containing it in the instrument to a temperature about 10-20 degrees lower than the known value. We can continue by lowering the intensity of the flame and observing the initial and final melting temperatures (fully melted compound). One can then proceed by extinguishing the flame and immersing the Thiele tube in water and ice, taking care not to allow any liquid to penetrate into it. The thermometer and capillary can then be removed and a second and, if necessary, third measurement can be taken to confirm the data collected so far.
Determination of the melting point of an unknown compound
A compound is unknown when its name is not known. To determine its melting point, regardless of the instrument used, one must start by heating the solid substance quickly until it melts completely. You will have a rough idea of the initial melting point from which you can start to determine the melting point as if it were a known compound by taking several measurements. It is always advisable to take at least two.