Melting points are used to determine the purity or impurity of a substance or a compound. For instance, when the heat is applied to a solid substance, it will reach the melting point with gradual heating. When excess heat is provided, the solid will convert into liquid without any temperature change.
Melting point determination of different elements is tested to understand the purity of the object. An extremely pure substance will have a melting range of one or two degrees. Impure substances have a much higher range and tend to depress and broaden the melting range, which is higher and smaller for the original, impure one. The melting occurs at a certain temperature change, but it depends on the structure of the molecule. For that very reason, different compounds have different melting properties.
In the melting point determination of organic compounds, these compounds are defined as a large set of chemical compounds where more than one atom is covalently linked with other atoms such as hydrogen, oxygen, and nitrogen. The melting point determination of organic compounds can be done by doing a small experiment.
A small experiment can be conducted to determine the melting point determination of organic compounds. One such is given below.
Thiele tube method – for this experiment, a tube known as Thiele tube is used, which is designed to contain oil and experiment with heat. A capillary, a thermometer, and a bunsen burner are required to complete the experiment a well. The substance whose melting point will be observed is placed in the capillary tube, and a rubber band is tied around it. Ensure that the rubber band is not getting dipped into the oil as with the rise of temperature; the rubber will tend to melt.
The Thiele tube is designed so that the heat that is being emitted from the bunsen burner is evenly distributed. During the experiment, the rate of the temperature is carefully controlled. In and around the melting point of the substance, the heating will be slow so that the rate of temperature increase is not faster than the ability of the heat to be transferred to the sample being observed. The thermometer bulb and the capillary tube sample must be at thermal equilibrium at the melting point.
The factors that affect the melting point of organic compounds are as follows:
The Size of the Molecule- Melting point determination of organic compounds helps to identify the organic compound properties, both physical and chemical. The structure of the molecule has a big role to play. The tighter the molecules are packed, the higher will be the melting point. If the structure of the molecules is loose, then the melting point would be lower.
For instance, in Isopentane, the molecules are loosely packed, so the melting point is lower. Whereas, in the case of symmetrical neopentane molecules, the melting point is higher. Another point is if all the other factors are equal, smaller molecules' melting point will be lower. For instance, ethanol melts at -114.1 degrees, while the bigger ethyl cellulose molecule's melting point is 151 degrees Celsius.
Force of Attraction- Another factor that influences the melting point is the force of attraction. If the force of attraction is strong among the molecules, then the organic compound's melting point will be higher. For this reason, the ionic compounds have a higher melting point due to its electrostatic force holding the ions together in a strong bond. The presence of polarity or hydrogen compounds may also lead to a great attraction force, which results in higher melting points. A weaker bond persists between butane molecules, so the melting point is lower.
Impurities- The melting point range of pure solids is quite narrow and usually ranges from one to two degrees, known as the sharp melting point. In the case of impure substances or elements, it can melt over lower temperatures and at a wider temperature. For instance, a pure organic crystal has uniform molecules, which is jam-packed together. However, the crystals are impure when they occur in a mixture of two different organic molecules because they don't fit together well. Thus, It takes more heat to melt the pure structure.
The melting point determination of organic compounds helps people understand the physical and chemical properties of the substance. Many different factors affect the melting point of any substance, such as the force of attraction, impurities present in the substance, and the molecules' size and structure.
1. Through what another well-known method, can we understand the melting point of a certain substance?
There are various experiments through which the melting point of a certain substance can be determined; one such method is the capillary method. The capillary method uses similar equipment like the Thiele melting point method, such as a thermometer, a capillary tube, Bunsen burner, and eyepiece through which it is observed. While the experiment is going, the heat needs to be controlled at proper intervals, and the sample or the substance is observed through the eyepiece. Record the temperature on the thermometer when the sample starts to melt and record.
2. Why do different solids have different melting points?
Different substances have different melting points due to their structure, composition, the force of attraction between the molecules of the organic compound, and the level of impure particles present in the compound. If the molecules' structure is tightly packed, then the heating range will be higher than for the loosely packed compounds. Similarly, if the force of attraction is greater between the compound molecules, the heating range will be higher.