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Information hidden in the melting point of matter
- Nov 26, 2018 -

1. Judging physical properties


The melting point of the applied substance can determine the state of the substance at a certain temperature, determine the difficulty of liquefying the gas, and the volatility of the liquid substance.


The ambient temperature is lower than the melting point of the substance, the substance is in a solid state; the ambient temperature is higher than the melting point, lower than the boiling point, and the substance is liquid; the ambient temperature is higher than the boiling point, and the substance is the boiling point. In addition, some substances can sublimate, and sublimation can be carried out at any temperature. The higher the temperature, the faster the rate of sublimation.


The lower the boiling point of the liquid substance, the more volatile it is. Such as liquid bromine (boiling point 58.8 ° C), benzene (boiling point 80.1 ° C) is volatile, while concentrated sulfuric acid (boiling point 338 ° C) is difficult to volatilize.


2. Inferring the structure of matter


Different substances have different melting points. For organic substances, different isomers have different melting points (excluding the melting point and boiling point of optical isomers may be the same). Thus different melting points and boiling points can indicate different spatial structures. The two monochlorinated compounds of butane, 1-chlorobutane and 2-chlorobutane, have different boiling points.


For compounds, the ionic compound has a higher melting point; while the general covalent compound has a lower melting point (such as carbon dioxide), and very few covalent compounds have a very high melting point (such as silica). Therefore, the type of the compound can be roughly determined depending on the melting point and boiling point of the substance. Of course, using the boiling point data can also determine whether there is a hydrogen bond in the substance.


3. Judging the method of substance separation


Separation of several different boiling point volatile components can be carried out by fractionation. The mixture is first distilled at the lowest boiling point until the distillate is collected as a component before the vapor temperature rises. An increase in vapor temperature indicates that the next higher boiling component in the mixture begins to distill. This component is then collected and collected. The petroleum industry utilizes the different boiling points of the various components in petroleum and separates the components by controlling the temperature of the heating.


For the mixed gas, it can be separated by first cooling and then increasing the temperature. The industrial method of oxygen, using the boiling point of nitrogen in the air is lower than the boiling point of oxygen, first pressurizing the air to a liquid state, and then heating, because the boiling point of liquid nitrogen (-196 ° C) is higher than the boiling point of oxygen (-183 ° C Low, nitrogen first evaporates from liquid air, leaving behind liquid oxygen.


4. Explain the direction of the reaction


The esterification reaction is a reversible reaction, and the reactants cannot be completely converted into a product. Therefore, in the preparation of ethyl acetate, using the boiling point of ethyl acetate (77 ° C) is lower than the boiling point of acetic acid (118 ° C) and ethanol (78.3 ° C), the product ethyl acetate is distilled off in time to facilitate the equilibrium to form ethyl acetate. The direction of movement is beneficial to increase the yield of ethyl acetate.


Another example is the industrial preparation of potassium, Na+KCl=NaCl+K. It is known that the boiling point of Na (882.9 °C) is higher than the boiling point of K (774 °C), so it can be controlled by the temperature K, Na is liquid, and the chemical equilibrium is applied. Principle, in the reaction, the potassium vapor is continuously removed from the reaction system, and the equilibrium moves to the right, and the reaction occurs.


5. Choose the right raw materials


Electrolytic molten metal compounds are required for the preparation of active metals, and metal compounds having a lower melting point are generally selected. Since the melting point of magnesium oxide (2800 ° C) is much higher than the melting point of magnesium chloride (712 ° C), when industrially smelting Mg by electrolysis, magnesium oxide is not selected as a raw material but magnesium chloride is selected as a raw material.


Since molten aluminum chloride is poor in electrical conductivity, metal aluminum is industrially smelted with electrolytic alumina. However, since the melting point of alumina is very high (2045 ° C), the addition of cryolite can cause alumina to dissolve in cryolite at about 1000 ° C.


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