The comparison of melting and boiling points mainly depends on the interaction forces between the substances. Here are some common ways to compare melting and boiling points:
Determination of the melting and boiling point of substances of different crystal types: atomic crystals > ionic crystals > molecular crystals (general). The melting and boiling point of metal crystals varies greatly, and some are lower than molecular crystals, such as mercury is liquid at room temperature. Some are higher than molecular crystals, such as tungsten melting point above 3000. The melting and boiling point of molecular crystals is also very different, and there are solid ones such as iodine at room temperature and the boiling point is only minus 05 of helium. In atomic crystals, silicon dioxide, diamond, silicon carbide, and metal crystals are more different.
Comparison method of atomic crystals: the smaller the atomic radius, the stronger the covalent bond and the higher the melting and boiling point. Such as diamond> silicon carbide > crystalline silicon.
Methods of comparison of ionic crystals:
The more charge and the smaller the ionic radius, the stronger the ionic bond and the higher the melting and boiling point. For example, kf > kcl > kbr > ki, cao > kcl.
When the difference between the ionic radius and the charge is not large, the electron shell structure of the ion is compared, and the more electron shells, the larger the ionic radius and the lower the melting and boiling point. Such as CSCL < NACL.
Methods of comparison of metal crystals: the more metal cation charges, the smaller the radius, the more free electrons, the stronger the metallic bonds, and the higher the melting and boiling point. For example, the melting point and boiling point are the strength of the metal bond of the metal crystal, and the radius and charge number of the metal cation. The particles present in metal crystals are metal cations and free electrons, and the more charges and the larger the number of charges, the stronger the metal bonds, and the higher the melting and boiling point of the metal. For example, in alloys formed by the same metal, the melting and boiling point of the metal generally increases with the increase of the number of metal cation charges in the alloy; The smaller the radius of the metal cation, the more free electrons, the stronger the metallic bond, and the higher the melting and boiling point of the metal. For example, the melting and boiling point of a metal crystal: metal (except mercury, cesium) > mercury > cesium.
Comparative Methods of Molecular Crystals:
When hydrogen bonds exist between molecules, the melting and boiling point of a substance tends to be abnormally high. Such as HF> Hi> HBR> HCl; h2o > h2te > h2se > h2s。
When intramolecular hydrogen bonds are formed, the melting and boiling point decreases. Such as o-hydroxybenzoic acid > paraben.
For molecules with similar composition and structure, the greater the relative molecular mass, the stronger the intermolecular forces, and the higher the melting and boiling point of the substance. For example, the melting and boiling point of halogen element: F2 < Cl2 < Br2 < I2.
When there is no active hydrogen in the molecule, the melting and boiling point is higher when the relative molecular mass is similar, and if there is a polar bond in the molecule. Such as melting boiling point: CO> N2, CH3OH > CH3CH3.
The boiling point of substances that can form intermolecular hydrogen bonds is higher (when hydrogen bonds are not considered, the stronger the non-metallicity of the elements forming hydrogen bonds, the stronger the hydrogen bonds formed, and the higher the boiling point; However, the boiling point decreases when hydrogen bonds are formed within the molecule, for example, the boiling point of o-hydroxybenzoic acid is less than that of paraben).
To sum up, the comparison of melting and boiling points needs to comprehensively consider many factors such as the crystal type, ionic radius, charge number, and intermolecular forces of the substance.