How to judge the melting and boiling point of a substance by looking at the state of the substance

1. Judgment of the melting and boiling point of different crystal types of substances

Atomic Crystals, Ionic Crystals, Molecular Crystals (General). The melting and boiling points of metal crystals have a wide range and a large span. Some are higher than atomic crystals, such as W melting point 3410, which is greater than Si. Some are lower than molecular crystals, such as Hg is liquid at room temperature.

2. Substances of the same crystal type:

Atomic crystals: compare the strength of covalent bonds. The smaller the atomic radius, the shorter the covalent bond, the greater the bond energy, and the ultra-high melting boiling point. Such as diamond, silicon carbide, crystalline silicon.

Ionic crystals: Compare the strength of ionic bonds. The more charge and the smaller the ionic radius of the anion and cation, the stronger the ionic bond and the higher the melting and boiling point. Such as MGO> NaCl.

Molecular crystals: 1) Molecular crystals with similar composition and structure, looking at the intermolecular forces. The greater the relative molecular mass, the greater the intermolecular forces and the higher the melting and boiling point. Such as Hi>HBR>HCl.

2) Molecular crystals with different compositions and structures also look at the intermolecular forces. Generally compare the state under the same conditions. At room temperature, the melting and boiling points of I2, H2O, and O2. Solid I2 is greater than liquid water and greater gas oxygen.

Metal crystals: The radius of the metal cation and the number of free electrons. The smaller the cation radius and the more free electrons there are, the higher the melting and boiling point.

Such as: Li>Na>K>RB>Cs, Al>mg>Na

First determine the type of crystal, the melting and boiling point of atomic crystals is generally the highest, the melting and boiling point of molecular crystals is the lowest, the ionic crystals are in the middle, and some metal crystals are particularly high such as metal tungsten, and some are particularly low, such as mercury. Moreover, according to the state of the substance at room temperature, the solid is the highest, followed by the liquid, and the lowest is the gas!

1) With the increase of atomic number, the melting point of the metal element composed of the element gradually increases, and the melting point of the non-metal element gradually decreases. (The melting point of the subgroup reaches the highest in the VIB group and decreases sequentially).

2) From top to bottom of the elements of the same main group, the melting point of the metal elements composed of the elements gradually decreases, and the melting point of the non-metal elements increases. (Parafamily irregular).

3) For substances with different crystal types, generally speaking: atomic crystals, ionic crystals, molecular crystals, and metal crystals have a wide range of melting points.

4) Atomic crystals: The shorter the bond length and the greater the bond energy between atomic crystals, the more stable the covalent bond is, the higher the melting and boiling point of the substance, and vice versa. Such as: diamond (C—C), silicon carbide (Si—C), crystalline silicon (Si—Si).

5) Ion crystal: The smaller the anion and cation radius and the higher the charge number in the ion crystal, the stronger the ionic bond, the higher the melting and boiling point, and vice versa.

1. Judgment of the melting and boiling point of substances of the same crystal type: generally atomic crystals, ion crystals, and molecular crystals. Metal crystals have different melting and boiling points depending on the type of metal (the same kind of metal has the same melting and boiling point) and metal (except for a few molecules).

2. The atomic radius in the atomic crystal is small, the bond length is short, the bond energy is large, and the melting point is high.

3. In the ion crystal, the more the charge number of the anion and the ion, the smaller the ionic radius, the stronger the inter-ion interaction, and the higher the melt skin spring point. The more the number of valence electrons of the metal atom in the metal crystal, the smaller the atomic radius, the stronger the electrostatic interaction between the metal cation and the free electron, the stronger the metal bond, the higher the melting point, and in general, the more active the metal, the lower the melting point. The greater the intermolecular force in a molecular crystal, the higher the melting point, and the abnormally high melting point of hydrogen bonds.

Answers. 1.There is a certain law in the boiling point of organic and substances, which is summarized as follows: first: the boiling point of homologues is judged, and the boiling point generally increases with the increase of the number of carbon atoms.

2.Second: the boiling point of chain hydrocarbon isomers is judged, generally the more branches, the smaller the boiling point.

3.Third: judging the boiling point of aromatic hydrocarbons, when the side chains are the same, the critical position is greater than the meta position and is greater than the paraposition.

4.Fourth: for the judgment of the boiling point of hydrocarbons with the same number of carbon atoms, alkenes are smaller than alkanes and less alkynes.

5.Fifth: judging the boiling point of the derivatives of aliphatic hydrocarbons with the same carbon atom, the boiling point of the derivatives of olefins is lower than that of similar derivatives of alkanes.

6.Sixth: Comparison of the boiling points of oxygenated derivatives of different types of hydrocarbons, the fatty carboxylic acids with similar relative molecular weight are greater than fatty alcohols and greater than fatty aldehydes.

7.Seventh: Phenol and carboxylic acids and their corresponding salt boiling point comparison, phenol and carboxylic acid are small at the boiling point of the corresponding salt.

In middle school, the following rules are mainly mastered: first, see whether there are hydrogen bonds between molecules, compounds with hydrogen in atoms connected to nitrogen or oxygen (such as acids, alcohols, etc.) contain hydrogen bonds between molecules, and substances with hydrogen bonds have higher melting and boiling points. Second, in the absence of hydrogen bonds, the greater the relative molecular mass, the greater the intermolecular forces, and the higher the melting and boiling point.

Thirdly, when the relative molecular mass is equal, the more branched chains, the lower the melting and boiling point; The more double and triple bonds, the lower the melting and boiling point; The more polar the substance, the higher the melting boiling point. Fourth, the boiling points of benzene homologues decreased sequentially, inter-ordination, and p-fusion.

1. Ionic crystal: the higher the charge number of the ion, the smaller the ionic radius, the higher the melting and boiling point;

2. Molecular crystals: For similar molecular crystals, the larger the formula, the higher the melting and boiling point. There are hydrogen bonds between the molecules of hydrofluoric acid, ammonia and other substances;

3. Atomic crystal: the smaller the bond length, the greater the bond energy, and the higher the melting and boiling point;

4. The state of pressing down at room temperature and constant before constant command, melting point: solid substance is greater than liquid substance; Boiling Point: Liquid substances are greater than gaseous substances.