How to judge the melting point of a substance, how to judge the melting point of a substance?

To judge the melting and boiling point of a substance, first look at the crystal type.

1. If the crystal shape is different, the atomic crystal is larger than the ionic crystal and larger than the molecular crystal (the melting and boiling point of the metal crystal is very different, there are particularly high such as tungsten, and there are also particularly low such as mercury, so the comparison with the three can not have a fixed law, and it is generally necessary to analyze it specifically).

2. If the crystal shape is the same, the strength of the interaction between ions inside the crystal is compared, and the stronger the interaction, the higher the melting and boiling point.

1) The strength of the ionic bond is generally the larger the ionic radius and the more charged the ionic bond is, the stronger the ionic bond and the higher the melting and boiling point.

2) The strength of the covalent bond of the atomic crystal is generally the stronger the non-metallicity and the smaller the radius, the stronger the covalent bond and the higher the melting and boiling point. For example, diamond has a higher melting and boiling point than crystalline silicon, because C is more non-metallic than Si and has a small atomic radius, so the carbon-carbon covalent bond is stronger than the silicon-silicon covalent bond.

3) Molecular crystals look at the strength of intermolecular forces, for substances with similar composition and structure (generally elementals, compounds or homologues of the same group of elements), the larger the relative molecular mass, the stronger the intermolecular force, and the higher the melting and boiling point.

4) The metal crystal depends on the strength of the metal bond, the metal ion radius is small, the number of charges is large, the metal bond is strong, and the melting and boiling point is high.

For the comparison of the melting and boiling points of the elemental elements of the same group in the periodic table, according to the above law, such as halogen, oxygen and nitrogen elemental elements are molecular crystals, the relative molecular mass increases from top to bottom, the intermolecular force increases, and the melting and boiling point increases; Alkali metals are all metal crystals, and the ionic radius increases from top to bottom, the metal bond weakens, and the melting and boiling point decreases.

As for the change with the oxidation or reduction strength, it is with the change of metallic and non-metallic properties, that is, the elemental oxidation of halogens, oxygen group elements and nitrogen group elements weakens from top to bottom, and the melting and boiling point increases; The reduction of alkali metals from top to bottom is enhanced, and the melting and boiling point is reduced.

First of all, the specific melting point of a substance cannot be judged!

It can only be judged by the crystalline form of the substance!

1. The melting and boiling points of the main group elements are seen from the periodic table.

The melting point of the same main group element is basically lower and lower as the lower the metal. The melting and boiling points of non-metallic elemental elements are gradually higher. However, the carbon group elements are special, that is, the lower the melting point of C, SI, GE, and Sn downwards, which is similar to the metal group. Also, the melting point of gallium in group A is lower than that of indium and thallium, and the melting point of tin in group A is lower than that of lead.

2. From the crystal type, the law of melting and boiling point is seen.

The melting and boiling points of atomic crystals are higher than those of ionic crystals and higher than those of molecular crystals. Elemental metals and alloys belong to metal crystals, in which the proportion of high melting and boiling points is large (but there are also low).

In atomic crystals, the shorter the covalent bond between the bonding elements, the greater the bond energy, the higher the melting point. When judging, the bond length and bond energy can be deduced from the atomic radius and then compared.

Principles of Physics

Melting point is the temperature at which a solid transforms (melts) its state of matter from a solid state to a liquid state, abbreviated as. The melting point of a DNA molecule can generally be expressed in terms of TM. The temperature at which the opposite action takes place (i.e., from a liquid to a solid state) is called the freezing point.

Unlike boiling point, melting point is less affected by pressure. In most cases, the melting point of an object is equal to the freezing point.

The melting point is essentially the temperature at which the solid and liquid phases of the substance can coexist and be in equilibrium, taking ice melting into water as an example, the melting point of ice at an atmospheric pressure is 0, and when the temperature is 0, ice and water can coexist, if there is no heat exchange with the outside world, the state of ice and water coexistence can remain stable for a long time.

The above content reference: Encyclopedia - Melting Point.

To determine the melting point of a substance, there are several ways to determine it:

1. Judgment of the melting and boiling point of different crystal types of substances: atomic crystals, ion crystals, and 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.

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.

The law of judging the high and low melting point is as follows:1. In atomic crystals, the shorter the bond length of the covalent bond, the higher the melting point is usually high.

2. When the type of elements and the number of carbon atoms in the reflex molecule are the same, the melting and boiling points of substances with unsaturated bonds in the molecule are lower.

3. The melting and boiling points of molecular crystals are related to the intermolecular forces.

4. Atomic crystal: the smaller the radius, the shorter the bond length, and the larger the bond energy, the higher the melting and boiling points.

Comparison of melting and boiling pointsGenerally speaking, the melting and boiling point: atomic crystals, ion crystals, molecular crystals, and metal crystals depend on the situation, some are very high, and some are very low. If there is no melting and boiling point between molecular crystals, first judge whether there is a hydrogen bond, and the melting and boiling point of the hydrogen bond is high, such as H2O, NH3, etc., if there is no hydrogen bond, then compare the van der Waals force (intermolecular force).

If the structure is similar and the relative molecular weight is larger, the van der Waals force is greater and the melting and boiling point is higher.

Determination of melting point.

1. Purpose: 1. What is the melting point? Understand the principle and application of melting point determination.

2. Master the capillary melting point determination method.

comprehension and textbook reflection questions. WiseCode.

2. Principle and application.

1. Melting point: When the crystal is heated to a certain temperature, it will then change from a solid state to a liquid state, and the temperature at this time can be regarded as the substance.

The melting point of the substance. Melting point range (melting distance, initial melting, total melting), mixed melting point.

2. Application: a. Determination of the melting point of solid compounds. b. Identify the purity of organic compounds.

c. Identify whether compounds with the same melting point are the same compound. d. Calibrate the thermometer.

3. Determination method:

1. Capillary determination. 2. Microscope hot plate determination method.

3. Automatic melting point determination method (digital melting point tester).

4. Capillary melting point determination method.

1. Device: hot bath, thermometer, melting point tube assembly. (Punching of rubber stoppers and installation of melting point measuring devices).

2. Measurement operation: (demonstration).

a Drying and grinding of samples.

b Loading (cut the middle of the melting point tube with a closed length of about 10 12 cm at the two ends, insert one end of the opening into the sample powder, the height of the sample is about 3 4 mm, and the sample height is 2 3 mm after inverting and filling).

c. Attach the capillary tube containing the sample to the thermometer (and the trapped sample part is in the middle of the mercury ball).

d Put it in a hot bath (thermometer mercury.

The ball is in the middle of the hot bath, and the mercury ball cannot touch the bottom of the bottle or leave the hot bath surface).

e Heating and temperature control (the temperature can be quickly raised at the beginning, and when it is close to the melting point of 10, the temperature rise speed is controlled for 1 2 min.)

f Observe the melting point (initial melting: the solid shrinks, when the sample begins to collapse and the liquid phase appears, it is the initial melting; Total melting: When the solid completely disappears into a transparent liquid, it is fully melted).

g Record the results (melting point range, i.e. temperature from the beginning to the full melt).

h There should be more than two replicates of the data (usually not averaged, the second time should use the melting point tube of the newly filled sample, and the bath temperature should be more than 20 below the melting point before putting it in.)

5. Problems that must be paid attention to in measuring the melting point:

1. Instrument factors: a thermometer should be calibrated; b The melting point tube should be clean and the wall of the tube should be thin.

2. Operation factors: a sample must be dry and finely ground and packed tightly; b. Strictly control the heating rate and observe accurately.

6. Introduction to the digital melting point tester (the melting point can be measured once by using the instrument).

7. Contents: 1. Known substances: cinnamic acid.

urea, mixture.

2. Unknowns: a, b, c (salicylic acid.

159, diphenylamine.

53, acetanilide 114).

3. Students can use an automatic melting point meter to determine the melting point of a compound (once).

8. Precautions:

precautions for using a thermometer; Remember the instructions for using the automatic melting point meter and operate it correctly.

<> judge the melting point of a substance first by looking at the crystal type. If the crystal form is different, the atomic crystal is larger than the ionic crystal and the molecular crystal is larger than the molecular crystal. If the crystal form is the same, the strength of the interaction between the ions inside the crystal is compared, and the stronger the interaction, the higher the melting point.

1. Look at the ion bond for the ion crystal.

Generally, the larger the ionic radius and the greater the number of charges, the stronger the ionic bond and the higher the melting point.

2. Look at covalent bonds for atomic crystals.

Generally, the stronger the non-metallic and the smaller the radius, the stronger the covalent bond and the higher the melting point.

3. Look at the intermolecular forces of molecular crystals.

The strength of the relative molecular mass of substances with similar composition and structure.

The larger the size, the stronger the intermolecular forces and the higher the melting point.

4. The metal crystal depends on the strength of the metal bond, the metal ion radius is small, the number of charges is large, the metal bond is strong, and the melting point is high.