Can density be understood as the number of molecules per unit area?

The accurate explanation should be: density is the mass of molecules contained in a unit volume; (not area, not number of molecules).

Density = mass volume, that is, directly proportional to mass and inversely proportional to volume; It is volume, not mass, that is compressed;

The water solidifies into ice, the mass does not change, and the volume becomes larger, which is explained as follows:

In general, objects obey thermal expansion and contraction, that is, for the same object, "the density of the solid should be greater than that of the liquid".

But water is the opposite, and the reason for this involves chemistry.

Since water molecules are highly polar molecules, they can be combined into associative molecules (groups of water molecules combined) by hydrogen bonding. Liquid water, in addition to simple water molecules (H2O), also contains association molecules, the most typical two are (H2O)2 and (H2O)3, the former is called bimolecular association water molecule. The density of a substance is determined by the average spacing of molecules within a substance.

When the temperature is 0 when the water is not frozen, most of the water molecules exist as (H2O)3 association molecules, and when the temperature rises, the water molecules mostly exist in the form of bimolecular association water molecules (in the process of water temperature rising from 0 to 4, the effect of increasing water density caused by the breaking of hydrogen bonds of associated water molecules is greater than the decrease of water density caused by the acceleration of molecular thermal movement, so in this process, the density of water increases with the increase of temperature. ), the molecules occupy a relatively small space, and the density of water is at its highest. If the temperature continues to rise above this, the law of thermal expansion and cold contraction of general matter will prevail.

When the water temperature drops to 0, the water freezes, and when the water freezes, almost all the molecules are associated together to become a huge association molecule, and the arrangement of water molecules in the ice is that each oxygen atom has four hydrogen atoms as close neighbors (two covalent bonds, two hydrogen bonds), so this arrangement leads to an open structure, that is to say, there are large voids in the structure of the ice, so the density of the ice is inversely smaller than that of water at the same temperature.

Can density be understood as the number of molecules per unit area? If this is the case, then the density of the compressed fine dust changes because the volume becomes smaller, but the mass does not change, so the density is inversely proportional to the mass, and the density becomes larger?

If this is the case, then the density of the compressed fine dust changes because the volume becomes smaller, but the mass does not change, so the density is inversely proportional to the mass, and the density becomes larger? That's true!

When water solidifies into a solid state, the reason why the volume will become larger is determined by the structure of the water molecules, just imagine, in the liquid state, the water molecules can be close to each other, and in the solid state, the water molecules must be arranged regularly, and the volume will naturally become larger!

Molecular number density The reciprocal of the molecular volume p=1 molecular volume, which is used for characterizationThe unit body digs Zheng ZhengThe number of Qi Xiang under the molecule.

Molecular number density is defined as: the number of molecules in the unit of volume occupied by these molecules in the International System of Units.

It is: m -3, which is "n (that is, how many cubic meters)".

The volume of a gas refers to the space occupied by the molecules it contains, and under normal conditions, the average distance between the molecules of a gas is about 10 times the diameter of the molecule, so when the number of molecules contained in the gas is determined, the volume of the gas is mainly determined by the average distance between the molecules rather than the size of the molecules themselves.

The average distance between molecules is determined by the temperature and pressure outside, when the temperature and pressure are the same, the average distance between the molecules of any gas is almost equal (the effect between the molecules of the gas is weak and negligible), this law is applied in chemical reactions with the participation of gas.

There are a wide range of applications such as inferring the molecular formula of unknown gases.

Precautions. 1. Since the density of the air is related to the pressure, it is important to make a beaker when collecting gas by draining and taking gas.

The water surface inside and outside is level, so that the pressure of the air in the cup is the same as that of the ambient air.

2. Because the container is filled with compressed air.

Therefore, it is necessary to deflate slowly to prevent the gas from rushing to the outside of the beaker.

3. This experiment also obviously shows that air has mass.

Molecular number density The reciprocal of the molecular volume p=1 molecular volume, which is used for characterizationUnit volumeThe number of lower molecules.

The formula p=nkt, n is the molecular number density, kshi Boltzmann constant.

Bring in the weight value; Multiplying n by the molecular mass of oxygen is fine, which is actually multiplied by the molar mass of oxygen.

Then divide by the tall Avogadro's constant.

Structural formula of molecular number density.

Generally, only the order of connection of atoms in the molecule is reflected, and the values of bond length and bond angle that determine the shape of the molecular number density need to be determined by experiments. The configuration that reflects the order and distribution of atoms in space in molecular number density is called molecular number density. The length and angle of the chemical bonds between the atoms in the molecular number density are called the stereoconfiguration parameters.

Characteristics of Molecular Number Density:

1.There is a gap between the number densities of molecules. For example, take 50 ml of alcohol and 50 ml of water and mix them to a volume of less than 100 ml.

2.The number density of all the molecules that make up matter is in a never-ending irregular motion. The higher the temperature, the faster the molecular diffusion, and in solids, liquids, and gases, the gas diffusion is the fastest.

Since the motion of the molecular number density is related to the temperature, this motion is called the thermal motion of the molecular number density. For example, clothes tend to dry in the sun when it's hot.

3.The general number of molecules, the density of the order of magnitude of diameter.

for 10 m.

4.The molecular number density is small but has a certain volume and mass.

5.The molecular number density properties of the same substance are the same, and the molecular number density properties of different substances are different.

Molecular number density and density are different concepts.

Density is the ratio of the mass of an object to its volume and is usually expressed using the formula = m v. where , denotes density, m denotes mass, and v denotes volume. Density is a scalar quantity that is usually expressed in units such as kg m or g cm.

Whereas, the number density of molecules is the number of molecules per unit volume, which is usually expressed using the formula n v. where n is the number of beats of the molecule per unit volume, and v is the volume. Molecular number density is also known as particle number density or molar number concentration.

Molecular number density is a scalar number and is usually expressed in units such as mol m or mol l.

Thus, density and molecular number density are two different concepts that differ in their unit silver chain and definition. Density describes the weight and volume of a substance, while molecular number density describes the number of molecules per unit volume. In some cases, we need to consider both of them, such as when studying the change in the concentration of a solution in a chemical reaction.

The mass of a substance per unit volume is called the density of that substance, and the greater the mass of the same volume of matter, the greater the density of that substance.

Total number of molecules: n = v v0, number of molecules per unit volume: 1 v0, volume of molecules: v0 = m p-na, the substitution formula is equal to p-na ma, (ps: na is the Avogadro constant, ma is the molar mass p is the density).