Aggregate state of matter

The physical or aggregate state of matter is characterized by its specific properties under specific conditions (pressure and temperature). That is, it can be solid, liquid or gaseous. The change in the aggregate states of a substance leads to a change in its physical properties (density, entropy, free energy). Being in a solid state, it is able to maintain a fixed volume and shape. Having passed into a liquid, with the same volume, changes shape, adapting to the vessel or container. Being gaseous, this same substance does not retain not only the form, but also the volume, since, expanding, it fills the entire space.

For example, the aggregate state of water can be solid, liquid or gaseous. In this case, the chemical formula (H2O) and the composition of the molecule (two hydrogen atoms and one oxygen atom) do not change. At temperatures from 0 ° C and below it is in a solid state and is called ice. From 0 ° С and up to 99,944 ° С is a colorless (for small volumes) transparent liquid, which has no smell and taste. At a temperature of 99.944 ° C passes into the gas phase and is called water vapor. This example shows that the aggregate state of a substance changes with changing external conditions. In this case, with an almost unchanged ambient pressure, but a change in temperature, the ice melts at 0 ° C and turns into water, which boils at 99.944 ° C and evaporates into steam.

Water, we can say, differs in its properties, being in different aggregate states. The ice density is 0.917 g / cm³, liquid water at standard conditions of 0.9982 g / cm³, water vapor (also under standard conditions) is less than 0.001 g / cm³. The dynamic viscosity of water at the melting point of ice (freezing point of water) is: μ = 1,793 · 103 Pa · s, and at 20 ° С the dynamic viscosity of water is: μ = 1,003 · 10ˉ³ Pa · s. Thus, the aggregate state of a substance and its properties change during a phase transition. This transition is characterized by the melting point (from solid matter becomes liquid) equal to the crystallization temperature (the liquid solidifies) and the boiling point (the liquid passes into the gas) equal to the condensation temperature (the gas becomes a liquid).

It should be emphasized that the aggregate state of a substance also depends on its composition. So, for example, under the same external conditions, different substances differ in their properties. Under standard conditions, water H2O is a liquid. Hydrogen H2 and oxygen O2 are gases. Iron Fe is a solid (its melting point is 1538.9 ° C, the boiling point is 2860.9 ° C). But the same aggregate state for different substances may have some similarities.

For example, relatively small intermolecular forces are characteristic of gases. Therefore, their molecules are at a great distance from each other. As a result, gases can strongly contract. The molecules of gases constantly move, and the motion is chaotic. This explains the ability of gaseous substances to fill evenly the entire volume they are provided with: gases acquire the shape and volume of the container in which they are located.

Liquids between gaseous and solid substances occupy an intermediate position. With the rise in temperature, they become more like gases, and with a decrease in temperature, they are more like solids. Molecules of liquids are more densely placed due to appreciable forces of attraction between them. X-ray studies have shown that they have some rudiments that resemble a crystalline structure.

Solids have always certain shapes and volumes. To change them, it is necessary to expend effort, since the molecules, atoms and ions are tightly connected. Particles of solid matter can not move freely, since they retain a mutual arrangement and perform ordered oscillations around the centers of equilibrium. Two states of solids are known - amorphous and crystalline. The crystals of the latter are characterized by a form for them. For example, crystals of sodium chloride NaCl resemble the shape of the cube, potassium nitrate KNO3 - prism and so on. The structure of amorphous molecules is randomly aggregated molecules. Unlike crystalline substances that melt at a certain temperature, amorphous ones are characterized by a wide range of melting points. Sometimes they are treated as a liquid with a very high viscosity. When the conditions under which the substance solidates change, it can be obtained both in the crystalline and in the amorphous state.

The aggregate state of matter is sometimes described as plasma or liquid crystals. But this is not quite a correct definition. The question requires a separate and more detailed study, so in this article this state of matter is not affected.