The physical or aggregative state of matter is characterized by its specific properties under specific conditions (pressure and temperature). That is, it can be solid, liquid or gaseous. A change in the state of aggregation 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. Passing into the liquid, with the same volume, changes shape, adapting to the vessel or container. Being gaseous, this same substance does not preserve not only its shape, but also its volume, since, expanding, it fills the entire space.
For example, the state of aggregation 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 ° C to 99.944 ° C it is a colorless (for small volumes) transparent liquid that does not have a smell or taste. At a temperature of 99.944 ° C, it 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, when the ambient pressure is almost unchanged, but the temperature changes, the ice melts at 0 ° C and turns into water, and it boils at 99.944 ° C and, evaporating, turns into steam.
Water, one can say, differs in its properties, being in different states of aggregation. The density of ice is 0.917 g / cm³, liquid water under standard conditions 0.9982 g / cm³, water vapor (also under standard conditions) less than 0.001 g / cm³. The dynamic viscosity of water at the melting temperature of ice (freezing point of water) is: μ = 1.793 · 103 Pa · s, and at 20 ° C 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 a melting temperature (from solid matter becomes liquid), equal to the crystallization temperature (liquid solidifies), and a boiling point (liquid passes into gas), equal to the condensation temperature (gas becomes liquid).
It should be emphasized that the state of aggregation of a substance also depends on its composition. So, for example, under the same external conditions, various 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, boiling point is 2860.9 ° C). But the same state of aggregation for different substances may have some similarities.
For example, gases are characterized by relatively small forces of intermolecular interaction. Therefore, their molecules are located at large distances from each other. As a result, the gases are able to strongly compress. Gas molecules are constantly moving, and the movement is chaotic. This explains the ability of gaseous substances to uniformly fill the entire volume provided to them: gases acquire the shape and volume of the container in which they are located.
Liquids between gaseous and solid substances are intermediate. With increasing temperature, they become more like gases, and with decreasing temperature, they are more like solids. Molecules of liquids are more densely placed due to the noticeable attractive forces between them. Their x-ray studies showed that they have some rudiments resembling a crystalline structure.
Solids always have certain shapes and volumes. To change them, it is necessary to expend effort, since molecules, atoms and ions are firmly connected with each other. Particles of solid matter cannot move freely, as they maintain mutual position and perform ordered oscillations near the centers of equilibrium. Two states of solids are known - amorphous and crystalline. The crystals of the latter are characterized by their specific shape. For example, crystals of sodium chloride NaCl resemble a cube in shape, potassium nitrate KNO3 - a prism and so on. The structure of amorphous is randomly aggregated molecules. In contrast to crystalline substances that melt at a certain temperature, amorphous substances are characterized by a wide range of melting points. Sometimes they are considered as a liquid with a very high viscosity. When the conditions under which the substance solidifies, it can turn out, both in the crystalline and in the amorphous state.
The aggregate state of a substance is sometimes described as plasma or liquid crystals. But this is not a completely correct definition. The question requires a separate and more detailed study, therefore, this state of matter is not affected in this article.