Before this century, people can only distinguish the state of matter from the macroscopic characteristics of objects; All states of matter that have a fixed shape and volume and are not easily deformed are called solid states; An object has a certain volume, but its shape changes with the container, and the state that it is easy to flow is called liquid; If the shape and volume of an object change with the container, when the container is opened, the material particles escape, and this state is gaseous. People often say that "matter has three states", which means that a matter can appear in the form of solid, liquid or gas.
But considering the internal structure of matter, there are far more than three states. Some solids, in which molecules or atoms appear in a regular, symmetrical and periodic structural state, are called crystalline. Other so-called solids, such as glass, asphalt, bakelite, plastic, etc. Although they also have a fixed volume and shape at normal temperature and pressure, they do not show obvious fluidity, but their internal structure is more like liquid, which is called glassy state. Many organic substances, which are between liquid state and crystalline state, have not only flow characteristics, but also some optical properties similar to crystals, which are called liquid crystal state or mesomorphic state. After the gas is heated to a high temperature of more than 10,000 degrees or irradiated, atoms may ionize, and the whole gas will become a collection of positively charged ions and negatively charged electrons, and the positive and negative charges are equal. The aggregation state of these two ions is called plasmon. If an object is kept at a very low temperature, such as a few degrees above absolute zero, the DC resistance of some metals will approach zero, which is called superconducting state. At extremely low temperature, the viscosity of some liquids (such as liquid helium) also disappears completely, which is called superfluid state. On the other hand, the state of matter can also be changed by changing the pressure. For example, under great pressure, hydrogen can be transformed into a solid state with metallic characteristics, which is called metallic hydrogen state.
These material states can be obtained in our lives or laboratories. What will happen if the pressure and temperature continue to increase?
Astronomers have found that in the space far away from the earth, there is a kind of star with large mass and small volume, called a white dwarf. Its internal pressure and temperature are so great that all the electrons of the substance atoms are separated from the pronucleus and become free electrons, and all the bare nuclei are piled up like crystals, and the free electrons move in disorder. Because of its high density, it is called supersolid. There is also a star whose internal temperature and pressure far exceed those of white dwarfs. Under strong pressure, all electrons outside the nucleus are "squeezed" into the nucleus, which combines with protons to form neutrons, while most of the electrons from the nucleus no longer maintain their original combined state, so the matter in the planetary shell almost becomes a fluid composed of neutrons, and its density greatly exceeds that of white dwarfs. This high-density state is called neutron state, so this star is called neutron star. However, the neutron state is not the highest density state of matter. Scientists believe that there may be some higher density states, such as hyperon states, anomalous neutron states, black holes or white holes. And the conditions for the stable existence of these States are calculated theoretically.
Both the low-density state and the high-density state of matter are formed by various physical particles, but when people study the properties of matter, they often regard most of the states of matter as a continuum, which is only an approximate method to deal with the problem. Even the vacuum occupied by nothingness is not empty, but a space full of "negative energy" particles that we can't directly perceive, forming a vast ocean of vacuum, so vacuum is also a form of material existence.
Now it is found that most elementary particles have particles with equal mass but opposite electromagnetic or other physical properties, which are called antiparticles, such as antiprotons, antineutrons and so on. So scientists speculate that there may be an "anti-world" composed of "anti-particles", in which the material state should correspond to the state of the "positive" world in which we are now, collectively known as the antimatter state.
In a word, from the macroscopic appearance of matter and whether it is easy to deform, we say that matter has three States: solid, liquid and gas. Judging from the internal structural characteristics of objects, there are actually more states of matter. And with the development of science, people's understanding of matter will be deeper and deeper, and there will be new discoveries of matter.