What is plasma - an unusual gas
Since childhood, we have known several states of aggregation of substances. Let's take water for example. Its usual state is known to all - liquid, it is distributed everywhere: rivers, lakes, seas, oceans. The second state of aggregation is gas. We don't see him often. Most easy way reach a gaseous state near water - boil it. Steam is nothing but the gaseous state of water. The third aggregate state - solid. We can observe a similar case, for example, in the winter months. Ice is frozen water, and there is a third state of aggregation.
This example clearly shows that almost any substance has three states of aggregation. For some, it is easy to achieve, for others it is more difficult (special conditions are required).
But modern physics highlights another, independent state of matter - plasma.
Plasma is an ionized gas with the same density of both positive and negative charges. As you know, with strong heating, any substance passes into the third state of aggregation - gas. If we continue to heat the resulting gaseous substance, then at the output we will get a substance with a sharply increased process of thermal ionization, the atoms that make up the gas decay to form ions. This condition can be observed with the naked eye. Our Sun is a star, like millions of other stars and galaxies in the universe, is nothing but a high-temperature plasma. Unfortunately, on Earth, plasma does not exist in natural conditions. But we can still observe it, for example, a flash of lightning. In laboratory conditions, plasma was first obtained by passing a high voltage through a gas. Today, many of us use plasma in everyday life - these are ordinary gas-discharge fluorescent lamps. On the streets, neon advertising is seen all the time, which is nothing more than low-temperature plasma in glass tubes.
In order to go from a gaseous state to a plasma, the gas must be ionized. The degree of ionization directly depends on the number of atoms. Another condition is temperature.
Until 1879, physics described and was guided by only three states of aggregation of substances. While the English scientist, chemist and physicist, William Crookes, did not begin to conduct experiments on the study of the conductivity of electricity in gases. His discoveries include the discovery of the Thalia element, the production of Helium in the laboratory, and, of course, the first experiments with the production of cold plasma in gas discharge tubes. The familiar term “plasma” was used for the first time in 1923 by the American scientist Langmuir, and later by Tonkson. Until that time, "plasma" meant only the colorless component of blood or milk.
Today's research shows, contrary to popular belief, about 99% of all matter in the universe is in the plasma state. All stars, all interstellar space, galaxies, nebulae, the solar fan are typical representatives of plasma.
On earth we can observe such natural phenomena like lightning northern lights, "St. Elmo's fire", the Earth's ionosphere and, of course, fire.
Man has also learned to use plasma for his own good. Thanks to the fourth aggregate state of matter, we can use gas discharge lamps, plasma TVs, electric arc welding, and lasers. Also, we can observe the phenomena of plasma during a nuclear explosion or the launch of space rockets.
One of the priority research in the direction of plasma can be considered the reaction of thermonuclear fusion, which should become a safe replacement for nuclear energy.
According to the classification, plasma is divided into low-temperature and high-temperature, equilibrium and non-equilibrium, ideal and non-ideal.
Low-temperature plasma is characterized by a low degree of ionization (about 1%) and a temperature of up to 100 thousand degrees. It is for this reason that plasma of this kind is often used in various technological processes (deposition of a diamond film on a surface, change in the wettability of a substance, ozonation of water, etc.).
High-temperature or “hot” plasma has almost 100% ionization (this is the state that is meant by the fourth state of aggregation) and temperatures up to 100 million degrees. In nature, they are stars. Under terrestrial conditions, it is high-temperature plasma that is used for thermonuclear fusion experiments. A controlled reaction is quite complex and energy-intensive, but an uncontrolled one has sufficiently proven itself as a weapon of colossal power - a thermonuclear bomb tested by the USSR on August 12, 1953.
But these are extremes. Cold plasma has firmly taken its place in human life, one can still dream of useful controlled thermonuclear fusion, weapons are not actually applicable.
But in everyday life, plasma is not always equally useful. Sometimes there are situations in which plasma discharges should be avoided. For example, in any switching processes, we observe a plasma arc between the contacts, which urgently needs to be extinguished.
Any substance consists of molecules, and its physical properties depend on how the molecules are ordered and how they interact with each other. In ordinary life, we observe three aggregate states of matter - solid, liquid and gaseous.
For example, water can be in solid (ice), liquid (water) and gaseous (steam) states.
Gas expands until it fills the entire volume allotted to it. If we consider a gas at the molecular level, we will see molecules randomly rushing about and colliding with each other and with the walls of the vessel, which, however, practically do not interact with each other. If you increase or decrease the volume of the vessel, the molecules will evenly redistribute in the new volume.
Unlike gas at a given temperature, it occupies a fixed volume, however, it also takes the form of a filled vessel - but only below its surface level. At the molecular level, the easiest way to think of a liquid is as spherical molecules that, although they are in close contact with each other, have the freedom to roll around each other, like round beads in a jar. Pour a liquid into a vessel - and the molecules will quickly spread and fill the lower part of the volume of the vessel, as a result, the liquid will take its shape, but will not spread in the full volume of the vessel.
Solid has its own shape, does not spread over the volume of the container
and does not take its form. At the microscopic level, atoms stick to each other chemical bonds, and their position relative to each other is fixed. At the same time, they can form both rigid ordered structures - crystal lattices - and a random heap - amorphous bodies (this is precisely the structure of polymers, which look like tangled and sticky pasta in a bowl).
Three classical aggregate states of matter have been described above. There is, however, a fourth state, which physicists tend to classify as aggregate. This is the plasma state. Plasma is characterized by partial or complete stripping of electrons from their atomic orbits, while the free electrons themselves remain inside the substance.
We can observe the change in the aggregate states of matter with our own eyes in nature. Water from the surface of water bodies evaporates and clouds form. So the liquid turns into a gas. In winter, the water in the reservoirs freezes, turning into a solid state, and in the spring it melts again, turning back into a liquid. What happens to the molecules of a substance when it changes from one state to another? Are they changing? Are, for example, ice molecules different from vapor molecules? The answer is unequivocal: no. The molecules remain exactly the same. Their kinetic energy changes, and, accordingly, the properties of the substance.
The energy of the vapor molecules is large enough to scatter in different directions, and when cooled, the vapor condenses into a liquid, and the molecules still have enough energy for almost free movement, but not enough to break away from the attraction of other molecules and fly away. With further cooling, the water freezes, becoming a solid body, and the energy of the molecules is no longer enough even for free movement inside the body. They oscillate about one place, held by the attractive forces of other molecules.
Definition
Aggregate states of matter (from the Latin aggrego - attach, connect) - these are the states of the same substance - solid, liquid, gaseous.
During the transition from one state to another, an abrupt change in energy, entropy, density and other characteristics of matter occurs.
Solid and liquid bodies
Definition
Solid bodies are bodies that are distinguished by the constancy of shape and volume.
In them, the intermolecular distances are small and the potential energy of the molecules is comparable to the kinetic one. Solids are divided into two types: crystalline and amorphous. Only crystalline bodies are in a state of thermodynamic equilibrium. Amorphous bodies, in fact, represent metastable states, which in their structure approach non-equilibrium, slowly crystallizing liquids. In an amorphous body, a very slow process of crystallization takes place, the process of a gradual transition of a substance into a crystalline phase. The difference between a crystal and an amorphous solid lies primarily in the anisotropy of its properties. The properties of a crystalline body depend on the direction in space. Various kinds of processes, such as thermal conductivity, electrical conductivity, light, sound, propagate in various directions solid body in different ways. Amorphous bodies (glass, resins, plastics) are isotopic, like liquids. The only difference between amorphous bodies and liquids is that the latter are fluid, static shear deformations are impossible in them.
Crystalline bodies have the correct molecular structure. The anisotropy of its properties is due to the correct structure of the crystal. The correct arrangement of the atoms of a crystal forms the so-called crystal lattice. In different directions, the arrangement of atoms in the lattice is different, which leads to anisotropy. Atoms (or ions, or whole molecules) in a crystal lattice make random oscillating motion near the middle positions, which are considered as nodes of the crystal lattice. The higher the temperature, the greater the energy of oscillations, and hence the average amplitude of oscillations. The size of the crystal depends on the amplitude of the oscillations. An increase in the amplitude of oscillations leads to an increase in the size of the body. This explains the thermal expansion of solids.
Definition
Liquid bodies are bodies that have a certain volume, but do not have elasticity of form.
Liquids are characterized by strong intermolecular interaction and low compressibility. A liquid occupies an intermediate position between a solid and a gas. Liquids, like gases, are isotopic. In addition, the liquid has fluidity. In it, as in gases, there are no tangential stresses (shear stresses) of bodies. Liquids are heavy, i.e. their specific gravity is comparable to the specific gravity of solids. Near the crystallization temperatures, their heat capacities and other thermal characteristics are close to those of solids. In liquids, to a certain extent, the correct arrangement of atoms is observed, but only in small areas. Here the atoms also oscillate near the nodes of a quasi-crystalline cell, but unlike the atoms of a solid body, they jump from one node to another from time to time. As a result, the motion of atoms will be very complex: it is oscillatory, but at the same time the center of vibrations moves in space.
Gas, evaporation, condensation and melting
Definition
A gas is a state of matter in which the distances between molecules are large.
The forces of interaction between molecules at low pressures can be neglected. Gas particles fill the entire volume that is provided to the gas. Gases can be considered as highly superheated or unsaturated vapors. Plasma is a special type of gas - it is partially or completely ionized gas, in which the density of positive and negative charges is almost the same. Plasma is a gas of charged particles that interact with each other using electrical forces at a great distance, but do not have near and far particles.
Substances can change from one state of aggregation to another.
Definition
Evaporation is the process of changing the state of aggregation of a substance, in which molecules fly out from the surface of a liquid or solid, the kinetic energy of which exceeds the potential energy of the interaction of molecules.
Evaporation is a phase transition. During evaporation, part of the liquid or solid passes into vapor. A substance in a gaseous state that is in dynamic equilibrium with a liquid is called saturated vapor. At the same time, the change internal energy bodies:
\[\triangle \ U=\pm mr\ \left(1\right),\]
where m is body weight, r is the specific heat of vaporization (J / kg).
Definition
Condensation is the reverse process of vaporization.
The calculation of the change in internal energy is carried out according to the formula (1).
Definition
Melting is the process of transition of a substance from a solid to a liquid state, the process of changing the state of aggregation of a substance.
When a substance is heated, its internal energy increases, therefore, the speed of thermal movement of molecules increases. In the event that the melting point of the substance is reached, the crystal lattice of the solid begins to break down. Bonds between particles are destroyed, the energy of interaction between particles increases. The heat transferred to the body goes to increase the internal energy of this body, and part of the energy goes to doing work to change the volume of the body when it melts. For most crystalline bodies, the volume increases when melted, but there are exceptions, for example, ice, cast iron. Amorphous bodies do not have a specific melting point. Melting is a phase transition, which is accompanied by an abrupt change in heat capacity at the melting temperature. The melting point depends on the substance and does not change during the process. In this case, the change in the internal energy of the body:
\[\triangle U=\pm m\lambda \left(2\right),\]
where $\lambda $ is the specific heat of fusion (J/kg).
The reverse process of melting is crystallization. The calculation of the change in internal energy is carried out according to the formula (2).
The change in the internal energy of each body of the system in the case of heating or cooling can be calculated by the formula:
\[\triangle U=mc\triangle T\left(3\right),\]
where c is the specific heat of the substance, J/(kgK), $\triangle T$ is the change in body temperature.
When studying the transitions of substances from one state of aggregation to another, it is impossible to do without the so-called heat balance equation, which says: the total amount of heat that is released in a thermally insulated system is equal to the amount of heat (total) that is absorbed in this system.
In its meaning, the heat balance equation is the law of conservation of energy for heat transfer processes in thermally insulated systems.
Example 1
Assignment: There are water and ice in a heat-insulated vessel at a temperature $t_i= 0^oС$. The masses of water ($m_(v\ ))$ and ice ($m_(i\ ))$ are 0.5 kg and 60 g respectively. Water vapor of mass $m_(p\ )=$10 g is let into the water. at temperature $t_p= 100^oС$. What will be the temperature of the water in the vessel after thermal equilibrium is established? The heat capacity of the vessel is ignored.
Solution: Let's determine what processes take place in the system, what aggregate states of matter we had and what we got.
Water vapor condenses, giving off heat.
This heat is used to melt the ice and, possibly, to heat the water available and obtained from the ice.
Let us first check how much heat is released during the condensation of the available mass of steam:
here, from reference materials, we have $r=2.26 10^6\frac(J)(kg)$ - specific heat of vaporization (also applicable for condensation).
Heat needed to melt ice:
here from reference materials we have $\lambda =3.3\cdot 10^5\frac(J)(kg)$ - specific heat of ice melting.
We get that the steam gives off more heat than required, only to melt the existing ice, therefore, we write the heat balance equation in the form:
Heat is released when steam of mass $m_(p\ )$ condenses and water, which is formed from steam, cools from temperature $T_p$ to the desired T. Heat is absorbed when ice of mass $m_(i\ )$ melts and water of mass $m_v+ is heated m_i$ from temperature $T_i$ to $T.\ $ Denote $T-T_i=\triangle T$, for the difference $T_p-T$ we get:
The heat balance equation will take the form:
\ \ \[\triangle T=\frac(rm_(p\ )+cm_(p\ )100-lm_(i\ ))(c\left(m_v+m_i+m_(p\ )\right))\left (1.6\right)\]
We will carry out calculations, taking into account that the heat capacity of water is tabular $c=4.2\cdot 10^3\frac(J)(kgK)$, $T_p=t_p+273=373K,$ $T_i=t_i+273=273K$:
$\triangle T=\frac(2,26\cdot 10^6\cdot 10^(-2)+4,2\cdot 10^3\cdot 10^(-2)10^2-6\cdot 10^ (-2)\cdot 3,3\cdot 10^5)(4,2\cdot 10^3\cdot 5,7\cdot 10^(-1))\approx 3\left(K\right)$then T=273+3=276 (K)
Answer: The temperature of the water in the vessel after the establishment of thermal equilibrium will be equal to 276 K.
Example 2
Task: The figure shows the section of the isotherm corresponding to the transition of a substance from a crystalline to a liquid state. What corresponds to this section on the p,T diagram?
The entire set of states depicted in the diagram p,V horizontal a straight line segment on the diagram p,T is represented by one point that determines the values of p and T, at which the transition from one state of aggregation to another takes place.
Aggregate state of matter
Substance- a real-life set of particles interconnected by chemical bonds and under certain conditions in one of the states of aggregation. Any substance consists of a combination of very a large number particles: atoms, molecules, ions, which can combine with each other into associates, also called aggregates or clusters. Depending on the temperature and behavior of particles in associates (the mutual arrangement of particles, their number and interaction in an associate, as well as the distribution of associates in space and their interaction with each other), a substance can be in two main states of aggregation - crystalline (solid) or gaseous, and in transitional states of aggregation - amorphous (solid), liquid crystal, liquid and vapor. Solid, liquid-crystal and liquid states of aggregation are condensed, and vaporous and gaseous are strongly discharged.
Phase- this is a set of homogeneous microregions, characterized by the same orderliness and concentration of particles and enclosed in a macroscopic volume of a substance bounded by an interface. In this understanding, the phase is characteristic only for substances that are in the crystalline and gaseous states, because they are homogeneous aggregate states.
metaphase- this is a set of heterogeneous microregions that differ from each other in the degree of ordering of particles or their concentration and are enclosed in a macroscopic volume of a substance bounded by an interface. In this understanding, metaphase is characteristic only for substances that are in inhomogeneous transition states of aggregation. Different phases and metaphases can mix with each other, forming one state of aggregation, and then there is no interface between them.
Usually do not separate the concept of "basic" and "transitional" state of aggregation. The concepts of "aggregate state", "phase" and "mesophase" are often used as synonyms. It is advisable to consider five possible aggregate states for the state of substances: solid, liquid crystal, liquid, vapor, gaseous. The transition of one phase to another phase is called a phase transition of the first and second order. Phase transitions of the first kind are characterized by:
An abrupt change in physical magnitudes that describe the state of matter (volume, density, viscosity, etc.);
A certain temperature at which a given phase transition occurs
A certain heat that characterizes this transition, because break intermolecular bonds.
Phase transitions of the first kind are observed during the transition from one state of aggregation to another state of aggregation. Phase transitions of the second kind are observed when the ordering of particles within a single state of aggregation changes, and are characterized by:
gradual change physical properties substances;
Change in the ordering of the particles of a substance under the action of a gradient of external fields or at a certain temperature, called the phase transition temperature;
The heat of phase transitions of the second order is equal to and close to zero.
The main difference between phase transitions of the first and second order is that during transitions of the first kind, first of all, the energy of the particles of the system changes, and in the case of transitions of the second kind, the ordering of the particles of the system changes.
The transition of a substance from a solid to a liquid state is called melting and is characterized by its melting point. The transition of a substance from a liquid to a vapor state is called evaporation and characterized by the boiling point. For some substances with a small molecular weight and weak intermolecular interaction, a direct transition from a solid state to a vapor state is possible, bypassing the liquid state. Such a transition is called sublimation. All of these processes can proceed in the opposite direction: then they are called freezing, condensation, desublimation.
Substances that do not decompose during melting and boiling can be, depending on temperature and pressure, in all four states of aggregation.
Solid state
At sufficiently low temperatures, almost all substances are in the solid state. In this state, the distance between the particles of a substance is comparable to the size of the particles themselves, which ensures their strong interaction and a significant excess of their potential energy over kinetic energy. . This leads to internal order in the arrangement of particles. Therefore, solids are characterized by their own shape, mechanical strength, constant volume (they are practically incompressible). Depending on the degree of ordering of the particles, solids are divided into crystalline and amorphous.
Crystalline substances are characterized by the presence of order in the arrangement of all particles. The solid phase of crystalline substances consists of particles that form a homogeneous structure, characterized by strict repeatability of the same unit cell in all directions. The elementary cell of a crystal characterizes a three-dimensional periodicity in the arrangement of particles, i.e. its crystal lattice. Crystal lattices are classified according to the type of particles that make up the crystal and the nature of the attractive forces between them.
Many crystalline substances, depending on the conditions (temperature, pressure), can have a different crystalline structure. This phenomenon is called polymorphism. Well-known polymorphic modifications of carbon: graphite, fullerene, diamond, carbine.
Amorphous (shapeless) substances. This state is typical for polymers. Long molecules easily bend and intertwine with other molecules, which leads to irregularities in the arrangement of particles.
The difference between amorphous particles and crystalline ones:
isotropy - the sameness of the physical and chemical properties of a body or medium in all directions, i.e. independence of properties from direction;
no fixed melting point.
Glass, fused quartz, and many polymers have an amorphous structure. Amorphous substances are less stable than crystalline ones, and therefore any amorphous body can eventually move into an energetically more stable state - a crystalline one.
liquid state
As the temperature rises, the energy of thermal vibrations of particles increases, and for each substance there is a temperature, starting from which the energy of thermal vibrations exceeds the energy of bonds. Particles can perform various movements, shifting relative to each other. They still remain in contact, although the correct geometric structure of the particles is violated - the substance exists in a liquid state. Due to the mobility of particles, the liquid state is characterized by Brownian motion, diffusion and volatility of particles. An important property of a liquid is viscosity, which characterizes interassociative forces that prevent the free flow of a liquid.
Liquids occupy an intermediate position between the gaseous and solid state of matter. More orderly structure than a gas, but less than a solid.
Steam and gaseous states
The vapor-gaseous state is usually not distinguished.
Gas - it is a highly rarefied homogeneous system, consisting of individual molecules far apart from each other, which can be considered as a single dynamic phase.
Steam - this is a highly discharged inhomogeneous system, which is a mixture of molecules and unstable small associates consisting of these molecules.
Molecular-kinetic theory explains the properties of an ideal gas, based on the following provisions: molecules make a continuous random motion; the volume of gas molecules is negligible compared to the intermolecular distances; there are no attractive or repulsive forces between gas molecules; the average kinetic energy of gas molecules is proportional to its absolute temperature. Due to the insignificance of the forces of intermolecular interaction and the presence of a large free volume, gases are characterized by: high speed of thermal motion and molecular diffusion, the desire of molecules to occupy as much volume as possible, as well as high compressibility.
An isolated gas-phase system is characterized by four parameters: pressure, temperature, volume, amount of substance. The relationship between these parameters is described by the equation of state for an ideal gas:
R = 8.31 kJ/mol is the universal gas constant.
DEFINITION
Substance- a collection of a large number of particles (atoms, molecules or ions).
Substances have a complex structure. Particles in matter interact with each other. The nature of the interaction of particles in a substance determines its state of aggregation.
Types of aggregate states
The following states of aggregation are distinguished: solid, liquid, gas, plasma.
In the solid state, the particles, as a rule, are combined into a regular geometric structure. The bond energy of particles is greater than the energy of their thermal vibrations.
If the body temperature is increased, the energy of thermal oscillations of the particles increases. At a certain temperature, the energy of thermal vibrations becomes greater than the bond energy. At this temperature, bonds between particles are destroyed and formed again. In this case, the particles make different kinds movements (oscillations, rotations, movements relative to each other, etc.). However, they are still in contact with each other. The correct geometric structure is broken. The substance is in a liquid state.
With a further increase in temperature, thermal fluctuations intensify, the bonds between particles become even weaker and practically absent. The substance is in a gaseous state. The simplest model of matter is an ideal gas, in which it is assumed that particles move freely in any direction, interact with each other only at the moment of collisions, while the laws of elastic impact are fulfilled.
It can be concluded that with increasing temperature, the substance passes from an ordered structure to a disordered state.
Plasma is a gaseous substance consisting of a mixture of neutral particles of ions and electrons.
Temperature and pressure in different states of matter
Different aggregate states of matter determine: temperature and pressure. Low pressure and high temperature correspond to gases. At low temperatures, usually the substance is in a solid state. Intermediate temperatures refer to substances in the liquid state. The phase diagram is often used to characterize the aggregate states of a substance. This is a diagram showing the dependence of the state of aggregation on pressure and temperature.
The main feature of gases is their ability to expand and compressibility. Gases do not have a shape, they take the shape of the vessel in which they are placed. The volume of the gas determines the volume of the vessel. Gases can mix with each other in any proportion.
Liquid has no shape, but has volume. Liquids compress poorly, only at high pressure.
Solids have shape and volume. In the solid state, there can be compounds with metallic, ionic and covalent bonds.
Examples of problem solving
EXAMPLE 1
| Exercise | Draw a phase diagram of states for some abstract substance. Explain its meaning. |
| Solution | Let's make a drawing. The state diagram is shown in Fig.1. It consists of three areas that correspond to the crystalline (solid) state of matter, liquid and gaseous state. These areas are separated by curves that indicate the boundaries of mutually inverse processes: 01 - melting - crystallization; 02 - boiling - condensation; 03 - sublimation - desublimation. The point of intersection of all curves (O) is a triple point. At this point, matter can exist in three states of aggregation. If the temperature of the substance is above the critical () (point 2), then the kinetic energy of the particles is greater than the potential energy of their interaction, at such temperatures the substance becomes a gas at any pressure. It can be seen from the phase diagram that if the pressure is greater than , then the solid melts as the temperature increases. After melting, an increase in pressure leads to an increase in the boiling point. If the pressure is less than , then an increase in the temperature of the solid leads to its transition directly to the gaseous state (sublimation) (point G). |
EXAMPLE 2
| Exercise | Explain what distinguishes one state of aggregation from another? |
| Solution | In various states of aggregation, atoms (molecules) have different arrangements. So the atoms (molecules or ions) of the crystal lattices are arranged in an orderly manner, they can make small vibrations around the equilibrium positions. Molecules of gases are in a disordered state and can move over considerable distances. In addition, the internal energy of substances in different aggregate states (for the same masses of matter) at different temperatures different. The processes of transition from one state of aggregation to another are accompanied by a change in internal energy. The transition: solid - liquid - gas, means an increase in internal energy, since there is an increase in the kinetic energy of the movement of molecules. |
