What is a "triple point" and how to determine its coordinates? Experiments show that for each substance there are conditions (pressure and temperature) under which vapor, liquid and crystal can coexist simultaneously for an arbitrarily long time. For example, if you place water with floating ice in a closed vessel at zero degrees, then both water and ice will evaporate into the free space. However, at a vapor pressure of 0.006 atm. (this is their "own" pressure, without taking into account the pressure created by the air) and a temperature of 0.01 ° C, the increase in the vapor mass will stop. From this moment on, ice, water and steam will retain their masses for as long as necessary. This is the triple point for water (left diagram). If water or steam is placed in the conditions of the left area, they will become ice. If a liquid or a solid is introduced into the "lower region", then steam will be obtained. In the right area, water will condense and ice will melt.
A similar diagram can be built for any substance. The purpose of such diagrams is to answer the question: what state of matter will be stable at such and such pressure and such and such temperature. For example, the diagram on the right is for carbon dioxide. The triple point for this substance has a coordinate "pressure" of 5.11 atm, that is, much more than normal atmospheric pressure. Therefore, under normal conditions (pressure 1 atm), we can only observe transitions "below the triple point", that is, the independent transformation of a solid into a gas. At a pressure of 1 atm, this will occur at a temperature of –78 ° C (see the dotted coordinate lines below the triple point).
We all live "near" the values of "normal conditions", that is, first of all, at a pressure close to one atmosphere. Therefore, if the atmospheric pressure is lower than the pressure corresponding to the triple point, when the body is heated, we will not see liquid, - the solid will immediately turn into vapor. This is exactly how “dry ice” behaves, which is very convenient for ice cream sellers. Ice cream briquettes can be shifted with pieces of "dry ice" without being afraid that the ice cream will get wet. If the pressure corresponding to the triple point is less than atmospheric, then the substance belongs to "melting" - when the temperature rises, it first turns into a liquid, and then boils.
As you can see, the features of the aggregate transformations of substances directly depend on how the current values of pressure and temperature correlate with the coordinates of the "triple point" on the "pressure-temperature" diagram.
And in conclusion, let us name the substances known to you, which always sublimate under normal conditions. This is iodine, graphite, "dry ice". At pressures and temperatures other than normal, these substances can be observed in a liquid and even in a boiling state.
(C) 2013. Physics.ru with the participation of A.V. Kuznetsova (Samara)
: [in 30 volumes] / ch. ed. A.M. Prokhorov; 1969-1978, vol. 1).
Under normal conditions, any substance is in one of three states - solid, liquid or gaseous ( cm. State of aggregation). Each of these conditions has its own structure of bonds between molecules and / or atoms, characterized by a certain bond energy between them. To change this structure, either an influx of heat energy from the outside is needed (for example, when a solid melts), or an outflow of energy outside (for example, during crystallization).
Taking, for a start, a solid substance, we understand speculatively that molecules / atoms in it are bound into some kind of rigid crystalline or amorphous structure - with a slight heating, they only begin to "shake" around their fixed position (the higher the temperature, the greater the amplitude of oscillations ). With further heating of the substance, the molecules are loosened more and more, until, finally, they are torn from their "familiar" place and are not sent to "free floating". That's what it is melting or melting solid to liquid. The flow of energy necessary for the melting of a substance is called heat of fusion.
The graph of the change in the temperature of a solid when it passes the melting point is very interesting in itself. Up to the melting point, as the atoms / molecules are heated, they swing around their fixed position more and more, and the arrival of each additional portion of thermal energy leads to an increase in the temperature of the solid. However, when a solid reaches its melting point, it remains at this temperature for some time, despite the ongoing influx of heat, until a sufficient amount of thermal energy is accumulated in it to break rigid intermolecular bonds. That is, in the process phase transition of a substance from a solid state to a liquid, energy is absorbed by it without increasing the temperature, since all of it goes to rupture of intermolecular bonds. That is why the ice cube in the cocktail remains icy in temperature even in the heat of the day until it is all melted. At the same time, melting, the ice cube takes away heat from the surrounding cocktail (and thereby cools it to a pleasant temperature), and itself gains the energy that it needs to break intermolecular bonds and final self-destruction.
The amount of heat required to melt or evaporate a unit volume of a solid or liquid is called, respectively, latent heat of fusion or latent heat of evaporation. And sometimes considerable figures figure here. For example, heating 1 kg of water from 0 ° С to 100 ° С requires "only" 420,000 joules (J) of thermal energy, and in order to convert this kilogram of water into 1 kg of steam with a temperature equal to the same 100 ° С , - as much as 2,260,000 Joules of energy.
After the solid mass has completely turned into a liquid, further heat input will again entail an increase in the temperature of the substance. In the liquid state, the molecules of a substance are still in close contact, but the rigid intermolecular bonds between them are broken, and the forces of interaction that hold the molecules together are several orders of magnitude weaker than in a solid, so the molecules begin to move relatively freely relative to each other. Further input of thermal energy brings the liquid to the phase boiling, and active evaporation or vaporization.
And, again, as it was described in the case of melting or melting, for some time all the additional incoming energy is spent on breaking liquid bonds between molecules and releasing them into a gaseous state (at a constant boiling point). The energy expended to break these seemingly weak bonds - the so-called. latent heat of vaporization - also requires a lot (see example above).
All the same processes during the outflow of energy (cooling) of a substance occur in the reverse order. At first, the gas cools down with decreasing temperature, and this happens until it reaches condensation points- the temperature at which the liquefaction, - and it is exactly equal to the evaporation (boiling) temperature of the corresponding liquid. During condensation, as the forces of mutual attraction between the molecules begin to gain the upper hand over the energy of thermal motion, the gas begins to turn into a liquid - "condense". In this case, the so-called specific heat of condensation - it is exactly equal to the latent specific heat of vaporization, which has already been mentioned. That is, how much energy you spent on the evaporation of a certain mass of liquid, exactly the same amount of energy the vapor will give in the form of heat during condensation back into the liquid.
The fact that the amount of heat released during condensation is very high is an easily verified fact: it is enough to bring your palm to the spout of a boiling kettle. In addition to the heat from the steam itself, your skin will also suffer from the heat generated as a result of its condensation into liquid water.
With further cooling of the liquid to freezing point(whose temperature is melting point), the process of transferring heat energy to the outside will once again begin without lowering the temperature of the substance itself. This process is called crystallization, and with it, exactly the same amount of thermal energy is released as is taken from the environment during melting (the transition of a substance from a solid to a liquid phase).
There is another type of phase transition - from a solid state of a substance directly to a gaseous state (bypassing liquid). This phase transformation is called sublimation, or sublimation... The most common example: damp linen hung out to dry in the frost. The water in it first crystallizes into ice, and then - under the influence of direct sunlight - microscopic ice crystals simply evaporate, bypassing the liquid phase. Another example: at rock concerts, "dry ice" (frozen carbon dioxide CO 2) is used to create a smoke screen - it evaporates directly into the air, enveloping the performing musicians and also bypassing the liquid phase. Accordingly, the conversion of a solid directly into a gas is spent sublimation energy.
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 states of aggregation of matter - solid, liquid and gaseous.
For example, water can be in solid (ice), liquid (water), and gaseous (vapor) states.
Gas expands until it fills the entire volume allocated to it. If we consider the gas at the molecular level, we will see molecules randomly tossing and colliding with each other and with the walls of the vessel, which, however, practically do not interact with each other. If the volume of the vessel is increased or decreased, the molecules will be evenly redistributed in the new volume.
Unlike a gas at a given temperature, it occupies a fixed volume, however, it also takes the form of a vessel to be filled - but only below its surface level. At the molecular level, a liquid is most easily represented in the form of spherical molecules, which, although they are in close contact with each other, have the freedom to roll relative to each other, like round beads in a jar. Pour liquid into the vessel - and the molecules will quickly spread and fill the lower part of the vessel volume, as a result, the liquid will take its shape, but will not spread to 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 are attached to each other by 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 disorderly heap - amorphous bodies (this is exactly the structure of polymers, which look like tangled and stuck together pasta in a bowl).
Above, three classical states of aggregation were described. There is, however, a fourth state, which physicists are inclined to attribute to the number of aggregates. This is a 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 reservoirs evaporates and clouds form. This is how the liquid turns into gas. In winter, water in reservoirs freezes, turning into a solid state, and in spring it melts again, turning back into a liquid. What happens to the molecules of a substance when it passes from one state to another? Are they changing? For example, are 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, and the energy of the molecules is no longer enough even for free movement inside the body. They vibrate about one place, held by the forces of attraction of other molecules.
All matter can exist in one of four types. Each of them is a certain aggregate state of matter. In the nature of the Earth, only one is represented at once in three of them. This is water. It is easy to see it evaporated, melted, and solidified. That is steam, water and ice. Scientists have learned how to change the state of aggregation of matter. The biggest challenge for them is only plasma. This state requires special conditions.
What is it, what does it depend on and how is it characterized?
If the body has passed into another state of aggregation of matter, this does not mean that something else has appeared. The substance remains the same. If the liquid had water molecules, then they will be the same for steam with ice. Only their location, speed of movement and forces of interaction with each other will change.
When studying the topic "State of aggregation (grade 8)", only three of them are considered. They are liquid, gas and solid. Their manifestations depend on the physical conditions of the environment. The characteristics of these states are presented in the table.
| State of aggregation name | solid | liquid | gas |
| Its properties | retains its shape with volume | has a constant volume, takes the form of a vessel | has no constant volume and shape |
| Molecule arrangement | at the nodes of the crystal lattice | disorderly | chaotic |
| Distance between them | comparable to molecular size | approximately equal to the size of the molecules | much larger than their size |
| How molecules move | oscillate around a lattice point | do not move from the point of equilibrium, but sometimes make large jumps | messy with occasional collisions |
| How they interact | strongly attracted | strongly attracted to each other | do not attract, repulsive forces are manifested upon impact |
First state: solid
Its fundamental difference from others is that the molecules have a strictly defined place. When they talk about a solid state of aggregation, they most often mean crystals. In them, the lattice structure is symmetric and strictly periodic. Therefore, it is always preserved, no matter how far the body extends. The vibrational motion of the molecules of the substance is not enough to destroy this lattice.
But there are also amorphous bodies. They lack a strict structure in the arrangement of atoms. They can be anywhere. But this place is as stable as in a crystalline body. The difference between amorphous and crystalline substances is that they do not have a specific melting (solidification) temperature and are characterized by fluidity. Striking examples of such substances: glass and plastic.
Second state: liquid
This state of aggregation of matter is a cross between a solid and a gas. Therefore, it combines some properties from the first and second. So, the distance between particles and their interaction is similar to what was the case with crystals. But the location and movement is closer to the gas. Therefore, the liquid does not retain its shape, but spreads over the vessel into which it is poured.
Third state: gas
For a science called "physics", the state of aggregation in the form of a gas is not in the last place. After all, she studies the world around her, and the air in it is very widespread.
The peculiarities of this state are that the forces of interaction between molecules are practically absent. This explains their free movement. Due to which the gaseous substance fills the entire volume provided to it. Moreover, everything can be transferred to this state, you just need to increase the temperature by the required amount.
Fourth state: plasma
This state of aggregation of matter is a gas that is fully or partially ionized. This means that the number of negatively and positively charged particles in it is practically the same. This situation arises when the gas is heated. Then there is a sharp acceleration of the thermal ionization process. It consists in the fact that molecules are divided into atoms. The latter are then converted into ions.
This state is very common within the Universe. Because it contains all the stars and the environment between them. It occurs extremely rarely within the boundaries of the Earth's surface. Apart from the ionosphere and the solar wind, plasma is only possible during a thunderstorm. In lightning flashes such conditions are created in which the gases of the atmosphere pass into the fourth state of matter.
But this does not mean that the plasma was not created in the laboratory. The first thing that was reproduced was a gas discharge. Plasma now fills fluorescent lights and neon advertisements.
How is the transition between states carried out?
To do this, you need to create certain conditions: constant pressure and a specific temperature. In this case, a change in the aggregate states of a substance is accompanied by the release or absorption of energy. Moreover, this transition does not occur with lightning speed, but requires a certain amount of time. During all this time, the conditions should be unchanged. The transition occurs with the simultaneous existence of a substance in two hypostases, which maintain thermal equilibrium.
The first three states of matter can mutually transform one into another. There are forward and reverse processes. They have the following names:
- melting(from solid to liquid) and crystallization eg melting ice and hardening of water;
- vaporization(from liquid to gaseous) and condensation, an example is the evaporation of water and obtaining it from steam;
- sublimation(from solid to gaseous) and desublimation, for example, the evaporation of dry flavoring for the first of them and frosty patterns on the glass for the second.
Melting and Crystallization Physics
If a solid is heated, then at a certain temperature, called melting point a specific substance, a change in the state of aggregation will begin, which is called melting. This process goes with the absorption of energy, which is called the amount of warmth and denoted by the letter Q... To calculate it, you need to know specific heat of fusion which is denoted λ ... And the formula takes this expression:
Q = λ * m, where m is the mass of the substance involved in melting.
If the opposite process occurs, that is, the crystallization of the liquid, then the conditions are repeated. The only difference is that energy is released, and a minus sign appears in the formula.
Physics of vaporization and condensation
As the substance continues to heat, it will gradually approach the temperature at which its intensive evaporation will begin. This process is called vaporization. It is again characterized by energy absorption. Only to calculate it you need to know specific heat of vaporization r... And the formula will be like this:
Q = r * m.
The reverse process or condensation occurs with the release of the same amount of heat. Therefore, a minus appears in the formula again.
