Aggregate states. Liquids. Phases in thermodynamics. Phase transitions.
Lecture 1.16
All substances can exist in three states of aggregation - solid, liquid And gaseous. Transitions between them are accompanied by abrupt changes in a number of physical properties (density, thermal conductivity, etc.).
The state of aggregation depends on the physical conditions in which the substance is located. The existence of several states of aggregation in a substance is due to differences in the thermal motion of its molecules (atoms) and in their interaction under different conditions.
Gas- the state of aggregation of a substance in which the particles are not connected or are very weakly connected by interaction forces; the kinetic energy of the thermal motion of its particles (molecules, atoms) significantly exceeds the potential energy of interactions between them, therefore the particles move almost freely, completely filling the vessel in which they are located and taking its shape. In the gaseous state, a substance has neither its own volume nor its own shape. Any substance can be converted into a gas by changing pressure and temperature.
Liquid- state of aggregation of a substance, intermediate between solid and gaseous. It is characterized by high mobility of particles and small free space between them. This causes liquids to maintain their volume and take the shape of the container. In a liquid, the molecules are located very close to each other. Therefore, the density of liquid is much greater than the density of gases (at normal pressure). The properties of a liquid are the same (isotropic) in all directions, with the exception of liquid crystals. When heated or the density decreases, the properties of the liquid, thermal conductivity, and viscosity change, as a rule, towards the properties of gases.
The thermal motion of liquid molecules consists of a combination of collective vibrational movements and jumps of molecules that occur from time to time from one equilibrium position to another.
Solid (crystalline) bodies- the state of aggregation of a substance, characterized by stability of shape and the nature of the thermal movement of atoms. This movement is the vibration of the atoms (or ions) that make up the solid. The vibration amplitude is usually small compared to the interatomic distances.
Properties of liquids.
The molecules of a substance in a liquid state are located almost close to each other. Unlike solid crystalline bodies, in which molecules form ordered structures throughout the entire volume of the crystal and can perform thermal vibrations around fixed centers, liquid molecules have greater freedom. Each molecule of a liquid, just like in a solid, is “sandwiched” on all sides by neighboring molecules and undergoes thermal vibrations around a certain equilibrium position. However, from time to time any molecule may move to a nearby vacant site. Such jumps in liquids occur quite often; therefore, the molecules are not tied to specific centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely located molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called close order.
Due to the dense packing of molecules, the compressibility of liquids, i.e., the change in volume with a change in pressure, is very small; it is tens and hundreds of thousands of times less than in gases. For example, to change the volume of water by 1%, you need to increase the pressure approximately 200 times. This increase in pressure compared to atmospheric pressure is achieved at a depth of about 2 km.
Liquids, like solids, change their volume with changes in temperature. For not very large temperature ranges, the relative change in volume Δ V / V 0 is proportional to the temperature change Δ T:
| |
The coefficient β is called temperature coefficient of volumetric expansion. This coefficient for liquids is tens of times greater than for solids. For water, for example, at a temperature of 20 °C β ≈ 2 10 –4 K –1, for steel - β st ≈ 3.6 10 –5 K –1, for quartz glass - β kV ≈ 9 10 – 6 K –1.
The thermal expansion of water has an interesting and important anomaly for life on Earth. At temperatures below 4 °C, water expands as the temperature decreases (β< 0). Максимум плотности ρ в = 10 3 кг/м 3 вода имеет при температуре 4 °С.
When water freezes, it expands, so ice remains floating on the surface of a freezing body of water. The temperature of freezing water under the ice is 0 °C. In denser layers of water at the bottom of the reservoir, the temperature is about 4 °C. Thanks to this, life can exist in the water of freezing reservoirs.
The most interesting feature of liquids is the presence free surface. Liquid, unlike gases, does not fill the entire volume of the container into which it is poured. An interface is formed between liquid and gas (or vapor), which is in special conditions compared to the rest of the liquid. Molecules in the boundary layer of a liquid, unlike molecules in its depth, are not surrounded by other molecules of the same liquid on all sides. The forces of intermolecular interaction acting on one of the molecules inside a liquid from neighboring molecules are, on average, mutually compensated. Any molecule in the boundary layer is attracted by molecules located inside the liquid (the forces acting on a given liquid molecule from gas (or vapor) molecules can be neglected). As a result, a certain resultant force appears, directed deep into the liquid. Surface molecules are drawn into the liquid by forces of intermolecular attraction. But all molecules, including molecules of the boundary layer, must be in a state of equilibrium. This equilibrium is achieved by slightly reducing the distance between the molecules of the surface layer and their nearest neighbors inside the liquid. As the distance between molecules decreases, repulsive forces arise. If the average distance between molecules inside a liquid is r 0, then the molecules of the surface layer are packed somewhat more densely, and therefore they have an additional reserve of potential energy compared to the internal molecules. It should be borne in mind that due to the extremely low compressibility, the presence of a more densely packed surface layer does not lead to any noticeable change in the volume of the liquid. If a molecule moves from the surface into the liquid, the forces of intermolecular interaction will do positive work. On the contrary, in order to pull a certain number of molecules from the depth of the liquid to the surface (i.e., increase the surface area of the liquid), external forces must do positive work A external, proportional to the change Δ S surface area:
| A ext = σΔ S. |
The coefficient σ is called the surface tension coefficient (σ > 0). Thus, the coefficient of surface tension is equal to the work required to increase the surface area of a liquid at constant temperature by one unit.
In SI, the coefficient of surface tension is measured in joules per meter square (J/m2) or in newtons per meter (1 N/m = 1 J/m2).
Consequently, the molecules of the surface layer of the liquid have an excess of potential energy. Potential energy E p of the liquid surface is proportional to its area: 
(1.16.1)
It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy. It follows that the free surface of the liquid tends to reduce its area. For this reason, a free drop of liquid takes on a spherical shape. The liquid behaves as if forces acting tangentially to its surface are contracting (pulling) this surface. These forces are called surface tension forces.
The presence of surface tension forces makes the surface of a liquid look like an elastic stretched film, with the only difference that the elastic forces in the film depend on its surface area (i.e., on how the film is deformed), and the surface tension forces do not depend on the surface area liquids.
Surface tension forces tend to reduce the surface of the film. Therefore we can write: (1.16.2)
Thus, the surface tension coefficient σ can be defined as the modulus of the surface tension force acting per unit length of the line bounding the surface ( l- the length of this line).
Due to the action of surface tension forces in liquid droplets and inside soap bubbles excess pressure Δ occurs p. If you mentally cut a spherical drop of radius R into two halves, then each of them must be in equilibrium under the action of surface tension forces applied to the cut boundary of length 2π R and strength overpressure, acting on area π R 2 sections (Fig. 1.16.1). The equilibrium condition is written as
Near the boundary between a liquid, a solid and a gas, the shape of the free surface of the liquid depends on the forces of interaction between liquid molecules and solid molecules (interaction with gas (or vapor) molecules can be neglected). If these forces are greater than the forces of interaction between the molecules of the liquid itself, then the liquid wets surface of a solid. In this case, the liquid approaches the surface of the solid at a certain acute angle θ, characteristic of a given liquid-solid pair. The angle θ is called contact angle. If the forces of interaction between liquid molecules exceed the forces of their interaction with solid molecules, then the contact angle θ turns out to be obtuse (Fig. 1.16.2(2)). In this case they say that the liquid does not wet surface of a solid. Otherwise (angle - acute) liquid wets surface (Fig. 1.16.2(1)). At full wettingθ = 0, at complete non-wettingθ = 180°.
Capillary phenomena called the rise or fall of liquid in small diameter tubes - capillaries. Wetting liquids rise through the capillaries, non-wetting liquids descend.
Figure 1.16.3 shows a capillary tube of a certain radius r, lowered at the lower end into a wetting liquid of density ρ. The upper end of the capillary is open. The rise of liquid in the capillary continues until the force of gravity acting on the column of liquid in the capillary becomes equal in magnitude to the resultant F n surface tension forces acting along the boundary of contact of the liquid with the surface of the capillary: F t = F n, where F t = mg = ρ hπ r 2 g, F n = σ2π r cos θ.
This implies: 
With complete wetting θ = 0, cos θ = 1. In this case
With complete non-wetting θ = 180°, cos θ = –1 and, therefore, h < 0. Уровень несмачивающей жидкости в капилляре опускается ниже уровня жидкости в сосуде, в которую опущен капилляр.
Water almost completely wets the clean glass surface. On the contrary, mercury does not completely wet the glass surface. Therefore, the level of mercury in the glass capillary drops below the level in the vessel.
State of matter
Substance- a really existing collection of particles connected by chemical bonds and under certain conditions in one of the states of aggregation. Any substance consists of a collection of a very large number of 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 relative 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 crystalline, liquid and vapor. Solid, liquid crystalline and liquid states of aggregation are condensed, while vapor and gaseous states are highly discharged.
Phase- this is a set of homogeneous microregions, characterized by the same ordering and concentration of particles and contained in a macroscopic volume of matter limited by the interface. In this understanding, the phase is characteristic only for substances in crystalline and gaseous states, because these are homogeneous states of aggregation.
Metaphase is a collection of heterogeneous microregions that differ from each other in the degree of ordering of particles or their concentration and are contained in a macroscopic volume of matter limited by the interface. In this understanding, metaphase is characteristic only of substances that are in heterogeneous 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 the concepts of “basic” and “transition” states of aggregation are not distinguished. The concepts of “aggregate state”, “phase” and “mesophase” are often used interchangeably. It is advisable to consider five possible states of aggregation for the state of substances: solid, liquid crystalline, liquid, vapor, gaseous. The transition of one phase to another phase is called a phase transition of the first and second order. First-order phase transitions are characterized by:
Abrupt changes in physical quantities that describe the state of a substance (volume, density, viscosity, etc.);
A certain temperature at which a given phase transition occurs
A certain heat that characterizes this transition, because intermolecular bonds are broken.
First-order phase transitions are observed during the transition from one state of aggregation to another state of aggregation. Phase transitions of the second order are observed when the order of particles changes within one state of aggregation and are characterized by:
Gradual change in the physical properties of a substance;
A change in the ordering of particles of a substance under the influence of a gradient of external fields or at a certain temperature, called the phase transition temperature;
The heat of second-order phase transitions is equal and close to zero.
The main difference between phase transitions of the first and second order is that during first-order transitions, first of all, the energy of the particles of the system changes, and in the case of second-order transitions, the ordering of the particles of the system changes.
The transition of a substance from solid to liquid 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 is characterized by boiling point. For some substances with low molecular weight and weak intermolecular interactions, a direct transition from the solid to the vapor state is possible, bypassing the liquid state. This transition is called sublimation. All of the above processes can also occur in the opposite direction: then they are called freezing, condensation, desublimation.
Substances that do not decompose upon melting and boiling can exist, depending on temperature and pressure, in all four states of aggregation.
Solid state
At a sufficiently low temperature, almost all substances are in a solid state. In this state, the distance between the particles of the 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. The movement of particles of solid matter is limited only by minor vibrations and rotations relative to their position, and they have no translational motion . This leads to internal order in the arrangement of particles. Therefore, solids are characterized by their own shape, mechanical strength, and 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 unit cell of a crystal characterizes three-dimensional periodicity in the arrangement of particles, i.e. his crystal lattice. Crystal lattices are classified depending on the type of particles that make up the crystal and the nature of the attractive forces between them.
Many crystalline substances, depending on conditions (temperature, pressure), can have different crystal structures. This phenomenon is called polymorphism. Well-known polymorphic modifications of carbon: graphite, fullerene, diamond, carbyne.
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 same physical and chemical properties of a body or environment 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, over time, transform into an energetically more stable state - crystalline.
Liquid state
As the temperature increases, 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, moving relative to each other. They still remain in contact, although the correct geometric structure of the particles is disrupted - 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 the inter-associate forces that impede the free flow of the liquid.
Liquids occupy an intermediate position between the gaseous and solid states of substances. More ordered structure than a gas, but less than a solid.
Vapor and gaseous states
The vapor-gaseous state is usually not distinguished.
Gas – this is a highly discharged 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.
The molecular kinetic theory explains the properties of an ideal gas based on the following principles: molecules undergo 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 rates of thermal movement 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, and amount of substance. The relationship between these parameters is described by the ideal gas equation of state:
R = 8.31 kJ/mol – universal gas constant.
Questions about what the state of aggregation is, what features and properties solids, liquids and gases have, are considered in several training courses. There are three classical states of matter, with their own characteristic structural features. Their understanding is important point in understanding the sciences of the Earth, living organisms, production activities. These questions are studied by physics, chemistry, geography, geology, physical chemistry and other scientific disciplines. Substances that, under certain conditions, are in one of three basic types of state can change with an increase or decrease in temperature and pressure. Let us consider possible transitions from one state of aggregation to another, as they occur in nature, technology and everyday life.
What is a state of aggregation?
The word of Latin origin "aggrego" translated into Russian means "to join". The scientific term refers to the state of the same body, substance. The existence of solids, gases and liquids at certain temperatures and different pressures is characteristic of all the shells of the Earth. In addition to the three basic states of aggregation, there is also a fourth. At elevated temperature and constant pressure, the gas turns into plasma. To better understand what a state of aggregation is, it is necessary to remember the smallest particles that make up substances and bodies.
The diagram above shows: a - gas; b—liquid; c is a solid body. In such pictures, circles indicate the structural elements of substances. This symbol, in fact, atoms, molecules, ions are not solid balls. Atoms consist of a positively charged nucleus around which negatively charged electrons move at high speed. Knowledge about the microscopic structure of matter helps to better understand the differences that exist between different aggregate forms.
Ideas about the microcosm: from Ancient Greece to the 17th century
The first information about the particles that make up physical bodies appeared in Ancient Greece. The thinkers Democritus and Epicurus introduced such a concept as the atom. They believed that these smallest indivisible particles of different substances have a shape, certain sizes, and are capable of movement and interaction with each other. Atomism became the most advanced teaching of ancient Greece for its time. But its development slowed down in the Middle Ages. Since then scientists were persecuted by the Inquisition of the Roman Catholic Church. Therefore, until modern times, there was no clear concept of what the state of matter was. Only after the 17th century did scientists R. Boyle, M. Lomonosov, D. Dalton, A. Lavoisier formulate the provisions of the atomic-molecular theory, which have not lost their significance today.
Atoms, molecules, ions - microscopic particles of the structure of matter
A significant breakthrough in understanding the microworld occurred in the 20th century, when it was invented electron microscope. Taking into account the discoveries made by scientists earlier, it was possible to put together a coherent picture of the microworld. Theories that describe the state and behavior of the smallest particles of matter are quite complex; they relate to the field of To understand the characteristics of different aggregate states of matter, it is enough to know the names and characteristics of the main structural particles that form different substances.
- Atoms are chemically indivisible particles. Saved in chemical reactions, but are destroyed in nuclear ones. Metals and many other substances of atomic structure have a solid state of aggregation under normal conditions.
- Molecules are particles that are broken down and formed in chemical reactions. oxygen, water, carbon dioxide, sulfur. The physical state of oxygen, nitrogen, sulfur dioxide, carbon, oxygen under normal conditions is gaseous.
- Ions are the charged particles that atoms and molecules become when they gain or lose electrons—microscopic negatively charged particles. Many salts have an ionic structure, for example table salt, iron sulfate and copper sulfate.
There are substances whose particles are located in space in a certain way. The ordered mutual position of atoms, ions, and molecules is called a crystal lattice. Typically, ionic and atomic crystal lattices are characteristic of solids, molecular - for liquids and gases. Diamond is distinguished by its high hardness. Its atomic crystal lattice is formed by carbon atoms. But soft graphite also consists of atoms of this chemical element. Only they are located differently in space. The usual state of aggregation of sulfur is solid, but at high temperatures the substance turns into a liquid and an amorphous mass.
Substances in a solid state of aggregation
Solids under normal conditions retain their volume and shape. For example, a grain of sand, a grain of sugar, salt, a piece of rock or metal. If you heat sugar, the substance begins to melt, turning into a viscous brown liquid. Let's stop heating and we'll get a solid again. This means that one of the main conditions for the transition of a solid into a liquid is its heating or increase internal energy particles of matter. The solid state of aggregation of salt, which is used for food, can also be changed. But to melt table salt, a higher temperature is needed than when heating sugar. The fact is that sugar consists of molecules, and salt- of charged ions that are more strongly attracted to each other. Solids in liquid form do not retain their shape because the crystal lattices are destroyed.
The liquid aggregate state of the salt upon melting is explained by the breaking of bonds between the ions in the crystals. Charged particles are released that can carry electric charges. Molten salts conduct electricity and are conductors. In the chemical, metallurgical and engineering industries, solid substances are converted into liquids to obtain new compounds from them or to give them different shapes. Metal alloys have become widespread. There are several ways to obtain them, associated with changes in the state of aggregation of solid raw materials.
Liquid is one of the basic states of aggregation
If you pour 50 ml of water into a round-bottomed flask, you will notice that the substance will immediately take the shape of a chemical vessel. But as soon as we pour the water out of the flask, the liquid will immediately spread over the surface of the table. The volume of water will remain the same - 50 ml, but its shape will change. The listed features are characteristic of the liquid form of existence of matter. Many organic substances are liquids: alcohols, vegetable oils, acids.
Milk is an emulsion, i.e. a liquid containing droplets of fat. A useful liquid resource is oil. It is extracted from wells using drilling rigs on land and in the ocean. Sea water is also a raw material for industry. Its difference from fresh water rivers and lakes lies in the content of dissolved substances, mainly salts. When evaporating from the surface of reservoirs, only H 2 O molecules pass into a vapor state, dissolved substances remain. Methods for obtaining useful substances from sea water and methods for its purification are based on this property.
When the salts are completely removed, distilled water is obtained. It boils at 100°C and freezes at 0°C. Brines boil and turn into ice at other temperatures. For example, water in the Arctic Ocean freezes at a surface temperature of 2 °C.
The physical state of mercury under normal conditions is liquid. This silvery-gray metal is commonly used to fill medical thermometers. When heated, the mercury column rises on the scale and the substance expands. Why is alcohol tinted with red paint used, and not mercury? This is explained by the properties of liquid metal. At 30-degree frosts, the state of aggregation of mercury changes, the substance becomes solid.
If the medical thermometer breaks and the mercury spills out, then collecting the silver balls with your hands is dangerous. It is harmful to inhale mercury vapor; this substance is very toxic. In such cases, children need to turn to their parents and adults for help.
Gaseous state
Gases are unable to maintain either their volume or shape. Fill the flask to the top with oxygen (its chemical formula O 2). As soon as we open the flask, the molecules of the substance will begin to mix with the air in the room. This occurs due to Brownian motion. Even the ancient Greek scientist Democritus believed that particles of matter are in constant motion. In solids, under normal conditions, atoms, molecules, and ions do not have the opportunity to leave the crystal lattice or free themselves from bonds with other particles. This is only possible when a large amount of energy is supplied from outside.
In liquids, the distance between particles is slightly greater than in solids; they require less energy to break intermolecular bonds. For example, the liquid state of oxygen is observed only when the gas temperature decreases to −183 °C. At −223 °C, O 2 molecules form a solid. When the temperature rises above these values, oxygen turns into gas. It is in this form that it is found under normal conditions. On industrial enterprises There are special installations for separating atmospheric air and obtaining nitrogen and oxygen from it. First, the air is cooled and liquefied, and then the temperature is gradually increased. Nitrogen and oxygen turn into gases under different conditions.
The Earth's atmosphere contains 21% oxygen and 78% nitrogen by volume. These substances are not found in liquid form in the gaseous shell of the planet. Liquid oxygen has a light blue color and is high blood pressure fill cylinders for use in medical institutions. In industry and construction, liquefied gases are needed to carry out many processes. Oxygen is needed for gas welding and cutting of metals, in chemistry - for oxidation reactions of inorganic and organic substances. If you open the valve of an oxygen cylinder, the pressure decreases and the liquid turns into gas.
Liquefied propane, methane and butane are widely used in energy, transport, industry and household activities. These substances are obtained from natural gas or during cracking (splitting) of petroleum feedstock. Carbon liquid and gaseous mixtures play an important role in the economies of many countries. But oil and natural gas reserves are severely depleted. According to scientists, this raw material will last for 100-120 years. Alternative source energy - air flow (wind). Fast-flowing rivers and tides on the shores of seas and oceans are used to operate power plants.
Oxygen, like other gases, can be in the fourth state of aggregation, representing a plasma. Unusual transition from solid to gaseous state - characteristic crystalline iodine. The dark purple substance undergoes sublimation - it turns into a gas, bypassing the liquid state.
How are transitions made from one aggregate form of matter to another?
Changes in the aggregate state of substances are not associated with chemical transformations, these are physical phenomena. As the temperature increases, many solids melt and turn into liquids. A further increase in temperature can lead to evaporation, that is, to the gaseous state of the substance. In nature and economy, such transitions are characteristic of one of the main substances on Earth. Ice, liquid, steam are states of water under different external conditions. The compound is the same, its formula is H 2 O. At a temperature of 0 ° C and below this value, water crystallizes, that is, turns into ice. As the temperature rises, the resulting crystals are destroyed - the ice melts, and liquid water is again obtained. When it is heated, evaporation is formed - the transformation of water into gas - occurs even at low temperatures. For example, frozen puddles gradually disappear because the water evaporates. Even in frosty weather, wet laundry dries, but this process takes longer than on a hot day.
All of the listed transitions of water from one state to another are of great importance for the nature of the Earth. Atmospheric phenomena, climate and weather are associated with the evaporation of water from the surface of the World Ocean, the transfer of moisture in the form of clouds and fog to land, and precipitation (rain, snow, hail). These phenomena form the basis of the World water cycle in nature.
How do the aggregate states of sulfur change?
Under normal conditions, sulfur is bright shiny crystals or light yellow powder, i.e. it is a solid substance. The physical state of sulfur changes when heated. First, when the temperature rises to 190 °C, the yellow substance melts, turning into a mobile liquid.
If you quickly pour liquid sulfur into cold water, then a brown amorphous mass is obtained. With further heating of the sulfur melt, it becomes more and more viscous and darkens. At temperatures above 300 °C, the state of aggregation of sulfur changes again, the substance acquires the properties of a liquid and becomes mobile. These transitions arise due to the ability of the atoms of an element to form chains of different lengths.
Why can substances be in different physical states?
The state of aggregation of sulfur, a simple substance, is solid under ordinary conditions. Sulfur dioxide is a gas sulfuric acid- an oily liquid is heavier than water. Unlike salt and nitric acids it is not volatile, molecules do not evaporate from its surface. What state of aggregation does plastic sulfur have, which is obtained by heating crystals?
In its amorphous form, the substance has the structure of a liquid, with insignificant fluidity. But plastic sulfur simultaneously retains its shape (as a solid). Exist liquid crystals, possessing a number of characteristic properties of solids. Thus, the state of a substance under different conditions depends on its nature, temperature, pressure and other external conditions.
What features exist in the structure of solids?
The existing differences between the basic aggregate states of matter are explained by the interaction between atoms, ions and molecules. For example, why does the solid state of matter lead to the ability of bodies to maintain volume and shape? In the crystal lattice of a metal or salt, structural particles are attracted to each other. In metals, positively charged ions interact with what is called an “electron gas,” a collection of free electrons in a piece of metal. Salt crystals arise due to the attraction of oppositely charged particles - ions. The distance between the above structural units of solids is much smaller than the sizes of the particles themselves. In this case, electrostatic attraction acts, it imparts strength, but repulsion is not strong enough.
To destroy the solid state of aggregation of a substance, effort must be made. Metals, salts, and atomic crystals melt at very high temperatures. For example, iron becomes liquid at temperatures above 1538 °C. Tungsten is refractory and is used to make incandescent filaments for light bulbs. There are alloys that become liquid at temperatures above 3000 °C. Many on Earth are in a solid state. These raw materials are extracted using technology in mines and quarries.
To separate even one ion from a crystal, a large amount of energy must be expended. But it is enough to dissolve salt in water for the crystal lattice to disintegrate! This phenomenon is explained by the amazing properties of water as a polar solvent. H 2 O molecules interact with salt ions, destroying the chemical bond between them. Thus, dissolution is not a simple mixing of different substances, but a physicochemical interaction between them.
How do liquid molecules interact?
Water can be a liquid, a solid, and a gas (steam). These are its basic states of aggregation under normal conditions. Water molecules consist of one oxygen atom to which two hydrogen atoms are bonded. Polarization of the chemical bond in the molecule occurs, and a partial negative charge appears on the oxygen atoms. Hydrogen becomes the positive pole in the molecule, attracted by the oxygen atom of another molecule. This is called "hydrogen bonding."
The liquid state of aggregation is characterized by distances between structural particles comparable to their sizes. Attraction exists, but it is weak, so the water does not retain its shape. Vaporization occurs due to the destruction of bonds that occurs on the surface of the liquid even at room temperature.
Do intermolecular interactions exist in gases?
The gaseous state of a substance differs from liquid and solid in a number of parameters. There are large gaps between the structural particles of gases, much larger than the sizes of molecules. In this case, the forces of attraction do not act at all. The gaseous state of aggregation is characteristic of substances present in the air: nitrogen, oxygen, carbon dioxide. In the picture below, the first cube is filled with gas, the second with liquid, and the third with solid.
Many liquids are volatile; molecules of the substance break off from their surface and go into the air. For example, if you bring a cotton wool soaked in hydrochloric acid to the opening of an open bottle of hydrochloric acid, ammonia, then white smoke appears. A chemical reaction between hydrochloric acid and ammonia occurs right in the air, producing ammonium chloride. What state of aggregation is this substance in? Its particles that form white smoke are tiny solid crystals of salt. This experiment must be carried out under a hood; the substances are toxic.
Conclusion
The state of aggregation of gas was studied by many outstanding physicists and chemists: Avogadro, Boyle, Gay-Lussac, Clayperon, Mendeleev, Le Chatelier. Scientists have formulated laws that explain behavior gaseous substances in chemical reactions, when external conditions change. Open patterns were not only included in school and university textbooks on physics and chemistry. Many chemical production based on knowledge about the behavior and properties of substances in different states of aggregation.
State of aggregation- this is the state of a substance in a certain range of temperatures and pressures, characterized by properties: the ability (solid) or inability (liquid, gas) to maintain volume and shape; the presence or absence of long-range (solid) or short-range (liquid) order and other properties.
A substance can be in three states of aggregation: solid, liquid or gaseous; currently, an additional plasma (ionic) state is distinguished.
IN gaseous In this state, the distance between the atoms and molecules of the substance is large, the interaction forces are small and the particles, moving chaotically in space, have a large kinetic energy that exceeds the potential energy. A material in a gaseous state has neither its own shape nor volume. Gas fills all available space. This state is typical for substances with low density.
IN liquid state, only short-range order of atoms or molecules is preserved, when individual areas with an ordered arrangement of atoms periodically appear in the volume of the substance, but the mutual orientation of these areas is also absent. Short-range order is unstable and under the influence of thermal vibrations of atoms it can either disappear or appear again. Liquid molecules do not have a specific position, and at the same time they do not have complete freedom of movement. The material in the liquid state does not have its own shape; it retains only its volume. The liquid can occupy only part of the volume of the vessel, but flow freely over the entire surface of the vessel. The liquid state is usually considered intermediate between a solid and a gas.
IN hard In a substance, the arrangement of atoms becomes strictly defined, naturally ordered, the forces of interaction between particles are mutually balanced, so the bodies retain their shape and volume. The regularly ordered arrangement of atoms in space characterizes the crystalline state; the atoms form a crystal lattice.
Solids have an amorphous or crystalline structure. For amorphous bodies are characterized only by short-range order in the arrangement of atoms or molecules, a chaotic arrangement of atoms, molecules or ions in space. Examples of amorphous bodies are glass, pitch, var, which are outwardly in a solid state, although in fact they flow slowly, like a liquid. Amorphous bodies, unlike crystalline ones, do not have a specific melting point. Amorphous solids occupy an intermediate position between crystalline solids and liquids.
Most solids have crystalline a structure characterized by the orderly arrangement of atoms or molecules in space. The crystal structure is characterized by long-range order, when the elements of the structure are periodically repeated; with short-range order there is no such correct repetition. Characteristic feature crystalline body is the ability to maintain shape. A sign of an ideal crystal, the model of which is a spatial lattice, is the property of symmetry. Symmetry refers to the theoretical ability of the crystal lattice of a solid body to align with itself when its points are mirrored from a certain plane, called the plane of symmetry. The symmetry of the external shape reflects the symmetry of the internal structure of the crystal. For example, all metals have a crystalline structure and are characterized by two types of symmetry: cubic and hexagonal.
In amorphous structures with a disordered distribution of atoms, the properties of the substance in different directions are the same, that is, glassy (amorphous) substances are isotropic.
All crystals are characterized by anisotropy. In crystals, the distances between atoms are ordered, but in different directions the degree of ordering may not be the same, which leads to differences in the properties of the crystal substance in different directions. The dependence of the properties of a crystal substance on the direction in its lattice is called anisotropy properties. Anisotropy manifests itself when measuring both physical and mechanical and other characteristics. There are properties (density, heat capacity) that do not depend on the direction in the crystal. Most of the characteristics depend on the choice of direction.
It is possible to measure properties of objects that have a certain material volume: sizes - from several millimeters to tens of centimeters. These objects with a structure identical to the crystal cell are called single crystals.
Anisotropy of properties manifests itself in single crystals and is practically absent in a polycrystalline substance, consisting of many small randomly oriented crystals. Therefore, polycrystalline substances are called quasi-isotropic.
Crystallization of polymers, the molecules of which can be arranged in an orderly manner with the formation of supramolecular structures in the form of packs, coils (globules), fibrils, etc., occurs in a certain temperature range. Complex structure molecules and their aggregates determines the specific behavior of polymers when heated. They cannot go into a liquid state with low viscosity and do not have a gaseous state. In solid form, polymers can be in glassy, highly elastic and viscous states. Polymers with linear or branched molecules can change from one state to another when the temperature changes, which manifests itself in the process of deformation of the polymer. In Fig. Figure 9 shows the dependence of deformation on temperature.
Rice. 9 Thermomechanical curve of an amorphous polymer: t c , t T, t p - glass transition, fluidity and onset of chemical decomposition temperatures, respectively; I - III - zones of glassy, highly elastic and viscous flow state, respectively; Δ l- deformation.
The spatial structure of the arrangement of molecules determines only the glassy state of the polymer. At low temperatures, all polymers deform elastically (Fig. 9, zone I). Above glass transition temperature t c amorphous polymer c linear structure goes into a highly elastic state ( zone II), and its deformation in the glassy and highly elastic states is reversible. Heating above the pour point t t transfers the polymer to a viscous flow state ( zone III). The deformation of a polymer in a viscous flow state is irreversible. An amorphous polymer with a spatial (network, cross-linked) structure does not have a viscous flow state; the temperature region of the highly elastic state expands to the temperature of polymer decomposition t R. This behavior is typical for materials such as rubber.
The temperature of a substance in any state of aggregation characterizes the average kinetic energy of its particles (atoms and molecules). These particles in bodies possess mainly the kinetic energy of vibrational movements relative to the center of equilibrium, where the energy is minimal. When a certain critical temperature is reached, the solid material loses its strength (stability) and melts, and the liquid turns into steam: it boils and evaporates. These critical temperatures are the melting and boiling points.
When a crystalline material is heated at a certain temperature, the molecules move so energetically that the rigid bonds in the polymer are broken and the crystals are destroyed - they turn into a liquid state. The temperature at which the crystals and liquid are in equilibrium is called the melting point of the crystal, or the solidification point of the liquid. For iodine, this temperature is 114 o C.
Every chemical element has an individual melting point t pl, separating the existence of a solid and a liquid, and the boiling point t kip, corresponding to the transition of liquid into gas. At these temperatures, substances are in thermodynamic equilibrium. A change in the state of aggregation can be accompanied by an abrupt change in free energy, entropy, density and others physical quantities.
To describe the various states in physics uses a broader concept thermodynamic phase. Phenomena that describe transitions from one phase to another are called critical.
When heated, substances undergo phase transformations. When copper melts (1083 o C) it turns into a liquid in which the atoms have only short-range order. At a pressure of 1 atm, copper boils at 2310 o C and turns into gaseous copper with randomly arranged copper atoms. At the melting point, the saturated vapor pressures of the crystal and the liquid are equal.
The material as a whole is a system.
System- a group of substances combined physical, chemical or mechanical interactions. Phase called a homogeneous part of a system, separated from other parts physical interface boundaries (in cast iron: graphite + iron grains; in water with ice: ice + water).Components systems are different phases that form this system. System components- these are the substances that form all the phases (components) of a given system.
Materials consisting of two or more phases are dispersed systems Dispersed systems are divided into sols, whose behavior resembles the behavior of liquids, and gels with characteristic properties solids In sols, the dispersion medium in which the substance is distributed is liquid; in gels, the solid phase predominates. Gels are semi-crystalline metal, concrete, a solution of gelatin in water at low temperatures (at high temperatures gelatin turns into a sol). A hydrosol is a dispersion in water, an aerosol is a dispersion in air.
Status diagrams.
In a thermodynamic system, each phase is characterized by parameters such as temperature T, concentration With and pressure R. To describe phase transformations, a single energy characteristic is used - the Gibbs free energy ΔG(thermodynamic potential).
Thermodynamics in describing transformations is limited to considering the equilibrium state. Equilibrium state thermodynamic system is characterized by the invariance of thermodynamic parameters (temperature and concentration, since in technological treatments R= const) in time and the absence of flows of energy and matter in it - with constant external conditions. Phase equilibrium- the equilibrium state of a thermodynamic system consisting of two or more phases
To mathematically describe the equilibrium conditions of a system, there is phase rule, derived by Gibbs. It connects the number of phases (F) and components (K) in an equilibrium system with the variability of the system, i.e., the number of thermodynamic degrees of freedom (C).
The number of thermodynamic degrees of freedom (variance) of a system is the number of independent variables as internal ( chemical composition phases) and external (temperature), which can be given various arbitrary (in a certain range) values so that new phases do not appear and old phases do not disappear.
Gibbs phase rule equation:
C = K - F + 1.
In accordance with this rule, in a system of two components (K = 2) it is possible the following options degrees of freedom:
For a single-phase state (F = 1) C = 2, i.e., you can change the temperature and concentration;
For a two-phase state (F = 2) C = 1, i.e., only one external parameter can be changed (for example, temperature);
For a three-phase state, the number of degrees of freedom is zero, i.e., the temperature cannot be changed without disturbing the equilibrium in the system (the system is invariant).
For example, for a pure metal (K = 1) during crystallization, when there are two phases (F = 2), the number of degrees of freedom is zero. This means that the crystallization temperature cannot be changed until the process is completed and one phase remains - the solid crystal. After the end of crystallization (Ф = 1), the number of degrees of freedom is 1, so you can change the temperature, i.e., cool the solid without disturbing the equilibrium.
The behavior of systems depending on temperature and concentration is described by a phase diagram. The phase diagram of water is a system with one component H 2 O, therefore greatest number phases that can simultaneously be in equilibrium is equal to three (Fig. 10). These three phases are liquid, ice, steam. The number of degrees of freedom in this case is zero, i.e. Neither the pressure nor the temperature can be changed without any of the phases disappearing. Ordinary ice, liquid water and water vapor can exist in equilibrium simultaneously only at a pressure of 0.61 kPa and a temperature of 0.0075 ° C. The point where three phases coexist is called the triple point ( O).
Curve OS separates the vapor and liquid regions and represents the dependence of saturated water vapor pressure on temperature. The OS curve shows those interrelated values of temperature and pressure at which liquid water and water vapor are in equilibrium with each other, therefore it is called the liquid-vapor equilibrium curve or boiling curve.
Fig 10 Diagram of the state of water
Curve OB separates the liquid region from the ice region. It is the solid-liquid equilibrium curve and is called the melting curve. This curve shows those interrelated pairs of temperature and pressure values at which ice and liquid water are in equilibrium.
Curve O.A. called a sublimation curve and shows the interrelated pairs of pressure and temperature values at which ice and water vapor are in equilibrium.
A phase diagram is a visual way of representing the regions of existence of different phases depending on external conditions, such as pressure and temperature. State diagrams are actively used in materials science at various technological stages of product production.
A liquid differs from a crystalline solid by low viscosity values (internal friction of molecules) and high fluidity values (the reciprocal of viscosity). A liquid consists of many aggregates of molecules, within which the particles are arranged in in a certain order, similar to the order in crystals. The nature of structural units and interparticle interactions determines the properties of the liquid. There are liquids: monoatomic (liquefied noble gases), molecular (water), ionic (molten salts), metallic (molten metals), liquid semiconductors. In most cases, liquid is not only a state of aggregation, but also a thermodynamic (liquid) phase.
Liquid substances are most often solutions. Solution homogeneous, but not chemically pure substance, consists of a solute and a solvent (examples of a solvent are water or organic solvents: dichloroethane, alcohol, carbon tetrachloride, etc.), therefore it is a mixture of substances. An example is a solution of alcohol in water. However, solutions are also mixtures of gaseous (for example, air) or solid (metal alloys) substances.
When cooled under conditions of low rate of formation of crystallization centers and a strong increase in viscosity, a glassy state may occur. Glasses are isotropic solid materials obtained by supercooling molten inorganic and organic compounds.
There are many known substances whose transition from a crystalline state to an isotropic liquid occurs through an intermediate liquid crystalline state. It is typical for substances whose molecules have the shape long rods(rods) with an asymmetrical structure. Such phase transitions, accompanied by thermal effects, cause abrupt changes in mechanical, optical, dielectric and other properties.
Liquid crystals, like a liquid, can take the form of an elongated drop or the shape of a vessel, have high fluidity, and are capable of merging. They are widely used in various fields of science and technology. Their optical properties are highly dependent on small changes in external conditions. This feature is used in electro-optical devices. In particular, liquid crystals are used in the manufacture of electronic wristwatch, visual equipment, etc.
The main states of aggregation include plasma- partially or fully ionized gas. Based on the method of formation, two types of plasma are distinguished: thermal, which occurs when gas is heated to high temperatures, and gaseous, which is formed during electrical discharges in a gaseous environment.
Plasma-chemical processes have taken a strong place in a number of branches of technology. They are used for cutting and welding refractory metals, synthesis of various substances, plasma light sources are widely used, the use of plasma in thermonuclear power plants etc.
The state of aggregation of a substance is usually called its ability to maintain its shape and volume. An additional feature is the methods of transition of a substance from one state of aggregation to another. Based on this, three states of aggregation are distinguished: solid, liquid and gas. Their visible properties are:
A solid body retains both shape and volume. It can pass either into a liquid by melting or directly into a gas by sublimation.
- Liquid – retains volume, but not shape, that is, it has fluidity. Spilled liquid tends to spread indefinitely over the surface on which it is poured. A liquid can become a solid by crystallization, and a gas by evaporation.
- Gas – does not retain either shape or volume. Gas outside any container tends to expand unlimitedly in all directions. Only gravity can prevent him from doing this, due to which the earth’s atmosphere does not dissipate into space. Gas passes into a liquid by condensation, and directly into a solid by sedimentation.
Phase transitions
The transition of a substance from one state of aggregation to another is called a phase transition, since the scientific state of aggregation is the phase of matter. For example, water can exist in the solid phase (ice), liquid (plain water) and gaseous phase (water vapor).
The example of water is also well demonstrated. Hung out in the yard to dry on a frosty, windless day, it immediately freezes, but after some time it turns out to be dry: the ice sublimates, directly turning into water vapor.
As a rule, a phase transition from a solid to a liquid and gas requires heating, but the temperature of the medium does not increase: thermal energy goes to break the internal bonds in the substance. This is the so-called latent heat. During reverse phase transitions (condensation, crystallization), this heat is released.
This is why steam burns are so dangerous. When it gets on the skin, it condenses. The latent heat of evaporation/condensation of water is very high: water in this regard is an anomalous substance; This is why life on Earth is possible. In a steam burn, the latent heat of condensation of water “scalds” the burned area very deeply, and the consequences of a steam burn are much more severe than from a flame on the same area of the body.
Pseudophases
The fluidity of the liquid phase of a substance is determined by its viscosity, and viscosity is determined by the nature of the internal bonds, which are discussed in the next section. The viscosity of the liquid can be very high, and such liquid can flow unnoticed by the eye.
A classic example is glass. It is not a solid, but a very viscous liquid. Please note that sheets of glass in warehouses are never stored leaning diagonally against the wall. Within a few days they will bend under their own weight and will be unfit for consumption.
Other pseudo-solids are shoe polish and construction pitch. If you forget the angular piece on the roof, over the summer it will spread into a cake and stick to the base. Pseudo-solid bodies can be distinguished from real ones by the nature of melting: real ones with it either retain their shape until they immediately spread (solder with), or float, releasing puddles and streams (ice). And very viscous liquids gradually soften, like pitch or bitumen.
Plastics are extremely viscous liquids, the fluidity of which is not noticeable for many years and decades. Their high ability to retain shape is ensured by the huge molecular weight of polymers, many thousands and millions of hydrogen atoms.
Phase structure of matter
In the gas phase, the molecules or atoms of a substance are very far apart from each other, many times greater than the distance between them. They interact with each other occasionally and irregularly, only during collisions. The interaction itself is elastic: they collided like hard balls and immediately scattered.
In a liquid, molecules/atoms constantly “feel” each other due to very weak bonds of a chemical nature. These bonds break all the time and are immediately restored again; the molecules of the liquid continuously move relative to each other, which is why the liquid flows. But to turn it into gas, you need to break all the bonds at once, and this requires a lot of energy, which is why the liquid retains its volume.
In this regard, water differs from other substances in that its molecules in the liquid are connected by so-called hydrogen bonds, which are quite strong. Therefore, water can be a liquid at a temperature normal for life. Many substances with a molecular weight tens and hundreds of times greater than that of water, in normal conditions– gases, like ordinary household gas.
In a solid, all its molecules remain firmly in place due to strong chemical bonds between them, forming a crystal lattice. Crystals of the correct shape require for their growth special conditions and therefore are rarely found in nature. Most solids are conglomerates of small and tiny crystals – crystallites – tightly coupled by mechanical and electrical forces.
If the reader has ever seen, for example, a cracked axle shaft of a car or a cast iron grate, then the grains of crystallites on scrap are visible to the naked eye. And on fragments of broken porcelain or earthenware they can be observed under a magnifying glass.
Plasma
Physicists also identify a fourth state of matter – plasma. In plasma, electrons are separated from atomic nuclei, and it is a mixture of electrically charged particles. Plasma can be very dense. For example, one cubic centimeter of plasma from the interior of stars - white dwarfs - weighs tens and hundreds of tons.
Plasma is isolated into a separate state of aggregation because it actively interacts with electromagnetic fields due to the fact that its particles are charged. In free space, plasma tends to expand, cooling and turning into gas. But under the influence, it can retain its shape and volume outside the vessel, like a solid body. This property of plasma is used in thermonuclear power reactors - prototypes of power plants of the future.
