Liquid A substance in a state intermediate between solid and gaseous. This is a state of aggregation of a substance in which the molecules (or atoms) are interconnected so much that it allows it to retain its volume, but not strongly enough to retain its shape.
Properties of liquids.
Liquids easily change their shape, but retain their volume. AT normal conditions they take the shape of the container they are in.
The surface of a liquid that is not in contact with the walls of the vessel is called free surface. It is formed as a result of the action of gravity on the molecules of the liquid.
The structure of liquids.
The properties of liquids are explained by the fact that the gaps between their molecules are small: the molecules in liquids are packed so tightly that the distance between each two molecules smaller sizes molecules. An explanation of the behavior of liquids on the basis of the nature of the molecular motion of the liquid was given by the Soviet scientist Ya. I. Frenkel. It consists in the following. The liquid molecule oscillates around the position of temporary equilibrium, colliding with other molecules from the nearest environment. From time to time, she manages to make a "jump" to leave her neighbors from the immediate environment and continue to oscillate among other neighbors. The time of the settled life of a water molecule, i.e., the time of oscillation around one equilibrium position at room temperature, is equal to an average of 10 -11 s. The time of one oscillation is much less - 10 -12 - 10 -13 .
Since the distances between the molecules of the liquid are small, an attempt to reduce the volume of the liquid leads to deformation of the molecules, they begin to repel each other, which explains the low compressibility of the liquid. The fluidity of a liquid is explained by the fact that the “jumps” of molecules from one settled position to another occur in all directions with the same frequency. The external force does not noticeably change the number of "jumps" per second, it only sets their predominant direction, which explains the fluidity of the liquid and the fact that it takes the form of a vessel.
All non-living matter consists of particles, the behavior of which may differ. The structure of gaseous, liquid and solid bodies has its own characteristics. Particles in solids are held together because they are very close to each other, which makes them very strong. Also, they can keep certain form, since their smallest particles practically do not move, but only vibrate. Molecules in liquids are quite close to each other, but they can move freely, so they do not have their own shape. Particles in gases move very fast, and there is usually a lot of space around them, which suggests that they are easily compressed.
Properties and structure of solids
What is the structure and features of the structure of solids? They are made up of particles that are very close to each other. They cannot move and therefore their shape remains fixed. What are the properties of a solid body? It does not shrink, but if it is heated, its volume will increase with increasing temperature. This is because the particles begin to vibrate and move, which leads to a decrease in density.
One of the features of solids is that they have a fixed shape. When a solid is heated, the movement of the particles increases. Faster moving particles collide more violently, causing each particle to push its neighbors. Therefore, an increase in temperature usually leads to an increase in the strength of the body.
Crystal structure of solids
Intermolecular forces of interaction between adjacent molecules of a solid are strong enough to keep them in a fixed position. If these smallest particles are in a highly ordered configuration, then such structures are usually called crystalline. The internal ordering of particles (atoms, ions, molecules) of an element or compound is dealt with by a special science - crystallography.
The solid state is also of particular interest. By studying the behavior of particles, how they are made, chemists can explain and predict how certain kinds of materials will behave under certain conditions. The smallest particles of a solid body are arranged in the form of a lattice. This is the so-called regular arrangement of particles, where various chemical bonds between them.
The band theory of the structure of a solid body considers it as a set of atoms, each of which, in turn, consists of a nucleus and electrons. In the crystal structure, the nuclei of atoms are located in the nodes of the crystal lattice, which is characterized by a certain spatial periodicity.
What is the structure of a liquid?
The structure of solids and liquids is similar in that the particles of which they are composed are at a close distance. The difference is that the molecules move freely, since the force of attraction between them is much weaker than in a solid.
What are the properties of a liquid? Firstly, it is fluidity, and secondly, the liquid will take the form of the container in which it is placed. If it is heated, the volume will increase. Due to the proximity of the particles to each other, the liquid cannot be compressed.
What is the structure and structure of gaseous bodies?
Gas particles are randomly arranged, they are so far apart that there can be no attractive force between them. What properties does a gas have and what is the structure of gaseous bodies? As a rule, the gas uniformly fills the entire space in which it was placed. It compresses easily. The speed of the particles of a gaseous body increases with increasing temperature. At the same time, there is also an increase in pressure.
The structure of gaseous, liquid and solid bodies is characterized by different distances between the smallest particles of these substances. The particles of a gas are much farther apart than in a solid or liquid state. In air, for example, the average distance between particles is about ten times the diameter of each particle. Thus, the volume of molecules occupies only about 0.1% of the total volume. The remaining 99.9% is empty space. In contrast, liquid particles fill about 70% of the total liquid volume.
Each gas particle moves freely along a straight path until it collides with another particle (gas, liquid or solid). The particles usually move fast enough that after two of them collide, they bounce off each other and continue on their way alone. These collisions change direction and speed. These properties of gas particles allow gases to expand to fill any shape or volume.
State change
The structure of gaseous, liquid and solid bodies can change if a certain external influence is exerted on them. They can even change into each other's states under certain conditions, such as during heating or cooling.
- Evaporation. The structure and properties of liquid bodies allow them, under certain conditions, to pass into a completely different physical state. For example, if you accidentally spill gasoline while refueling a car, you can quickly smell its pungent smell. How does this happen? Particles move throughout the liquid, as a result, a certain part of them reaches the surface. Their directional motion can carry these molecules off the surface and into the space above the liquid, but the attraction will pull them back. On the other hand, if a particle is moving very fast, it can break away from others by a decent distance. Thus, with an increase in the speed of particles, which usually happens when heated, the process of evaporation occurs, that is, the transformation of liquid into gas.
Behavior of bodies in different physical states
The structure of gases, liquids, solids is mainly due to the fact that all these substances are composed of atoms, molecules or ions, but the behavior of these particles can be completely different. Gas particles are chaotically distant from each other, liquid molecules are close to each other, but they are not as rigidly structured as in a solid. Gas particles vibrate and move at high speeds. The atoms and molecules of a liquid vibrate, move, and slide past each other. Particles of a solid body can also vibrate, but motion as such is not characteristic of them.
Features of the internal structure
In order to understand the behavior of matter, one must first study the features of its internal structure. What are the internal differences between granite, olive oil and helium in balloon? simple model structure of matter will help to find the answer to this question.
A model is a simplified version of a real object or substance. For example, before actual construction begins, architects first construct a model construction project. Such a simplified model does not necessarily imply an exact description, but at the same time it can give a rough idea of what this or that structure will be like.
Simplified Models
In science, however, models are not always physical bodies. The last century has seen a significant increase in human understanding about the physical world. However most of accumulated knowledge and experience is based on extremely complex representations, for example in the form of mathematical, chemical and physical formulas.
In order to understand all this, you need to be quite well versed in these exact and complex sciences. Scientists have developed simplified models to visualize, explain, and predict physical phenomena. All this greatly simplifies the understanding of why some bodies have permanent form and volume at certain temperature, while others can change them, and so on.
All matter is made up of tiny particles. These particles are in constant motion. The volume of movement is related to temperature. Elevated temperature indicates an increase in speed. The structure of gaseous, liquid and solid bodies is distinguished by the freedom of movement of their particles, as well as by how strongly the particles are attracted to each other. Physical depend on his physical condition. Water vapor, liquid water and ice have the same Chemical properties, but they physical properties differ significantly.
Lesson #2/5 2
Topic No. 26: “Model of the structure of a liquid. Saturated and unsaturated pairs. Air humidity."
1 Fluid structure model
Liquid one of aggregate states of matter. The main property of a liquid, which distinguishes it from other states of aggregation, is the ability to change its shape indefinitely under the action of tangential mechanical stresses, even arbitrarily small, while practically maintaining volume.
Fig.1
The liquid state is usually considered intermediate between solid and gas : a gas retains neither volume nor shape, but a solid retains both.
molecules liquids do not have a definite position, but at the same time they do not have full freedom of movement. There is an attraction between them, strong enough to keep them close.
A substance in a liquid state exists in a certain interval temperatures , below which it goes intosolid state(crystallization occurs or transformation into a solid-state amorphous state glass), above into gaseous (evaporation takes place). The boundaries of this interval depend on pressure .
All liquids are usually divided into pure liquids and mixtures . Some mixtures of liquids have great importance for life: blood, sea water etc. Liquids can perform the function solvents.
Fluidity is the main property of liquids. If you apply to a section of a fluid in equilibrium external force , then there is a flow of fluid particles in the direction in which this force is applied: the fluid flows. Thus, under the action of unbalanced external forces, the liquid does not retain the shape and relative arrangement of the parts, and therefore takes the form of the vessel in which it is located.
Unlike plastic solids, liquids do not haveyield strength: it is enough to apply an arbitrarily small external force for the liquid to flow.
One of characteristic properties liquid is what it has certain amount ( under constant external conditions). Liquid is extremely difficult to compress mechanically because, unlike gas There is very little free space between molecules. The pressure exerted on a liquid enclosed in a vessel is transmitted without change to each point of the volume of this liquid ( pascal's law , also valid for gases). This feature, along with very low compressibility, is used in hydraulic machines.
Liquids typically increase in volume (expand) when heated and decrease in volume (contract) when cooled. However, there are exceptions, for example, water shrinks when heated, at normal pressure and at temperatures between 0°C and approximately 4°C.
In addition, liquids (like gases) are characterized by viscosity . It is defined as the ability to resist the movement of one of the parts relative to the other that is, as internal friction.
When adjacent layers of a liquid move relative to each other, a collision of molecules inevitably occurs in addition to that due tothermal motion. There are forces that slow down the ordered movement. In this case, the kinetic energy of ordered motion is converted into thermal energy of the chaotic motion of molecules.
The liquid in the vessel, set in motion and left to itself, will gradually stop, but its temperature will rise.In a vapor, like a gas, one can hardly take into account the cohesion forces and consider the movement as a free flight of molecules and their collision with each other and with the surrounding bodies (walls and liquid covering the bottom of the vessel). In a liquid, molecules, as in a solid, strongly interact, holding each other. However, while in a solid body each molecule retains an indefinitely long definite position of equilibrium inside the body and its motion is reduced to oscillation around this equilibrium position, the nature of motion in a liquid is different. Liquid molecules move much more freely than solid molecules, though not as freely as gas molecules. Each molecule in a liquid moves back and forth for some time, without moving away, however, from its neighbors. This movement is reminiscent of the oscillation of a solid molecule around an equilibrium position. However, from time to time a liquid molecule breaks out of its environment and moves to another place, falling into a new environment, where again for some time it makes a movement similar to oscillation.
Thus, the movement of liquid molecules is something like a mixture of movements in a solid body and in a gas: "oscillatory" movement in one place is replaced by a "free" transition from one place to another. In accordance with this, the structure of a liquid is something in between the structure of a solid body and the structure of a gas. The higher the temperature, i.e., the greater the kinetic energy of the molecules of the liquid, the greater the role played by "free" motion: the shorter the intervals of the "oscillatory" state of the molecule and the more often "free" transitions, i.e., the more the liquid is likened to a gas. When enough high temperature, characteristic of each liquid (the so-called critical temperature), the properties of a liquid do not differ from those of a highly compressed gas.
2 Saturated and unsaturated vapors and their properties
Above free surface liquids always have vapors of this liquid. If the vessel with the liquid is not closed, then the concentration of vapor particles at a constant temperature can vary over a wide range in the direction of decrease and increase.
The evaporation process in closed space (closed container with liquid)can occur at a given temperature only up to a certain limit. This is due to the fact that vapor condensation occurs simultaneously with the evaporation of the liquid. First, the number of molecules emitted from the liquid in 1 s, more number molecules returning back, and the density, and hence the vapor pressure, increases. This leads to an increase in the rate of condensation. After some time, dynamic equilibrium sets in, at which the vapor density over the liquid becomes constant.
Steam that is in dynamic equilibrium with its liquid is called saturated steam. A vapor that is not in dynamic equilibrium with its liquid is called unsaturated.
Experience shows that unsaturated vapors obey all gas laws , and the more accurate, the farther they are from saturation. For saturated vapors, the following properties are characteristic:
- density and pressure of saturated steam at a given temperature these are the maximum density and pressure that steam can have at a given temperature;
- the density and pressure of saturated vapor depend on the type of substance. The less specific heat vaporization of a liquid, the faster it evaporates and the greater the pressure and density of its vapors;
- the pressure and density of saturated steam are uniquely determined by its temperature (they do not depend on how the steam reached this temperature: during heating or during cooling);
- vapor pressure and density increase rapidly with increasing temperature (Fig. 1, a, b).
Experience shows that when a liquid is heated, the level of the liquid in a closed vessel decreases. Consequently, the mass and density of the vapor increase. A stronger increase in the pressure of saturated vapor compared to an ideal gas (the Gay-Lussac law is not applicable to saturated vapor) is explained by the fact that here the pressure increases not only due to an increase in the average kinetic energy of the molecules (as in an ideal gas), but also due to increasing the concentration of molecules;
- at constant temperature, the pressure and density of saturated vapor do not depend on volume. Figure 2 shows for comparison the isotherms of ideal gas (a) and saturated steam (b).
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Experience shows that during isothermal expansion, the level of the liquid in the vessel decreases; the number of vapor molecules changes so that the vapor density remains constant.
3 Humidity
Air containing water vapor is called wet . To characterize the content of water vapor in the air, a number of quantities are introduced: absolute humidity, water vapor pressure and relative humidity.
absolute humidityρ air is called a value numerically equal to the mass of water vapor contained in 1 m 3 air (i.e. the density of water vapor in air under given conditions).
Water vapor pressure p is the partial pressure of water vapor contained in the air. The SI units for absolute moisture and elasticity are, respectively, kilograms per cubic meter(kg/m 3) and pascal (Pa).
If only absolute humidity or water vapor pressure is known, it is still impossible to judge how dry or humid the air is. To determine the degree of air humidity, it is necessary to know whether the water vapor is close or far from saturation.
relative humidity air φ called the percentage ratio of absolute humidity to densityρ 0 saturated steam at a given temperature (or the ratio of water vapor pressure to pressure p0 saturated steam at a given temperature):
The lower the relative humidity, the further the steam from saturation, the more intense the evaporation. Saturated steam pressure p0 at a given temperature tabular value. The elasticity of water vapor (and hence the absolute humidity) is determined by the dew point.
With isobaric cooling to a temperature tp the steam becomes saturated and its state is represented by a dot AT . Temperature tp at which water vapor becomes saturated is called dew point . When cooled below the dew point, vapor condensation begins: fog appears, dew falls, windows fog up.
4 Humidity measurement
Used to measure air humidity measuring instruments hygrometers. There are several types of hygrometers, but the main ones are: hair and psychrometric.
Since it is difficult to directly measure the pressure of water vapor in the air, the relative humidity of the air is measuredin an indirect way.
Operating principlehair hygrometerbased on the property of defatted hair (human or animal)change its lengthdepending on the humidity of the air in which it is located.
Hair stretched over a metal frame. The change in the length of the hair is transmitted to the arrow moving along the scale. Hair hygrometer in winter time are the main instrument for measuring outdoor humidity.
A more accurate hygrometer is a psychrometric hygrometer psychrometer
(according to other Greek "psychros" means cold).
It is known that relative humidity depends evaporation rate.
The lower the air humidity, the easier it is for moisture to evaporate.
The psychrometer has two thermometers . One is ordinary, it is called dry. It measures the temperature of the surrounding air. The flask of another thermometer is wrapped in a fabric wick and lowered into a container of water. The second thermometer does not show the temperature of the air, but the temperature of the wet wick, hence the name moistened thermometer. The lower the air humidity, the more intense moisture evaporates from the wick, the large quantity of heat per unit time is removed from the wetted thermometer, the smaller its readings, therefore, the greater the difference between the readings of the dry and wetted thermometers.
The dew point is determined using hygrometers. The condensation hygrometer is a metal box BUT , front wall To which is well polished (Fig. 2) An easily evaporating liquid ether is poured inside the box and a thermometer is inserted. Passing air through the box with a rubber bulb G , cause strong evaporation of the ether and rapid cooling of the box. The thermometer measures the temperature at which dew drops appear on the polished surface of the wall. To . The pressure in the area adjacent to the wall can be considered constant, since this area communicates with the atmosphere and the decrease in pressure due to cooling is compensated by an increase in the vapor concentration. The appearance of dew indicates that the water vapor has become saturated. Knowing the air temperature and dew point, you can find the partial pressure of water vapor and relative humidity.
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5 Tasks for independent solution
Task 1
On the street goes cold fall rain. In which case will the laundry hung in the kitchen dry faster: when the window is open, or when it is closed? Why?
Task 2
The humidity is 78% and the dry bulb reading is 12°C. What temperature does a wet bulb thermometer show?(Answer: 10 °C.)
Task 3
The difference between dry and wet thermometer readings is 4°C. Relative Humidity air 60%. What are the dry and wet bulb readings?(Answer: t c -l9 ° С, t m \u003d 10 ° С.)
In the two previous paragraphs, we considered the structure and properties of solids - crystalline and amorphous. Let us now turn to the study of the structure and properties of liquids.
The hallmark of a liquid is fluidity- the ability to change shape in a short time under the influence of even small forces. Due to this, liquids pour in jets, flow in streams, take the shape of a vessel into which they are poured.
The ability to change shape in different liquids is expressed in different ways. Take a look at the picture. Under roughly equal gravity, honey takes longer to change its shape than water. Therefore, these substances are said to have unequal viscosity: honey has more than water. This is explained differently complex structure molecules of water and honey. Water is made up of molecules that look like balls with tubercles, while honey is made up of molecules that look like tree branches. Therefore, when honey moves, the “branches” of its molecules are hooked onto each other, giving it a greater viscosity than water.
Important: changing shape, the liquid retains its volume. Consider the experience (see figure). The liquid in the beaker has the shape of a cylinder and a volume of 300 ml. After pouring into the bowl, the liquid took on a flat shape, but retained its previous volume: 300 ml. This is due to the attraction and repulsion of its particles: on average, they continue to be kept at the same distances from each other.
one more a common property of all liquids is their obedience to Pascal's law. In grade 7, we learned that it describes the property of liquids and gases to transfer the pressure exerted on them in all directions (see § 4-c). Now we note that less viscous liquids do this quickly, and viscous liquids take a long time.
The structure of liquids. In molecular kinetic theory, it is believed that in liquids, as in amorphous bodies, there is no strict order in the arrangement of particles, that is, they are not equally dense. Gaps have various sizes, including such that one more particle can fit there. This allows them to jump from "densely populated" places to more free ones. Jumps of each liquid particle occur very often: several billion times per second.
If some external force acts on the liquid (for example, gravity), the movement and jumps of particles will occur mainly in the direction of its action (down). This will cause the liquid to take the form of an elongated drop or flowing jet (see figure). So, the fluidity of liquids is explained by the jumps of their particles from one stable position to another.
Jumps of particles of liquids occur frequently, but much more often their particles, as in solids, oscillate in one place, continuously interacting with each other. Therefore, even a small compression of the liquid leads to a sharp "hardening" of the interaction of particles, which means a sharp increase in the pressure of the liquid on the walls of the vessel in which it is compressed. That's how it's explained the transfer of pressure by liquids, that is, Pascal's law, and, at the same time, the property of liquids to resist compression, that is, to maintain volume.
Note that the preservation of its volume by a liquid is a conditional representation. This means that, compared to gases that are easy to compress even with the force of a child’s hand (for example, in a balloon), liquids can be considered incompressible. However, at a depth of 10 km in the World Ocean, water is under such great pressure that each kilogram of water reduces its volume by 5% - from 1 liter to 950 ml. Using high pressures, liquids can be compressed even more.
Molecular-kinetic ideas about the structure of matter explain the whole variety of properties of liquids, gases and solids. Between the particles of matter there are electromagnetic interactions- they attract and repel each other with the help of electromagnetic forces. At very large distances between molecules, these forces are negligible.
Interaction forces of molecules
But the picture changes if the distance between the particles decreases. Neutral molecules begin to orient themselves in space in such a way that their surfaces facing each other begin to have charges opposite in sign, and attractive forces begin to act between them. This occurs when the distance between the centers of molecules is greater than the sum of their radii.
If we continue to reduce the distance between the molecules, then they begin to repel as a result of the interaction of like-charged electron shells. This happens when the sum of the radii of the interacting molecules more distance between particle centers.
That is, at large intermolecular distances, attraction prevails, and at close distances, repulsion prevails. But there is a certain distance between the particles when they are in a position of stable equilibrium (the forces of attraction are equal to the forces of repulsion). In this position, the molecules have the minimum potential energy. Molecules also have kinetic energy because they are in constant motion all the time.
Thus, the strength of interaction bonds between particles distinguishes three states of matter: solid, gas and liquid, and explains their properties.
Let's take water as an example. Size, shape and chemical composition particles of water remains the same, whether it is solid (ice) or gaseous (steam). But the way these particles move and are arranged is different for each state.
Solids
Solids retain their structure and can be split or broken with force. You cannot pass through the table because both you and the table are solid. Solid particles have least amount energies from the three traditional states of matter. Particles are arranged in a certain structural sequence with very small space between them.
They are held together in balance and can only vibrate around a fixed position. As a result, solids are high density and fixed shape and volume. If a table is left alone for a few days, it won't expand, and a thin layer of wood all over the floor won't fill the room!
Liquids
Just like in a solid, the particles in a liquid are packed close together, but are arranged randomly. Unlike solids, a person can pass through a liquid, this is due to the weakening of the attractive force acting between the particles. In a liquid, particles can move relative to each other.
Liquids have a fixed volume, but do not have a fixed shape. They will gravitational flow. But some liquids are more viscous than others. In a viscous liquid, the interaction between molecules is stronger.
Liquid molecules have much more kinetic energy (energy of motion) than a solid body, but much less than a gas.
gases
Particles in gases are far apart and randomly arranged. This state of matter has the highest kinetic energy, since there are practically no attractive forces between the particles.
Gas molecules are in constant motion in all directions (but only in a straight line), collide with each other, and with the walls of the vessel in which they are located - this causes pressure.
Gases also expand to completely fill the volume of a vessel, regardless of its size or shape - gases do not have a fixed shape or volume.
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