General properties of liquids and gases physics. Structure of gases, liquids and solids. Features of the structure of solutions. The concept of a “reactive field.” Evaporation and condensation of liquids

• Atoms (or molecules) in crystals are arranged in order to form crystal lattice.

Questions and tasks

First level

1. What states of matter do you know?
2. How can you experimentally verify that an “empty” glass is filled with air?
3. Why can't you fill only half of a vessel with gas that has no partitions?
4. What is the molecular structure of gases? What properties of gases does it explain?
5. What observations about the properties of a liquid can be made by pouring water from one container to another?
6. What is the molecular structure of liquids? What properties of liquids does it explain?
7. What properties solids do you know? Give examples illustrating the differences in the properties of solids.

   Second level

8. Give examples of gases, liquids and solids that you know.
9. What are general properties liquids and gases? Liquid and solid?
10. What are the main differences between a gas and a liquid and a solid?
11. What explains the low compressibility of liquids and solids?
12. What's happened crystalline bodies? What is their molecular structure? Give examples of crystalline solids.
13. Give examples amorphous bodies. What is their difference from crystalline ones?
14. What do amorphous bodies and crystalline bodies have in common? In amorphous bodies and liquids?
15. Make up a problem about states of matter, the answer to which would be: “Only gas.”

Home laboratory

1. Fill plastic bottle water to about halfway and close it tightly with a stopper. Try squeezing the bottle. Then repeat the same experiment, filling the bottle to the top. What difference did you notice? What does it indicate?
2. Examine the crystals of granulated sugar and table salt under a magnifying glass. Compare them to very small pieces of broken glass. What is the difference? Can you explain it?

The main thing in this chapter

• All bodies around us are made of atoms. Scientists know today more than 100 various types atoms.
• By attracting each other, atoms form molecules. Scientists know several million types of molecules.
• The properties of a substance are determined by the type of molecules that make up the substance.
• The sizes of molecules are measured in millionths of a millimeter.
• Molecules of gases, liquids and solids are in constant chaotic motion - this is indicated, for example, by Brownian motion and the phenomenon of diffusion.
• The speed of chaotic (thermal) movement of molecules increases with increasing temperature.
• Molecules interact with each other: at very short distances they repel, and at slightly larger distances they attract. The repulsion of molecules explains the incompressibility of liquids and solids in which the molecules are located close to each other.

• A substance can be in a solid, liquid or gaseous state.
• Gas occupies the entire volume provided to it. Gas is easily compressible. The molecules in a gas are not located close to each other.
• The liquid takes the shape of the container in which it is located. This is due to its fluidity. The liquid is practically incompressible. Molecules in a liquid are located close to each other, but there is no specific order in this arrangement.
• Solids retain volume and shape.
• Solids are crystalline and amorphous.
• The atoms (or molecules) in crystals are arranged in an orderly manner, forming a crystal lattice.
• The properties of crystalline solids are determined not only by the type of atoms or molecules, but also by the structure of the crystal lattice.

Liquid - state of aggregation a substance occupying an intermediate position between its solid and gaseous states.

The most common liquid on Earth is water. Her solid state- ice, and gaseous - steam.

In liquids, molecules are located almost close to each other. They have greater freedom than solid molecules, although they cannot move completely freely. The attraction between them, although weaker than in solids, is still sufficient to keep the molecules at a close distance from each other. Each molecule of a liquid can vibrate around some center of equilibrium. But under the influence of an external force, molecules can jump to a free space in the direction of the applied force. This explains fluidity of liquid .

Fluidity

The main physical property of a liquid is fluidity . When an external force is applied to a liquid, a flow of particles appears in it, the direction of which coincides with the direction of this force. By tilting a kettle of water, we will see how water flows down from its spout under the influence of gravity. In the same way, water flows out of a watering can when we water plants in the garden. We see a similar phenomenon in waterfalls.

Due to its fluidity, a liquid can change shape in a short time under the influence of even a small force. All liquids can flow in a stream or splash in drops. They are easy to pour from one vessel to another. At the same time they do not retain shape , but take the form of the vessel in which they are located. This property of the liquid is used, for example, when casting metal parts. Molten liquid metal is poured into molds of a certain configuration. As it cools, it turns into a solid that retains this configuration.

Fluidity increases as the temperature of the liquid increases and decreases as it decreases. This is explained by the fact that with increasing temperature, the distance between liquid particles also increases, and they become more mobile. Fluidity also depends on the structure of the molecules. The more complex their shape, the less fluidity the liquid has.

Viscosity

Different liquids have different fluidity. So, water flows out of a bottle faster than vegetable oil. Honey pours out of a glass more slowly than milk. These fluids are subject to the same gravitational forces. So why are their turnover rates different? The thing is that they have different viscosity . The higher the viscosity of a liquid, the less fluid it is.

What is viscosity, and what is its nature? Viscosity is also called internal friction . This is the ability of a liquid to resist the movement of different layers of liquid relative to each other. Molecules located in one of the layers and colliding with each other during thermal motion also collide with molecules of neighboring layers. Forces arise that slow down their movement. They are directed in the direction opposite to the movement of the layer in question.

Viscosity - important characteristic liquids. It is taken into account in various technological processes, for example, when it is necessary to pump liquid through pipelines.

The viscosity of a liquid is measured using an instrument called viscometer. The simplest is considered capillary viscometer. The principle of its operation is not complicated. The time during which a given volume of liquid flows through a thin tube (capillary) under the influence of a pressure difference at its ends is calculated. Since the diameter and length of the capillary and the pressure difference are known, calculations can be made based on Poiseuille's law , Whereby volume of liquid passing per second (second volumetric flow rate) is directly proportional to the pressure drop per unit length of the pipe and the fourth power of its radius and inversely proportional to the viscosity coefficient of the liquid .

Where Q - second fluid flow rate, m 3 /s;

p 1 - p 2 = ∆р - pressure difference at the ends of the capillary, Pa;

R - capillary radius, m;

d - capillary diameter, m;

ƞ - coefficient of dynamic viscosity, Pa/s;

l - capillary length, m.

Volume

The distance between molecules inside a liquid is very small. It is smaller than the size of the molecules themselves. Therefore, the liquid is very difficult to compress mechanically. The pressure exerted on a liquid enclosed in a vessel is transmitted to any point without changes in all directions. This is how it is formulated Pascal's law . The operation of brake systems, hydraulic presses and other hydraulic devices is based on this feature of fluids.

The liquid retains its volume if external conditions (pressure, temperature) do not change. But when heated, the volume of liquid increases, and when cooled, it decreases. However, there is an exception here. At normal pressure and an increase in temperature from 0 to 4 o, the volume of water does not increase, but decreases.

Density waves

It is very difficult to compress liquid. But if the pressure changes, it is still possible. And in this case, its density and volume changes. If compression occurs in one area of ​​the liquid, then it will be gradually transferred to other areas. This means that elastic waves will propagate in the liquid. If the density changes slightly, then we get sound wave. And if it’s strong enough, a shock wave occurs.

Surface tension of a liquid

We observe the phenomenon of surface tension every time water slowly drips from a tap. First we see a thin transparent film that stretches under the weight of water. But it does not break, but embraces a small amount of water and forms a droplet falling from the tap. It is created by surface tension forces, which pull the water into a small ball.

How do these forces arise? Unlike gas, liquid fills only part of the volume of the container in which it is located. Its surface is the interface between the liquid itself and the gas (air or steam). On all sides, a molecule located inside a liquid is surrounded by other molecules of the same liquid. It is acted upon by intermolecular forces. They are mutually balanced. The resultant of these forces is zero.

And on molecules located in the surface layer of a liquid, the forces of attraction from the molecules of the same liquid can act only from one side. On the other hand, they are acted upon by the attractive forces of air molecules. But since they are very small, they are neglected.

The resultant of all forces acting on a molecule located on the surface is directed into the liquid. And in order not to be drawn into the liquid and remain on the surface, the molecule does work against this force. As a result, the molecules of the upper layer receive an additional supply of potential energy. The larger the surface of a liquid, the more molecules there are, and the greater the potential energy. But in nature everything is arranged in such a way that any system tries to reduce its potential energy to a minimum. Investigator, there is a force that will tend to reduce the free surface of the liquid. This force is called surface tension force .

The surface tension of the liquid is very high. And it takes quite a lot of force to break it. The undisturbed surface of water can easily hold a coin, razor blade or steel needle, although these objects are much heavier than water. The force of gravity acting on them turns out to be less than the force of surface tension of water.

The sphere has the smallest surface of all geometric volumetric bodies. Therefore, if only surface tension forces act on a liquid, then it takes the shape of a sphere. This is the shape of water drops in zero gravity. Soap bubbles or bubbles of boiling liquid also try to take on a spherical shape.

Miscibility

Liquids can dissolve in each other. This ability of theirs is called miscibility . If you place two mixed liquids in one vessel, then as a result of thermal movement their molecules will gradually cross the interface. As a result, mixing will occur. But not all liquids can mix. For example, water and vegetable oil never mix. And it’s very easy to mix water and alcohol.

Adhesion

We all know that geese and ducks come out of the water dry. Why don't their feathers get wet? It turns out that they have a special gland that secretes fat, which waterfowl use to lubricate their feathers with their beaks. And they remain dry because the water drips off them in droplets.

Place a drop of water on a polystyrene plate. It takes the shape of a flattened ball. Let's try placing the same drop on a glass plate. We will see that it spreads on the glass. What happens to the water? The thing is that attractive forces act not only between the molecules of the liquid itself, but also between the molecules of different substances in the surface layer. These forces are called forces adhesion (from Latin adhaesio- adhesion).

The interaction of a liquid with a solid is called wetting . But the surface of a solid body is not always wetted. If it turns out that the molecules of the liquid itself are attracted to each other more strongly than to the solid surface, then the liquid will gather into a droplet. This is exactly how water behaves on a polystyrene plate. She does not wet this record. In the same way, droplets do not spread morning dew on the leaves of plants. And for the same reason, water flows from the fat-covered feathers of waterfowl.

And if the attraction of liquid molecules to a solid surface stronger than strength attraction between the molecules themselves, the liquid spreads on the surface. Therefore, our droplet on the glass also spread. In this case, water wets glass surface.

Pour water into a polystyrene container. Looking at the surface of the water, we will see that it is not horizontal. At the edges of the vessel it bends downward. This happens because the forces of attraction between water molecules are greater than the forces of adhesion (sticking). And in a glass vessel, the surface of the water at the edges curves upward. In this case, the adhesion forces are greater than the intramolecular forces of water. In wide vessels, this curvature is observed only at the walls of the vessels. And if the vessel is narrow, then this curvature is noticeable over the entire surface of the water.

The phenomenon of adhesion is widely used in various industries - paint and varnish, pharmaceutical, cosmetics, etc. Wetting is necessary when gluing, dyeing fabrics, applying to surfaces paints, varnishes. When constructing swimming pools, their walls, on the contrary, are covered with a material that is not wetted by water. The same materials are used for umbrellas, raincoats, waterproof shoes, and awnings.

Capillarity

Another one interesting feature liquids - capillary effect . This is the name given to its ability to change its level in tubes, narrow vessels, and porous bodies.

If you lower a narrow glass tube (capillary) into water, you can see how the water column rises in it. The narrower the tube, the higher the water column. If you lower the same tube into liquid mercury, the height of the mercury column will be lower than the level of the liquid in the vessel.

Liquid in capillaries is able to rise through a narrow channel (capillary) only if it wets its walls. This happens in soil, sand, and glass tubes through which moisture easily rises. For the same reason, the wick in a kerosene lamp is soaked in kerosene, the towel absorbs moisture from wet hands, and various chemical processes. In plants, nutrients and moisture travel through capillaries to the leaves. Thanks to the capillary effect, the vital activity of living organisms is possible.

It is known that everything that surrounds a person, including himself, is a body consisting of substances. Those, in turn, are built from molecules, the latter from atoms, and these from even smaller structures. However, the surrounding diversity is so great that it is difficult to even imagine any commonality. This is true. There are millions of compounds, each of them is unique in its properties, structure and role. In total, several phase states are distinguished, according to which all substances can be correlated.

Aggregate states of substances

There are four options for the aggregate state of connections.

1. Gases.
2. Solids.
3. Liquids.
4. Plasma is highly rarefied ionized gases.

In this article we will consider the properties of liquids, features of their structure and possible performance parameters.

Classification of liquid bodies

The basis this division the properties of liquids, their structure and chemical structure, as well as types of interactions between the constituent particles of the compound.

1. These are liquids that consist of atoms held together by van der Waals forces. Examples include liquid gases(argon, methane and others).
2. Such substances that consist of two identical atoms. Examples: gases in liquefied form - hydrogen, nitrogen, oxygen and others.
3. - mercury.
4. Substances consisting of elements bonded by covalent bonds polar bonds. Examples: hydrogen chloride, hydrogen iodide, hydrogen sulfide and others.
5. Compounds that contain hydrogen bonds. Examples: water, alcohols, ammonia in solution.

There are also special structures - such as non-Newtonian liquids, which have special properties.

We will consider the basic properties of a liquid that distinguish it from all other states of aggregation. First of all, these are what are commonly called physical.

Properties of liquids: shape and volume

In total, we can distinguish about 15 characteristics that allow us to describe what the substances in question are and what their value and features are.

The very first liquids that come to mind when mentioning this state of aggregation are the ability to change shape and occupy a certain volume. So, for example, if we talk about the form of liquid substances, it is generally accepted to consider it absent. However, it is not.

Under the influence of a well-known drop, substances undergo some deformation, so their shape is disrupted and becomes indeterminate. However, if you place a drop in conditions under which gravity does not act or is very limited, then it will take the ideal shape of a ball. Thus, having received the task: “Name the properties of liquids,” a person who considers himself sufficiently knowledgeable in physics should mention this fact.

As for volume, the general properties of gases and liquids should be noted. Both are capable of occupying the entire volume of space in which they are located, limited only by the walls of the vessel.

Viscosity

The physical properties of liquids are very diverse. But what is unique is viscosity. What is it and how is it determined? The main parameters on which the value under consideration depends are:

• shear stress;
• movement speed gradient.

The dependence of these quantities is linear. If we explain more in simple words, then viscosity, like volume, are those properties of liquids and gases that are common to them and imply unlimited movement regardless of external forces impact. That is, if water flows out of a vessel, it will continue to do so under any influence (gravity, friction and other parameters).

This is in contrast to non-Newtonian fluids, which have greater viscosity and can leave holes behind their movement that fill over time.

What will this indicator depend on?

1. From temperature. With increasing temperature, the viscosity of some liquids increases, while others, on the contrary, decreases. It depends on the specific compound and its chemical structure.
2. From pressure. An increase causes an increase in the viscosity index.
3. From chemical composition substances. Viscosity changes in the presence of impurities and foreign components in a sample of a pure substance.

Heat capacity

This term defines the ability of a substance to absorb a certain amount of heat to increase its own temperature by one degree Celsius. There are different connections for this indicator. Some have greater, others less, heat capacity.

For example, water is a very good heat accumulator, which allows it to be widely used for heating systems, cooking and other needs. In general, the heat capacity indicator is strictly individual for each individual liquid.

Surface tension

Often, when given the task: “Name the properties of liquids,” they immediately remember surface tension. After all, children are introduced to it in physics, chemistry and biology lessons. And each subject explains this important parameter from its own side.

The classic definition of surface tension is: it is the interface between phases. That is, at the time when the liquid has occupied a certain volume, it borders on the outside with a gaseous medium - air, steam or some other substance. Thus, phase separation occurs at the point of contact.

At the same time, molecules strive to surround themselves as much as possible. a large number particles and thus lead to a compression of the liquid as a whole. Consequently, the surface seems to be stretched. The same property can explain the spherical shape of liquid droplets in the absence of gravity. After all, it is precisely this form that is ideal from the point of view of the energy of the molecule. Examples:

• bubble;
• boiling water;
• liquid drops in zero gravity.

Some insects have adapted to “walking” on the surface of water precisely due to surface tension. Examples: water striders, waterfowl, some larvae.

Fluidity

There are common properties of liquids and solids. One of them is fluidity. The whole difference is that for the former it is unlimited. What is the essence of this parameter?

If you attach external influence to a liquid body, then it will divide into parts and separate them from each other, that is, it will flow. In this case, each part will again fill the entire volume of the vessel. For solids, this property is limited and depends on external conditions.

Dependence of properties on temperature

These include three parameters that characterize the substances we are considering:

• overheat;
• cooling;
• boiling.

Properties of liquids such as superheating and supercooling are directly related to the boiling and freezing points, respectively. A liquid is called overheated if it has passed the threshold of the critical heating point when exposed to temperature, but external signs didn't bring it to a boil.

Supercooled, accordingly, is a liquid that has overcome the threshold of the critical point of transition to another phase under the influence of low temperatures, but has not become solid.

In both the first and second cases there are conditions for the manifestation of such properties.

1. No mechanical influences on the system (movement, vibration).
2. Uniform temperature, without sudden jumps and changes.

An interesting fact is that if you throw a foreign object into a superheated liquid (for example, water), it will instantly boil. It can be obtained by heating under the influence of radiation (in a microwave oven).

Coexistence with other phases of substances

There are two options for this parameter.

In general, the discipline of hydroaeromechanics studies the interaction of liquids with compounds in other states of aggregation.

Compressibility

The basic properties of a liquid would be incomplete if we did not mention compressibility. Of course, this parameter is more typical for gas systems. However, the ones we are considering can also be compressed under certain conditions.

The main difference is the speed of the process and its uniformity. While gas can be compressed quickly and under low pressure, liquids are compressed unevenly, for quite a long time and under specially selected conditions.

Evaporation and condensation of liquids

These are two more properties of liquid. Physics gives them the following explanations:

1. Evaporation - uh is a process that characterizes the gradual transition of a substance from a liquid state of aggregation to a solid state. This happens under the influence of thermal influences on the system. The molecules begin to move and, changing their crystal lattice, pass into a gaseous state. The process can continue until all the liquid turns into vapor (for open systems). Or until equilibrium is established (for closed vessels).
2. Condensation- a process opposite to that indicated above. Here the vapor turns into liquid molecules. This happens until equilibrium or a complete phase transition is established. The steam releases more particles into the liquid than it does to it.

Typical examples of these two processes in nature are the evaporation of water from the surface of the World Ocean, its condensation into upper layers atmosphere and then falls out as precipitation.

Mechanical properties of liquid

These properties are the subject of study of such a science as fluid mechanics. Specifically, its section, the theory of fluid and gas mechanics. The main mechanical parameters characterizing the considered state of aggregation of substances include:

• density;
• specific gravity;
• viscosity.

The density of a liquid body is understood as its mass, which is contained in one unit of volume. This indicator varies for different compounds. There are already calculated and experimentally measured data on this indicator, which are entered into special tables.

Why you should study mechanical properties liquids? This knowledge is important for understanding the processes occurring in nature, inside human body. Also when creating technical means, various products. After all, it is one of the most common aggregate forms on our planet.

Non-Newtonian liquids and their properties

The properties of gases, liquids, and solids are the object of study of physics, as well as some related disciplines. However, in addition to traditional liquid substances, there are also so-called non-Newtonian substances, which are also studied by this science. What are they and why did they get such a name?

To understand what such connections are, here are the most common everyday examples:

• "lizun" that children play with;
• "hand gum", or chewing gum for hands;
• ordinary construction paint;
• a solution of starch in water, etc.

That is, these are liquids whose viscosity is subject to a velocity gradient. The faster the impact, the higher the viscosity index. Therefore, when a hand-gum hits the floor sharply, it turns into a completely solid substance that can split into pieces.

If you leave it alone, then in just a few minutes it will spread into a sticky puddle. - substances that are quite unique in their properties, which have found application not only for technical purposes, but also for cultural and everyday purposes.

Liquid- a physical body that has property of fluidity, i.e., not having the ability to independently maintain its shape. The fluidity of a liquid is due to the mobility of the molecules that make up the liquid.

Liquid is a state of aggregation of a substance, intermediate between solid and gaseous.. The liquid is characterized by the following properties: 1) retains volume; 2) forms a surface; 3) has tensile strength; 4) takes the form of a vessel; 5) has fluidity. Properties of liquids from 1) to 3) are similar to the properties of solids, and property 4) is similar to the properties of liquids.

Fluids whose laws of motion and equilibrium are studied in hydraulics (fluid and fluid mechanics), are divided into two classes: compressible liquids or gases, almost incompressible - droplet liquids.

In hydraulics, both ideal and real fluids are considered.

Ideal liquid- a liquid between the particles of which there are no internal friction forces. As a result, such a liquid does not resist tangential shear forces and tensile forces. An ideal fluid does not compress at all; it offers infinitely great resistance to compression forces. Such a liquid does not exist in nature - it is a scientific abstraction necessary to simplify the analysis of the general laws of mechanics as applied to liquid bodies.

Real liquid- a liquid that does not perfectly possess the properties of an ideal liquid; it resists tangential and tensile forces to some extent, and is also partially compressed. To solve many hydraulic problems, this difference in the properties of ideal and real fluids can be neglected. Due to this physical laws, derived for an ideal liquid, can be applied to real liquids with appropriate corrections.

Briefly presented below general information concerning physical properties of liquids. Specific physical properties of different liquids are found in the subsections of our website. These sections will be gradually updated with new information, which may be useful to engineers and designers when carrying out calculations.

Liquid density

Kilogram per cubic meter [kg/m3] is equal to homogeneous density gaseous substance , the mass of which with a volume of 1 m 3 is equal to 1 kg.

dm is the mass of the liquid element, volume dV;

dV is the volume of the liquid element.

Dynamic viscosity of liquid

F is the force of internal friction of the fluid.

ΔS is the surface area of ​​the liquid layer for which the internal friction force is calculated.

The reciprocal of the fluid velocity gradient.

Pascal second [Pa. c] is equal dynamic viscosity of liquid, the tangential stress in which during laminar flow at a distance of 1 m normal to the direction of velocity is equal to 1 Pa.

Surface tension of a liquid

dF is the force acting on a section of the free surface contour normal to the contour and tangential to the surface to the length dl of this section.

dl is the length of the liquid surface area.

Newton per meter [N/m] is equal to surface tension of the liquid, created by a force of 1 N acting on a section of the free surface contour 1 m long, normal to the contour and tangential to the surface.

Kinematic viscosity of liquid

μ - dynamic viscosity of the liquid;

ρ - fluid density;

Square meter per second [m 2 /s] is equal to kinematic viscosity of liquid with a dynamic viscosity of 1 Pa s and a density of 1 kg/m 3.

Thermal conductivity coefficient of liquid

S - surface area;

Q is the amount of heat [J] transferred during time t through a surface of area S.

The reciprocal of the liquid temperature gradient.

Watt per meter-Kelvin [W/(m.K)] is equal to thermal conductivity coefficient of liquid, in which in stationary mode with surface density heat flow 1 W/m2 sets a temperature gradient of 1 K/m.

To the main physical properties liquids and gases include: density, specific gravity, compressibility, thermal expansion, viscosity. For liquids, additionally important properties are vaporization, surface tension and capillarity. Gases have the property of solubility in liquids.

Density r– mass of liquid or gas contained in a unit volume (kg/m3). For a homogeneous liquid

Where m- liquid mass, kg; V– volume of liquid, m3.

Specific gravity g– weight of liquid or gas per unit volume (N/m3):

, (2.2)

where G is the weight of the liquid, N.

Density and specific gravity are related by the relationship:

, (2.3)

where g is the acceleration of gravity: g = 9.81 m/s 2 .

With increasing temperature, the density of liquids and gases decreases (except for water). For water, the maximum density occurs at 4 0 C; as its temperature decreases from 4 0 C to 0 0 C and the temperature increases >4 0 C, the density decreases. The dependence of gas density on temperature will be discussed in more detail below.

Also, the density of liquids and gases depends on pressure. For a liquid, this dependence is insignificant, but the density of a gas depends significantly on pressure. These dependencies will be given below.

Compressibility– the property of a liquid to reversibly change its volume when external pressure changes. The compressibility of a liquid is characterized by the volumetric compression ratio b r(Pa -1), which is equal to:

Where V 0– initial volume of liquid, m3; D.V.– change in initial volume (m 3) with a change in initial pressure p 0 by the amount Dр(Pa) .

The minus sign in formula (2.4) means that as the pressure increases (positive increment), the initial volume decreases (negative increment).

It's obvious that D.V.=V to־ V 0, A Dр=р к- р 0 (V к,r k– final values ​​of volume and pressure). Substituting these values ​​into formula (2.4) we get:

. (2.5)

Let's substitute the value V into formula (2.1) and obtain the dependence for determining the density of the liquid on pressure:

, (2.6)

Where r 0 – initial density of the liquid, kg/m3.

Liquids, cleared of bubbles of undissolved air and other gases, are compressed extremely slightly. Thus, with an increase in pressure by 0.1 MPa, the volume of water decreases by only 0.005%.

Reciprocal value b r, is called the bulk modulus of elasticity of the liquid E(Pa):

Distinguish adiabatic And isothermal fluid elastic moduli. In calculations, the adiabatic module is used in cases where heat exchange with the environment can be neglected, for example, in fast processes (water hammer, rapid compression of a liquid, etc.). In other cases, the isothermal modulus of elasticity of the liquid is used, which is slightly less than the adiabatic one.

The isothermal modulus of elasticity of a liquid decreases with increasing temperature and increases with increasing pressure.

Temperature expansion– the property of a liquid to reversibly change its volume when the temperature changes. For liquids, it is characterized by the coefficient of thermal expansion β T(K -1 or 0 C -1):

Where DT– temperature change: ( DT = T c – T 0); T 0 and T k- initial and final temperatures, respectively, K or 0 C.

, (2.9)

. (2.10)

Gases, unlike liquids, are characterized by significant compressibility and thermal expansion. Relationship between volume V, pressure p and absolute temperature T ideal gas is described by the Clapeyron equation, which combines the Boyle–Mariotte and Gay-Lussac equations:

DI. Mendeleev combined Clapeyron's equation with Avogadro's law and obtained the following equation:

Where R– gas constant, J/(kg K): for air R=287 J/(kg K). Physical entity R– work of expansion of 1 kg of gas when heated by 1 K. This equation is called the Clapeyron–Mendeleev equation.

Real gases and their mixtures, under conditions far from liquefaction, practically obey the same laws as ideal ones. Therefore, when designing ventilation systems for buildings and structures, you can use equations (1.11 and 1.12).

Viscosity– the property of liquids and gases to resist relative motion(shift) of their particles. For the first time, the hypothesis about the forces of internal friction in a liquid was expressed by I. Newton in 1686. Almost 200 years later, in 1883, Prof. N.P. Petrov experimentally confirmed this hypothesis and expressed it mathematically. In a layered flow of a viscous fluid along a solid wall, the speed of movement of its layers is u are different (Fig. 2.1). The maximum speed will be at the top layer, the speed of the layer in contact with the wall will be zero. Due to the difference in speeds, a relative shift of neighboring layers will occur, and tangential stresses will arise at their boundary τ . For homogeneous liquids and gases, the equation for determining shear stresses is τ (Pa) with layered motion has the following form and is called the Newton–Petrov equation:

, (2.13)

Where m– proportionality coefficient, called dynamic viscosity, Pa s; du/dn- velocity gradient, i.e. elementary speed change u along the normal n, drawn to the layer velocity vectors, s -1 . The velocity gradient can be positive or negative. Therefore, in equation (2.13) before m there is a ± sign.

With constancy τ tangential stresses over the entire surface of the contacting layers, total tangential force (friction force) T will be equal to:

, (2.14)

Where S– surface area of ​​contacting layers, m2.

In fluid and gas mechanics, kinematic viscosity is most often used when performing calculations. ν (m/s 2):

Viscosity depends on temperature and pressure. As temperature increases, the viscosity of liquids decreases and that of gases increases. In liquids, viscosity is caused by molecular cohesion forces, which weaken with increasing temperature. For water, the dependence of kinematic viscosity on temperature is determined using the empirical Poiseuille formula (m 2 /s):

Where T- water temperature, 0 C.

In gases, viscosity is mainly caused by the chaotic thermal movement of molecules, the speed of which increases with increasing temperature. There is a constant exchange of molecules between layers of gas moving relative to each other. The transition of molecules from one layer to the neighboring one, which moves at a different speed, leads to the transfer of a certain amount of movement. As a result, the slow layers speed up and the faster layers slow down. Therefore, as the temperature increases, the viscosity of gases increases. The dynamic viscosity of gases depending on temperature can be determined using the Sutherland formula:

, (2.17)

Where μ 0 – dynamic viscosity of gas at 0 o C; T g– gas temperature, K; C g– constant, depending on the type of gas: for air C g=130,5.

As pressure increases, the viscosity of the liquid increases, which can be calculated using the following formula:

, (2.18)

Where m And m 0- dynamic viscosity of liquid at pressure r k And p 0, respectively, Pa∙s; a- coefficient depending on the temperature of the liquid (at high temperatures a=0.02, low – a = 0,03).

For gases m slightly depends on pressure when it changes from 0 to 0.5 MPa. With a further increase in pressure, the viscosity of the gas increases according to a dependence close to exponential. For example, when gas pressure increases from 0 to 9 MPa m increases almost fivefold.

Tensile strength for liquids, due to the presence of intermolecular attractive forces, can reach significant values. Thus, in water purified from impurities and degassed, tensile stresses of up to 28 MPa were briefly obtained. Technically pure liquids containing gas bubbles and solid particles of impurities practically do not resist stretching. In gases, the distances between molecules are significant and the intermolecular forces of attraction are extremely small. Therefore, in fluid and gas mechanics it is generally accepted that the tensile strength in liquids and gases is zero.

Solubility of gases in liquids is the ability of gas molecules to environment penetrate into the liquid through its free surface. This process continues until the liquid is completely saturated with gas or a mixture of gases. The amount of dissolved gas per unit volume of liquid depends on the type of gas and liquid, its temperature and pressure on the free surface. This phenomenon was first studied by the English chemist W. Henry in 1803 and derived the law that currently bears his name: in a state of saturation, the mass of a gas dissolved in a certain volume of liquid at a constant temperature is directly proportional to the partial pressure of this gas above the liquid.

As the pressure decreases, dissolved gas is released from the liquid. Bubbles are formed in it, filled with liquid vapor and gas released from this liquid.

As the temperature increases, the solubility of a gas in a liquid almost always decreases. So, when boiling water, the gases dissolved in it can be almost completely removed.

Vaporization– the property of liquids to turn into vapor, i.e. into a gaseous state. Vaporization that occurs on the surface of a liquid is called evaporation . All liquids, without exception, evaporate. The evaporation of a liquid depends on the type of liquid, temperature and external pressure at the free surface. The higher the temperature and the lower the pressure on the surface of the liquid, the faster the evaporation process occurs. The amount of steam that can be contained in the surrounding gas environment is not infinite. It is limited to some level called state saturation. In this case, the amount of evaporated liquid is equal to the amount of liquid that has turned from vapor into droplets (condensation process). Density and pressure saturated steam depends on the temperature and type of liquid; at a fixed temperature, the density and saturated vapor pressure for a certain liquid are constant values. There are always tiny gas bubbles in a liquid; when the liquid is heated near the walls of the vessel, since the temperature is highest there, the liquid evaporates into these bubbles until the pressure saturated vapors in the bubble will not become equal to the external pressure. With a further increase in temperature, the size of the bubble increases; under the action of a buoyant force (Archimedes' force), it breaks away from the wall, reaches the free surface and bursts. The vapor-gas mixture enters the surrounding gas environment. When a certain temperature is reached, the formation of vapor-gas bubbles occurs throughout the entire volume of the liquid. As noted above, the amount of gas dissolved in a liquid also depends on pressure. Therefore, liquid boiling can occur when the pressure on the free surface decreases. The process of vaporization throughout the entire volume of liquid with the formation of vapor-gas bubbles is called boiling. Boiling occurs at a certain temperature and pressure. This temperature is called boiling point, and the pressure is saturated vapor pressure р n.p.. (in reference books r n.p.. is given in the absolute pressure reference system). For example, at a temperature of 100 0 C for water, the saturated vapor pressure is approximately 0.1 MPa, and at 20 0 C - 0.0024 MPa. Thus, to boil water whose temperature is 20 0 C, it is necessary either to heat it at atmospheric pressure up to 100 0 C, or reduce the absolute pressure on the free surface to 0.0024 MPa without heating.

In some hydraulic devices, it is possible to reduce the pressure below atmospheric pressure, for example, at the inlet to the pump when sucking in liquid. When the pressure there decreases to r n.p.. the formation of vapor-gas bubbles and disruption of the continuity of the liquid begins. In the vast majority of cases, bubbles are carried away by the flow of liquid into an area of ​​​​high pressure. The vapor begins to condense inside the bubbles, and the gas located there dissolves again in the liquid. The so-called “collapse” of bubbles occurs, which is accompanied by local water hammer, noise and vibration. As a result, the efficiency and flow of the pump or the performance of the turbine decreases. The surface of the streamlined body may be destroyed. This process is called cavitation (from lat. сavitas– emptiness) (Fig. 2.2). The phenomenon of cavitation has been known in science and technology for a little over a hundred years. This phenomenon was first discovered by the English engineer R. Froude in 1894 while testing English destroyers. It was then that he coined the term “cavitation.”

Cavitation also has useful applications. For example, when drilling rocks and treating surfaces due to cavitation erosion.

Surface tension– stresses arising in the surface layer of a liquid and caused by the forces of intermolecular attraction. Let's compare the effect on the molecule A, located inside a liquid, with a molecule IN, located near the interface between liquid and gas (Fig. 2.3). Molecule A surrounded by other molecules on all sides and the attractive forces from the surrounding molecules are balanced. Molecule IN, located at the border with the gas, is surrounded by other molecules only on the liquid side; there are practically no molecules on the gas side. Therefore, for a molecule IN the resultant of all forces is directed downwards into the liquid. As a result, additional compressive stresses arise in the surface layer of the liquid. As a result, the liquid tends to take a shape in which its free surface is minimal. For example, in zero gravity, a liquid takes on a spherical shape; drops of water and oil on a hot stove tend to take the same shape.

In the case of contact of a liquid with a solid body, the liquid may or may not wet the surface of this body. The behavior of a liquid will depend on the magnitude of the adhesion forces between liquid molecules and solid molecules. In the first case, if the adhesion forces between the molecules of the liquid itself are greater than the adhesion forces between the molecules of the liquid and the solid, then a drop of liquid on the surface given body will form a slightly flattened sphere (for example, a drop of mercury on the surface of glass). In the second case, when the interaction forces between the molecules of the liquid and the solid are greater than the interaction forces between the molecules of the liquid itself, then a drop of liquid spreads over the surface of the solid. So a drop of water spreads on the same glass surface, and the total outer surface of the former drop of water increases. In the first case, the liquid wets surface of a solid body, and in the second - does not wet. If you place a thin tube (capillary) in a sufficiently large vessel, then due to non-wetting or wetting of the capillary walls by the liquid, the surface of the liquid (meniscus) has a convex shape in the first case and a concave shape in the second case (Fig. 2.4).

Interaction forces between liquid molecules and wall molecules cause additional pressure on the surface of the liquid. This pressure is caused by surface tension forces and for a convex surface it is positive and directed towards the inside of the liquid, for a concave surface it is negative and directed towards the opposite side. As a result, with a concave meniscus, the liquid, under the influence of the pressure difference on the surface of the vessel and on the surface of the meniscus, will rise in the capillary to a height h(Fig. 2.4) . With a convex meniscus, the liquid, on the contrary, will sink in the capillary. Physical phenomenon, which consists in the ability of liquids to change the level in tubes, narrow channels of arbitrary shape, porous bodies, is called capillarity (from lat. capillaris - hair).

Height of rise or fall of liquid in a capillary h(m) is determined by the formula:

Where σ – surface tension, N/m; ρ– liquid density, kg/m3; d to– capillary diameter, m.

For water at 20 0 C, formula (1.19) will take the form: h=0, 0298/d to.

Capillary phenomena occur both in nature (moisture exchange in soil and plants) and in technology (the action of wicks, moisture absorption by porous media, unbrakable control microcracks, etc.). This phenomenon can lead to dampness in the basement and first floors of buildings if the waterproofing is performed poorly.

Ideal liquid

An ideal liquid called a non-existent liquid in which there are no forces of internal friction, it does not change its volume with changes in pressure and temperature and does not resist rupture at all. Thus, an ideal fluid is a simplified model of a real fluid. Using an ideal fluid model can significantly simplify the methods for solving hydraulic problems. At the same time, the use of this model does not allow one to obtain an objective picture of the processes occurring during the movement of a real fluid. Therefore, to achieve the required accuracy in calculations, the resulting equations for an ideal liquid are corrected by correction factors.

Non-Newtonian fluids

Non-Newtonian fluids are fluids that do not obey Newton's law of internal friction (see Equation 2.13). Such liquids include polymer, cement, clay and lime mortars, sapropels, paints, adhesives, wastewater with large amounts of impurities, etc.

The movement of such liquids begins after the tangential stresses in them reach a certain value. These voltages are called initial shear stress. In a non-Newtonian fluid, the shear stress is determined by the Shvedov–Bingham formula:

, (2.20)

Where τ 0 – initial shear stress, Pa; μpl– Bingham (plastic) viscosity, Pa∙s.

Values τ 0 And μpl for each non-Newtonian fluid are different.