Special properties of liquids, gases and solids. Basic properties of liquids

Liquids:

Unlike a solid, a liquid is characterized by low adhesion between particles, as a result of which it has fluidity and takes the form of a vessel in which it is placed.

Liquids are divided into two types: drip and gaseous. Droplet liquids have high resistance to compression (practically incompressible) and low resistance to tangential and tensile forces (due to insignificant adhesion of particles and small forces of friction between particles). Gaseous liquids are characterized by almost no resistance to compression. Droplet liquids include water, gasoline, kerosene, oil, mercury, and others, and gaseous liquids - all gases.

Hydraulics studies droplet fluids. When solving practical problems in hydraulics, the concept of an ideal fluid is often used - an incompressible medium that does not have internal friction between individual particles.

The main physical properties of a liquid include density, pressure, compressibility, thermal expansion, and viscosity.

Density is the ratio of mass to the volume occupied by that mass. Density is measured in SI units in kilograms per cubic meter (kg / m3). The density of the water is 1000 kg / m3.

The enlarged indicators are also used: - kilopascal - 1 kPa \u003d 103 Pa; - megapascal - 1 MPa \u003d 106 Pa.

The compressibility of a fluid is its property to change its volume when pressure changes. This property is characterized by the coefficient of volumetric compression or compressibility, which expresses the relative decrease in the volume of liquid with increasing pressure per unit area. For calculations in the field of construction hydraulics, water is considered incompressible. In this regard, when solving practical problems, the compressibility of a liquid is usually neglected.

The reciprocal of the volumetric compression ratio is called the modulus of elasticity. The modulus of elasticity is measured in pascals.

The thermal expansion of a liquid when it is heated is characterized by the coefficient of thermal expansion, which shows the relative increase in the volume of the liquid when the temperature changes by 1 C.

Unlike other bodies, the volume of water when it is heated from 0 to 4 ° C decreases. At 4 ° C, water has the highest density and the highest specific gravity; with further heating, its volume increases. However, in the calculations of many structures with insignificant changes in water temperature and pressure, the change in this coefficient can be neglected.

The viscosity of a liquid is its property to resist the relative movement (shear) of liquid particles. The forces resulting from the sliding of layers of liquid are called internal friction forces, or viscous forces.

The forces of viscosity appear when a real fluid moves. If the liquid is at rest, then its viscosity can be taken to be zero. With increasing temperature, the viscosity of the liquid decreases rapidly; remains almost constant as the pressure changes.

Gases:

The physical properties of gases, like any substance, begin with the definitions associated with its mass and energy. So the density of a gas, in a sense, is equally valid, is determined as follows: if the final values \u200b\u200bof the mass and dimensions of the volume are known, then for infinitely small volumes of matter the limiting value of the density is equal. When calculating the commercial gas consumption, use the relative density of the gas, i.e. the ratio of r - gas density to dry air density - ra under standard conditions. Relative density gas through air is equal to Gas density at 0 ° С and atmospheric pressure can be determined by its molar mass - The density is recalculated for different physical parameters of the gas using the formula. The density of the gas mixture is determined by the mixing (additivity) rule ai - volumetric concentration of gas components in the mixture (0 ai 1), - density of the mixture components. Specific gas volume is calculated as follows Average molar mass mixture is equal In thermal calculations, depending on the ongoing process, the concept of heat capacity of a substance is used - at constant pressure cp, and at constant volume cv, for which Mayer's formula is valid.The ratio of heat capacities is called the adiabatic exponent Another important physical property of a real gas is its compressibility. In fact, the compressibility of a gas is the determining factor in distinguishing the deviation of a gas from ideal. The compressibility characteristic is determined by the compressibility factor, or Z - factor, in foreign terminology, in the real gas model. The compressibility factor depends on the reduced temperature and pressure (Tm, pm), which are determined as follows: T, Tcr - current and critical gas temperature, p, pcr - current and critical gas pressure, for example, in a pipeline Compressibility factor calculation (according to the ONTP 51- 1-85): According to Gubkin University: Consider physical properties real gases associated with its viscosity. As is known, the viscosity of a continuous medium determines its internal friction between layers of liquid or gas during their relative motion. Determined from experimental relationships between voltage and velocity gradient. To calculate the shear stresses, the concept of the dynamic viscosity coefficient is used, which is used when calculating the shear stresses by the formula: v, n - the relative flow velocity and its normal to streamlines; - coefficient of dynamic viscosity of gas (Pa s); - internal friction stress (Pa). The designation has been introduced for the kinematic viscosity: Almost all natural gases contain water vapor. The presence of water vapor in the gas promotes the formation of hydrates on the pipe surface. Distinguish between w - absolute mass and - volumetric moisture These formulas do not take into account the deviation of the laws of a real gas from the laws of an ideal gas. Therefore, the concept of relative gas humidity is introduced. The relative humidity of the gas is the ratio of the actual amount of water vapor to the maximum possible (at the same pressures and temperatures) per unit volume: mw, T - the maximum possible amount of water vapor that can be at a given temperature T; mw is the vapor density; w, T is the density of saturated steam; pw is the partial pressure of water vapor in the gas mixture; pw, T is the pressure of saturated water vapor in the gas mixture. The temperature at which a gas becomes saturated at a certain pressure is called the dew point. During technological calculations of a gas pipeline, the gas must be dried so that the temperature of its transportation would be several degrees below its dew point.

Mechanics of liquids and gases

Basic physical properties of liquid and gas. Parameters that determine the properties of liquids and gases. Forces acting on a liquid.

Liquid - physical a body that has great resistance to change in its volume and little resistance to change in its shape. Zh. Differ from solids in the small force of cohesion between particles and their ease of mobility, due to which f. takes the form of the vessel into which it is poured. This property - fluidity ... J. are: drip- incompressible (water, oil) and gaseous - compressible. (vapors, gases).

Physical properties:

1) Density :. For distilled water at

2) Specific gravity - weight of liquid per unit volume:.

3) Relative density (relative specific gravity) - density ratio (specific weight) w. to density (specific gravity) at:

4) Compressibility - ability w. decrease volume with increasing pressure.

Volume Compression Ratio - the relative change in the volume of liquid with a single change in pressure: .

Bulk modulus - the opposite value:.

5) Thermal expansion - ability w. change the volume when the temperature changes.

Thermal expansion coefficient - relative change in volume when the temperature changes by: . .

6) Viscosity - in the liquid to resist the movement of its particles and develop internal shear stresses during movement: ,

where is the internal force. friction, N; S is the area of \u200b\u200brubbing layers, m 2;

- dynamic viscosity of the liquid, [Pa ∙ s] \u003d - poise.

τ - shear stress: (for Newtonian) and (for non-Newtonian f.), is the shear stress of a fluid at rest.

Dynamic viscosity is numerically equal to the unit friction force (τ) at a velocity gradient equal to one... The ± sign says that two adjacent layers interact: a layer with a higher speed accelerates the other (+), a layer with a lower speed slows down (-).

Kinematic viscosity - the ratio of µ to the density of the liquid:.

du / dy Is the velocity gradient characterizing the rel. speed change du between separate layers of thickness dy, s -1. du / dy \u003d tg β, where β is the angle of inclination of the tangent to the diagram.

Viscosity of ordinary (Newtonian) oil. depends on the genus. and temperature. Viscosity tester - viscometer. For non-Newtonian railways the viscosity depends on the velocity gradient (mortars, oil products).

Forces acting on a liquid

1) Surface forces (forces of hydrodynamic pressure, forces of elasticity, friction) are distributed over the surface of the rail. and are proportional to its area:

, where p - unit force or stress, N / m 2; ω - area of \u200b\u200baction of force, m 2.

A liquid is a substance that is in an aggregate state, which is intermediate between solid and gaseous. Moreover, its state, as in the case of solids, is condensable, that is, it implies a bond between particles (atoms, molecules, ions). The liquid has properties that radically distinguish it from substances that are in other states of aggregation. The main one is the ability to repeatedly change shape under the influence of mechanical stress without loss of volume. Today we will find out what properties liquids have, and what they are in general.

general characteristics

Gas does not retain volume and shape, a solid retains both, and a liquid - only volume. That is why the liquid state of aggregation is considered intermediate. The surface of the liquid is like an elastic membrane and determines its shape. The molecules of such bodies, on the one hand, do not have a definite position, and on the other hand, they cannot receive complete freedom of movement. They can collect in droplets and flow under their own surface. There is an attraction between the molecules of the liquid, which is enough to keep them close.

The substance is in a liquid state in a certain temperature range. If the temperature drops below it, there is a transition to a solid form (crystallization), and if it rises above it, into a gaseous form (evaporation). The boundaries of this interval for the same fluid can fluctuate depending on the pressure. For example, in the mountains, where the pressure is significantly lower than on the plains, the water boils at a lower temperature.

Usually, a liquid has only one modification, therefore it is both an aggregate state and a thermodynamic phase. All liquids are divided into pure substances and mixtures. Some of these mixtures are of decisive importance in human life: blood, sea \u200b\u200bwater and others.

Let's consider the basic properties of liquids.

Fluidity

Liquid differs from other substances, first of all, by its fluidity. If an external force is applied to it, a stream of particles arises in the direction of its application. Thus, under the influence of external unbalanced forces, the liquid is not capable of maintaining the shape and relative position of particles. For the same reason, it takes the form of a vessel into which it falls. Unlike solid plastic bodies, liquids do not have a yield point, that is, they flow at the slightest exit from an equilibrium state.

Volume preservation

One of the characteristic physical properties of liquids is the ability to retain volume under mechanical stress. They are extremely difficult to compress due to their high molecular density. According to Pascal's law, the pressure that is exerted on a liquid enclosed in a vessel is transmitted without change to each point of its volume. Along with the minimum compressibility, this feature is widely used in hydraulics. Most liquids increase in volume when heated, and decreases when cooled.

Viscosity

Among the main properties of liquids, as in the case of gases, it is worth noting viscosity. Viscosity is the ability of particles to resist movement relative to each other, that is, internal friction. When adjacent layers of liquid move relative to each other, an inevitable collision of molecules occurs, and forces arise that slow down the ordered movement. The kinetic energy of ordered motion is converted into heat energy of chaotic motion. If the liquid placed in the vessel is moved and then left alone, then it will gradually stop, but its temperature will rise.

Free surface and surface tension

If you look at a drop of water lying on a flat surface, you can see that it is rounded. This is due to such properties of liquids as the formation free surface and surface tension. The ability of liquids to preserve volume determines the formation of a free surface, which is nothing more than an interface between the phases: liquid and gaseous. When these phases of one and the same substance come into contact, forces arise, aimed at reducing the area of \u200b\u200bthe interface plane. They are called surface tension. The phase boundary is an elastic membrane that tends to contract.

Surface tension is also explained by the attraction of liquid molecules to each other. Each molecule seeks to "surround" itself with other molecules and leave the interface. Because of this, the surface is rapidly decreasing. This explains the fact that bubbles and bubbles formed during boiling tend to take a spherical shape. If only the surface tension force acts on the liquid, it will certainly take this shape.

Small objects, the density of which exceeds the density of the liquid, are able to remain on its surface due to the fact that the force that prevents the increase in surface area is greater than the force of gravity.

Evaporation and condensation

Evaporation is the gradual transition of a substance from a liquid to a gaseous state. In the process of thermal motion, some of the molecules leave the liquid, passing through its surface, and are converted into vapor. In parallel with this, another part of the molecules, on the contrary, passes from vapor to liquid. When the number of compounds leaving the liquid exceeds the number of compounds entering it, the process of evaporation takes place.

Condensation is the reverse process of evaporation. During condensation, the liquid receives more molecules from the vapor than it gives off.

Both processes described are nonequilibrium and can continue until local equilibrium is established. In this case, the liquid can completely evaporate or enter into equilibrium with its vapor.

Boiling

Boiling is the process of internal transformations of a liquid. When the temperature rises to a certain value, the vapor pressure exceeds the pressure inside the substance, and bubbles begin to form in it. Under gravity, they float upward.

Wetting

Wetting is a phenomenon that occurs when a liquid comes into contact with a solid in the presence of steam. Thus, it occurs at the interface between the three phases. This phenomenon characterizes the "sticking" of a liquid substance to a solid, and its spreading over the surface of a solid. There are three types of wetting: limited, full and non-wetting.

Miscibility

It characterizes the ability of liquids to dissolve in each other. Examples of miscible liquids are water and alcohol, and immiscible liquids are water and oil.

Diffusion

When two mixed liquids are in one vessel, due to the thermal movement, the molecules begin to overcome the interface, and the liquids gradually mix. This process is called diffusion. It can also occur in substances that are in other states of aggregation.

Overheating and hypothermia

Among the fascinating properties of liquids are overheating and hypothermia. These processes often form the basis of chemical foci. With uniform heating, without strong temperature drops and mechanical influences, the liquid can heat up above the boiling point without boiling. This process is called overheating. If an object is thrown into the superheated liquid, it will instantly boil.

In a similar way, the liquid is supercooled, that is, it is cooled to a temperature below the freezing point, bypassing the freezing itself. With a light blow, the supercooled liquid instantly crystallizes and turns into ice.

Waves on the surface

If the equilibrium of a section of the surface of the liquid is disturbed, then it, under the action of restoring forces, will move back to equilibrium. This movement is not limited to one cycle, but turns into vibrations and spreads to other areas. This produces waves that can be observed on the surface of any liquid.

When the force of gravity acts predominantly as the restoring force, the waves are called gravitational. They can be seen everywhere on the water. If the restoring force is formed mainly from the surface tension force, then the waves are called capillary. Now you know what property of liquids causes the familiar excitement of water.

Density Waves

The liquid is extremely difficult to compress, however, with a change in temperature, its volume and density do change. This does not happen instantly: when one area is compressed, others are compressed with a delay. Thus, elastic waves propagate inside the liquid, which are called density waves. If, as the wave propagates, the density changes slightly, then I call it sound, and if it is strong enough - shock.

We got acquainted with the general properties of liquids. All basic characteristics depend on the type and composition of the fluids.

Classification

Having covered the basic physical properties of liquids, let's find out how they are classified. The structure and properties of liquid substances depend on the individuality of the particles that make up their composition, as well as the nature and depth of interaction between them. Based on this, there are:

Atomic liquids. They consist of atoms or spherical molecules that are linked by central van der Waals forces. A prime example are liquid argon and liquid methane.
Liquids consisting of diatomic molecules with identical atoms, the ions of which are bound by Coulomb forces. Examples include: liquid hydrogen, liquid sodium and liquid mercury.
Liquids that are composed of polar molecules linked by dipole-dipole interactions, such as liquid hydrogen bromide.
Associated liquids. They have hydrogen bonds (water, glycerin).
Liquids that are made up of large molecules. For the latter, internal degrees of freedom play an important role.

Substances of the first two (less often three) groups are called simple. They are better studied than everyone else. Among the difficult liquids, water is the most studied. This classification does not include liquid crystals and quantum fluids, since they are special cases and are considered separately.

From the point of view of hydrodynamic properties, liquids are divided into Newtonian and non-Newtonian. The flow of the former obeys Newton's law. This means that their shear stress is linearly dependent on the velocity gradient. The proportionality coefficient between these values \u200b\u200bis called viscosity. In non-Newtonian fluids, the viscosity fluctuates with the velocity gradient.

Study of

The study of the motion and mechanical equilibrium of liquids and gases, as well as their interaction, including with solids, is dealt with by such a branch of mechanics as hydroaeromechanics. It is also called hydrodynamics.

Incompressible fluids are studied in a subsection of fluid mechanics, which is simply called fluid mechanics. Since the compressibility of liquids is very small, in many cases it is simply neglected. Compressible liquids are studied by gas dynamics.

Hydromechanics is additionally subdivided into hydrostatics and hydrodynamics (in the narrow sense). In the first case, the equilibrium of incompressible fluids is studied, and in the second, their motion.

Magnetic hydrodynamics deals with the study of magnetic and electrically conductive fluids, while hydraulics are engaged in applied problems.

The basic law of hydrostatics is Pascal's law. The motion of ideal incompressible fluids is described by the Euler equation. For their stationary flow, Bernoulli's law is satisfied. And the Torricelli formula describes the outflow of liquid substances from the holes. The motion of viscous fluids obeys the Navier-Stokes equation, which, among other things, can take into account the compressibility.

Elastic waves and oscillations in a liquid (as well as in other media) are studied by such a science as acoustics. Hydroacoustics is a subsection devoted to the study of sound in an aquatic environment for solving problems of underwater communication, location and other things.

Finally

Today we got acquainted with the general physical properties of liquids. We also learned what such substances are in general, and how they are classified. Concerning chemical properties liquid, then they directly depend on its composition. Therefore, it is worth considering them separately for each substance. It is difficult to answer which property of a liquid is important and which is not. It all depends on the task in the context of which this fluid is considered.

We already know that liquids have a fixed volume and take the form of the vessel in which they are located. We also know that liquids are much more dense than gases. In general, the densities of liquids have values \u200b\u200bsimilar to those of solids. The compressibility of liquids is very low, since very little free space remains between the liquid particles.

Free falling drop of water. Its spherical shape is due to surface tension.

There are three other important properties of liquids to consider. All these properties can be explained based on representations kinetic theory liquids.

Fluidity and viscosity. Like gases, liquids can flow, and this property is called fluidity. Resistance to flow is called viscosity... A number of factors affect fluidity and viscosity. The most important of these are the forces of attraction between liquid molecules and the shape, structure, and relative molecular weight of these molecules. The fluidity of a liquid made up of large molecules is lower than that of a liquid made up of small molecules. The viscosity of liquids is approximately 100 times that of gases.

Surface tension. The forces of intermolecular attraction evenly act on a molecule in the depth of the liquid from all sides. However, on the surface of the liquid, these forces are unbalanced, and as a result, the surface molecules experience the effect of the resulting force directed into the liquid. Therefore, the surface of the liquid is in a state of tension - it strives to contract all the time. The surface tension of a liquid is the minimum force required to overcome the pushing of liquid particles inward and thereby keep the surface of the liquid from contracting. The existence of surface tension explains the spherical shape of freely falling liquid droplets.

Diffusion. This is the name of the process by which a substance is redistributed from an area of \u200b\u200bhigh concentration or high pressure to an area of \u200b\u200blower concentration or lower pressure. Diffusion in liquids is much slower than in gases, because liquid particles are packed much more densely than gas particles. A particle diffusing in a liquid is subject to frequent collisions and therefore difficult to move. There is a lot of free space between particles in gases, and they can be redistributed much faster. Diffusion is carried out between mutually soluble, or miscible, liquids. It does not occur between immiscible fluids. Unlike liquids, all gases mix with each other and therefore can diffuse into one another.

Liquids:

Unlike a solid, a liquid is characterized by low adhesion between particles, as a result of which it has fluidity and takes the form of a vessel in which it is placed.

The main physical properties of a liquid include density, pressure, compressibility, thermal expansion, and viscosity.

Density is the ratio of mass to the volume occupied by that mass. Density is measured in SI units in kilograms per cubic meter (kg / m3). The density of the water is 1000 kg / m3.

The enlarged indicators are also used: - kilopascal - 1 kPa \u003d 103 Pa; - megapascal - 1 MPa \u003d 106 Pa.

The reciprocal of the volumetric compression ratio is called the modulus of elasticity. The modulus of elasticity is measured in pascals.

Gases:

The physical properties of gases, like any substance, begin with the definitions associated with its mass and energy. So the density of a gas, in a sense, is equally valid, is determined as follows: if the final values \u200b\u200bof the mass and dimensions of the volume are known, then for infinitely small volumes of a substance we have the limiting value of the density. the ratio of r - gas density to dry air density - ra under standard conditions. The relative density of the gas in air is equal to The density of the gas at 0 ° C and atmospheric pressure can be determined by its molar mass - The density is recalculated for different physical parameters of the gas using the formula. The density of the gas mixture is determined by the mixing (additivity) rule ai - volumetric concentration of gas components in the mixture (0 ai 1), - density of the mixture components. The specific volume of gas is calculated as follows.The average molar mass of the mixture is equal to In thermal calculations, depending on the process, the concept of heat capacity of a substance is used - at constant pressure cp, and at constant volume cv, for which Mayer's formula is valid The ratio of heat capacities is called the adiabatic index. Another important physical the property of a real gas is its compressibility. In fact, the compressibility of a gas is the determining factor in distinguishing the deviation of a gas from ideal. The compressibility characteristic is determined by the compressibility factor, or Z - factor, in foreign terminology, in the real gas model. The compressibility factor depends on the reduced temperature and pressure (Tm, pm), which are determined as follows: T, Tcr - current and critical gas temperature, p, pcr - current and critical gas pressure, for example, in a pipeline Compressibility factor calculation (according to the ONTP 51- 1-85): According to Gubkin University: Consider the physical properties of real gases associated with its viscosity. As is known, the viscosity of a continuous medium determines its internal friction between layers of liquid or gas during their relative motion. Determined from experimental relationships between voltage and velocity gradient. To calculate the shear stresses, the concept of the dynamic viscosity coefficient is used, which is used to calculate the shear stresses by the formula: v, n is the relative flow velocity and its normal to streamlines; - coefficient of dynamic viscosity of gas (Pa s); - internal friction stress (Pa). The designation has been introduced for the kinematic viscosity: Almost all natural gases contain water vapor. The presence of water vapor in the gas promotes the formation of hydrates on the pipe surface. Distinguish between w - absolute mass and - volumetric moisture These formulas do not take into account the deviation of the laws of a real gas from the laws of an ideal gas. Therefore, the concept of relative gas humidity is introduced. The relative humidity of the gas is the ratio of the actual amount of water vapor to the maximum possible (at the same pressures and temperatures) per unit volume: mw, T - the maximum possible amount of water vapor that can be at a given temperature T; mw is the vapor density; w, T is the density of saturated steam; pw is the partial pressure of water vapor in the gas mixture; pw, T is the pressure of saturated water vapor in the gas mixture. The temperature at which a gas becomes saturated at a certain pressure is called the dew point. During technological calculations of a gas pipeline, the gas must be dried so that the temperature of its transportation would be several degrees below its dew point.