Why does a liquid have fluidity. Basic physical properties of liquid and gas

A characteristic property of liquid and gaseous bodies is their fluidity, that is, low resistance to shear deformation: if the shear rate tends to zero, then the forces of resistance of the liquid or gas to this deformation also tend to zero. In other words, liquid and gaseous substances do not have elasticity of form- they easily take the form of the vessel in which they are located.

To change the volume V of a liquid or gas, finite external forces are required. When the volume changes as a result of external influences, elastic forces arise in the liquid and gas, which balance the action of external forces. The elastic properties of liquids and gases are determined by the fact that their individual parts act on each other (interact) or on bodies in contact with them with a force that depends on the degree of compressibility of the liquid or gas. The corresponding interaction is characterized by a quantity called pressure P.

Let us consider a liquid that is in equilibrium, that is, under conditions when its individual parts do not move relative to each other. Select an elementary area in the liquid D.S.(see figure 5.1). On the D.S. forces acting from other parts of the fluid are equal in magnitude but opposite in direction. To clarify the nature of these forces, let us mentally remove the liquid above D.S., and replace it with the resultant force Df, so that the state of other parts is not disturbed. These forces must be perpendicular D.S., since otherwise the tangential component of the forces would set the fluid particles in motion along D.S., and the balance would be disturbed. Therefore, the equilibrium of the liquid will take place when the resultant of all forces Df perpendicular D.S..

Strength Df, referred to the unit of the site surface D.S., is called pressure P, i.e

Atoms (or molecules) in crystals are ordered to form a crystal lattice.

Questions and tasks

First level

What states of matter do you know?
How can you be sure by experience that an “empty” glass is filled with air?
Why is it impossible to fill with gas only half of the vessel, in which there are no partitions?
What is the molecular structure of gases? What properties of gases does it explain?
What observations about the properties of a liquid can be made by pouring water from one vessel into another?
What is the molecular structure of liquids? What properties of liquids does it explain?
What properties of solids do you know? Give examples illustrating the differences in the properties of solids.
Second level
Give examples of gases, liquids and solids known to you.
What are the general properties of liquid and gas? Liquids and solids?
What are the main differences between a gas, a liquid and a solid?
What explains the low compressibility of liquids and solids?
What are crystalline bodies? What is their molecular structure? Give examples of crystalline bodies.
Give examples of amorphous bodies. What is their difference from crystalline?
What do amorphous and crystalline bodies have in common? Have amorphous bodies and liquids?
Write a problem about the states of matter, the answer to which would be: "Only gas."

home laboratory

Fill a plastic bottle about halfway with water and cap tightly. Try squeezing the bottle. Then repeat the same experiment, filling the bottle to the top. What difference did you notice? What does it indicate?
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?

Key points in this chapter

All the bodies around us are made up of atoms. Scientists today know more than 100 different types of atoms.
Attracting to 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 this substance.
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 rate of chaotic (thermal) motion of molecules increases with increasing temperature.
Molecules interact with each other: at very small distances they repel, and at several large 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. The gas is easily compressible. Molecules in a gas are not located close to each other.
The liquid takes the shape of the container it is in. 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 definite order in this arrangement.
Solid bodies retain volume and shape.
Solids are either crystalline or amorphous.
Atoms (or molecules) in crystals are ordered, 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.

Liquids:

Unlike a solid body, a liquid is characterized by low cohesion 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. Dropping liquids have a high resistance to compression (practically incompressible) and low resistance to tangential and tensile forces (due to insignificant adhesion of particles and low friction forces between particles). Gaseous liquids are characterized by an almost complete absence of resistance to compression. Dropping liquids include water, gasoline, kerosene, oil, mercury and others, and gaseous - all gases.

Hydraulics studies dropping liquids. 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 a mass to the volume occupied by that mass. Density is measured in the SI system in kilograms per cubic meter (kg/m3). The density of water is 1000 kg/m3.

Aggregated indicators are also used: - kilopascal - 1 kPa = 103 Pa; - megapascal - 1 MPa = 106 Pa.

The compressibility of a liquid is its property to change volume with a change in pressure. This property is characterized by volumetric compression or compressibility, which expresses the relative decrease in the volume of a liquid with an increase in pressure per unit area. For calculations in the field of building 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 with a change in temperature by 1 C.

Unlike other bodies, the volume of water decreases when it is heated from 0 to 4 °C. 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 minor changes in water temperature and pressure, the change in this coefficient can be neglected.

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

Viscous forces are manifested during the motion of a real fluid. If the liquid is at rest, then its viscosity can be taken equal to zero. With increasing temperature, the viscosity of the liquid decreases rapidly; remains almost constant as pressure changes.

Gases:

The physical properties of gases, like any substance, begin with definitions related to its mass and energy. So the density of the gas, in a certain sense, equally, is determined as follows: if the finite values of the mass and the dimensions of the volume are known, then for infinitely small volumes of matter the limit value of the density is the ratio of r - the density of the gas to the density of dry air - ra under standard conditions. The relative density of a gas in air is The density of a gas at 0°C and atmospheric pressure can be determined by its molar mass - We recalculate the density for different physical parameters of the gas using the formula. The density of the gas mixture is determined by the rule of mixing (additivity) ai - volumetric concentrations 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 In thermal calculations, depending on the ongoing process, the concept of the heat capacity of a substance is used - at a constant pressure cp, and at a constant volume cv, for which the Mayer 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 that distinguishes the deviation of a gas from an ideal one. The compressibility characteristic is determined by the compressibility factor, or Z - factor, in foreign terminology, in a real gas model. The compressibility coefficient 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 Calculation of the compressibility coefficient (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 shear stresses, the concept of the dynamic viscosity coefficient is used, which is used in the calculation of shear stresses according to the formula: v, n - relative flow velocity and its normal to the streamlines; - coefficient of dynamic viscosity of the gas (Pa s); - stresses of internal friction (Pa). For kinematic viscosity, a designation has been introduced: Almost all natural gases contain water vapor. The presence of water vapor in the gas contributes to the formation of hydrates on the surface of the pipe. There are w - absolute mass and - volumetric humidity. 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 humidity of the gas is introduced. The relative humidity of a 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 is the maximum possible amount of water vapor that can be at a given temperature T; mw - vapor density; w,T - saturation vapor density; 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 the gas pipeline, the gas must be dried so that the temperature of its transportation would be several degrees below its dew point.

Liquid is a state of aggregation of a substance, which occupies an intermediate position between its solid and gaseous states.

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

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

Fluidity

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

Due to fluidity, the liquid is able to change shape in a short time under the action of even a small force. All liquids can pour in a jet, splash with drops. They are easy to pour from one vessel to another. At the same time they do not keep their 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 body that retains this configuration.

Fluidity increases with increasing liquid temperature and decreases with its decrease. This is because as the temperature rises, the distance between the particles of the liquid also increases, and they become more mobile. The 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. The same forces of gravity act on these fluids. So why is their fluidity different? The point is that they have different viscosity . The higher the viscosity of a liquid, the lower its fluidity.

What is viscosity, and what is its nature? Viscosity is also called internal friction . This is the ability of a fluid to resist the movement of different layers of fluid 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. There are forces that slow down their movement. They are directed in the direction opposite to the motion of the considered layer.

Viscosity is an important characteristic of 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 action is not complicated. The time is calculated during which a given volume of liquid flows through a thin tube (capillary) under the influence of the pressure difference at its ends. Since the diameter and length of the capillary, the pressure difference are known, it is possible to make calculations based on Poiseuille's law , Whereby volume of liquid passing per second (second volume flow) 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 of the liquid .

where Q - second fluid flow, m 3 / s;

r 1 - r 2 = ∆p - 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 change in all directions. This is how it is worded pascal's law . The operation of brake systems, hydraulic presses and other hydraulic devices is based on this feature of liquids.

The liquid retains its volume if external conditions (pressure, temperature) do not change. But when heated, the volume of the 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 a liquid. But it's still possible if the pressure changes. And in this case, its density and volume change. If compression occurs in one section of the fluid, then it will be transferred to other sections gradually. This means that elastic waves will propagate in the liquid. If the density changes slightly, then we get a sound wave. And if it is strong enough, then a shock wave arises.

Surface tension of liquid

We observe the phenomenon of surface tension every time water slowly drips from a faucet. First, we see a thin transparent film that stretches under the weight of water. But it does not break, but covers 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 semblance of a ball.

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

And on the molecules that are 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, the forces of attraction of air molecules act on them. But since they are very small, they are neglected.

The resultant of all forces acting on a molecule located on the surface is directed inside 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 the liquid, the more molecules are there, 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 the water can easily hold a coin, a razor blade, or a steel needle, although these objects are much heavier than water. The force of gravity acting on them is less than the surface tension of water.

The ball has the smallest surface of all geometric solids. Therefore, if only surface tension forces act on a liquid, then it takes the form of a sphere. This is the shape of water drops in weightlessness. Soap bubbles or bubbles of boiling liquid also tend to take on a spherical shape.

Miscibility

Liquids can dissolve in each other. This ability is called miscibility . If two miscible liquids are placed in one vessel, then as a result of thermal motion, their molecules will gradually pass through the interface. The result will be mixing. But not all liquids can be mixed. For example, water and vegetable oil never mix. Water and alcohol are very easy to mix.

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 lubricate their feathers with their beaks. And they stay dry because the water drips off them in droplets.

Place a drop of water on a polystyrene plate. It takes the form of a flattened ball. Let's try to place the same drop on a glass plate. We will see that it spreads on the glass. What happens to 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 a solid surface, then the liquid will collect into a droplet. This is how water behaves on a polystyrene plate. She is does not wet this plate. In the same way, droplets of morning dew do not spread 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 is stronger than the forces of attraction between the molecules themselves, then 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 vessel. Looking at the surface of the water, we can see that it is not horizontal. At the edges of the vessel, it curves down. 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 is bent upwards. In this case, the sticking 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, cosmetic, etc. Wetting is necessary when gluing, dyeing fabrics, applying to the surface paints, varnishes. And during the construction of 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, awnings.

Capillarity

Another interesting feature of the liquid - capillary effect . This is the name of its ability to change its level in tubes, narrow vessels, 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 column of water. If you lower the same tube into liquid mercury, then the height of the mercury column will be below 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, glass tubes, through which moisture easily rises. For the same reason, the wick in a kerosene lamp is impregnated with kerosene, the towel absorbs moisture from wet hands, various chemical processes take place. In plants, nutrients and moisture enter the leaves through the capillaries. Due to the capillary effect, the vital activity of living organisms is possible.