Boiling is the process of transition of a substance from a liquid to a gaseous state (vaporization in a liquid). Boiling is not evaporation: it differs in that it can happen only at a certain pressure and temperature.

Boiling - heating water to the boiling point.

Boiling water is a complex process that takes place in four stages... Consider an example of boiling water in an open glass vessel.

In the first stage boiling water, small air bubbles appear at the bottom of the vessel, which can also be seen on the surface of the water on the sides.

These bubbles are formed as a result of the expansion of small air bubbles that are found in small cracks in the vessel.

In the second stage an increase in the volume of bubbles is observed: more and more air bubbles break to the surface. There is saturated steam inside the bubbles.

As the temperature rises, the pressure of the saturated bubbles increases, as a result of which they increase in size. As a result, the Archimedean force acting on the bubbles increases.

It is thanks to this force that the bubbles tend to the surface of the water. If the top layer of water did not have time to warm up up to 100 degrees C (and this is the boiling point of pure water without impurities), then the bubbles descend into the hotter layers, after which they again rush back to the surface.

Due to the fact that the bubbles are constantly decreasing and increasing in size, sound waves appear inside the vessel, which create the noise characteristic of boiling.

In the third stage a huge number of bubbles rise to the surface of the water, which initially causes a slight turbidity of the water, which then "turns pale". This process does not last long and is called “boiling with a white key”.

Finally, at the fourth stage boiling, the water begins to boil intensively, large bursting bubbles and splashes appear (as a rule, splashes mean that the water has boiled over too much).

Water vapor begins to form from the water, and the water makes specific sounds.

Why do walls “bloom” and windows “cry”? Very often, builders are to blame for this, having incorrectly calculated the dew point. Read the article to find out how important this physical phenomenon is, and how can you still get rid of excessive dampness in the house?

What benefits can melt water bring to those who want to lose weight? You will find out about this, it turns out that you can lose weight without much effort!

Steam temperature at boiling water ^

Steam is the gaseous state of water. When steam enters the air, it, like other gases, exerts a certain pressure on it.

During vaporization, the temperature of the steam and water will remain constant until all the water has evaporated. This phenomenon is explained by the fact that all the energy (temperature) is directed to the conversion of water into steam.

In this case, dry saturated steam is formed. There are no highly dispersed particles of the liquid phase in such a vapor. Also steam can be saturated wet and overheated.

Saturated steam containing suspended fine particles of the liquid phase, which are evenly distributed over the entire mass of steam, is called wet saturated steam.

At the beginning of water boiling, just such a steam is formed, which then turns into dry saturated. Steam, the temperature of which is higher than the temperature of boiling water, or rather superheated steam, can only be obtained using special equipment. In this case, such a vapor will be close in its characteristics to a gas.

Boiling point of salt water ^

The boiling point of salt water is higher than the boiling point of fresh water... Consequently salt water boils later fresh... Salt water contains Na + and Cl- ions, which occupy a certain area between water molecules.

In salt water, water molecules attach to salt ions - this process is called "hydration". The bond between water molecules is much weaker than the bond formed during hydration.

Therefore, when boiling from fresh water molecules, vaporization occurs faster.

Boiling water with dissolved salt will require more energy, which in this case is temperature.

As the temperature rises, the molecules in the salt water begin to move faster, but there are fewer of them, so they collide less often. As a result, less steam is produced, the pressure of which is lower than that of fresh water steam.

In order for the pressure in salt water to rise above atmospheric pressure and the boiling process to begin, a higher temperature is required. When 60 grams of salt is added to 1 liter water, the boiling point will increase by 10 C.

Oleg

And here they were mistaken by 3 orders of magnitude "The specific heat of vaporization of water is 2260 J / kg." Correct kJ, i.e. 1000 times more.

Nastya

What explains the high boiling point of water?
What causes water to boil at high temperatures?

IamJiva

Superheated steam is steam with a temperature above 100C (well, if you are not in the mountains or in a vacuum, but under normal conditions), it is obtained by passing steam through hot tubes, or, more simply, from a boiling solution of salt or alkali (dangerous - alkali is stronger than Na2CO3 (for example potash - K2CO3 why NaOH residues in a day or two become not dangerous for the eyes, unlike KOH residues carbonated in the air) saponifies the eyes, do not forget to wear swimming goggles!), but such solutions boil in jerks, you need boiling water and a thin layer on bottom, water can be added when boiling, only it boils away.
so from salty water, steam with a temperature of about 110C can be obtained by boiling, no worse than the same from a hot 110C pipe, this steam contains only water and is heated, in which way it does not remember, but at 10C it has a "power reserve" in comparison with steam from a teapot of fresh water.
It can be called dry because warming (contacting as in a pipe, or even by radiation characteristic not only of the sun but also of any body to a certain (temperature-dependent) degree) a certain object, after cooling down to 100C, it can still remain a gas, and only further cooling below 100C will cause it to condense into a drop of water, and almost a vacuum (the saturated vapor pressure of water is about 20 mm Hg out of 760 mm Hg (1 atm), that is, 38 times lower than atmospheric pressure, this also happens with unheated, saturated steam with a temperature of 100 C in a warmed-up vessel (a kettle from a spout steam), and not only with water, but with any boiling substance, for example, medical ether boils already at body temperature, and can boil in a flask in the palm of your hand, from the neck of which its vapors will "gush", noticeably refracting light, if now Close the flask with the second palm, and remove the heating of the lower palm, replacing it with a stand with a temperature below 35C, the ether will stop boiling, and its saturated vapor, which pushed all the air out of the flask during boiling, will condense are drawn into a drop of ether, creating a vacuum no stronger than the one from which the ether boils, that is, approximately equal to the pressure of the saturated vapor of the ether at the temperature of the coldest point inside the flask, or a second vessel or hose connected to it without leaks with a closed far end, this is how Kriophor device, demonstrating the principle of a cold wall, like a sweet sticky-stick - bees, capturing all vapor molecules in the system. ("vacuum alcohol" is driven this way, without heating)

And at more than 1700 Celsius, water decomposes very well into oxygen and hydrogen ... it turns out bad-boom, no need to splash it on all sorts of burning metal-sycamber structures

Boiling liquids

At a sufficiently low temperature, liquid evaporation occurs from its free surface and is calm. Upon reaching a certain temperature, called boiling point, vaporization begins to occur not only from the free surface, but also in the bulk of the liquid. Inside it, steam bubbles appear, increase in size and rise to the surface. Vaporization becomes violent and is called boiling.The boiling mechanism is as follows.

There are always tiny air bubbles in a liquid, which, like Brownian particles, make slow random movements in the volume of the liquid. Inside the bubbles, along with air, there is also a saturated vapor of the surrounding liquid. The condition for the stability of the bubble size is the equality of the internal and external pressures on its surface. External pressure is equal to the sum of atmospheric pressure and hydrostatic pressure at the depth where the bubble is located. The internal pressure is equal to the sum of the partial pressures of air and vapor inside the bubble. Thus,

.

For shallow depths, at which the hydrostatic pressure is small compared to the atmospheric pressure, we can put, and the last equality will take the form:

If the temperature is slightly increased, then the saturated vapor pressure in the bubble will increase and the bubble size will increase, the air pressure inside it will decrease, so that the sum will remain unchanged and the equilibrium condition (13.19) will be fulfilled at an increased temperature for a bubble with an increased size. However, if the temperature is increased so much that the saturated vapor pressure in the bubble becomes equal to atmospheric pressure,

then equality (13.19) ceases to hold. The size of the bubble and the mass of vapor in it will increase, the bubble, under the action of the buoyancy (Archimedean) force, will rush to the surface of the liquid, and the liquid will begin to boil. So, equality (13.20) is the condition for the boiling of a liquid in a vessel at a shallow depth: boiling of a liquid at a shallow depth occurs at a temperature at which the pressure of saturated vapor of this liquid becomes equal to atmospheric pressure. Thus, the boiling point is dependent on atmospheric pressure.

Example 13.4. Water at normal atmospheric pressure boils at temperature. Consequently, the pressure of saturated water vapor at this temperature is equal to normal atmospheric pressure.

Example 13.5. At temperature, the volume of a bubble in water at a shallow depth is equal to. The water temperature became equal. What is the volume of the bubble at temperature? Atmospheric pressure is normal. The pressure of saturated water vapor at a temperature is , and at temperature it is equal to.

Let us denote by the mass of air in the bubble. We have:

,

where is the molar mass of air, is the air pressure in the volume bubble at temperature. In accordance with the equilibrium condition, the bubble size (13.19) should be set. We get:

Applying the last equality at two different temperatures and, we get:

From the last equalities we find:

.

Example 13.6. Consider a solution of a non-volatile substance in a certain solvent. Applying Raoult's law (13.3), we obtain for the saturated vapor pressure over the solution:

.

In view of the non-volatility of the substance, we have, and the last equality takes the form:

.

So, the saturated vapor pressure over the solution is less than over the pure solvent (at the same temperature). It follows that the solution must be heated to a higher temperature than the pure solvent in order for the saturated vapor pressure to equal atmospheric pressure and boiling begins. Thus, the boiling point of the solution in question is higher than the boiling point of the pure solvent.

Task 13.5. Find the boiling point of water in the mountains at an altitude above sea level. The atmospheric pressure at sea level is considered normal. The temperature of the atmosphere is taken equal.

Answer: , where is the boiling point of water at normal atmospheric pressure, - molar mass of air, - latent molar heat of vaporization of water at temperatures close to.

Indication. To find the atmospheric pressure at the level, use the barometric formula. To find the saturated vapor pressure at temperature, use the formula (13.17). Use the boiling condition (13.20).

13.7. Transformations "liquid - solid"

At sufficiently low temperatures, all liquids, with the exception of liquid helium, become solid.

Consider the transformation of a one-component liquid, that is, one consisting of atoms of the same type, into a solid. This process is called crystallization... Crystallization is the transition of a system of atoms to a state with a higher degree of order and occurs at a certain temperature, called melting point(hardening). At this temperature, the kinetic energy of the thermal motion of atoms becomes sufficiently small and the forces of interaction between the atoms can hold the atoms in certain positions - the nodes of the crystal lattice.

The process of converting a solid into a liquid is called meltingand is the reverse process of crystallization. This process takes place at the same temperature as melting.

If heat is continuously supplied to a solid, then its temperature will change over time, as shown in Fig. 13.4 a. The section corresponds to the heating of a solid, the section corresponds to the two-phase state of a substance, in which the solid and liquid phases of this substance are in equilibrium. Thus, the area corresponds to the melting of the solid. At the point, all matter becomes liquid and further heat input is accompanied by an increase in the temperature of the liquid.

The heat supplied to the "solid - liquid" system at the stage of melting does not lead to a change in the temperature of the system and goes to the destruction of bonds between atoms. This warmth is called latent heat of fusion.

If the liquid gives off heat, then its temperature depends on time, as shown in Fig. 13.4 b. The stage corresponds to the cooling of the liquid, the stage to its crystallization (two-phase states of the system), and the stage to the cooling of the solid. The heat that the system gives off at the crystallization stage is called latent heat of crystallization... It is equal to the latent heat of fusion.

The time dependences of the temperature of the system shown in Fig. 13.4, are characteristic precisely for crystalline bodies. For amorphous substances, when they are heated (cooled), the graph of the temperature versus time is a monotonic curve, which corresponds to the gradual softening (solidification) of the amorphous substance with an increase (decrease) in its temperature.

Crystallization begins in a liquid near the center or centers of crystallization. They are random associations of atoms, to which other atoms are then attached, lining up, until all the liquid turns into a solid. Foreign macroscopic particles can also play the role of crystallization centers if they are present in the liquid.

Usually, many crystallization centers arise in a liquid when it is cooled. Atomic structures are formed around these centers, which ultimately form polycrystalcomposed of many small crystals. A schematic diagram of a polycrystal is shown in Fig. 13.5.

Under special conditions, it turns out to be possible to obtain ("grow") a single crystal - single crystalformed around a single crystallization center. If, in this case, for all directions the same conditions are provided for the attachment of particles from a liquid to the resulting crystal, then it will turn out properly facetedaccording to its symmetry properties.

The melting point generally depends on the pressure to which the solid is subjected; the possible course of this dependence is shown graphically in Fig. 13.6. The experimental dependence can be removed, for example, by placing a crucible with a molten substance in a gas atmosphere, the pressure of which can be changed. The curve of dependence is the curve of equilibrium of liquid and solid phases. Points under the curve correspond to the solid state of the substance, and above the curve - to the liquid state. If, at a constant temperature, the pressure above the liquid is increased from a point, then at pressure (point) a solid phase will appear in the liquid, and with a further increase in pressure, all the liquid will solidify (point).

The theoretical connection between pressure and melting point can be established by considering the Carnot cycle, performed by the two-phase system "solid - liquid" in exactly the same way as the relationship (13.12) was established between the pressure of a saturated vapor over a liquid and temperature. Making formal substitutions in (13.12),,, where is the latent molar heat of fusion, is the molar volume of the solid phase, is the molar volume of the liquid phase, we get:

. (13.21)

If the substance is not pure, but is alloy, that is, contains dissimilar atoms, then, in the general case, solidification can occur in a certain temperature range, and not at a certain temperature, as in pure substances.

Target 13.6... Acetic acid melts at atmospheric pressure at temperature. The difference in specific volumes (i.e. volumes of a unit mass of acid) of liquid and solid phases ... The melting point of acetic acid shifts by when the pressure changes by ... Find the specific (that is, per unit mass) heat of fusion of acetic acid.

Answer: .

Indication. Use formula (13.21). Take into account that the molar volume is related to the specific volume by the ratio, where is the molar mass. The molar heat of fusion is related to the specific heat of fusion by the ratio.

Boiling - the process of intense vaporization of a liquid, including the birth of vapor bubbles, their growth and movement to the surface of the liquid. Boiling, characterized by the formation of vapor bubbles on the surface of contact of a liquid with a solid, is called surface boiling. In real conditions, we are always dealing with surface boiling, which occurs at the interface between a liquid and a solid heated above the boiling point (heater). When the liquid is heated to the beginning of boiling, the bulk of the supplied heat is spent on heating, and the rest on evaporation. Let the temperature of the bottom of the vessel T 1, the temperature of the liquid on the free surface T 2. As long as the temperature difference is small, heat is transferred in a liquid medium only by thermal conduction. In this case, as we know, the stationary temperature distribution in the liquid satisfies the one-dimensional heat conduction equation (4.5.21). The solution to this equation is function (4.5.23), i.e., the temperature of the liquid drops linearly from the bottom of the vessel ( x\u003d 0) to the free surface ( x= d). In this case, the temperature gradient is constant and equal to (fig. 78, and).

a B C

With a further increase in the temperature of the bottom of the vessel T 1, the temperature gradient in the liquid medium also grows. When the latter reaches a certain value, free convection occurs, and heat in the liquid begins to be transferred more intensively (free convection of heat arises under the action of Archimedean forces and consists in the transfer of masses of a more heated liquid vertically upward and lowering a less heated liquid in its place). Now the stationary temperature distribution is determined by the well-known convective heat transfer equation

, (5.7.1)

where is the velocity of the liquid during convection, a - coefficient of temperature conductivity. Assuming the fluid velocity to be constant in the first approximation, we arrive at an exponential decrease in temperature with height (Fig. 78, b). This leads to a significant increase in the temperature gradient in the liquid at the boundary with the hot bottom, and thus the heat transfer to the liquid increases. Finally, let the temperature of the bottom become so significant that vapor bubbles begin to appear on its surface, which gradually increase, break off and float. The boiling process is established in the liquid. Experiments show that heat transfer in this case becomes even more intense. As a result, the drop in the temperature of the liquid near a hot solid surface will occur even more steeply than during convection (Fig. 78, in).

The surface boiling process begins at the bottom of the vessel adjacent to the heater. In the pores of the bottom of the vessel, there is always air or other dissolved gas, which is the generator of future steam bubbles. As the liquid evaporates inside the bubbles, the vapor pressure in them rises, the bubble begins to grow. The increase in bubble size occurs especially rapidly when at a certain temperature T S pressure p(T S) of saturated steam in it becomes equal to or slightly higher than the external pressure, i.e. p(T S) = p ext. Then the bubble breaks off from the bottom and, under the action of the Archimedean force, rises to the surface of the liquid.

External pressure p externally composed of atmospheric pressure p 0, hydrostatic pressure (ρ is the density of the liquid, h Is the depth at which the bubble forms) and the Laplace pressure ( R - bubble radius, - liquid surface tension coefficient). Thus, the boiling process will begin provided that the saturated vapor pressure at a given temperature T S

Temperature T Sfluid at which pressure p(T S) of its saturated vapor becomes equal to the external pressure p externally to a liquid is called the boiling point of this liquid. From equality

(5.7.3)

it follows that the boiling point is a function of external pressure. Therefore, to say that the boiling point of a given substance is T S, without specifying at what external pressure it was obtained, it is incorrect.

We know that the saturated vapor pressure of a liquid decreases with decreasing temperature and increases with increasing temperature, therefore, the boiling point of a liquid also decreases with a decrease in external pressure and rises with an increase. Thus, if some function expresses the dependence of the saturated vapor pressure on temperature, then the inverse function determines the dependence of the boiling point on external pressure. Since the Clapeyron-Clausis equation

in differential form expresses the dependence of the saturated vapor pressure on temperature, then the equation

(5.7.4)

defines in differential form the dependence of the boiling point

from external pressure, i.e., equation (5.7.4) is the equation of the boiling curve in differential form. In this equation dT - change in the boiling point of the liquid when the external pressure changes by dp.

In conclusion, we note that if air or other dissolved gas is removed from a liquid by prolonged boiling, then this liquid can be heated to a temperature significantly higher than its boiling point at a given external pressure. So, the resulting liquid is called superheated. If inhomogeneities are introduced into a superheated liquid, for example, by throwing grains of sand into it, in the pores of which there is air, then the liquid boils violently, resembling an explosion.

73. Amorphous and crystalline state of matter. Symmetry of solids. Basic elements of symmetry of solids.

In physics, amorphous and crystalline solids are distinguished. On the basis of shape retention, amorphous bodies are classified as solid, in all other respects they do not differ from liquids. Amorphous bodies are considered as supercooled liquids with an abnormally high coefficient of viscosity, due to which they cannot flow at ordinary temperatures. However, as the temperature rises, they gradually soften without having a definite melting point, and acquire the ability to flow, which is usual for liquids. The properties of amorphous bodies are the same in all directions, that is, they are isotropic. For example, if a ball is made of glass (an amorphous body), then its properties will be the same in different directions. So, when it is compressed with the same force in different directions, it will decrease by the same amount. If we measure the thermal conductivity of glass by heating the ball from above and cooling it from below, or heating it from the left and cooling it from the right, we find that the thermal conductivity of glass in all directions is also the same. For light rays penetrating the glass in all directions, the refractive index is also the same. If you place a glass ball between two plates of a charged capacitor and rotate the ball around its center, then there will be no change in the capacitance of the capacitor; this means that the dielectric constant does not depend on the direction of the electric field inside it.

Crystalline solids behave quite differently. Crystals have a definite melting point depending on external pressure. The speed of propagation of light, isothermal coefficient of compression, coefficient of thermal conductivity, modulus of elasticity, dielectric constant, and many other physical properties of a crystal are strongly dependent on the direction in it.

Crystals can be obtained in various ways, for example, by cooling a liquid. With such cooling, if no special measures are taken, many crystallization centers appear in the liquid phase, around which the formation of a solid phase occurs. A lot of small crystals appear, merging with each other chaotically and forming the so-called polycrystal. Although each of the crystals forming a polycrystal is anisotropic, due to the random orientation of these crystals, the polycrystalline body as a whole is isotropic.

If a seed is introduced into the cooled liquid - a small crystal, then crystallization will begin on it, and a large single crystal of the correct shape can be grown. For this, it is necessary that the conditions for the growth of the crystal are the same on all its surfaces, which can be achieved by rotating the seed in the solution. When growing large single crystals of metals and semiconductors, the seed is pushed very slowly out of the heating furnace in a vertical direction at a rate of several millimeters per hour.

According to the law, discovered in 1783 by Rome de Lille, in all crystals of the same substance, the angles between the corresponding faces are equal. So, for example, in crystals of rock salt (NaCl) all the angles between the faces are 90˚. If a ball is cut out of such a crystal and placed in a saturated solution of rock salt, then the cubic shape of the crystal will tend to recover. The reason for this restoration of the crystal shape is the well-known condition for the stability of the equilibrium of a thermodynamic system: the condition for the minimum potential energy. For crystals, this condition is expressed in the principle formulated by Gibbs, Curie, and Wolfe: the surface energy should be minimal. This minimum should be found under the condition that the angles between the crystal faces are specified.

When a crystal is placed in a saturated solution or in a melt, a dynamic equilibrium is established between the solid and liquid phases: atoms from the solid phase pass into the liquid phase, and from the liquid phase - into the solid; but the deposition from the liquid phase proceeds in such a way that a system with a minimum of potential energy is formed, that is, a crystal form characteristic of a given substance is formed and all former disturbances of this form disappear, therefore the ball in the described experiment tends to turn again into a cubic structure or another characteristic crystalline shape.

If the conditions for the growth of a crystal are not the same at different points of its surface, then the shape of the growing crystal may differ from the characteristic shape, although the angles between the main faces remain the same as in the case of the correct shape.

Place a glass jar of cold water on the burner and watch. Soon the bottom and walls of the vessel will be covered with bubbles; their origin was mentioned in § 260. These bubbles, as we know, contain air and water vapor. Bubbles appear in those places of the vessel walls where there is no complete wetting. Such places may be traces of fat on the wall or small cracks on it.

Observing a bubble at a constant temperature, we see that it retains its size; this means that the pressures from the inside and from the outside on its surface are mutually balanced. Since there is air inside the bubble, the amount of which must be considered constant, this equilibrium is stable. Indeed, if for some random reason the bubble expanded, then the air pressure in it, according to the Boyle - Mariotte law, would decrease and the external pressure, which remains almost unchanged, would again reduce the bubble. Reasoning in the same way, it is easy to find out why an accidentally reduced bubble will immediately expand again to its previous volume. As the temperature rises, the bubble gradually expands so that the sum of the air and steam pressure in it remains equal to the external pressure. However, when the bubble becomes large enough, the buoyancy force of the water will cause it to come off, just like a too heavy drop of water that hangs on a roof comes off (Fig. 372). In this case, an ever narrowing air bridge is formed between the bubble and the wall of the vessel (Fig. 483) and, finally, the bubble breaks off, leaving a small amount of air at the wall from which a new bubble will develop over time.

Fig. 483. Adhered to the bottom of a vessel with liquid and detaching gas bubbles

Rising upward, the detached bubbles decrease in size again. Why is this happening? These bubbles contain water vapor and some air. When the bubble reaches the upper layers of water that have not yet had time to heat up, a significant part of the water vapor condenses into water and the bubble decreases. This alternating increase and decrease of bubbles is accompanied by sounds: boiling water "makes noise". Finally, all the water is warmed up sufficiently. Then the rising bubbles no longer decrease in size and burst on the surface, ejecting steam into the outer space. The “noise” stops, and the “gurgling” begins - we say that the water has boiled. A thermometer placed in steam over boiling water, while the water is boiling, shows the same temperature, about.

Obviously, during boiling, the vapor pressure formed inside the bubbles at the bottom of the vessel is such that the bubbles can expand, overcoming the atmospheric pressure acting on the free surface of the water, as well as the pressure of the water column. We come to the conclusion that boiling occurs at a temperature at which the saturated vapor pressure of the liquid is equal to the external pressure. The vapor temperature of a boiling liquid is called the boiling point.

It is clear from the above reasoning that the boiling point must depend on the external pressure. This can be easily observed. We put a glass of warm water under the bell of the air pump. By pumping out the air, we can make the water boil at a temperature much lower (Fig. 484). On the contrary, with increasing external pressure, the boiling point rises. So, in boilers of steam engines, water is heated under a pressure of several atmospheres. The boiling point is significantly higher. At a pressure of about 15 atm, the boiling point of water is close to. When talking about the boiling point of a liquid without indicating pressure, they always mean the boiling point at normal pressure. .

Fig. 484. When pumping out air from under the bell, the water, which has a temperature much lower, boils

Boiling point versus pressure gives us a new way to measure atmospheric pressure. By measuring the boiling point of water, it is possible to judge atmospheric pressure from the steam pressure tables at different temperatures. If, for example, being in the mountains, we determined that the boiling point of water is about, then from this we can conclude (Table 18) that the air pressure is. Thermometers specially adapted for such measurements are called gypsum thermometers. They are designed in such a way that they make it possible to read the temperature about with great accuracy (Fig. 485).

Fig. 485. Hypsothermometer

The boiling points of various liquids (at normal pressure) vary greatly among themselves. This can be seen from table. 19.

Table 19. Boiling points of some liquids at

Liquid	Boiling temperature	Liquid	Boiling temperature
Liquid helium	-269	Alcohol	78
\u003e\u003e hydrogen	-253	Water	100
\u003e\u003e oxygen	-183	Mercury	357
\u003e\u003e nitrogen	-196	Molten zinc	906
Chlorine	-34	Molten iron	2880
Ether	-35

The difference in boiling points of different substances is widely used in technology, for example, in the separation of petroleum products. When oil is heated, the most valuable, volatile parts (gasoline) evaporate first of all, which can thus be separated from the "heavy" residues (oils, fuel oil).

The difference in boiling points of substances is due to the difference in vapor pressure at the same temperature. We have seen that ether vapor has a pressure exceeding half the atmospheric pressure even at room temperature. Therefore, in order for the vapor pressure of the ether to reach atmospheric, a slight increase in temperature (up to) is needed. The situation is different, for example, with mercury, which has absolutely negligible pressure at room temperature. The vapor pressure of mercury becomes equal to atmospheric only with a significant increase in temperature (up to).

294.1. Where is the boiling water hotter: at sea level, on a mountain or in a deep mine?

294.2. Some industrial processes in the food industry (eg, cooking beets) require higher water temperatures. By what means can this be achieved?

294.3. Using the table. 18, determine the highest temperature that water can have under pressure and.

294.4. In fig. 486 depicts an autoclave (a device used in chemical industries for processes that require a higher temperature than the boiling point of the liquid in it). This is a durable boiler with a pressure gauge 1, tightly closed with a lid so that steam can escape from it only through the safety valve 2. What temperature will the water in it reach when the boiler is heated if the valve base area is equal and the distance from support 3 to valve 2 is 6.5 cm, and up to a weight 4-18 cm? Weight 1 kg. The mass of the rod is negligible.

Figure 486. To exercise 294.4

294.5. Try boiling water in a narrow tube that is full to the brim, warming it to the bottom. Why, in this case, are bubbles ejecting water from the test tube?

Note. Something of the kind occurs on a huge scale in nature in the so-called geysers (in the USSR in Kamchatka, as well as in a number of other countries, for example, in Iceland). A geyser is a periodically operating fountain that throws hot water out of a narrow vertical vent in the ground (Fig. 487). Steam is generated at a depth of several tens of meters. The pressure at such a depth of the reservoir can reach several atmospheres and the water can be heated much higher. When steam bubbles form at the bottom, some of the water flows out, the pressure drops and the vaporization of superheated water proceeds with such intensity that the remaining water is thrown out to a great height.

Fig. 487. Geyser

294.6. Boil water in a round-bottomed flask and cap. Turn the flask over. If now you put a little snow on the bottom of the flask or pour cold water over it, then the water in the flask will boil. Explain the phenomenon.

Boiling is the process of changing the state of aggregation of a substance. When we talk about water, we mean a change from a liquid state to a vapor state. It is important to note that boiling is not evaporation, which can occur even at room temperature. Also, not to be confused with boiling, which is the process of heating water to a certain temperature. Now that we have figured out the concepts, we can determine at what temperature the water boils.

Process

The very process of converting the state of aggregation from liquid to gaseous is complex. And although people don't see it, there are 4 stages:

1. In the first stage, small bubbles form at the bottom of the heated container. They can also be seen on the sides or on the surface of the water. They are formed due to the expansion of air bubbles, which are always present in the cracks of the container where the water is heated.
2. In the second stage, the volume of the bubbles increases. All of them begin to tear to the surface, since they contain saturated steam, which is lighter than water. With an increase in the heating temperature, the pressure of the bubbles increases, and they are pushed out to the surface due to the known force of Archimedes. At the same time, you can hear the characteristic boiling sound, which is formed due to the constant expansion and decrease in the size of the bubbles.
3. In the third stage, a large number of bubbles can be seen on the surface. This initially creates a cloudy water. This process is popularly called "boiling with a white key", and it lasts for a short period of time.
4. At the fourth stage, the water boils intensively, large bursting bubbles appear on the surface, and splashes may appear. Most often, splashing means that the liquid has reached its maximum temperature. Steam will start to come out of the water.

It is known that water boils at a temperature of 100 degrees, which is possible only at the fourth stage.

Steam temperature

Steam is one of the states of water. When it enters the air, it, like other gases, puts a certain pressure on it. During vaporization, the temperatures of steam and water remain constant until all the liquid changes its state of aggregation. This phenomenon can be explained by the fact that during boiling all the energy is spent on converting water into steam.

At the very beginning of boiling, moist saturated steam is formed, which, after evaporation of all the liquid, becomes dry. If its temperature begins to exceed the temperature of water, then such steam is superheated, and by its characteristics it will be closer to gas.

Salt water boiling

It is interesting enough to know at what temperature water with a high salt content boils. It is known that it should be higher due to the content of Na + and Cl- ions in the composition, which occupy a region between water molecules. This is how the chemical composition of water with salt differs from ordinary fresh liquid.

The fact is that in salt water a hydration reaction takes place - the process of attaching water molecules to salt ions. The bond between fresh water molecules is weaker than those formed during hydration, so the boiling of a liquid with dissolved salt will take longer. As the temperature rises, the molecules in the salt-containing water move faster, but there are fewer of them, which makes collisions between them less frequent. As a result, less steam is generated, and its pressure is therefore lower than the steam pressure of fresh water. Consequently, more energy (temperature) is required for full steam generation. On average, to boil one liter of water containing 60 grams of salt, it is necessary to raise the boiling degree of water by 10% (that is, by 10 C).

Boiling pressure versus pressure

It is known that in the mountains, regardless of the chemical composition of the water, the boiling point will be lower. This is due to the fact that the atmospheric pressure is lower at altitude. Pressure with a value of 101.325 kPa is considered normal. With it, the boiling point of water is 100 degrees Celsius. But if you go up the mountain, where the pressure is on average 40 kPa, then the water there boils at 75.88 C. But this does not mean that cooking in the mountains will have to spend almost half the time. For thermal processing of products, a certain temperature is required.

It is believed that at an altitude of 500 meters above sea level, water will boil at 98.3 C, and at an altitude of 3000 meters, the boiling temperature will be 90 C.

Note that this law also works in the opposite direction. If you place a liquid in a closed flask through which steam cannot pass, then with an increase in temperature and the formation of steam, the pressure in this flask will increase, and boiling at an increased pressure will occur at a higher temperature. For example, at a pressure of 490.3 kPa, the boiling point of water will be 151 C.

Boiling distilled water

Distilled water is purified water without any impurities. It is often used for medical or technical purposes. Given that there are no impurities in such water, it is not used for cooking. It is interesting to note that distilled water boils faster than ordinary fresh water, but the boiling point remains the same - 100 degrees. However, the difference in boiling time will be minimal - only a fraction of a second.

In the teapot

Often people are interested in the temperature at which water boils in a kettle, since it is these devices that they use to boil liquid. Taking into account the fact that the atmospheric pressure in the apartment is equal to the standard, and the water used does not contain salts and other impurities that should not be there, then the boiling temperature will also be standard - 100 degrees. But if the water contains salt, then the boiling point, as we already know, will be higher.

Conclusion

Now you know at what temperature water boils, and how atmospheric pressure and fluid composition affect this process. There is nothing difficult in this, and children receive such information at school. The main thing is to remember that with a decrease in pressure, the boiling point of the liquid also decreases, and with its increase, it also increases.

On the Internet, you can find many different tables, which indicate the dependence of the boiling point of a liquid on atmospheric pressure. They are available to everyone and are actively used by schoolchildren, students and even teachers at institutes.