Vaporization can occur not only as a result of evaporation, but also during boiling. Let's consider boiling from an energy point of view.
There is always some air dissolved in a liquid. When a liquid is heated, the amount of gas dissolved in it decreases, as a result of which some of it is released in the form of small bubbles at the bottom and walls of the vessel and on undissolved solid particles suspended in the liquid. Liquid evaporates into these air bubbles. Over time, the vapors in them become saturated. With further heating, the saturated vapor pressure inside the bubbles and their volume increase. When the vapor pressure inside the bubbles becomes equal to atmospheric pressure, they rise to the surface of the liquid under the influence of the buoyant force of Archimedes, burst, and steam comes out of them. Vaporization that occurs simultaneously both from the surface of the liquid and inside the liquid itself into air bubbles is called boiling. The temperature at which the pressure of saturated vapor in the bubbles becomes equal to the external pressure is called boiling point.
Since at the same temperatures the saturated vapor pressures of various liquids are different, at different temperatures they become equal to atmospheric pressure. This causes different liquids to boil at different temperatures. This property of liquids is used in the sublimation of petroleum products. When oil is heated, the most valuable, volatile parts (gasoline) evaporate first, which are thus separated from the “heavy” residues (oils, fuel oil).
From the fact that boiling occurs when the pressure of saturated vapors is equal to the external pressure on the liquid, it follows that the boiling point of the liquid depends on the external pressure. If it is increased, then the liquid boils at a higher temperature, since saturated vapor requires a higher temperature to achieve this pressure. On the contrary, at reduced pressure the liquid boils at a lower temperature. This can be verified by experience. Heat the water in the flask to a boil and remove the alcohol lamp (Fig. 37, a). The water stops boiling. Having closed the flask with a stopper, we will begin to remove air and water vapor from it with a pump, thereby reducing the pressure on the water, which as a result boils. Having forced it to boil in the open flask, by pumping air into the flask we will increase the pressure on the water (Fig. 37, b) It stops boiling under pressure. 1 atm water boils at 100° C, and at 10 atm- at 180° C. This dependence is used, for example, in autoclaves, in medicine for sterilization, in cooking to speed up the cooking of food products.
For a liquid to begin to boil, it must be heated to boiling temperature. To do this, you need to impart energy to the liquid, for example, the amount of heat Q = cm(t° to - t° 0). When boiling, the temperature of the liquid remains constant. This happens because the amount of heat reported during boiling is spent not on increasing the kinetic energy of liquid molecules, but on the work of breaking molecular bonds, i.e., on vaporization. During condensation, steam, according to the law of conservation of energy, releases into the environment the same amount of heat that was expended on steam formation. Condensation occurs at the boiling point, which remains constant during the condensation process. (Explain why).
Let's create a heat balance equation for vaporization and condensation. Steam, taken at the boiling point of the liquid, enters the water in the calorimeter through tube A (Fig. 38, a), condenses in it, giving it the amount of heat spent on its production. Water and the calorimeter receive an amount of heat not only from the condensation of steam, but also from the liquid that is obtained from it. Data of physical quantities are given in table. 3.
The condensing steam gave off the amount of heat Q p = rm 3(Fig. 38, b). The liquid obtained from steam, having cooled from t° 3 to θ°, gave up an amount of heat Q 3 = c 2 m 3 (t 3 ° - θ °).
The calorimeter and water, heating from t° 2 to θ° (Fig. 38, c), received the amount of heat
Q 1 = c 1 m 1 (θ° - t° 2); Q 2 = c 2 m 2 (θ° - t° 2).
Based on the law of conservation and transformation of energy
Q p + Q 3 = Q 1 + Q 2,
From the above considerations it is clear that the boiling point of a liquid must depend on the external pressure. Observations confirm this.
The greater the external pressure, the higher the boiling point. Thus, in a steam boiler at a pressure reaching 1.6 × 10 6 Pa, water does not boil even at a temperature of 200 °C. In medical institutions, water boiling in hermetically sealed vessels - autoclaves (Fig. 6.11) also occurs at elevated pressure. Therefore, the boiling point is significantly higher than 100 °C. Autoclaves are used to sterilize surgical instruments, dressings, etc.
And vice versa, by reducing external pressure, we thereby lower the boiling point. Under the bell of an air pump, you can make water boil at room temperature (Fig. 6.12). As you climb mountains, the atmospheric pressure decreases, therefore the boiling point decreases. At an altitude of 7134 m (Lenin Peak in the Pamirs) the pressure is approximately 4 10 4 Pa (300 mm Hg). Water boils there at about 70 °C. It is impossible to cook meat, for example, under these conditions.
Figure 6.13 shows a curve of the boiling point of water versus external pressure. It is easy to understand that this curve is also a curve expressing the dependence of saturated water vapor pressure on temperature.
Differences in boiling points of liquids
Each liquid has its own boiling point. The difference in boiling points of liquids is determined by the difference in the pressure of their saturated vapors at the same temperature. For example, ether vapors already at room temperature have a pressure greater than half atmospheric. Therefore, in order for the ether vapor pressure to become equal to atmospheric pressure, a slight increase in temperature (up to 35 ° C) is necessary. In mercury, saturated vapors have a very negligible pressure at room temperature. The pressure of mercury vapor becomes equal to atmospheric pressure only with a significant increase in temperature (up to 357 ° C). It is at this temperature, if the external pressure is 105 Pa, that mercury boils.
The difference in boiling points of substances is widely used in technology, for example, in the separation of petroleum products. When oil is heated, its most valuable, volatile parts (gasoline) evaporate first, which can thus be separated from “heavy” residues (oils, fuel oil).
A liquid boils when its saturated vapor pressure equals the pressure inside the liquid.
§ 6.6. Heat of vaporization
Is energy required to change liquid into vapor? Probably yes! Is not it?
We noted (see § 6.1) that the evaporation of a liquid is accompanied by its cooling. To maintain the temperature of the evaporating liquid unchanged, it is necessary to supply heat from outside. Of course, heat itself can be transferred to the liquid from surrounding bodies. Thus, the water in the glass evaporates, but the temperature of the water, slightly lower than the ambient temperature, remains unchanged. Heat is transferred from air to water until all the water has evaporated.
To maintain the boiling of water (or other liquid), heat must also be continuously supplied to it, for example, by heating it with a burner. In this case, the temperature of the water and the vessel does not increase, but a certain amount of steam is produced every second.
Thus, to convert a liquid into vapor by evaporation or by boiling, an input of heat is required. The amount of heat required to convert a given mass of liquid into vapor at the same temperature is called the heat of vaporization of this liquid.
What is the energy supplied to the body spent on? First of all, to increase its internal energy during the transition from a liquid to a gaseous state: after all, this increases the volume of the substance from the volume of liquid to the volume of saturated vapor. Consequently, the average distance between molecules increases, and hence their potential energy.
In addition, as the volume of a substance increases, work is done against external pressure forces. This part of the heat of vaporization at room temperature is usually several percent of the total heat of vaporization.
The heat of vaporization depends on the type of liquid, its mass and temperature. The dependence of the heat of vaporization on the type of liquid is characterized by a value called the specific heat of vaporization.
The specific heat of vaporization of a given liquid is the ratio of the heat of vaporization of a liquid to its mass:
(6.6.1)
Where r- specific heat of liquid vaporization; T- mass of liquid; Q n- its heat of vaporization. The SI unit of specific heat of vaporization is joule per kilogram (J/kg).
The specific heat of vaporization of water is very high: 2.256·10 6 J/kg at a temperature of 100 °C. For other liquids (alcohol, ether, mercury, kerosene, etc.) the specific heat of vaporization is 3-10 times less.
Boiling –This is vaporization that occurs in the volume of the entire liquid at a constant temperature.
The process of evaporation can occur not only from the surface of the liquid, but also inside the liquid. Vapor bubbles inside a liquid expand and float to the surface if the saturated vapor pressure is equal to or greater than the external pressure. This process is called boiling. While the liquid boils, its temperature remains constant.
At a temperature of 100 0 C, the pressure of saturated water vapor is equal to normal atmospheric pressure, therefore, at normal pressure, water boils at 100 ° C. At a temperature of 80 °C, the saturated vapor pressure is approximately half the normal atmospheric pressure. Therefore, water boils at 80 °C if the pressure above it is reduced to 0.5 normal atmospheric pressure (figure).
When the external pressure decreases, the boiling point of the liquid decreases; when the pressure increases, the boiling point increases.
Liquid boiling point- This is the temperature at which the pressure of saturated vapor in the bubbles of a liquid is equal to the external pressure on its surface.
Critical temperature.
In 1861 D.I. Mendeleev established that for each liquid there must be a temperature at which the difference between the liquid and its vapor disappears. Mendeleev named it absolute boiling point (critical temperature). There is no fundamental difference between gas and steam. Usually gas a substance in a gaseous state is called when its temperature is above critical, and ferry- when the temperature is below critical.
The critical temperature of a substance is the temperature at which the density of the liquid and the density of its saturated vapor become the same.
Any substance that is in a gaseous state can turn into a liquid. However, each substance can experience such a transformation only at temperatures below a certain value specific to each substance, called the critical temperature Tc. At temperatures above the critical temperature, the substance does not turn into a liquid at any pressure.
The ideal gas model is applicable to describe the properties of gases that actually exist in nature in a limited range of temperatures and pressures. When the temperature drops below the critical one for a given gas, the action of attractive forces between molecules can no longer be neglected, and at a sufficiently high pressure, the molecules of the substance are connected to each other.
If a substance is at a critical temperature and critical pressure, then its state is called a critical state.
(When water is heated, the air dissolved in it is released at the walls of the vessel and the number of bubbles continuously increases, and their volume increases. If the volume of the bubble is sufficiently large, the Archimedes force acting on it tears it off from the bottom surface and lifts it up, and in the place of the detached bubble there remains the embryo of a new one bubble, since when the liquid is heated from below, its upper layers are colder than the lower ones, when the bubble rises, the water vapor in it condenses, and the air dissolves in the water again and the volume of the bubble decreases, many bubbles disappear before reaching the surface of the water, and some reach the surface. At this point, there is very little air and steam left in them. This happens until, due to convection, the temperature in the entire liquid becomes the same. When the temperature in the liquid is equalized, the volume of the bubbles will increase as it rises. . This is explained as follows. When the same temperature has established throughout the liquid and the bubble rises, the pressure of the saturated vapor inside the bubble remains constant, and the hydrostatic pressure (the pressure of the upper layer of the liquid) decreases, so the bubble grows. As the bubble grows, the entire space inside the bubble is filled with saturated steam. When such a bubble reaches the surface of the liquid, the pressure of the saturated vapor in it is equal to the atmospheric pressure at the surface of the liquid.)
TASKS
1.Relative humidity at 20° C is 58%. At what maximum temperature will dew fall?
2. How much water must be evaporated in 1000 ml of air, the relative humidity of which is 40% at 283 K, in order to humidify it to 40% at 290 K?
3. Air at a temperature of 303 K has a dew point at 286 K. Determine the absolute and relative humidity of the air.
4.At 28° C, relative air humidity is 50%. Determine the mass of dew that fell from 1 km3 of air when the temperature drops to 12° C.
5. In a room with a volume of 200 m3, the relative air humidity at 20° C is 70%. Determine the mass of water vapor in the air of the room.
Boiling is the process of changing the state of aggregation of a substance. When we talk about water, we mean the 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. It should also not be confused with boiling, which is the process of heating water to a certain temperature. Now that we have understood the concepts, we can determine at what temperature water boils.
Process
The process of transforming the state of aggregation from liquid to gaseous is complex. And although people don't see it, there are 4 stages:
- At 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.
- In the second stage, the volume of bubbles increases. They all begin to rush to the surface, since inside them there is saturated steam, which is lighter than water. As the heating temperature increases, the pressure of the bubbles increases, and they are pushed to the surface thanks to the well-known Archimedes force. In this case, you can hear the characteristic sound of boiling, which is formed due to the constant expansion and reduction in the size of the bubbles.
- At the third stage, a large number of bubbles can be seen on the surface. This initially creates cloudiness in the water. This process is popularly called “white boiling,” and it lasts a short period of time.
- At the fourth stage, the water boils intensely, 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 begin to emanate from 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, exerts a certain pressure on it. During vaporization, the temperature of steam and water remains constant until the entire 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 becomes dry after all the liquid has evaporated. If its temperature begins to exceed the temperature of water, then such steam is overheated, and its characteristics will be closer to gas.
Boiling salt water
It is quite interesting 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 the area 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 adding water molecules to salt ions. The bonds between fresh water molecules are weaker than those formed during hydration, so it will take longer for a liquid with dissolved salt to boil. As the temperature rises, the molecules in salty water move faster, but there are fewer of them, causing collisions between them to occur less often. As a result, less steam is produced, and its pressure is therefore lower than the steam pressure of fresh water. Consequently, more energy (temperature) will be required for complete vaporization. On average, to boil one liter of water containing 60 grams of salt, it is necessary to increase the boiling degree of water by 10% (that is, by 10 C).
Dependence of boiling on pressure
It is known that in the mountains, regardless of the chemical composition of the water, the boiling point will be lower. This occurs because the atmospheric pressure is lower at altitude. Normal pressure is considered to be 101.325 kPa. With it, the boiling point of water is 100 degrees Celsius. But if you climb a mountain, where the pressure is on average 40 kPa, then the water there will boil at 75.88 C. But this does not mean that you will have to spend almost half as much time cooking in the mountains. Heat treatment of foods requires a certain temperature.
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 point will be 90 C.
Note that this law also applies in the opposite direction. If you place a liquid in a closed flask through which steam cannot pass, then as the temperature rises and steam forms, the pressure in this flask will increase, and boiling at 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. Considering 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 a teapot
People often wonder at what temperature water boils in a kettle, since these are the devices they use to boil liquids. Taking into account the fact that the atmospheric pressure in the apartment is equal to standard, and the water used does not contain salts and other impurities that should not be there, then the boiling point 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 the composition of the liquid affect this process. There is nothing complicated about this, and children receive such information at school. The main thing is to remember that as the pressure decreases, the boiling point of the liquid also decreases, and as it increases, it also increases.
On the Internet you can find many different tables that 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.
Since the saturation vapor pressure is uniquely determined by temperature, and the boiling of a liquid occurs at the moment when the saturation vapor pressure of this liquid is equal to the external pressure, the boiling point must depend on the external pressure. With the help of experiments it is easy to show that when the external pressure decreases, the boiling point decreases, and when the pressure increases, it increases.
The boiling of a liquid at reduced pressure can be demonstrated using the following experiment. Water from the tap is poured into a glass and a thermometer is lowered into it. A glass of water is placed under the glass cover of the vacuum unit and the pump is turned on. When the pressure under the hood drops sufficiently, the water in the glass begins to boil. Since energy is spent on steam formation, the temperature of the water in the glass begins to drop as it boils, and when the pump is working well, the water finally freezes.
Heating of water to high temperatures is carried out in boilers and autoclaves. The structure of the autoclave is shown in Fig. 8.6, where K is a safety valve, is a lever pressing the valve, M is a pressure gauge. At pressures greater than 100 atm, water is heated to temperatures above 300 °C.
Table 8.2. Boiling points of some substances
The boiling point of a liquid at normal atmospheric pressure is called the boiling point. From the table 8.1 and 8.2 it is clear that the saturation vapor pressure for ether, water and alcohol at the boiling point is 1.013 105 Pa (1 atm).
From the above it follows that in deep mines water should boil at a temperature above 100 °C, and in mountainous areas - below 100 °C. Since the boiling point of water depends on the altitude above sea level, on the thermometer scale, instead of temperature, you can indicate the height at which water boils at this temperature. Determining height using such a thermometer is called hypsometry.
Experience shows that the boiling point of a solution is always higher than the boiling point of a pure solvent, and increases with increasing concentration of the solution. However, the temperature of the vapor above the surface of the boiling solution is equal to the boiling point of the pure solvent. Therefore, to determine the boiling point of a pure liquid, it is better to place the thermometer not in the liquid, but in the vapor above the surface of the boiling liquid.
The boiling process is closely related to the presence of dissolved gas in the liquid. If the gas dissolved in it is removed from a liquid, for example, by prolonged boiling, then this liquid can be heated to a temperature significantly higher than its boiling point. Such a liquid is called superheated. In the absence of gas bubbles, the formation of tiny vapor bubbles, which could become centers of vaporization, is prevented by Laplace pressure, which is high at a small radius of the bubble. This explains the overheating of the liquid. When it does boil, the boiling occurs very violently.
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