Effect of negative pressure on the boiling point. At what temperature does water boil? The dependence of the boiling point on pressure. What determines the boiling point of water
Boiling- This is an intense transition of liquid to vapor, occurring with the formation of vapor bubbles throughout the entire volume of the liquid at a certain temperature.
During boiling, the temperature of the liquid and vapor above it does not change. It remains unchanged until all the liquid boils away. This is because all the energy supplied to the liquid is spent on turning it into vapor.
The temperature at which a liquid boils is called boiling point.
The boiling point depends on the pressure exerted on the free surface of the liquid. This is due to the dependence of saturated vapor pressure on temperature. A vapor bubble grows as long as the pressure of the saturated vapor inside it slightly exceeds the pressure in the liquid, which is the sum of the external pressure and the hydrostatic pressure of the liquid column.
The greater the external pressure, the more boiling temperature.
Everyone knows that water boils at 100 ºC. But we should not forget that this is true only at normal atmospheric pressure (about 101 kPa). With an increase in pressure, the boiling point of water increases. So, for example, in pressure cookers, food is cooked under a pressure of about 200 kPa. The boiling point of water reaches 120°C. In water of this temperature, the cooking process is much faster than in ordinary boiling water. This explains the name "pressure cooker".
Conversely, by reducing the external pressure, we thereby lower the boiling point. For example, in mountainous regions (at an altitude of 3 km, where the pressure is 70 kPa), water boils at a temperature of 90 ° C. Therefore, the inhabitants of these areas, using such boiling water, require much more time for cooking than the inhabitants of the plains. And to cook in this boiling water, for example, a chicken egg is generally impossible, since at a temperature below 100 ° C the protein does not coagulate.
Each liquid has its own boiling point, which depends on the saturation vapor pressure. The higher the saturated vapor pressure, the lower the boiling point of the corresponding liquid, since at lower temperatures the saturated vapor pressure becomes equal to atmospheric pressure. For example, at a boiling point of 100 ° C, the pressure of saturated water vapor is 101,325 Pa (760 mm Hg), and vapor pressure is only 117 Pa (0.88 mm Hg). Mercury boils at 357°C at normal pressure.
The heat of vaporization.
Heat of vaporization (heat of vaporization)- the amount of heat that must be reported to the substance (at constant pressure and constant temperature) for the complete transformation of a liquid substance into vapor.
The amount of heat required for vaporization (or released during condensation). To calculate the amount of heat Q, necessary for the transformation into vapor of a liquid of any mass, taken at the boiling point, you need the specific heat of vaporization r mind-knife to the mass m:
When steam condenses, the same amount of heat is released.
Why did a person begin to boil water before its direct use? Correctly, to protect yourself from many pathogenic bacteria and viruses. This tradition came to the territory of medieval Russia even before Peter the Great, although it is believed that it was he who brought the first samovar to the country and introduced the rite of unhurried evening tea drinking. In fact, our people used some kind of samovars back in ancient Rus' for making drinks from herbs, berries and roots. Boiling was required here mainly for the extraction of useful plant extracts, rather than for disinfection. Indeed, at that time it was not even known about the microcosm where these bacteria and viruses live. However, thanks to boiling, our country was bypassed by global pandemics of terrible diseases such as cholera or diphtheria.
Celsius
The great meteorologist, geologist and astronomer from Sweden originally used 100 degrees to indicate the freezing point of water under normal conditions, and the boiling point of water was taken as zero degrees. And after his death in 1744, no less famous person, botanist Carl Linnaeus and Celsius receiver Morten Strömer, reversed this scale for ease of use. However, according to other sources, Celsius himself did this shortly before his death. But in any case, the stability of the readings and the understandable graduation influenced the widespread use of it among the most prestigious scientific professions at that time - chemists. And, despite the fact that, in an inverted form, the scale mark of 100 degrees set the point of stable boiling of water, and not the beginning of its freezing, the scale began to bear the name of its primary creator, Celsius.
Below the atmosphere
However, not everything is as simple as it seems at first glance. Looking at any state diagram in P-T or P-S coordinates (entropy S is a direct function of temperature), we see how closely temperature and pressure are related. Similarly, water, depending on the pressure, changes its values. And any climber is well aware of this property. Everyone who at least once in his life comprehended heights over 2000-3000 meters above sea level knows how hard it is to breathe at altitude. This is because the higher we go, the thinner the air becomes. Atmospheric pressure falls below one atmosphere (below N.O., that is, below "normal conditions"). The boiling point of water also drops. Depending on the pressure at each of the heights, it can boil both at eighty and at sixty
pressure cookers
However, it should be remembered that although the main microbes die at temperatures above sixty degrees Celsius, many can survive at eighty degrees or more. That is why we achieve boiling water, that is, we bring its temperature to 100 ° C. However, there are interesting kitchen appliances that allow you to reduce time and heat the liquid to high temperatures, without boiling it and losing mass through evaporation. Realizing that the boiling point of water can change depending on pressure, engineers from the United States, based on a French prototype, introduced the world to a pressure cooker in the 1920s. The principle of its operation is based on the fact that the lid is tightly pressed against the walls, without the possibility of steam removal. Increased pressure is created inside, and water boils at higher temperatures. However, such devices are quite dangerous and often led to an explosion and serious burns to users.
Ideally
Let's look at how the process comes and goes. Imagine an ideally smooth and infinitely large heating surface, where the distribution of heat is uniform (the same amount of thermal energy is supplied to each square millimeter of the surface), and the surface roughness coefficient tends to zero. In this case, at n. y. boiling in a laminar boundary layer will begin simultaneously over the entire surface area and occur instantly, immediately evaporating the entire unit volume of liquid located on its surface. This ideal conditions, V real life this does not happen.
In real
Let's find out what is the initial boiling point of water. Depending on the pressure, it also changes its values, but the main point here lies in this. Even if we take the smoothest, in our opinion, pan and bring it under a microscope, then in its eyepiece we will see uneven edges and sharp, frequent peaks protruding above the main surface. The heat to the surface of the pan, we will assume, is supplied evenly, although in reality this is also not a completely true statement. Even when the pan is on the largest burner, the temperature gradient is unevenly distributed on the stove, and there are always local overheating zones responsible for the early boiling of water. How many degrees are at the same time at the peaks of the surface and in its lowlands? The peaks of the surface with an uninterrupted supply of heat warm up faster than the lowlands and the so-called depressions. Moreover, surrounded on all sides by water with a low temperature, they better give energy to water molecules. The thermal diffusivity of the peaks is one and a half to two times higher than that of the lowlands.
Temperatures
That is why the initial boiling point of water is about eighty degrees Celsius. At this value, the peaks of the surface bring up enough that is necessary for instantaneous boiling of the liquid and the formation of the first bubbles, visible to the eye, which timidly begin to rise to the surface. And what is the boiling point of water at normal pressure - many people ask. The answer to this question can be easily found in the tables. At atmospheric pressure, stable boiling is established at 99.9839 °C.
Boiling is the process of changing the aggregate state of a substance. When we talk about water, we mean the change from liquid to vapor. It is important to note that boiling is not evaporation, which can occur even at room temperature. Also, do not confuse 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 very process of transforming the state of aggregation from liquid to gaseous is complex. Although people don't see it, there are 4 stages:
- 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 tank, where the water is heated.
- In the second stage, the volume of the bubbles increases. All of them begin to rush to the surface, as there is saturated steam inside them, which is lighter than water. With an increase in the heating temperature, the pressure of the bubbles increases, and they are pushed to the surface due to known power Archimedes. 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, on the surface one can see a large number of bubbles. This initially creates cloudiness in the water. This process is popularly called "boiling with a white key", and it lasts a short period of time.
- 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 heated up to 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, then, like other gases, it 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, after the evaporation of all the liquid, becomes dry. If its temperature begins to exceed the temperature of water, then such steam is superheated, and in terms of its characteristics it will be closer to gas.
Boiling salt water
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 an area between water molecules. This chemical composition of water with salt differs from the usual fresh liquid.
The fact is that in salt water a hydration reaction takes place - the process of attaching water molecules to salt ions. Communication between molecules fresh water weaker than those formed during hydration, so boiling of a liquid with dissolved salt will take longer. As the temperature rises, the molecules in water containing salt move faster, but there are fewer of them, which is why collisions between them occur less frequently. As a result, less steam is produced and its pressure is therefore lower than the steam head of fresh water. Therefore, more energy (temperature) is required for full vaporization. On average, to boil one liter of water containing 60 grams of salt, it is necessary to raise the boiling point of water by 10% (that is, by 10 C).
Boiling pressure dependences
It is known that in the mountains, regardless of chemical composition boiling point of water will be lower. This is 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 will boil there at 75.88 C. But this does not mean that cooking in the mountains will take almost half the time. For heat treatment of products, a certain temperature is needed.
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 works in the opposite direction. If a liquid is placed in a closed flask through which vapor cannot pass, then with an increase in temperature and the formation of steam, the pressure in this flask will increase, and boiling at high blood 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 a teapot
Often people are interested in what temperature water boils in a kettle, since it is these devices that they use to boil liquids. Taking into account the fact that the atmospheric pressure in the apartment is equal to the standard one, 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 in this, and children receive such information at school. The main thing to remember is 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 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 in institutes.
Boiling- this is vaporization that occurs simultaneously both from the surface and throughout the volume of the liquid. It consists in the fact that numerous bubbles pop up and burst, causing a characteristic seething.
As experience shows, the boiling of a liquid at a given external pressure begins at a quite definite temperature that does not change during the boiling process and can only occur when energy is supplied from the outside as a result of heat transfer (Fig. 1):
where L is the specific heat of vaporization at the boiling point.
Boiling mechanism: there is always a dissolved gas in a liquid, the degree of dissolution of which decreases with increasing temperature. In addition, there is adsorbed gas on the walls of the vessel. When the liquid is heated from below (Fig. 2), the gas begins to evolve in the form of bubbles near the walls of the vessel. The liquid evaporates into these bubbles. Therefore, in addition to air, they contain saturated steam, the pressure of which increases rapidly with increasing temperature, and the bubbles grow in volume, and, consequently, the Archimedes forces acting on them increase. When the buoyant force becomes greater than the gravity of the bubble, it begins to float. But until the liquid is uniformly heated, as it rises, the volume of the bubble decreases (the saturated vapor pressure decreases with decreasing temperature) and, before reaching the free surface, the bubbles disappear (collapse) (Fig. 2, a), which is why we hear a characteristic noise before boiling. When the temperature of the liquid equalizes, the volume of the bubble will increase as it rises, since the saturated vapor pressure does not change, and the external pressure on the bubble, which is the sum of the hydrostatic pressure of the liquid above the bubble and the atmospheric pressure, decreases. The bubble reaches the free surface of the liquid, bursts, and the saturated vapor comes out (Fig. 2, b) - the liquid boils. The saturation vapor pressure in the bubbles is practically equal to the external pressure.
The temperature at which the saturated vapor pressure of a liquid is equal to the external pressure on its free surface is called boiling point liquids.
Since the pressure of saturated vapor increases with increasing temperature, and during boiling it should be equal to the external pressure, the boiling temperature increases with an increase in external pressure.
The boiling point also depends on the presence of impurities, usually increasing with increasing concentration of impurities.
If the liquid is first freed from the gas dissolved in it, then it can be overheated, i.e. heat above boiling point. This is an unstable state of the liquid. Sufficient small shaking and the liquid boils, and its temperature immediately drops to the boiling point.
Vaporization centers. For the boiling process, it is necessary that inhomogeneities exist in the liquid - the nuclei of the gaseous phase, which play the role of centers of vaporization. Usually, dissolved gases are present in the liquid, which are released by bubbles on the bottom and walls of the vessel and on dust particles suspended in the liquid. When heated, these bubbles increase both due to a decrease in the solubility of gases with temperature, and due to the evaporation of liquid in them. Bubbles that have increased in volume float up under the action of the Archimedean buoyancy force. If the upper layers of the liquid have more low temperature, then due to vapor condensation, the pressure in them drops sharply and the bubbles "collapse" with a characteristic noise. As the entire liquid warms up to the boiling point, the bubbles stop collapsing and float to the surface: the entire liquid boils.
Ticket number 15
1. Temperature distribution along the radius of a cylindrical fuel element.
Vaporization can occur not only as a result of evaporation, but also during boiling. Let us consider boiling from the energetic point of view.
A certain amount of air is always dissolved in a liquid. When a liquid is heated, the amount of gas dissolved in it decreases, as a result of which part of it is released in the form of small bubbles on 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 pressure of saturated vapor 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 action of the buoyant force of Archimedes, burst, and steam escapes from them. Vaporization, which 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 saturated vapor pressure in the bubbles becomes equal to the external pressure is called boiling point.
Since at the same temperature the pressures of saturated vapors of various liquids are different, then at various temperatures they become equal atmospheric pressure. This causes different liquids to boil at different temperatures. This property liquids are used in the sublimation of petroleum products. When oil is heated, its most valuable, volatile parts (gasoline) are the first to evaporate, which are thus separated from the "heavy" residues (oils, fuel oil).
From the fact that boiling occurs when the saturated vapor pressure 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 in order to achieve such a pressure saturated steam more heat. Conversely, at reduced pressure, the liquid boils at a lower temperature. This can be verified by experience. We heat the water in the flask to a boil and remove the spirit lamp (Fig. 37, a). The boiling of water stops. 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 "boils as a result of this. Making it boil in an open flask, pumping air into the flask will increase the pressure on the water (Fig. 37, b) Its boiling stops. 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.
In order for a liquid to begin to boil, it must be heated to the boiling point. To do this, it is necessary to impart energy to the liquid, for example, the amount of heat Q \u003d cm (t ° to - t ° 0). When boiling, the temperature of a liquid remains constant. This happens because the amount of heat reported during boiling is spent not on increasing the kinetic energy of the molecules of the liquid, but on the work of breaking molecular bonds, i.e., on vaporization. When condensing steam, according to the law of conservation of energy, it gives off to environment the amount of heat that was spent on vaporization. Condensation takes place at the boiling point, which remains constant during the condensation process. (Explain why).
Let us compose the 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 to obtain it. In this case, water and the calorimeter receive an amount of heat not only from the condensation of steam, but also from the liquid, which is obtained from it. The data of physical quantities are given in table. 3.
The condensing steam gave off the amount of heat Q p \u003d rm 3(Fig. 38, b). The liquid obtained from steam, having cooled from t ° 3 to θ °, gave up the amount of heat Q 3 \u003d 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 \u003d c 1 m 1 (θ ° - t ° 2); Q 2 \u003d c 2 m 2 (θ ° - t ° 2).
Based on the law of conservation and transformation of energy
Q p + Q 3 \u003d Q 1 + Q 2,