How does the boiling point of water change? Why is the boiling point of water different under different conditions? Effect of pressure on melting point
home Boiling
- this is an intensive transition of liquid into vapor, which occurs 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 the vapor above it does not change. It remains unchanged until all the liquid has boiled away. This happens because all the energy supplied to the liquid is used to convert 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 explained by the pressure dependence saturated steam
on temperature. The vapor bubble grows until 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 a temperature of 100 ºC. But we should not forget that this is only true at normal atmospheric pressure (approximately 101 kPa). As pressure increases, the boiling point of water increases. For example, in pressure cookers, food is cooked under pressure of about 200 kPa. The boiling point of water reaches 120°C. In water at this temperature, the cooking process occurs much faster than in ordinary boiling water. This explains the name “pressure cooker”.
Each liquid has its own boiling point, which depends on the saturated 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 saturated vapor pressure of water is 101,325 Pa (760 mm Hg), and the vapor pressure is only 117 Pa (0.88 mm Hg). Mercury boils at 357°C at normal pressure.
Heat of vaporization.
Heat of vaporization (heat of evaporation)- the amount of heat that must be imparted to a 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 required to transform any mass of liquid taken at boiling point into vapor, the specific heat of vaporization is required r mind-knife to mass m:
When steam condenses, the same amount of heat is released.
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 The temperature at which a liquid boils is called.
Since at the same temperatures the pressures of saturated vapors of various liquids are different, then at different temperatures they become equal to 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, 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 more high temperature, since saturated vapor requires a higher temperature to achieve such 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) . 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.
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. When condensing, according to the law of conservation of energy, steam releases environment the amount of heat that was expended for vaporization. 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,
When boiling, the liquid begins to intensively transform into steam, and steam bubbles form in it and rise to the surface. When heated, steam first appears only on the surface of the liquid, then this process begins throughout the entire volume. Small bubbles appear on the bottom and walls of the pan. As the temperature rises, the pressure inside the bubbles increases, they increase in size and rise upward.
When the temperature reaches the so-called boiling point, rapid formation of bubbles begins, there are many of them, and the liquid begins to boil. Steam is formed, the temperature of which remains constant until all water is present. If vaporization occurs under normal conditions, at a standard pressure of 100 mPa, its temperature is 100°C. If you artificially increase the pressure, you can get superheated steam. Scientists managed to heat water vapor to a temperature of 1227 ° C; with further heating, the dissociation of ions turns the steam into plasma.
At a given composition and constant pressure, the boiling point of any liquid is constant. In textbooks and manuals you can see tables indicating the boiling point of various liquids and even metals. For example, water boils at a temperature of 100°C, at 78.3°C, ether at 34.6°C, gold at 2600°C, and silver at 1950°C. This data is for a standard pressure of 100 mPa, it is calculated at sea level.
How to change the boiling point
If the pressure decreases, the boiling point decreases, even if the composition remains the same. This means that if you climb a mountain 4000 meters high with a pot of water and put it on a fire, the water will boil at 85°C, and this will require much less firewood than below.
Housewives will be interested in a comparison with a pressure cooker, in which the pressure is artificially increased. At the same time, the boiling point of water also increases, due to which food cooks much faster. Modern pressure cookers allow you to smoothly change the boiling temperature from 115 to 130°C or more.
Another secret to the boiling point of water lies in its composition. Hard water, which contains various salts, takes longer to boil and requires more energy to heat. If you add two tablespoons of salt to a liter of water, its boiling point will increase by 10°C. The same can be said about sugar, 10% sugar syrup boils at a temperature of 100.1°C.
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 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 you can see on the surface a large number of bubbles. 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 heated up to 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. Communication between molecules fresh water weaker than those formed during hydration, so the boiling of liquid with dissolved salt will take longer. 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 chemical composition water will have a lower boiling point. 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 with increasing temperature and the formation of steam, the pressure in this flask will increase, and boiling at high blood pressure will occur at higher temperatures. 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.
home- this is vaporization that occurs simultaneously both from the surface and throughout the entire volume of the liquid. It consists in the fact that numerous bubbles float up and burst, causing a characteristic seething.
As experience shows, the boiling of a liquid at a given external pressure begins at a well-defined 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 exchange (Fig. 1):
where L is the specific heat of vaporization at the boiling point.
Boiling mechanism: a liquid always contains a dissolved gas, 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), gas begins to be released in the form of bubbles at the walls of the vessel. Liquid evaporates into these bubbles. Therefore, in addition to air, they contain saturated steam, the pressure of which quickly increases 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 evenly heated, as it ascends, the volume of the bubble decreases (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 saturated steam comes out (Fig. 2, b) - the liquid boils. The saturated vapor pressure in the bubbles is almost 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 The temperature at which a liquid boils is called liquids.
Since the saturated vapor pressure increases with increasing temperature, and during boiling it must be equal to the external pressure, then with increasing external pressure the boiling point increases.
The boiling point also depends on the presence of impurities, usually increasing with increasing concentration of impurities.
If you first free the liquid from the gas dissolved in it, then it can be overheated, i.e. heat above boiling point. This is an unstable state of liquid. Small shocks are enough 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 - nuclei of the gaseous phase, which play the role of centers of vaporization. Typically, a liquid contains dissolved gases, which are released in bubbles at the bottom and walls of the container and on dust particles suspended in the liquid. When heated, these bubbles increase both due to the decrease in solubility of gases with temperature and due to the evaporation of liquid in them. Bubbles that have increased in volume float up under the influence of the Archimedean buoyancy force. If the upper layers of liquid have more low temperature, then due to steam condensation, the pressure in them drops sharply and the bubbles “collapse” with a characteristic noise. As the entire liquid warms up to boiling temperature, the bubbles stop collapsing and float to the surface: the entire liquid boils.
Ticket No. 15
1. Temperature distribution along the radius of a cylindrical fuel rod.