Boiling is the process of vaporization that occurs when a liquid is brought to the boiling point. Every person knows from his school desk that water boils at t=100˚С. But many are interested in the question of which water boils faster: salted or fresh?
Boiling is a rather complex process consisting of four stages:
- First stage characterized by the appearance of small air bubbles that appear both on the surface of the liquid and on the side. Their occurrence is the result of the expansion of air bubbles located in microscopic cracks in the container.
- During the second stage you can see that the bubbles increase in volume and more and more of them appear on top. This phenomenon is explained by an increase in temperature, at which the pressure on the bubbles increases. Thanks to Archimedean force, they appear on the surface. If it does not have time to warm up to the boiling point (100˚C), then the bubbles again go to the bottom, where the water is hotter. The noise characteristic of boiling is created as the size of the bubbles increases and decreases.
- At the third stage a mass of bubbles is observed, which, rising to the surface, causes short-term turbidity of the water.
- Fourth stage characterized by intense seething and the appearance of large bubbles, which, when bursting, create splashes. The latter indicate that the water has boiled. Water vapor appears, and the water makes sounds characteristic of boiling.
Fresh water boiling
Boiling water is water brought to a boil. During this process, abundant steam formation occurs, which is accompanied by the release of free oxygen molecules from the boiling liquid. Due to prolonged exposure to high temperatures, microbes and pathogenic bacteria die in boiling water. Therefore, if the quality of tap water is poor, it is undesirable to consume it raw.
Fresh but hard water contains salts. During boiling, they form a coating on the walls of the kettle, which is more often called scale. Boiling water is commonly used to prepare hot drinks or disinfect fruits or vegetables.
When salt water boils
Experiments show that the boiling point of salt water is higher than the boiling point of fresh water. Therefore, we can conclude that fresh water boils faster. Salt water contains chlorine and sodium ions, which are found among the water molecules. Between them, the process of hydration occurs - the addition of water molecules to salt ions.
It is worth noting that the hydration bond is much stronger than the water intermolecular bond. Therefore, when fresh water boils, the process of vaporization begins faster. A liquid with salts dissolved in it requires a little more energy to boil, which in this situation is temperature.
When it increases, molecules in salt water move much faster, but their number decreases, which means they collide less often. This is what can explain the smaller amount of steam - after all, its pressure is less than that of fresh water. To achieve greater than atmospheric pressure in salt water and begin to boil, a higher temperature is required.
Another justification
When cooking, many housewives salt the water at the beginning of the process, citing the fact that it will boil faster this way. And some find an explanation for why salt water boils faster, relying on school knowledge of a physics course, namely the topic related to heat transfer. As is known, heat transfer is of three types: heat transfer, which is characteristic of solids, convection, which is present in gaseous and liquid bodies, and radiation.
The latter type of heat transfer exists even in space. This is confirmed by the stars and, of course, the sun. But still, the main factor in this matter is density. Since salt water has a higher density than fresh water, it boils faster. At the same time, it takes more time to freeze. Consequently, with a denser liquid, heat transfer will be more active and boiling will occur faster.
Boiling water at reduced pressure: Video
Everyone knows that the boiling point of water at normal atmospheric pressure (about 760 mm Hg) is 100 °C. But not everyone knows that water can boil at different temperatures. The boiling point depends on a number of factors. If certain conditions are met, water can boil at +70 °C, and at +130 °C, and even at 300 °C! Let's look at the reasons in more detail.
What determines the boiling point of water?
Boiling of water in a container occurs according to a certain mechanism. As the liquid heats up, air bubbles appear on the walls of the container into which it is poured. There is steam inside each bubble. The temperature of the steam in the bubbles is initially much higher than the heated water. But its pressure during this period is higher than inside the bubbles. Until the water warms up, the steam in the bubbles is compressed. Then, under the influence of external pressure, the bubbles burst. The process continues until the temperatures of the liquid and vapor in the bubbles are equal. It is now that the steam balls can rise to the surface. The water begins to boil. Then the heating process stops, as excess heat is removed by steam to the atmosphere. This is thermodynamic equilibrium. Let's remember physics: water pressure consists of the weight of the liquid itself and the air pressure above the vessel with water. Thus, by changing one of two parameters (liquid pressure in the vessel and atmospheric pressure), you can change the boiling point.
What is the boiling point of water in the mountains?
In the mountains, the boiling point of a liquid gradually drops. This is due to the fact that the atmospheric pressure gradually decreases when climbing a mountain. For water to boil, the pressure in the bubbles that appear during the heating process must be equal to atmospheric pressure. Therefore, with every 300 m increase in altitude in the mountains, the boiling point of water decreases by approximately one degree. This type of boiling water is not as hot as boiling liquid on flat terrain. At high altitudes it is difficult, and sometimes impossible, to brew tea. The dependence of boiling water on pressure looks like this:
Height above sea level | ||||||||
Boiling point |
What about in other conditions?
What is the boiling point of water in a vacuum? A vacuum is a rarefied environment in which the pressure is significantly lower than atmospheric pressure. The boiling point of water in a rarefied environment also depends on the residual pressure. At a vacuum pressure of 0.001 atm. the liquid will boil at 6.7 °C. Typically the residual pressure is about 0.004 atm, so at this pressure water boils at 30 °C. With increasing pressure in a rarefied environment, the boiling point of the liquid will increase.
Why does water boil at a higher temperature in a sealed container?
In a hermetically sealed container, the boiling point of the liquid is related to the pressure inside the container. During the heating process, steam is released, which settles as condensation on the lid and walls of the vessel. Thus, the pressure inside the vessel increases. For example, in a pressure cooker the pressure reaches 1.04 atm, so the liquid boils in it at 120 °C. Typically, in such containers, the pressure can be regulated using built-in valves, and therefore the temperature too.
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.
If a liquid is heated, it will boil at a certain temperature. When a liquid boils, bubbles form, rise to the top and burst. The bubbles contain air containing water vapor. When the bubbles burst, steam escapes, and thus the liquid evaporates intensely.
Various substances in a liquid state boil at their own characteristic temperature. Moreover, this temperature depends not only on the nature of the substance, but also on atmospheric pressure. So water at normal atmospheric pressure boils at 100 °C, and in the mountains, where the pressure is lower, water boils at a lower temperature.
When a liquid boils, further supply of energy (heat) to it does not increase its temperature, but simply maintains the boil. That is, energy is spent on maintaining the boiling process, and not on raising the temperature of the substance. Therefore, in physics such a concept as specific heat of vaporization(L). It is equal to the amount of heat required to completely boil away 1 kg of liquid.
It is clear that different substances have their own specific heat of vaporization. So for water it is equal to 2.3 · 10 6 J/kg. For ether, which boils at 35 °C, L = 0.4 10 6 J/kg. For mercury boiling at 357 °C, L = 0.3 10 6 J/kg.
What is the boiling process? When water heats up but has not yet reached its boiling point, small bubbles begin to form. They usually form at the bottom of the container, since they are usually heated under the bottom, and the temperature is higher there.
The bubbles are lighter than the water surrounding them and therefore begin to rise to the upper layers. However, the temperature here is even lower than at the bottom. Therefore, the steam condenses, the bubbles become smaller and heavier, and fall down again. This happens until all the water is heated to boiling point. At this time, a noise is heard that precedes boiling.
When the boiling point is reached, the bubbles no longer sink down, but float to the surface and burst. Steam comes out of them. At this time, it is no longer a noise that is heard, but the gurgling of the liquid, which indicates that it has boiled.
Thus, during boiling, just like during evaporation, a transition of liquid into vapor occurs. However, unlike evaporation, which occurs only at the surface of the liquid, boiling is accompanied by the formation of bubbles containing steam throughout the volume. Also, unlike evaporation, which occurs at any temperature, boiling is possible only at a certain temperature characteristic of a given liquid.
Why does the higher the atmospheric pressure, the higher the boiling point of a liquid? The air presses on the water and therefore creates pressure inside the water. When bubbles form, the steam also presses into them, and more strongly than external pressure. The greater the external pressure on the bubbles, the stronger the internal pressure within them. Therefore they are formed at a higher temperature. This means that water boils at a higher temperature.
Boiling water process consists of three stages:
- the beginning of the first stage - tiny air bubbles jumping from the bottom of the kettle or any other vessel in which water is brought to a boil and new bubble formations appear on the surface of the water. Gradually the number of such bubbles increases.
- On the second boiling water stage there is a massive rapid rise of bubbles upward, causing at first a slight turbidity of the water, which then turns into “whitening”, in which the water looks like a stream of a spring. This phenomenon is called boiling white key and extremely short-lived.
– the third stage is accompanied by intense processes of water boiling, the appearance of large bursting bubbles and splashes on the surface. A large amount of splashing means that the water has boiled too much.
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Ordinary observers have long noticed the fact that all three stages of boiling water are accompanied by various sounds. Water at the first stage makes a barely audible thin sound. In the second stage, the sound turns into noise, reminiscent of the hum of a swarm of bees. At the third stage, the sounds of boiling water lose their uniformity and become sharp and loud, growing chaotically.
All boiling water stage are easily verified by experience. Having started heating water in an open glass container and periodically measuring the temperature, after a short period of time we will begin to observe bubbles covering the bottom and walls of the container.
Let's take a closer look at the bubble that appears near the bottom. Gradually increasing its volume, the bubble also increases the area of contact with the warming water, which has not yet reached a high temperature. As a result of this, the steam and air inside the bubble are cooled, as a result of which their pressure decreases, and the gravity of the water bursts the bubble. It is at this moment that the water makes a sound characteristic of boiling, which occurs due to collisions of water with the bottom of the container in those places where the bubbles burst.
As the temperature in the lower layers of water approaches 100 degrees Celsius, the intrabubble pressure equalizes with the water pressure on them, as a result of which the bubbles gradually expand. An increase in the volume of bubbles also leads to an increase in the buoyancy force on them, under the influence of which the most voluminous bubbles break away from the walls of the container and rapidly rise upward. If the top layer of water has not yet reached 100 degrees, then the bubble, falling into colder water, loses some of the water vapor that condenses and goes into the water. In this case, the bubbles again decrease in size and fall down under the influence of gravity. Near the bottom, they gain volume again and rise upward, and it is these changes in bubble size that create the characteristic noise of boiling water.
By the time the entire volume of water reaches 100 degrees, the rising bubbles no longer decrease in size, but burst on the very surface of the water. In this case, steam is released outward, accompanied by a characteristic gurgling sound - this means that water is boiling. The temperature at which a liquid reaches boiling depends on the pressure experienced by its free surface. The higher this pressure, the higher the temperature required, and vice versa.
That water boils at 100 degrees Celsius is a well-known fact. But it is worth considering that this temperature is only valid under normal atmospheric pressure (about 101 kilopascals). As pressure increases, the temperature at which the liquid reaches boiling also increases. For example, in pressure cookers, food is cooked under pressure approaching 200 kilopascals, at which the boiling point of water is 120 degrees. In water at this temperature, cooking proceeds much faster than at normal boiling temperature - hence the name of the pan.
Accordingly, a decrease in pressure also lowers the boiling point of water. For example, residents of mountainous regions, living at an altitude of 3 kilometers, achieve boiling water faster than residents of the plains - all stages of boiling water occur faster, since this requires only 90 degrees at a pressure of 70 kilopascals. But mountain residents cannot boil, for example, a chicken egg, since the minimum temperature at which the white coagulates is exactly 100 degrees Celsius.
