Boiling rate. Why is the boiling point of water different under different conditions? How to change the boiling point

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 triggered, water can boil at +70 °C, and at +130 °C, and even at 300 °C! Let's consider the reasons in more detail.

What does the boiling point of water depend on?

Boiling water in a container occurs according to a certain mechanism. In the process of heating the liquid, air bubbles appear on the walls of the container into which it is poured. Inside each bubble is steam. 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 vapor in the bubbles compresses. Then under the influence 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 balls with steam can rise to the surface. The water starts to boil. Further, the heating process stops, since excess heat is removed by steam to the outside into the atmosphere. This is thermodynamic equilibrium. Recall physics: the pressure of water consists of the weight of the liquid itself and the air pressure above the vessel with water. Thus, by changing one of the two parameters (the pressure of the liquid in the vessel and the pressure of the atmosphere), it is possible to 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 Atmosphere pressure when climbing a mountain, it gradually decreases. In order for water to boil, the pressure in the bubbles that appear during the heating of water must be equal to atmospheric pressure. Therefore, with an increase in altitude in the mountains for every 300 m, the boiling point of water decreases by approximately one degree. Such boiling water is not as hot as boiling liquid in the flat country. At high altitude it is difficult and sometimes impossible to make tea. The dependence of boiling water on pressure looks like this:

And in other conditions?

What is the boiling point of water in vacuum? Vacuum is a rarefied medium in which the pressure is much lower than atmospheric pressure. The boiling point of water in a rarefied medium also depends on the residual pressure. At a vacuum pressure of 0.001 atm. liquid will boil at 6.7°C. Typically, the residual pressure is about 0.004 atm., Therefore, at this pressure, water boils at 30 ° C. As pressure increases in a rarefied medium, the boiling point of a liquid will increase.

Why does water boil at a higher temperature in a sealed container?

In a hermetically sealed vessel, the boiling point of a liquid is related to the pressure inside the vessel. In the process of heating, steam is released, which settles as condensate 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., Therefore, 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.

The process of boiling water consists of three stages:
- the beginning of the first stage - slipping from the bottom of the kettle or any other vessel in which water is brought to a boil, tiny air bubbles and the appearance of new bubble formations on the surface of the water. Gradually, the number of such bubbles increases.

- On the second stages of water boiling there is a massive rapid rise of the bubbles upward, causing at first a slight turbidity of the water, which then turns into a “whitening”, in which the water looks like a stream of a spring. This phenomenon is called boiling white key and extremely short.

- the third stage is accompanied by intense processes of water seething, the appearance of large bursting bubbles and splashes on the surface. A large amount of splashing means that the water has boiled strongly.

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Simple observers have long paid attention to the fact that all three stages of boiling water are accompanied by different sounds. Water in the first stage makes a subtle subtle sound. In the second stage, the sound turns into noise, reminiscent of the hum of a swarm of bees. In the third stage, the sounds of boiling water lose their uniformity and become sharp and loud, growing chaotically.

All stages of water boiling 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 occurs near the bottom. Gradually increasing the 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 vapor 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 emits a sound characteristic of boiling, which occurs due to collisions of water with the bottom of the tank 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 action of 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 upwards. In the event that the upper layer of water has not yet reached 100 degrees, then the bubble, falling into colder water, loses part 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 again gain volume and rise up, and it is these changes in the size of the bubbles 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 to the outside, accompanied by a characteristic gurgling - 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 the pressure, the higher the required temperature, and vice versa.

That water boils at 100 degrees Celsius- a well-known fact. But it is worth considering that such a temperature is valid only under the condition of normal atmospheric pressure (about 101 kilopascals). As pressure increases, the temperature at which a 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 with this temperature, boiling proceeds much faster than at a normal boiling point - hence the name of the pan.

Accordingly, lowering the pressure lowers the boiling point of water. For example, residents of mountainous regions living at an altitude of 3 kilometers achieve water boiling faster than inhabitants of the plains - all stages of boiling water occur faster, since it requires only 90 degrees at a pressure of 70 kilopascals. But to cook, for example, egg the inhabitants of the mountains cannot, since the minimum temperature at which the protein folds is just 100 degrees Celsius.

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. And although people do not 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 tank, where the water is heated.
2. 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 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.
3. In 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 "boiling with a white key", and it lasts 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, splashes mean 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, 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 often. 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, where the dependence of the boiling point of a liquid on atmospheric pressure is indicated. They are available to everyone and are actively used by schoolchildren, students and even teachers in institutes.

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 what can happen only at certain pressures and temperatures.

Boiling - heating water to boiling point.

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

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

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

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

As the temperature rises, the pressure of the saturated bubbles increases, causing them to 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 clean water without impurities), then the bubbles sink down 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 are generated inside the vessel, which create the noise characteristic of boiling.

At 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 water begins to boil intensely, large bursting bubbles and splashes appear (as a rule, splashes mean that the water has boiled strongly).

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

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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.

In the process of 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.

AT this case dry saturated steam is produced. There are no highly dispersed particles of the liquid phase in such a pair. Also steam can be saturated wet and overheated.

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

At the beginning of boiling water, just such 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 steam will be close in its characteristics to gas.

Boiling point of salt water^

The boiling point of salt water is higher than the boiling point of fresh water. Consequently salty 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, a process 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 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 become higher than atmospheric pressure and the boiling process to begin, a higher temperature is needed. When adding 60 grams of salt to 1 liter of water, the boiling point will increase by 10 C.

If you are asked at what temperature water boils, you will most likely answer that at 100 ° C. And your answer will be correct, but this value is only true at normal atmospheric pressure - 760 mm Hg. Art. In fact, water can boil at both 80°C and 130°C. In order to explain the reason for such discrepancies, it is first necessary to clarify what boiling is.

To figure out how many degrees are needed for the water to boil, studying the mechanism of this will help. physical phenomenon. Boiling is the process of converting liquid into vapor and takes place in several stages:

1. When the liquid is heated, bubbles with air and water vapor come out of the microcracks in the walls of the vessel.
2. The bubbles expand a little, but the liquid in the vessel is so cold that the vapor in the bubbles condenses.
3. The bubbles begin to burst until the entire thickness of the liquid becomes hot enough.
4. After some time, the pressure of water and steam in the bubbles equalize. At this stage, individual bubbles can rise to the surface and release steam.
5. The bubbles begin to rise intensively, seething begins with a characteristic sound. Starting from this stage, the temperature in the vessel does not change.
6. The boiling process will continue until all the liquid passes into a gaseous state.

Steam temperature

The temperature of the steam when water boils is the same as that of the water itself. This value will not change until all the liquid in the vessel has evaporated. During the boiling process, wet steam is formed. It is saturated with liquid particles uniformly distributed throughout the entire gas volume. Further, highly dispersed particles of the liquid condense, and the saturated steam turns into dry.

There is also superheated steam, which is much hotter than boiling water. But it can only be obtained with the help of special equipment.

Pressure influence

We have already found out that for a liquid to boil, it is necessary to equalize the pressure of a liquid substance and vapor. Since water pressure is the sum of atmospheric pressure and the pressure of the liquid itself, there are two ways to change the boiling time:

• change in atmospheric pressure;
• change in pressure in the vessel itself.

We can observe the first case in territories located at different heights above sea level. On the coasts, the boiling point will be 100 ° C, and at the top of Everest - only 68 ° C. The researchers calculated that when climbing mountains, the boiling point of water drops by 1 °C every 300 meters.

These values ​​may vary depending on the chemical composition of the water and the presence of impurities (salts, metal ions, soluble gases).

Kettles are most often used to get boiling water. The boiling point of water in a kettle also depends on where you live. Mountain dwellers are advised to use autoclaves and pressure cookers, which help to make boiling water hotter and speed up the cooking process.

Boiling salt water

The temperature at which water boils determines the presence of impurities in it. As part of sea ​​water sodium and chloride ions are present. They are located between the H2O molecules and attract them. This process is known as hydration.

The bond between water and salt ions is much stronger than between water molecules. It takes more energy to boil salt water so that these bonds can be broken. This energy is temperature.

Also, salty liquid differs from fresh water by a low concentration of H2O molecules. In this case, when heated, they begin to move faster, but they cannot form a sufficiently large vapor bubble, since they collide less often. The pressure of small bubbles is not enough to bring them to the surface.

To equalize water and atmospheric pressure, you need to increase the temperature. Therefore, salt water takes much longer to boil than fresh water, and the boiling point will depend on the salt concentration. It is known that adding 60 g of NaCl to 1 liter of liquid increases the boiling point by 10 °C.

How to change the boiling point

In the mountains it is very difficult to cook food, it takes too much time. The reason is not enough hot boiling water. At very high altitudes, it is almost impossible to boil an egg, let alone cook meat that needs a good heat treatment.

Changing the temperature at which the liquid boils is important for residents not only of mountainous areas.

For sterilization of products and equipment, it is desirable to use a temperature higher than 100 °C, as some microorganisms are heat resistant.

This is important information not only for housewives, but also for professionals working in laboratories. Also, increasing the boiling point can significantly save time spent on cooking, which is important in our time.

To increase this figure, you need to use a tightly closed container. Pressure cookers are best suited for this, in which the lid does not allow steam to pass through, increasing the pressure inside the vessel. During heating, steam is released, but since it cannot escape, it condenses on the inside of the lid. This leads to a significant increase in internal pressure. In autoclaves, the pressure is 1–2 atmospheres, so the liquid in them boils at a temperature of 120–130 °C.

The maximum boiling point of water is still unknown, since this figure can increase as long as atmospheric pressure increases. It is known that water cannot boil in steam turbines even at 400 °C and a pressure of several tens of atmospheres. The same data were obtained at great depths of the ocean.

Boiling Water Under Reduced Pressure: Video