Ordinary water boils at 100 degrees - we do not doubt the validity of this statement, and the thermometer easily confirms this. However, there are people who can smile skeptically, because they know - water does not always and everywhere boil at exactly 100 degrees.

Is this possible? Yes, it is possible, but only under certain conditions.

It must be said right away that water can boil at temperatures both below and above +100 ° C. So do not be surprised by the expression "Water boiled at + 73 ° C" or "Water began to boil at + 130 ° C" - both of these situations are not only possible, but also relatively easy to implement.

But in order to understand how to achieve the effects just described, it is necessary to understand the mechanism of boiling water and any other liquids.

When the liquid is heated near the bottom and on the walls of the vessel, bubbles filled with steam and air begin to form. However, the temperature of the surrounding water is too low, which causes the vapor in the bubbles to condense and shrink, and under the pressure of the water, these bubbles burst. This process continues until the entire volume of the liquid does not warm up to the boiling point- at this moment, the pressure of steam and air inside the bubbles is compared with the pressure of water. Such bubbles are already able to rise to the surface of the liquid, releasing steam into the atmosphere there - this is boiling. During boiling, the temperature of the liquid no longer rises, since thermodynamic equilibrium sets in: how much heat is spent on heating, the same amount of heat is removed by steam from the surface of the liquid.

The key point in boiling water and any other liquid is the equality of the vapor pressure in the bubbles and the water pressure in the vessel. A simple conclusion can be drawn from this rule - a liquid can boil at completely different temperatures, and this can be achieved by changing the pressure of the liquid. As you know, the pressure in liquids consists of two components - its own weight and air pressure above it. It turns out that it is possible to lower or raise the boiling point of water change in atmospheric pressure or pressure inside a vessel with a heated liquid.

In fact, this is what happens. For example, in the mountains, boiling water is not at all as hot as on the plains - at an altitude of 3 km, where the air pressure drops to 0.7 atmospheres, the water boils already at +89.5 degrees. And on Everest (height - 8.8 km, pressure - 0.3 atmospheres), water boils at a temperature of slightly more than +68 degrees. Yes, cooking at such temperatures is a very difficult task, and if it were not for special means, then at such heights it would be completely impossible.

To increase the boiling point, it is necessary to increase the pressure of the atmosphere, or at least tightly close the vessel with water. This effect is used in the so-called pressure cookers- a tightly closed lid does not allow steam to escape, due to which the pressure in it rises, which means that the boiling point also rises. In particular, at a pressure of 2 atmospheres, water boils only at +120 degrees. And in steam turbines, where a pressure of tens of atmospheres is maintained, water does not boil even at + 300-400 ° C!

However, there is another possibility of heating water to high temperatures without boiling. It has been observed that the formation of the first bubbles begins at the roughness of the vessel, as well as around more or less large particles of contaminants present in the liquid. Therefore, if you heat an absolutely pure liquid in perfectly polished vessel, then at normal atmospheric pressure it is possible to make this liquid not boil at very high temperatures. The so-called superheated liquid, characterized by extreme instability - a minimal push or a speck of dust is enough for the liquid to instantly boil (and in fact, literally explode) immediately in its entire volume.

Ordinary water, with some effort, can be heated to +130 ° C and it will not boil. To obtain high temperatures, it is already necessary to use special equipment, but the limit occurs at +300 ° C - superheated water at this temperature can exist for a fraction of a second, after which it occurs explosive effervescence.

Interestingly, a superheated liquid can also be obtained in another way - by heating it to a relatively low temperatures(slightly below +100 °C) and sharply reduce the pressure in the vessel (for example, by a piston). In this case, a superheated liquid is also formed, capable of boiling with minimal impact. This method is used in bubble chambers registering charged elementary particles. When flying through a superheated liquid, a particle causes its local boiling up, and externally this is displayed as the appearance of a track (trace, thin line) from microscopic bubbles. However, it is not water that is used in bubble chambers, but various liquefied gases.

So, water does not always boil at +100 ° C - it all depends on pressure. external environment or inside a vessel. Therefore, in the mountains special means it is impossible to get "normal" boiling water, and in the boilers of thermal power plants, water does not boil even at +300 °C.

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 pure 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, inside the vessel there are sound waves, which create the noise characteristic of boiling.

At the third stage rises to the surface of the water great amount bubbles, 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.

In this case, dry saturated steam is formed. 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 fresh water . Consequently salt water boils later than fresh water. 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.

But it is equally important to heat it correctly - under-boiled and over-boiled water equally spoil the taste of tea.

boiled water

Have you ever run, leaving everything to the kettle, as soon as you heard the sound that another second - and the water will boil? Do your non-tea friends look at you like crazy at this time? :)

At first, for tea lovers, the problem of boiled water is very acute - electric kettles automatically turn off when the water is pretty boiled, and this is not attached special attention. It is also easy to forget the kettle on fire to the point where a powerful stream of steam the size of a cumulus cloud comes out of the spout.

Boiled water leaves little oxygen, so the tea becomes flat and tasteless. For the same reason, water cannot be boiled repeatedly - always only fresh water.

How to properly heat water, we will describe below.

Underboiled water

Not enough hot water- the other extreme and the same problem as the boiled one.
Often people quite consciously choose colder water for brewing to avoid bitterness and astringency in taste. More cold water, indeed, reduces bitterness and astringency. But by brewing your tea with such water, you do not get everything that it can give you (to a greater extent this applies to "dark" teas).

The best way to manage astringency/bitterness is to adjust the steeping time and the amount of steeping. Lowering the temperature often reduces the richness of the taste, making it thinner and lighter. For green teas and oolongs of weak fermentation, this is all possible, but not for dark teas, and especially shu pu-erh. You just don't reach their full potential.

Water heating devices

Coolers

There is absolutely nothing to please people who use coolers. The problem with coolers is that the water in them is not hot enough to brew dark teas. If you like red teas, pu-erh, strongly fermented oolong teas, then the only way out is to buy an electric kettle.

Electric kettles with thermometer

These kettles allow you to heat water to the desired temperature. They have sensors - 70C, 80C, 90C, 95C, 100C.
Alas, 70-80-90C is not boiled water, and it is not suitable for tea.

How to heat water for tea

Remember, friends, for any tea water must be boiled. And only then cool, if required: on average, in 5 minutes, water at room temperature cools down to 80C.

First, you need to boil if you are using spring water, especially if you are not sure about its safety.

Secondly, boiling helps to reduce the hardness of water, reduce the chlorine content. Many teas, experimentally brewed with underboiled water, suddenly acquired a fishy flavor.

The kettle must be removed from the fire / turned off as soon as the noise of water subsides in it, and the first large air bubbles appear on the surface, which rise from the bottom of the kettle - that is, at the very, very beginning of seething. It is very important not to miss this moment.

In the old tea texts, this is called "watching the boiling water".

Stages of boiling water

They were described again by Lu Yu in his "Tea Canon":

1. "Crab eye" - small air bubbles appear at the bottom, and a barely perceptible crack appears in the water.

2. "Fish eye" - the bubbles increase, the crackle grows.

3. "Pearl threads" - strings of bubbles begin to rise from the bottom to the surface, the water makes noise.

4. The threads become thick, the water begins to seethe - "the noise of the wind in the pines." At the very beginning of this stage, the kettle must be removed from the heat.

Boiling water over live fire.

Water on fire boils slowly, so all stages of boiling are easily tracked. Not everything is transmitted in the photo, but the sequence can be traced. A glass heat-resistant teapot and a gas tourist burner were used.

Boiling water in an electric kettle

It's a little harder to trace the water in electric kettles. First, many teapots are opaque. Secondly, water boils rapidly in them, and it turns off automatically only after it has boiled strongly.

We photographed the main stages of boiling water in a kettle:

What to boil water in?

As you can see, in both cases we use glass. It is chemically inert and allows you to observe the water.

Other materials:

Plastic(electric kettles) - the most unsuitable option. Plastic is not chemically inert. In addition, you should avoid kettles that prevent the formation of scale - the heating element will remain clean and shiny, but the water will remain hard, and calcium enters the body and can provoke the formation of kidney stones.

Iron(metal kettles for heating on fire). Not particularly suitable for boiling water option. Metal somehow comes into contact with water, changing its taste. That is why it is better not to get rid of scale on the walls of metal teapots or use enamelware.

Fire-clay- the most canonical (based on the old treatises on tea) option for boiling water. But also the rarest in a city apartment. Clay passes oxygen, enriches water, keeps heat for a long time. And although through the clay walls you can’t see the stages of boiling water, by the sounds made by such a kettle, you can easily determine at what stage of boiling the water is.

To prepare various delicious dishes, water is often needed, and if it is heated, it will boil sooner or later. Every educated person at the same time, he knows that water begins to boil at a temperature equal to one hundred degrees Celsius, and with further heating its temperature does not change. It is this property of water that is used in cooking. However, not everyone knows that this is not always the case. Water may boil at different temperature depending on the conditions in which it is located. Let's try to figure out what the boiling point of water depends on, and how to use it.

When heated, the temperature of the water approaches the boiling point, and numerous bubbles form throughout the volume, inside which there is water vapor. The vapor density is less than the density of water, so the Archimedes force acting on the bubbles lifts them to the surface. At the same time, the volume of bubbles either increases or decreases, so boiling water makes characteristic sounds. Reaching the surface, the bubbles with water vapor burst, for this reason, boiling water gurgles intensely, releasing water vapor.

The boiling point explicitly depends on the pressure exerted on the surface of the water, which is explained by the dependence of the pressure of saturated vapor in the bubbles on temperature. In this case, the amount of vapor inside the bubbles, and with it their volume, increase until the saturation vapor pressure exceeds the water pressure. This pressure is the sum of the hydrostatic pressure of water, due to the gravitational attraction to the Earth, and the external atmospheric pressure. Therefore, the boiling point of water increases with increasing atmospheric pressure and decreases with its decrease. Only in the case of normal atmospheric pressure of 760 mm Hg. (1 atm.) water boils at 100 0 C. The graph of the dependence of the boiling point of water on atmospheric pressure is presented below:

It can be seen from the graph that if we increase Atmosphere pressure up to 1.45 atm, then the water will boil already at 110 0 C. At an air pressure of 2.0 atm. water will boil at 120 0 C and so on. Increasing the boiling point of water can be used to speed up and improve the cooking process of hot foods. To do this, they invented pressure cookers - pans with a special hermetically sealed lid, equipped with special valves to regulate the boiling temperature. Due to the tightness, the pressure in them rises to 2-3 atm., which provides a boiling point of water of 120-130 0 C. However, it must be remembered that the use of pressure cookers is fraught with danger: the steam coming out of them has high pressure and high temperature. Therefore, you need to be as careful as possible so as not to get burned.

The opposite effect is observed if the atmospheric pressure decreases. In this case, the boiling point also decreases, which happens with an increase in altitude above sea level:

On average, when climbing 300 m, the boiling point of water decreases by 1 0 C and quite high in the mountains drops to 80 0 C, which can lead to some difficulties in cooking.

If, however, the pressure is further reduced, for example, by pumping air out of a vessel with water, then at an air pressure of 0.03 atm. water will boil already at room temperature, and this is quite unusual, since the usual boiling point of water is 100 0 C.

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 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 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 atmospheric pressure gradually decreases when climbing a mountain. 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. On high altitude 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.