At what temperature does water boil in an electric kettle? At what temperature does water boil? Boiling water how many degrees

Let's follow the boiling process, starting from the moment when the first bubbles form on the heated bottom of the vessel (pot or). By the way, do they form? Yes, because a thin layer of water, directly in contact with the bottom of the vessel, heated up to 100 degrees. And, according to physical properties water, began to turn from into gaseous.

So, the first bubbles, while still small, begin to slowly float up - they are affected by a buoyant force, otherwise called Archimedean - and almost immediately sink to the bottom again. Why? Yes, because the water is still not warm enough from above. Having come into contact with colder layers, the bubbles, as it were, “wrinkle”, lose their volume. And, accordingly, the Archimedean force immediately decreases. Bubbles sink to the bottom, and "burst" from gravity.

But the heating continues, more and more layers of water take on a temperature close to 100 degrees. The bubbles no longer sink to the bottom. They strive to reach the surface, but the most upper layer even much colder, therefore, in contact with it, each bubble again decreases in size (due to the fact that part of the water vapor contained in it, cooling, turns into water). Because of this, it begins to sink down, but, once in the hot layers, which have already taken a temperature of 100 degrees, again increases in size. Because the condensed steam becomes steam again. A huge number of bubbles scurry up and down, alternately decreasing and increasing in size, producing a characteristic noise.

And then, finally, the moment comes when the entire water column, including the uppermost layer, has taken a temperature of 100 degrees. What will happen at this stage? The bubbles, rising up, freely reach the surface. And it is here, at the interface between two media, that “seething” occurs: they burst, releasing water vapor. And this process, subject to constant heating, will continue until all the water boils away, turning into a gaseous state.

It should be noted that the boiling point depends on atmospheric pressure. For example, high in the mountains, water boils at temperatures less than 100 degrees. Therefore, the inhabitants of the highlands take much longer to cook their own food.

Boiling water is one of the frequent daily activities. However, in mountainous areas this process has its own characteristics. At different altitudes above sea level, water boils at different temperatures.

How does the boiling point of water vary with atmospheric pressure?

Boiling water is characterized by pronounced external: seething liquid, the formation of small bubbles inside the dishes and rising steam. When heated, water molecules receive additional energy from a heat source. They become more mobile and begin to vibrate.

Ultimately, the liquid reaches a temperature at which steam bubbles form on the walls of the dish. This temperature is called the boiling point. Once the water begins to boil, the temperature does not change until all the liquid has turned into a gas.

Water molecules escaping as vapor exert pressure on the atmosphere. This is called steam pressure. With an increase in the temperature of water, it increases, and the molecules, moving faster, overcome the intermolecular forces that bind them. The vapor pressure is opposed by another force created air mass: . When the vapor pressure reaches or exceeds the ambient pressure, overcoming it, the water begins to boil.

The boiling point of water also depends on its purity. Water that contains impurities (salt, sugar) will boil at a higher temperature than pure water.

Features of boiling water in the mountains

The air atmosphere exerts pressure on all objects on. At sea level, it is the same everywhere and is equal to 1 atm., Or 760 mm Hg. Art. It's normal Atmosphere pressure and water boils at 100°C. The vapor pressure at this water temperature is also 760 mm Hg. Art.

The higher you are above sea level, the thinner the air becomes. In the mountains, its density and pressure decrease. Due to the decrease in external pressure on the water, it is required less energy to break intermolecular bonds. This implies less heat and the water will boil at a lower temperature.

With each kilometer of altitude, water boils at a temperature that is 3.3 ° C lower than the initial one (or about minus 1 for every 300 meters). At an altitude of 3 km above sea level, atmospheric pressure is about 526 mm Hg. Art. Water will boil when the vapor pressure is equal to atmospheric, namely 526 mm Hg. Art. This condition is reached at a temperature of 90°C. At an altitude of 6 km, the pressure is about two times less than normal, and - about 80 ° C.

At the top of Everest, whose height is 8848 m, the water boils at a temperature of about 72°C.

In the mountains at an altitude of 600 m, where water boils at 98°C, understanding the boiling process is especially important when cooking. Some products can be brought to readiness by increasing the cooking time. However, for foods that require good thermal processing and long cooking times, it is best to use a pressure cooker.

Boiling is a seemingly simple physical process known to everyone who has boiled a kettle at least once in their life. However, he has many features that physicists study in laboratories, and housewives - in the kitchens. Even the boiling point is far from constant, but varies depending on various factors.

boiling liquid

When boiling, the liquid begins to intensively turn into steam, steam bubbles are formed in it, rising to the surface. When heated, at first steam appears only on the surface of the liquid, then this process begins throughout the volume. Small bubbles appear on the bottom and walls of the dish. When the temperature rises, the pressure inside the bubbles increases, they increase and rise up.

When the temperature reaches the so-called boiling point, the rapid formation of bubbles begins, there are a lot of them, the liquid boils. Steam is formed, the temperature of which remains constant until all the water is gone. 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 vapor into plasma.

For 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 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 is reduced, the boiling point decreases even though 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, 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 is cooked much faster. Modern pressure cookers allow you to smoothly change the boiling point from 115 to 130 ° C or more.

Another secret of 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 100.1°C.

One of the important steps for obtaining a tasty, healthy and fragrant infusion is getting boiling water. But remember, boiled water, as well as re-boiled water, is dead water!

Water usually contains a lot of microscopic salts, and if it is boiled, their concentration will increase. Boiling water must be young. If the water does not have time to boil, the tea leaves will not unroll, will not fall to the bottom, but will float on the surface. The tea will not brew and the aroma of the tea will not be revealed either. And each tea has its own temperature requirements. So after the water has boiled, if a temperature lower than 100 degrees is needed, it is allowed to cool. When there is no water thermometer at hand, they use the rule that water cools down to about 85 degrees in five minutes.

To get young boiling water, you need to monitor the water in the kettle. In the treatise of the famous Lu Yu, it was said that when the "crab eye" first appears - small bubbles at the bottom and at the same time a slight clicking begins - this is the first stage of boiling water. The water temperature is about 70-80 C.

Then the bubbles increase, the crackling becomes more frequent and merges into a slight noise and the second short stage called the "fisheye" begins. The temperature is about 80-85C.

Then "pearl threads" begin to rise along the walls of the teapot - sort of strings of bubbles, the water begins to boil, the noise changes a little and becomes, as it were, muffled - this is the third stage. It is she who is considered the most suitable for pouring tea into water (if you brew tea using the Lu Yu method) or removing water from the fire. The temperature is about 85-92C. Also behind this stage there is a very short one - this stage is called "Wind noise in the pines" - if you listen to the water at this moment, you will understand why. But since you need to practice to catch it, we recommend that you shoot the kettle not at the third stage.

When stormy waves go over the surface of the water - the so-called "bulk boiling" - this is the fourth stage of boiling water brewing. The fourth stage of boiling water, according to Lu Yu, is not suitable for brewing tea. And the thing is that the oxygen contained in the water is lost, they leave the water with steam, from which the water changes its taste.

If the water is hard or not clean, then there will be no classic boiling stages or they will be smeared.

The water boiled, and we got young boiling water. Then, if necessary, let the water cool down. If we don’t remember what temperature was recommended in the description for tea, then we adhere to the general rule:

Water temperature from 90 degrees to 95 is suitable for brewing black teas, for example pu-erh, fully fermented(these are red teas) and also highly fermented oolong teas.

Water temperature from 80 to 90 degrees is brewed mainly lightly fermented Taiwanese oolong teas.

Low water temperature, which is below 80 degrees, suitable for green, white and yellow teas.

The importance of making tea desired temperature, because if you brew tender green or white tea with boiling water, then there will be no freshness, there will be no lightness, there will be no sweetness, there will be no rich aftertaste, but there will be a taste of bitterness and unpleasant astringency. Only correctly brewed tea will give us amazing sensations, feelings of pleasant lightness, purity of thought and, finally, pleasant communication, if brewed not only for ourselves.

Happy tea!

Boiling water is accompanied by changes in the features of its phase state and the acquisition of a vaporous consistency when certain temperature indicators are reached.

In order to boil water and contribute to the release of steam, a temperature of 100 degrees Celsius is required. Today we will try to deal with the question of how to understand that the water has boiled.

Ever since childhood, we have all heard parental advice regarding what can only be consumed boiled water. Today, one can meet both supporters and opponents of such recommendations.

On the one hand, boiling water is actually a necessary and useful procedure, because it is accompanied by the following positive aspects:

• Reaching water temperatures of 100 degrees and above is accompanied by the death of many pathogens, so boiling can be called a kind of purification of the liquid. For effective fight With bacteria, experts recommend boiling water for at least 10 minutes.
• When boiling water, various impurities are also eliminated, which can pose a certain danger to human health. A sign of getting rid of impurities is the formation of scale, which we often see on the walls of kettles and pots. But keep in mind that brewing tea only with boiled water, there is a high probability of regularly filling the body with crystallized deposits, which is fraught with the development of urolithiasis in the future.

The harm of boiling water may be due to non-compliance with the indicated recommendations regarding the time of boiling.

If you brought the liquid to 100 degrees and immediately removed it from the fire, there is no doubt that the prevailing number of microorganisms were not adversely affected. To avoid this, be sure to boil water for 10 to 15 minutes.

One more negative side boiling water enters the loss of oxygen, which is a vital element for any living organism.

Thanks to large oxygen molecules, the distribution of useful elements is ensured through circulatory system. Of course, the lack of oxygen is not detrimental to health, but it does not represent any benefit.

There are several methods for bringing water to a boil. They differ, first of all, in what pud you use to boil the liquid. Kettles are most often used to make tea or coffee, but pots are most often used in cooking.

So, first you need to fill the kettle cold water from the tap and place the container on fire. As it warms up, crackling sounds will be clearly audible, which will be replaced by an increasing hiss.

The next stage is the fading of the hiss, which is replaced by a faint noise, the appearance of which is accompanied by the release of steam. These signs will indicate that the water in the kettle has boiled. It remains only to wait about 10 minutes and remove the kettle from the heat.

It is much easier to determine the boiling of water in open containers. Fill the pan with the required amount of cold water and place the container on the fire. The first signs that the water will soon boil will be the appearance of small bubbles that form at the bottom of the container and rise to the top.

The next step is an increase in the size and number of bubbles, which is accompanied by the formation of steam above the surface of the container. If the water begins to boil, then the liquid has reached the temperature required for boiling.

The following facts will be quite useful for you:

• If you want to bring water to a boil as quickly as possible using a saucepan, be sure to cover the container with a lid to retain heat. You also need to remember that in large containers, water reaches a boil longer, which is associated with the expenditure of more time to heat such a pan.
• Use only cold water from the tap. The fact is that hot water may contain impurities of lead found in the plumbing system. According to many experts, such water is not suitable for consumption and use in cooking, even after boiling.
• Never fill containers to the brim, as water will overflow from the pot as it boils.
• As the altitude increases, the boiling point decreases. In such a case, more boiling time may be required to ensure that all pathogens are killed. This fact should be taken into account when going hiking in the mountains.

You should also take all precautions when dealing with not only hot water, capacity, but also with the steam generated, which can cause serious burns.

Of course, at 100° Celsius, each of us will answer. In answering this question in this way, we often forget that our answer is correct only for water under air pressure on the surface of the earth.

Boiling of a liquid occurs when the vapor pressure above it becomes equal to the pressure of air or other gas above the surface of the liquid. The boiling point, therefore, is a variable and depends on the pressure under which the liquid is located. As soon as a liquid is placed in a rarefied space, its boiling point will decrease.

We will climb to the top of Mount Kazbek (5043 m above sea level), where the air pressure is 405 mm Hg, and try to measure the temperature of the "boiling water" - the thermometer will show only 83 °. In a rarefied space, you can get quite "cold" boiling water. For example, at a pressure of 17.5 mm mercury column water will boil at 20°. It will be really "cold" boiling water.

In the chemical, food and other industries, it is sometimes necessary to evaporate huge amounts of liquids. This evaporation is especially effective in vacuum. In some cases, the ability to quickly evaporate water at low temperature is of decisive importance: the dissolved product is protected from decomposition. When evaporating milk, fruit and berry juices, yeast, organic dyes in a vacuum, their most important properties are preserved.

In a dairy plant, vacuum is used not only to evaporate milk and dry it, but also to ensure that milk and its products do not get contaminated during pumping. In order to supply milk from one vat to another or to a tank truck, a vacuum is created and the milk itself rushes in the right direction.

Vacuum is also used at the cannery. In order to kill the bacteria that got into the tin during packaging, it is heated and kept at an elevated temperature. If there is air left in the jar before sealing, it will expand as it warms up and may break the jar. To prevent this from happening, the jar is evacuated before capping.

The most perfect way to keep food fresh is to quickly freeze them and then dry them - freezing out the moisture under vacuum. This is the most advanced way of food preservation.

Is it possible to create a vacuum without a pump? Yes, you can. To get a vacuum without a pump, you need to turn part of the gas into a liquid by strong cooling.

This technique is used in vacuum evaporation. On fig. 30 shows an evaporator plant in a sugar factory, which consists of several, usually three, units connected in series. The first of them is heated by steam coming from the boiler house, the second - by the steam of the first, the third - by the steam of the second. The first apparatus receives the pre-evaporated syrup, which has passed through the second and third apparatuses. The syrup boils, some of the water evaporates from it,

And, when the concentration becomes sufficient, the syrup is released for sugar crystallization or the crystallization process is carried out in the apparatus itself. The resulting mixture of molasses and crystals is released for further processing. Steam from the third apparatus enters the condenser, where it is cooled by water and condensed. When the steam condenses, a vacuum is created, under which the syrup is located in the third body of the residue. The boiling point of the syrup in the evaporation chambers depends on the vacuum value. Since air can enter the evaporation apparatus, a vacuum pump is connected to the condenser to maintain the vacuum. The water formed in the condenser, as it accumulates, flows down the barometric tube, the degree of filling of which with water is determined by the vacuum value. In each of the evaporators, the solution boils at a lower temperature, since the pressure in them is below atmospheric. This allows better use of the heat of the heating steam.

In the chemical industry, not only evaporation, but also drying and crystallization of many products is carried out in a vacuum.

In any industry, we will see the use of vacuum. Many readers probably have not heard that even in the production of bricks, vacuum can play a role. important role. In brick production there is a type of marriage, which is figuratively called "dragon's tooth". In this case, the brick comes out of the press with a torn edge. It depends on the properties of clay, and it is difficult to get rid of this type of marriage. And this is where the vacuum helps! As soon as a vacuum is created in the brick press chamber, the marriage is terminated. This is because air bubbles are removed from the clay, the clay mass becomes denser and more cohesive and better molded.

Vacuum presses are widely used in the ceramic industry, where the requirements for processing plastic mass are especially high.

metallurgy also began to make extensive use of vacuum, which promises a significant improvement in the quality of metals. A fiery jet of molten iron is released from a blast furnace. A huge ladle containing tens of tons of metal is filled, the ladle is fed to the filling machine. Sparks, the hiss of water, the noise of mechanisms, and now an endless chain pulls molds with still fiery red, but gradually fading, solidifying cast iron. At the other end of the machine, a cast-iron bar is removed from the molds - an ingot. The same picture for a powerful open-hearth furnace. Here, steel, sparkling in all shades - from dazzling white to orange-red, is poured into huge molds, solidifies into an ingot, which will go to a powerful rolling mill, will be compressed, stretched, rolled and turned into hundreds of meters of beams or rails.

But what is it? After so much effort has been spent on obtaining steel - melted, poured, cooled, reheated, rolled - the finished rails are thrown aside and sent back to the open-hearth furnace for remelting along with rusty scrap.

This is a marriage! Thin - thinner than a hair - cracks, bubbles, cavities turned out to be in the casting in an unacceptable amount, and the finished product was rejected, it cannot work reliably.

What's the matter, where is the reason for marriage? It turns out that the main cause of various defects in steel are gases dissolved in the metal. When the metal is smelted, a series of complex processes take place in the furnace, which in some cases are accompanied by the release of large quantities gases. Some gases remain in the molten metal. During cooling, when the molten metal solidifies into a strong and dense ingot, gases remain in it, creating defects. Hydrogen, nitrogen, oxygen can be dissolved in steel. Their weight is small. Hydrogen, for example, is contained in an amount of about 0.001%; but in terms of volume it is 4-10 cubic meters. cm at normal pressure for every 100 grams of steel. Hydrogen fills small voids in the steel ingot. In the process of cooling, the metal contracts and in the space filled with gas, it can develop high pressure up to several thousand

atmosphere. Such pressure forms small cracks in the metal - flocks. Metallurgists have long been fighting their enemy - gases dissolved in cast iron, steel and other metals. To reduce their number, various substances are introduced into the metal during melting, which could chemically bind gases. Aluminum, silicon, titanium and other substances are added to steel for this purpose, but this is not in vain. Non-metallic compounds are formed that lower the quality of the metal, even if they are contained in hundredths of a percent.

And here, with the help of vacuum, metallurgists managed to improve the quality of the metal. If a ladle of molten steel is placed in a vacuum, gases will rapidly come out of it. In vacuum, the solubility of gases in the metal decreases sharply. The quality of castings is increasing.

The methods of short-term degassing of steel directly in ladles and molds, developed by Soviet scientists, reduce the content of gases in it by several times.

In a vacuum, not only gas impurities are removed, but also during casting and cooling, the metal is protected from the action of active gases, primarily oxygen.

High quality chromium molybdenum alloys for turbine blades and nickel alloys for radio equipment melted in a vacuum to avoid oxidation.

Degassing under vacuum is especially important for special steels. Vacuum steel bearings last three to four times longer than conventional steel bearings. Electricity losses in magnetic steels for transformer cores are reduced. The main defect of heat-resistant steels - brittleness - is reduced. Increases the chemical resistance of stainless steels. One enumeration of the advantages that the use of evacuation gives in the melting of metals speaks of high efficiency this process.

For vacuum melting of high-quality steels, induction furnaces have been created, in which the entire process, including casting, takes place in a vacuum. The whole furnace is placed in a hermetically sealed casing connected to powerful vacuum pumps.

Of great practical interest is not only melting in a vacuum, but also the distillation of metals in a vacuum.

We observe daily how liquids evaporate. You pour a few drops of ether into your palm, wave your hand - a feeling of cold appears, and the liquid disappears, evaporates, the smell of ether spreads in the air. Ether molecules were distributed among the molecules of air gases.

It is hard to imagine that, like ether, steel or other strong and stable metals can evaporate. And indeed, no matter how much we hold a steel plate in air at ordinary temperature, its weight will not decrease, unless, of course, the air is dry and the possibility of rusting is excluded. However, it is possible to create conditions under which even the most refractory metals will gradually evaporate. Pay attention to the old burnt out light bulb. The surface of its glass container is covered from the inside with a dark metallic coating. Where could he come from? Indeed, in the lamp there is only a thread made of a very refractory and resistant tungsten metal. Analysis shows that this plaque consists of tungsten, which evaporated when the filament was heated and settled on the cold surface of the glass container, just like water vapor, falling on a cold surface, condenses and the surface fogs up.

At high temperatures, metals evaporate in the same way as water or ether at room temperature. Of course, a very high temperature is needed for evaporation to be noticeable.

Relatively easy volatile metals are zinc, magnesium, chromium and some others. Yes, steam pressure

1 10 "" 2 mm Hg is achieved for zinc at 350 °, magnesium at 439 °, chromium at 917 °. At the same time, iron at 750° has a vapor pressure of only 1 x 10~8 mm of mercury, while tungsten has the same vapor pressure at temperatures above 2100°.

The possibility of evaporation of metals in vacuum is widely used in modern technology. This property is used to apply protective coatings of metallic chromium to the surface of metals. Who among you has not admired the silvery sheen of the coating of car parts that do not fade in the rain and in the sun, durable and beautiful. This coating is a thin film of metallic chromium.

The chromium film can be applied by electrolysis, but the use of vacuum has contributed to the expansion of the use of so-called thermochromizing. With this method, parts and crushed chromium with certain additives are placed in a furnace. The furnace is filled with chlorine gas, then heating is started. The chlorine is taken up by the additives and a vacuum is created in the oven. Chromium begins to evaporate and deposit a thin layer on the surface of the parts.

The vacuum method of thermochromizing simplifies the preparation of parts for coating, reduces the consumption of chromium, and simplifies equipment. When high purity metal is needed, vacuum helps to remove traces of impurities of various substances, for example, in magnetic, heat-resistant, stainless steels. A high vacuum is needed to remove volatile impurities (lead, cadmium, bismuth) from copper.

To obtain pure volatile metals, melting and distillation in high vacuum are used. Just as alcohol is distilled in order to increase its strength and separate it from impurities, they distill, for example, mercury, zinc, cadmium, and sometimes magnesium.

Even silicic acid, which makes up such a seemingly stable material as quartz sand, evaporates noticeably in a high vacuum. And chromium is so volatile in a high vacuum that it evaporates intensely before it melts.

Vacuum distillation produces extremely pure metals. It is possible to obtain aluminum that is purer than by electrolysis, with an iron content of less than one thousandth of a percent. It is known that aluminum is easily oxidized in air, the more active is the aluminum film obtained by distillation, and only a high vacuum protects the metal from oxidation. The role of vacuum is the same in the melting of molybdenum. Only in a furnace with high vacuum was it possible to melt without oxidation this refractory metal, melting at temperatures above 2600 ° C.

The use of vacuum in metallurgy has led to the development of techniques for obtaining vacuum in large volumes and with high speed. Increasing the performance of the pumps makes it possible to accommodate ever larger equipment in the evacuated space.

At present, furnaces have already been created for the simultaneous melting of 1 ton of steel at a vacuum of 1-10 "2-

1 1SG3 mm Hg.

Molding and casting under vacuum give very precise castings.

For the use of vacuum in metallurgy, oil diffusion pumps with an inlet diameter of 80 cm and a pumping speed of 14,000 liters were built! sec, at a theoretical speed of up to 60,000 liters! sec.

Even a cursory review of the use of vacuum in metallurgy shows that this most important branch of technology makes extensive use of the ability to control the properties of the gaseous medium surrounding the metal at all stages of its "life" from melting to processing. The prospects here are even wider. Powerful vacuum units will soon become as much an integral part of a steel plant as blowers are for supplying air to furnaces.

The process of boiling water is quite interesting and at the same time a very complex process. Boiling is the process by which a substance (in this case water) changes from a liquid state to a gaseous state. For water to boil, you need a suitable temperature, otherwise the process will not start. Under normal conditions, the boiling point of water is 100 degrees Celsius. It is at this temperature that water will begin to turn into a gas.

How water boils

As soon as the water reaches 100 degrees, the liquid will begin to turn into steam. To make it easier to imagine the whole transformation process, fill a small metal saucepan with water and put it on fire. Here's what will happen:

• the water in the pot will start to heat up;
• when the water temperature reaches 100 degrees, bubbles with steam will begin to form at the very bottom of the pan;
• reaching the surface, these bubbles burst, releasing steam to freedom;
• the amount of water in the pan will gradually decrease.

Thus, after some certain time, the water in the pan will completely disappear, turning into steam. By the way, do not confuse boiling and evaporation, these processes differ from each other. Evaporation can occur at any temperature, while boiling only at a certain temperature. Also, the process of boiling occurs throughout the liquid, and during evaporation, water turns into steam, starting from the surface of the water. As it evaporates, the liquid will gradually cool.

What other conditions affect the boiling process

In fact, boiling can occur at lower or higher temperatures than 100 degrees. In addition to temperature, pressure is equally important. So, for example, if we start to climb mountains, the pressure will decrease, and therefore the boiling point will decrease. If we go down into a deep mine, the pressure will increase, hence the boiling point will also increase. In addition to pressure, it is also important that the water is constantly heated, otherwise the temperature will drop and the process will stop.