Secrets of boiling salt and fresh water. Why does salt water boil faster than ordinary water, is it true?

Boiling is the process of vaporization that occurs when a liquid is brought to its boiling point. Everyone knows from the school desk that water boils at t = 100˚С. But many are interested in the question, which water boils faster: salty or fresh?

What is the boiling process

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 are on top. This phenomenon is explained by an increase in temperature at which the pressure on the bubbles increases. Thanks to the Archimedean force, they end up on the surface. If it did not manage to warm up to the boiling point (100˚С), then the bubbles again go to the bottom, where the water is hotter. The boiling noise is generated by increasing and decreasing bubble size.
• 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 bubbling and the appearance of large bubbles, which, when bursting, create splashes. The latter say that the water has boiled over. Water vapor is generated, and the water emits sounds characteristic of boiling.

Boiling fresh water

Water brought to a boil is called boiling water. During this process, abundant vapor formation occurs, which is accompanied by the release of free oxygen molecules from the composition of 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 use it raw.

Fresh but hard water contains salt. During the boil, they form a deposit on the walls of the kettle, which is often called scale. Boiling water is commonly used to prepare hot drinks or to 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 chloride and sodium ions, which are found among the water molecules. A process of hydration occurs between them - the attachment of water molecules to salt ions.

It is worth noting that the hydration bond is much stronger than the aqueous intermolecular bond. Therefore, during the boiling of fresh water, the vaporization process begins faster. A liquid with salts dissolved in it requires a little more energy to boil, which in this situation is the temperature.

When it rises, the molecules in the salt water move much faster, but their number decreases, which means they collide less often. This can explain the smaller amount of steam - after all, its pressure is lower than that of fresh water. To achieve a pressure in salt water above atmospheric pressure and the beginning of boiling, a higher temperature is required.

Another justification

When preparing food, many housewives salt water at the beginning of the process, arguing that this way it will boil faster. And some find an explanation for why salt water boils faster, based on school knowledge of a physics course, namely, the topic of heat transfer. As you know, there are three types of heat transfer: heat transfer, 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. Moreover, it takes more time for it to freeze. Consequently, with a denser liquid, heat transfer will be more active, and boiling will occur faster.

Boiling water under reduced pressure: Video

Many housewives, trying to speed up the cooking process, salt water immediately after placing the pot on the stove. They firmly believe that they are doing the right thing, and are ready to bring many arguments in their defense. Is this really so and which water boils faster - salty or fresh? To do this, it is not at all necessary to set up experiments in laboratory conditions; it is enough to dispel the myths that have been reigning in our kitchens for decades with the help of the laws of physics and chemistry.

Common myths about boiling water

In the matter of boiling water, people can be conditionally divided into two categories. The former are convinced that salt water boils much faster, while the latter absolutely disagree with this statement. The arguments for bringing salt water to a boil take less time, the following arguments are made:

• the density of the water in which the salt is dissolved is much higher, therefore the heat transfer from the burner is greater;
• during dissolution in water, the crystal lattice of table salt is destroyed, which is accompanied by the release of energy. That is, if you add salt to cold water, the liquid will automatically become warmer.

Those who refute the hypothesis that salt water boils faster argue this: during the dissolution of salt in water, a hydration process takes place.

At the molecular level, stronger bonds are formed, which require more energy to break. Therefore, it takes longer for the salt water to boil.

Who is right in this debate, and is it really so important to salt water at the very beginning of cooking?

Boiling process: physics "on the fingers"

To understand what exactly happens to salt and fresh water when heated, you need to understand what the boiling process is. Regardless of whether the water is salty or not, it boils in the same way and goes through four stages:

• the formation of small bubbles on the surface;
• an increase in bubbles in volume and their settling at the bottom of the container;
• turbidity of water caused by the intensive movement of air bubbles up and down;
• directly the boiling process, when large bubbles rise to the surface of the water and burst with noise, releasing steam - the air that is inside and heats up.

The theory of heat transfer, to which advocates of salting water at the beginning of cooking, appeal, in this case, "works", but the effect of heating water due to its density and heat release during the destruction of the crystal lattice is insignificant.

Much more important is the process of hydration, in which stable molecular bonds are formed.

The stronger they are, the more difficult it is for an air bubble to rise to the surface and sink to the bottom of the container, which takes more time. As a result, if salt is added to the water, then the circulation of air bubbles slows down. Consequently, salt water boils more slowly, since molecular bonds hold air bubbles in salt water a little longer than in fresh water.

To salt or not to salt? That's the question

Kitchen disputes about which water boils salty or unsalted faster can be endless. As a result, from the point of view of practical application, there is not much difference whether you salted the water at the very beginning or after it has boiled. Why doesn't it really matter? To understand the situation, you need to turn to physics, which provides comprehensive answers to this seemingly difficult question.

Everyone knows that at a standard atmospheric pressure of 760 mm Hg, water boils at 100 degrees Celsius. Temperature parameters can change subject to changes in air density - everyone knows that in the mountains, water boils at a lower temperature. Therefore, when it comes to the domestic aspect, in this case, an indicator such as the intensity of combustion of a gas burner or the degree of heating of an electric kitchen surface is much more important.

The process of heat exchange depends on this, that is, the rate of heating of the water itself. And, accordingly, the time spent for it to boil.

For example, over an open fire, if you decide to cook dinner over a campfire, the water in the pot will boil in a matter of minutes due to the fact that the wood, when burned, emits more heat than the gas in the stove, and the surface heating area is much larger. Therefore, it is not at all necessary to salt the water in order for it to boil faster - it is enough to turn on the burner of the stove to the maximum.

The boiling point of salt water is exactly the same as that of fresh and distilled water. That is, it is 100 degrees at normal atmospheric pressure. But the boiling rate under equal conditions (for example, if the usual burner of a gas stove is taken as a basis) will differ. It will take longer for the salt water to boil because it is harder for air bubbles to break stronger molecular bonds.

By the way, there is a difference in boiling time between tap and distilled water - in the second case, the liquid without impurities and, accordingly, without "heavy" molecular bonds, will heat up faster.

True, the time difference is only a few seconds, which does not make the weather in the kitchen and practically does not affect the speed of cooking. Therefore, you need to be guided not by the desire to save time, but by the laws of cooking, prescribing to salt each dish at a certain moment in order to preserve and enhance its taste.

Boiling is the process of changing the state of aggregation of a substance. When we talk about water, we mean a 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. Also, not to be confused with boiling, which is the process of heating water to a certain temperature. Now that we have figured out the concepts, we can determine at what temperature the water boils.

Process

The very process of converting the state of aggregation from liquid to gaseous is complex. And although people don't 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 in the container where the water is heated.
2. In the second stage, the volume of the bubbles increases. All of them begin to tear to the surface, since they contain saturated steam, 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 known force of Archimedes. At the same time, you can hear the characteristic boiling sound, which is formed due to the constant expansion and decrease 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 a cloudy water. This process is popularly called "boiling with a white key", and it lasts for 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, splashing means 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, it, like other gases, puts a certain pressure on it. During vaporization, the temperatures of steam and water remain 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 evaporation of all the liquid, becomes dry. If its temperature begins to exceed the temperature of water, then such steam is superheated, and by its characteristics it will be closer to gas.

Salt water boiling

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 a region 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 attaching water molecules to salt ions. The bond between fresh water molecules is weaker than those formed during hydration, so the boiling of a liquid with dissolved salt will take longer. As the temperature rises, the molecules in the salt-containing water move faster, but there are fewer of them, which makes collisions between them less frequent. As a result, less steam is generated, and its pressure is therefore lower than the steam pressure of fresh water. Consequently, more energy (temperature) is required for full steam generation. On average, to boil one liter of water containing 60 grams of salt, it is necessary to raise the boiling degree of water by 10% (that is, by 10 C).

Boiling pressure versus pressure

It is known that in the mountains, regardless of the chemical composition of the water, the boiling point will be lower. This is due to the fact that the atmospheric pressure is lower at altitude. Pressure with a value of 101.325 kPa is considered normal. With it, the boiling point of water is 100 degrees Celsius. But if you go up the 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 have to spend almost half the time. For thermal processing of products, a certain temperature is required.

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 temperature will be 90 C.

Note that this law also works in the opposite direction. If you place a liquid in a closed flask through which steam cannot pass, then with an increase in temperature and the formation of steam, the pressure in this flask will increase, and boiling at an 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. 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 the teapot

Often people are interested in the temperature at which water boils in a kettle, since it is these devices that they use to boil liquid. 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 temperature 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 fluid composition affect this process. There is nothing difficult in this, and children receive such information at school. The main thing is to remember 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, which 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.

Why is it easier to swim in salt water than in fresh water?

It is easier to swim in salt water than in fresh water, because salt makes the water heavier: if you take two cylinders of the same capacity, one with salt water and the other fresh water, then a salt water cylinder will weigh a little more. And the higher the density (weight) of the water, the easier it is to swim in it.

An object can float in a liquid if its weight is equal to the weight of the water it displaces or pushes out (water is forced out to make room for the object). You can look at it from the other side: when you sit down in the bath, you see that the water level in it rises. If you knock down the water that your body has displaced, the weight of this water will be equal to the weight of your body. If the water has a higher density, like salt water, then your body will displace less of it (i.e. less water is required to equalize with your body weight), and you will be higher when you float than if you floated in fresh water.

In the first glass, ordinary fresh water, in the second - salty,
in the third it is very salty.

Which keeps you warm better: fresh or salt water?

Two vessels were filled with fresh water. They were heated for about 10 minutes. Then 2 tablespoons of salt was added to one of the vessels and the label "salt water" was pasted on it. On the first try, no significant difference was noticed, the temperature was 120 degrees. On the second try, 2 more tablespoons of salt was added and the difference became noticeable. Salt water cooled significantly faster than regular tap water. As part of the experiment, the amount of salt in the water was monitored. When the water temperature reached 90 degrees, data collection began. The same thermometers were used in the experiment.

Why is the water in the ocean salty?

Salt from the Earth's surface is constantly dissolving and entering the ocean.
If all the oceans were dried out, the remaining salt could be used to build a wall 230 km high and almost 2 km thick. Such a wall would be able to circle the entire globe along the equator. Or another comparison. The salt of all the dried up oceans is 15 times the volume of the entire European continent!
Ordinary salt is obtained from seawater, salt sources or from the development of rock salt deposits. Sea water contains 3-3.5% salt. Inland seas such as the Mediterranean Sea and the Red Sea contain more salt than the open seas. Dead Sea, occupying only 728 sq. km., contains approximately 10 523 million tons of salt.
On average, a liter of sea water contains about 30 g of salt. Rock salt deposits in various parts of the earth were formed many millions of years ago as a result of the evaporation of sea water. For the formation of rock salt, it is necessary that nine-tenths of the volume of sea water evaporate; It is believed that inland seas were located at the site of modern deposits of this salt. They evaporated faster than new sea water entered - and that is how rock salt deposits appeared.
The main amount of edible salt is extracted from rock salt. Usually, mines are laid to the salt deposits. Clean water is pumped through the pipes, which dissolves the salt. Through the second pipe, this solution rises to the surface.

Why does fresh water boil faster than salty water?

Salt water boils at a higher temperature than fresh water, therefore, under the same heating conditions, fresh water will boil faster, salty water will boil later. There is a whole physicochemical theory why this is the case, "on the fingers" it can be explained as follows. Water molecules bind with salt ions - a hydration process takes place. The bond between water molecules is weaker than the bond formed by hydration. Therefore, a molecule of fresh water is easier (at a lower temperature) to detach from its "environment" - ie. roughly speaking evaporates. And in order for a water molecule with a dissolved salt to "break free from the embrace" of salt and other water molecules, more energy is required, i.e. high temperature.

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 that it can happen only at a certain pressure and temperature.

Boiling - heating water to the boiling point.

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

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

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

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

As the temperature rises, the pressure of the saturated bubbles increases, as a result of which they 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 of pure water without impurities), then the bubbles descend 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 appear inside the vessel, which create the noise characteristic of boiling.

In 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, the water begins to boil intensively, large bursting bubbles and splashes appear (as a rule, splashes mean that the water has boiled over too much).

Water vapor begins to form from the water, and 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.

During 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 vapor. Also steam can be saturated wet and overheated.

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

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

Boiling point of salt water ^

The boiling point of salt water is higher than the boiling point of fresh water... Consequently salt 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 - this process is 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 rise above atmospheric pressure and the boiling process to begin, a higher temperature is required. When 60 grams of salt is added to 1 liter water, the boiling point will increase by 10 C.