What is boiling? Specific heat of vaporization

What is boiling is known from the school curriculum. Nevertheless, this knowledge quickly disappears, and gradually people stop paying attention to the essence of the usual phenomena. Sometimes it is useful to recall theoretical knowledge.

Definition

What is boiling? This is a physical process, during which intense vaporization occurs both on the free surface of the liquid and inside its structure. One of the signs of boiling is the formation of bubbles, which consist of saturated vapor and air.

It is worth noting the existence of such a concept as the boiling point. The rate of steam formation also depends on the pressure. It must be permanent. Typically, the main characteristic of liquid chemicals is the boiling point at normal atmospheric pressure. Nevertheless, factors such as the intensity of sound waves, ionization of air, can also influence this process.

The stages of boiling water

The steam begins to form during such a procedure as heating. Boiling involves the passage of liquid through 4 stages:

1. At the bottom of the vessel, as well as on its walls, small bubbles begin to form. This is the result of the fact that the cracks in the material from which the container is made contain air that expands under the influence of high temperature.
2. Bubbles begin to increase in volume, as a result of which they break out onto the surface of the water. If the upper layer of the liquid has not yet reached the boiling point, the cavities sink to the bottom, after which they again begin to tilt upward. This process leads to the formation of sound waves. That is why during the boiling of water we can hear the noise.
3. The largest number of bubbles come to the surface, which creates the impression of turbidity of water. After that, the liquid turns pale. Given the visual effect, this stage of boiling is called a "white key".
4. There is intense turbulence, which is accompanied by the formation of large bubbles, which quickly burst. This process is accompanied by the appearance of splashes, as well as intensive formation of steam.

Specific heat of vaporization

Practically every day we encounter such phenomena as boiling. Specific heat of vaporization is a physical quantity that determines the amount of heat. With its help, the liquid substance can be converted to steam. In order to calculate this parameter, it is necessary to divide the heat value of evaporation by mass.

How is the measurement

The indicator of specific heat of formation is measured in laboratory conditions by carrying out appropriate experiments. They include the following:

• The required amount of liquid is measured, which is then poured into the calorimeter;
• The initial measurement of the water temperature;
• A flask with a test substance placed in it is installed on the burner;
• The vapor emitted by the test substance is launched into the calorimeter;
• Repeated water temperature measurement;
• The calorimeter is weighed, which makes it possible to calculate the mass of the condensed vapor.

Bubble boiling mode

Understanding the question of what is boiling, it is worth noting that it has several modes. Thus, when heated, the vapor can form in the form of bubbles. They periodically grow and burst. Such a boiling regime is called bubbly. Usually cavities filled with steam are formed just at the walls of the vessel. This is due to the fact that they are usually overheated. This is a necessary condition for boiling, because otherwise the bubbles will collapse, not reaching large sizes.

Film boiling mode

What is boiling? The easiest way to explain this process is as vaporization at a certain temperature and constant pressure. In addition to the bubble regime, a film is also distinguished. Its essence is that when the heat flow increases, individual bubbles unite, forming a vapor layer on the walls of the vessel. When they reach a critical level, they break through to the surface of the water. This boiling mode is different in that the degree of heat transfer from the walls of the vessel to the liquid itself is significantly reduced. The reason for this is the same steam film.

Boiling temperature

It should be noted that there is a dependence of the boiling point on the pressure that appears on the surface of the heated liquid. So, it is commonly believed that water boils when heated to 100 degrees Celsius. Nevertheless, this indicator can be considered valid only if the atmospheric pressure index is considered normal (101 kPa). If it increases, the boiling point also changes in the direction of increase. For example, in popular saucepans-pressure cookers, the pressure is approximately 200 kPa. Thus, the boiling point rises by 20 points (up to 20 degrees).

An example of low atmospheric pressure can be considered mountainous areas. So, given that there it is small enough, the water begins to boil at a temperature of about 90 degrees. Residents of such areas have to spend much more time preparing food. So, for example, to boil an egg, you have to heat water no less than 100 degrees, otherwise the protein will not curdle.

The boiling point of the substance depends on the saturated vapor pressure index. Its influence on temperature is inversely proportional. For example, mercury boils when heated to 357 degrees Celsius. This can be explained by the fact that the saturated vapor pressure is only 114 Pa (for water this figure is 101 325 Pa).

Boiling in different conditions

Depending on the conditions and the state of the liquid, the boiling point may differ substantially. For example, it is worth adding salt to the liquid. Ions of chlorine and sodium are placed between the water molecules. Thus, boiling requires an order of magnitude more energy, and accordingly - of time. In addition, such water forms much less steam.

The kettle is used for boiling water in domestic conditions. If a clean liquid is used, then the process temperature is standard 100 degrees. Under similar conditions, distilled water boils. Nevertheless, a little less time will be spent if we take into account the absence of foreign impurities.

What distinguishes boiling from evaporation

Whenever the boiling of water occurs, the steam is released into the atmosphere. But these two processes can not be identified. They are only ways of vaporization, which occurs under certain conditions. So, boiling is a first-order phase transition. This process is more intense than evaporation. This is due to the formation of steam foci. It is also worth noting that the evaporation process occurs exclusively on the water surface. The boiling point concerns the entire volume of the liquid.

What does evaporation depend on?

Evaporation is the process of converting a liquid or solid substance into a gaseous state. There is a "flight" of atoms and molecules, the connection of which with the rest of the particles is weakened under the influence of certain conditions. The rate of evaporation can be influenced by the following factors:

• The surface area of the liquid;
• The temperature of the substance itself, as well as the environment;
• The velocity of the molecules;
• Kind of substance.

Interesting facts about boiling

The energy of boiling water is widely used by man in everyday life. This process has become so commonplace and familiar that no one thinks about its nature and features. Nevertheless, a number of interesting facts are connected with the boiling:

• Probably everyone noticed that there is a hole in the lid of the kettle, but few people think about its purpose. It is done for the purpose of partially releasing steam. Otherwise, water can splash through the spout.
• The duration of cooking potatoes, eggs and other food products does not depend on how powerful the heater is. It matters only the fact how long they were under the influence of boiling water.
• At a rate such as the boiling point, does not affect the power of the heating device. It can only affect the rate of evaporation of the liquid.
• Boiling is associated not only with the heating of water. With this process, you can also make the liquid freeze. Thus, during the boiling process, continuous evacuation of air from the vessel must be performed.
• One of the most pressing problems for housewives is that milk can "run away". Thus, the risk of this phenomenon increases significantly during weather deterioration, which is accompanied by a fall in atmospheric pressure.
• The hottest boiling water is obtained in deep underground mines.
• Through experimental research, scientists were able to establish that on Mars water boils at a temperature of 45 degrees Celsius.

Can water boil at room temperature?

By simple calculations scientists have found that water can boil at room temperature at the level of the stratosphere. Similar conditions can be reconstructed using a vacuum pump. Nevertheless, similar experience can be conducted in more simple, mundane conditions.

In a liter flask you need to boil 200 ml of water, and when the container is filled with steam, it must be tightly closed, removed from the fire. Putting it above the crystallizer, you need to wait until the boiling process ends. Then flask poured cold water. After that, the vessel will again begin intense boiling. This is due to the fact that under the influence of low temperature, the vapor at the top of the bulb is lowered.