Specific heat capacity of water at different temperatures. Thermophysical properties of water vapor: density, heat capacity, thermal conductivity

The table shows the thermophysical properties of water vapor on the saturation line depending on temperature. The properties of steam are given in the table in the temperature range from 0.01 to 370°C.

Each temperature corresponds to the pressure at which water vapor is in a state of saturation. For example, at a water vapor temperature of 200°C, its pressure will be 1.555 MPa or about 15.3 atm.

The specific heat capacity of steam, thermal conductivity, and steam increase as the temperature rises. The density of water vapor also increases. Water vapor becomes hot, heavy and viscous, with a high specific heat capacity, which has a positive effect on the choice of steam as a coolant in some types of heat exchangers.

For example, according to the table, specific heat water vapor C p at a temperature of 20°C it is 1877 J/(kg deg), and when heated to 370°C, the heat capacity of steam increases to a value of 56520 J/(kg deg).

The table shows the following thermophysical properties of water vapor on the saturation line:

• vapor pressure at specified temperature p·10 -5, Pa;
• vapor density ρ″ , kg/m 3 ;
• specific (mass) enthalpy h″, kJ/kg;
• r, kJ/kg;
• specific heat capacity of steam C p, kJ/(kg deg);
• coefficient of thermal conductivity λ·10 2, W/(m deg);
• thermal diffusivity coefficient a·10 6, m 2 /s;
• dynamic viscosity μ·10 6, Pa·s;
• kinematic viscosity ν·10 6, m 2 /s;
• Prandtl number Pr.

The specific heat of vaporization, enthalpy, thermal diffusivity and kinematic viscosity of water vapor decrease with increasing temperature. The dynamic viscosity and Prandtl number of the steam increase.

Be careful! Thermal conductivity in the table is indicated to the power of 10 2. Don't forget to divide by 100! For example, the thermal conductivity of steam at a temperature of 100°C is 0.02372 W/(m deg).

Thermal conductivity of water vapor at various temperatures and pressures

The table shows the thermal conductivity values ​​of water and water vapor at temperatures from 0 to 700°C and pressure from 0.1 to 500 atm. Thermal conductivity dimension W/(m deg).

The line under the values ​​in the table means the phase transition of water into steam, that is, the numbers below the line refer to steam, and those above it refer to water. According to the table, it can be seen that the value of the coefficient and water vapor increases as the pressure increases.

Note: thermal conductivity in the table is indicated in powers of 10 3. Don't forget to divide by 1000!

Thermal conductivity of water vapor at high temperatures

The table shows the thermal conductivity values ​​of dissociated water vapor in the dimension W/(m deg) at temperatures from 1400 to 6000 K and pressure from 0.1 to 100 atm.

According to the table, the thermal conductivity of water vapor at high temperatures increases noticeably in the region of 3000...5000 K. At high values pressure, the maximum thermal conductivity coefficient is achieved at higher temperatures.

Be careful! Thermal conductivity in the table is indicated to the power of 10 3. Don't forget to divide by 1000!

Water is one of the most amazing substances. Despite its widespread and widespread use, it is a real mystery of nature. Being one of the oxygen compounds, water, it would seem, should have very low characteristics such as freezing, heat of vaporization, etc. But this does not happen. The heat capacity of water alone is, despite everything, extremely high.

Water is capable of absorbing a huge amount of heat, while practically not heating up - this is its physical feature. water is approximately five times higher than the heat capacity of sand, and ten times higher than that of iron. Therefore, water is a natural coolant. Its ability to accumulate a large number of energy makes it possible to smooth out temperature fluctuations on the Earth’s surface and regulate the thermal regime throughout the entire planet, and this happens regardless of the time of year.

This unique property water allows it to be used as a coolant in industry and at home. In addition, water is a widely available and relatively cheap raw material.

What is meant by heat capacity? As is known from the course of thermodynamics, heat transfer always occurs from a hot to a cold body. In this case, we are talking about the transfer of a certain amount of heat, and the temperature of both bodies, being a characteristic of their state, shows the direction of this exchange. In the process of a metal body with water of equal mass at the same initial temperatures, the metal changes its temperature several times more than water.

If we take as a postulate the basic statement of thermodynamics - of two bodies (isolated from the others), during heat exchange one gives off and the other receives an equal amount of heat, then it becomes clear that metal and water have completely different heat capacities.

Thus, the heat capacity of water (as well as of any substance) is an indicator characterizing the ability of a given substance to give (or receive) something when cooling (heating) per unit temperature.

The specific heat capacity of a substance is the amount of heat required to heat a unit of this substance (1 kilogram) by 1 degree.

The amount of heat released or absorbed by a body is equal to the product of specific heat capacity, mass and temperature difference. It is measured in calories. One calorie is exactly the amount of heat that is enough to heat 1 g of water by 1 degree. For comparison: the specific heat capacity of air is 0.24 cal/g ∙°C, aluminum - 0.22, iron - 0.11, mercury - 0.03.

The heat capacity of water is not constant. As the temperature increases from 0 to 40 degrees, it decreases slightly (from 1.0074 to 0.9980), while for all other substances this characteristic increases during heating. In addition, it can decrease with increasing pressure (at depth).

As you know, water has three states of aggregation - liquid, solid (ice) and gaseous (steam). At the same time, the specific heat capacity of ice is approximately 2 times lower than that of water. This is the main difference between water and other substances, the specific heat capacity of which does not change in the solid and molten states. What's the secret?

The fact is that ice has a crystalline structure, which does not immediately collapse when heated. Water contains small ice particles consisting of several molecules called associates. When water is heated, part of it is spent on the destruction of hydrogen bonds in these formations. This explains the unusually high heat capacity of water. The bonds between its molecules are completely destroyed only when water transforms into steam.

The specific heat capacity at a temperature of 100° C is almost no different from that of ice at 0° C. This once again confirms the correctness of this explanation. The heat capacity of steam, like the heat capacity of ice, is currently much better studied than water, regarding which scientists have not yet reached a consensus.

Today we will talk about what heat capacity is (including water), what types it comes in, and where this physical term is used. We will also show how useful the value of this value is for water and steam, why you need to know it and how it affects our daily life.

The concept of heat capacity

This physical quantity it is used so often in the outside world and science that first of all we need to talk about it. The very first definition will require the reader to have some preparedness, at least in differentials. So, the heat capacity of a body is defined in physics as the ratio of increments of an infinitesimal amount of heat to the corresponding infinitesimal amount of temperature.

Quantity of heat

Almost everyone understands what temperature is, one way or another. Let us recall that “amount of heat” is not just a phrase, but a term denoting the energy that a body loses or gains in exchange with the environment. This value is measured in calories. This unit is familiar to all women who are on diets. Dear ladies, now you know what you burn on the treadmill and what each piece of food you eat (or leave on your plate) is worth. Thus, any body whose temperature changes experiences an increase or decrease in the amount of heat. The ratio of these quantities is the heat capacity.

Application of heat capacity

However, a strict definition of the physical concept we are considering is rarely used on its own. We said above that it is very often used in Everyday life. Those who didn’t like physics at school are probably perplexed now. And we will lift the veil of secrecy and tell you that hot (and even cold) water in the tap and in heating pipes appears only thanks to heat capacity calculations.

Weather conditions, which determine whether the swimming season can already be opened or whether it is worth staying on the shore for now, also take this value into account. Any device associated with heating or cooling (oil radiator, refrigerator), all energy costs when preparing food (for example, in a cafe) or street soft-serve ice cream are influenced by these calculations. As you can understand, we are talking about such a quantity as the heat capacity of water. It would be foolish to assume that this is done by sellers and ordinary consumers, but engineers, designers, and manufacturers took everything into account and put the appropriate parameters into household appliances. However, heat capacity calculations are used much more widely: in hydraulic turbines and cement production, in testing alloys for aircraft or railways, in construction, smelting, and cooling. Even space exploration relies on formulas containing this value.

Types of heat capacity

So, in all practical applications use relative or specific heat capacity. It is defined as the amount of heat (note, no infinitesimal quantities) required to heat a unit amount of a substance by one degree. The degrees on the Kelvin and Celsius scales are the same, but in physics it is customary to call this value in the first units. Depending on how the unit of quantity of a substance is expressed, mass, volume and molar specific heat capacities are distinguished. Recall that one mole is an amount of substance that contains approximately six to ten to the twenty-third power molecules. Depending on the task, the corresponding heat capacity is used; their designation in physics is different. Mass heat capacity is designated as C and is expressed in J/kg*K, volumetric heat capacity is C` (J/m 3 *K), molar heat capacity is C μ (J/mol*K).

Ideal gas

If the problem of an ideal gas is being solved, then the expression for it is different. Let us recall that in this substance, which does not exist in reality, the atoms (or molecules) do not interact with each other. This quality radically changes any properties of an ideal gas. That's why traditional approaches calculations will not give the desired result. An ideal gas is needed as a model to describe electrons in a metal, for example. Its heat capacity is defined as the number of degrees of freedom of the particles of which it is composed.

State of aggregation

It seems that for a substance all physical characteristics are the same under all conditions. But that's not true. When transitioning to another state of aggregation (during melting and freezing of ice, evaporation or solidification of molten aluminum), this value changes abruptly. Thus, the heat capacity of water and water vapor are different. As we will see below, significantly. This difference greatly affects the use of both the liquid and gaseous components of this substance.

Heating and heat capacity

As the reader has already noticed, most often in real world the heat capacity of water appears. She is the source of life, without her our existence is impossible. A person needs it. Therefore, from ancient times to the present, the task of delivering water to homes and industries or fields has always been a challenge. Good for those countries that have all year round positive temperature. The ancient Romans built aqueducts to supply their cities with this valuable resource. But where there is winter, this method would not be suitable. Ice, as is known, has a larger specific volume than water. This means that when it freezes in pipes, it destroys them due to expansion. Thus, before the engineers central heating and delivery hot and cold water The challenge at home is how to avoid this.

The heat capacity of water, taking into account the length of the pipes, will give the required temperature to which the boilers must be heated. However, our winters can be very cold. And at one hundred degrees Celsius, boiling already occurs. In this situation, the specific heat capacity of water vapor comes to the rescue. As noted above, the state of aggregation changes this value. Well, the boilers that bring heat to our homes contain highly superheated steam. Because it has a high temperature, it creates incredible pressure, so the boilers and the pipes leading to them must be very durable. In this case, even a small hole or a very small leak can lead to an explosion. The heat capacity of water depends on temperature, and nonlinearly. That is, heating it from twenty to thirty degrees will require a different amount of energy than, say, from one hundred fifty to one hundred sixty.

For any actions that involve heating water, this should be taken into account, especially if we are talking about large volumes. The heat capacity of steam, like many of its properties, depends on pressure. At the same temperature as the liquid state, the gaseous state has almost four times less heat capacity.

Above we gave many examples of why it is necessary to heat water and how it is necessary to take into account the magnitude of the heat capacity. However, we have not yet told you that among all the available resources of the planet, this liquid has enough high rate energy costs for heating. This property is often used for cooling.

Since the heat capacity of water is high, it will effectively and quickly absorb excess energy. This is used in production, in high-tech equipment (for example, in lasers). And at home we probably know that the most effective method cool hard-boiled eggs or a hot frying pan - rinse under cold running tap.

And the operating principle of atomic nuclear reactors is generally based on the high heat capacity of water. The Hot Zone, as the name suggests, has incredible high temperature. By heating itself, the water cools the system, preventing the reaction from getting out of control. Thus, we receive the necessary electricity (heated steam rotates the turbines), and no catastrophe occurs.

In this short article we will briefly consider one of the most important properties of water for our planet, its Heat capacity.

Specific heat capacity of water

Let's make a brief interpretation of this term:

Heat capacity a substance is its ability to accumulate heat. This value is measured by the amount of heat absorbed by it when heated by 1°C. For example, the heat capacity of water is 1 cal/g, or 4.2 J/g, and the heat capacity of soil at 14.5-15.5°C (depending on soil type) ranges from 0.5 to 0.6 cal (2 .1-2.5 J) per unit volume and from 0.2 to 0.5 cal (or 0.8-2.1 J) per unit mass (grams).

The heat capacity of water has a significant impact on many aspects of our lives, but in this material we will focus on its role in the formation temperature regime of our planet, namely...

Heat capacity of water and Earth's climate

Heat capacity water in its absolute value is quite large. From the above definition we see that it significantly exceeds the heat capacity of the soil of our planet. Due to this difference in heat capacity, the soil, compared to the waters of the world's oceans, heats up much faster and, accordingly, cools down faster. Thanks to the more inert oceans, fluctuations in the Earth's daily and seasonal temperatures are not as great as they would be in the absence of oceans and seas. That is, in the cold season, water warms the Earth, and in the warm season it cools. Naturally, this influence is most noticeable in coastal areas, but in global average terms it affects the entire planet.

Naturally, fluctuations in daily and seasonal temperatures are influenced by many factors, but water is one of the most important.

An increase in the amplitude of fluctuations in daily and seasonal temperatures would radically change the world around us.

For example, everyone is fine known fact— stone loses its strength and becomes brittle during sharp temperature fluctuations. Obviously, we ourselves would be “somewhat” different. At a minimum, the physical parameters of our body would be different.

Anomalous properties of the heat capacity of water

The heat capacity of water has anomalous properties. It turns out that as the temperature of water increases, its heat capacity decreases; this dynamics persists up to 37°C; with a further increase in temperature, the heat capacity begins to increase.

This fact contains one interesting statement. Relatively speaking, nature itself, in the person of Water, determined 37°C as the most comfortable temperature for the human body, provided, of course, that all other factors are met. For any temperature change dynamics environment The water temperature tends to 37°C.

Enthalpy is a property of a substance that indicates the amount of energy that can be converted into heat.

Enthalpy is a thermodynamic property of a substance that indicates energy level, preserved in its molecular structure. This means that although a substance may have energy based on , not all of it can be converted into heat. Part internal energy always remains in the substance and maintains its molecular structure. Some of the substance is inaccessible when its temperature approaches the ambient temperature. Hence, enthalpy is the amount of energy that is available to be converted into heat at a certain temperature and pressure. Enthalpy units- British thermal unit or joule for energy and Btu/lbm or J/kg for specific energy.

Enthalpy quantity

Quantity enthalpy of matter based on its given temperature. This temperature- this is the value that is chosen by scientists and engineers as the basis for calculations. It is the temperature at which the enthalpy of a substance is zero J. In other words, the substance has no available energy that can be converted into heat. This temperature is different for different substances. For example, this temperature of water is the triple point (0 °C), nitrogen is -150 °C, and refrigerants based on methane and ethane are -40 °C.

If the temperature of a substance is higher than its given temperature or changes state to gaseous state at a given temperature, enthalpy is expressed as a positive number. Conversely, at a temperature below this, the enthalpy of a substance is expressed as a negative number. Enthalpy is used in calculations to determine the difference in energy levels between two states. This is necessary to configure the equipment and determine useful action process.

Enthalpy often defined as total energy of matter, since it is equal to the sum of its internal energy (u) in this state along with his ability to get the job done (pv). But in reality, enthalpy does not indicate the total energy of a substance at a given temperature above absolute zero(-273°C). Therefore, instead of defining enthalpy as the total heat of a substance, it is more accurately defined as the total amount of available energy of a substance that can be converted into heat.
H = U + pV