In this short material, 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 substance is its ability to accumulate heat in itself. 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 soil - at 14.5-15.5 ° C (depending on the type of soil) 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 life, but in this material we will focus on its role in shaping the 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 capacities, the soil, in comparison with the waters of the world ocean, heats up much faster and, accordingly, cools down faster. Thanks to a more inert world ocean, fluctuations in the daily and seasonal temperatures of the Earth 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 a global average, it affects the entire planet.

Naturally, many factors influence fluctuations in daily and seasonal temperatures, 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, a well-known fact is that a stone loses its strength and becomes brittle during sharp temperature fluctuations. Obviously, we ourselves would be “somewhat” different. At least the physical parameters of our body would be exactly different.

Anomalous heat capacity properties of water

The heat capacity of water has anomalous properties. It turns out that with an increase in the temperature of water, 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, represented by Water, has determined 37°C as the most comfortable temperature for the human body, provided, of course, that all other factors are observed. With any dynamics of change in ambient temperature, the water temperature tends to 37°C.

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

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

This unique property of water makes it possible to use it 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 transition 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 main statement of thermodynamics - from two bodies (isolated from 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 any substance) is an indicator that characterizes the ability of a given substance to give (or receive) some during 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 is 0.22, iron is 0.11, and mercury is 0.03.

The heat capacity of water is not a constant. With an increase in temperature from 0 to 40 degrees, it slightly decreases (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 in the solid and molten state does not change. What is the secret here?

The fact is that ice has a crystalline structure, which does not immediately collapse when heated. Water contains small particles of ice, which consist of several molecules and are called associates. When water is heated, a part 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 passes into steam.

The specific heat capacity at a temperature of 100°C almost does not differ 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 now much better understood than that of water, on which scientists have not yet come to a consensus.

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

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

Enthalpy quantity

Quantity enthalpies of matter based on its given temperature. Given temperature is the value chosen by scientists and engineers as the basis for calculations. This 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 above its given temperature, or changes state to gaseous at a given temperature, the enthalpy is expressed as a positive number. Conversely, at a temperature below a given 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 set up the equipment and determine the beneficial effect of the process.

enthalpy often defined as the total energy of matter, since it is equal to the sum of its internal energy (u) in a given state, along with its ability to do work (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, more precisely define it as the total amount of available energy of a substance that can be converted into heat.
H=U+pV

The table shows the thermophysical properties of water vapor on the saturation line depending on the temperature. Steam properties 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 its increase with increasing temperature. 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 heat carrier in some types of heat exchangers.

For example, according to the table, the specific heat of water vapor Cp at a temperature of 20°C it is equal to 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 gives the following thermophysical properties of water vapor at the saturation line:

• vapor pressure at a 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 Cp, kJ/(kg deg);
• coefficient of thermal conductivity λ 10 2, W/(m deg);
• thermal diffusivity a 10 6, m2/s;
• dynamic viscosity μ 10 6, Pa s;
• kinematic viscosity v 10 6, m2/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 the Prandtl number of the steam increase in this case.

Be careful! The thermal conductivity in the table is given 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 values ​​of thermal conductivity of water and steam at temperatures from 0 to 700°C and pressure from 0.1 to 500 atm. The unit of thermal conductivity is W/(m deg).

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

Note: the thermal conductivity in the table is given to the power 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 W/(m deg) at temperatures from 1400 to 6000 K and pressures from 0.1 to 100 atm.

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

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

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

The concept of heat capacity

This physical quantity is so often used in the surrounding world and science that, first of all, it is necessary 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

One way or another, almost everyone understands what temperature is. Recall that the "amount of heat" is not just a phrase, but a term denoting the energy that the 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 are burning on the treadmill and what each piece of food eaten (or left on a plate) is equal to. 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.

Heat capacity application

However, a rigorous definition of the physical concept we are considering is rarely used by itself. We said above that it is very often used in everyday life. Those who did not 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 the heating pipes appears only thanks to heat capacity calculations.

Weather conditions, which determine whether it is possible to open the swimming season already or whether it is worth staying on the shore for now, also take this value into account. Any appliance associated with heating or cooling (oil cooler, refrigerator), all energy costs for food preparation (for example, in a cafe) or street soft ice cream are affected 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 sellers and ordinary consumers do this, but engineers, designers, manufacturers took everything into account and invested the appropriate parameters in household appliances. However, heat capacity calculations are used much more widely: in hydraulic turbines and the production of cements, in testing alloys for aircraft or railway trains, in construction, smelting, and cooling. Even space exploration is based on formulas containing this value.

Types of heat capacity

So, in all practical applications, relative or specific heat capacity is used. It is defined as the amount of heat (no infinitesimals, mind you) required to raise a unit amount of matter by one degree. Degrees on the Kelvin and Celsius scales coincide, 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, there are mass, volume and molar specific heat capacities. Recall that one mole is such an amount of a substance that contains approximately six times ten to the twenty-third degree of molecules. Depending on the task, the corresponding heat capacity is used, their designation in physics is different. Mass heat capacity is denoted as C and is expressed in J / kg * K, volume - C` (J / m 3 * K), molar - C μ (J / mol * K).

Ideal gas

If the problem of an ideal gas is being solved, then the expression for it is different. Recall that in this substance that does not exist in reality, atoms (or molecules) do not interact with each other. This quality radically changes any properties of an ideal gas. Therefore, traditional approaches to calculations will not give the desired result. An ideal gas is needed as a model for describing 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 in all conditions. But it's not. Upon transition to another state of aggregation (during the melting and freezing of ice, during 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 liquid and gaseous constituents of this substance.

Heating and heat capacity

As the reader has already noticed, most often in the real world the heat capacity of water appears. It is the source of life, without it our existence is impossible. She needs a person. 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 a positive temperature all year round. The ancient Romans built aqueducts to supply their cities with this valuable resource. But where there is winter, this method would not work. Ice, as you know, has a larger specific volume than water. This means that, freezing in pipes, it destroys them due to expansion. Thus, the challenge for central heating engineers and the delivery of hot and cold water to homes 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 are very cold. And at one hundred degrees Celsius, boiling is already occurring. 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, in the boilers that bring heat to our homes, there is strongly superheated steam. Due to the fact that it has a high temperature, it creates incredible pressure, so the boilers and the pipes leading to them must be very strong. In this case, even a small hole, a very small leak can lead to an explosion. The heat capacity of water depends on temperature, and non-linearly. That is, to heat it from twenty to thirty degrees, a different amount of energy will be required than, say, from one hundred and fifty to one hundred and sixty.

With any action that affects the heating of water, this should be taken into account, especially when it comes to 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 have given many examples of why it is necessary to heat water and how it is necessary to take into account the value of heat capacity. However, we have not yet told that, among all the available resources of the planet, this liquid has a fairly high rate of energy consumption for heating. This property is often used for cooling.

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

And the principle of operation of atomic nuclear reactors is generally based on the high heat capacity of water. The hot zone, as the name implies, has an incredibly high temperature. By heating itself, the water thereby cools the system, preventing the reaction from getting out of control. Thus, we receive the necessary electricity (heated steam rotates the turbines), and there is no catastrophe.