Evaporation and condensation
At a given temperature, liquid molecules have different speeds. The velocities of most molecules are close to a certain average value (characteristic of this temperature). But there are molecules whose speeds differ significantly from the average, both smaller and larger.
In Fig. Figure 1 shows an approximate graph of the distribution of liquid molecules by speed. The blue background shows the majority of molecules whose velocities are grouped around the average value. The red “tail” of the graph is a small number of “fast” molecules, the speeds of which significantly exceed the average speed of the bulk of liquid molecules.
Rice. 1. Distribution of molecules by speed
When such a very fast molecule finds itself on the free surface of the liquid (i.e., at the interface between liquid and air), the kinetic energy of this molecule may be enough to overcome the attractive forces of other molecules and fly out of the liquid. This process is evaporation, and the molecules that leave the liquid form vapor.
So, evaporation is the process of converting liquid into vapor, occurring on the free surface of the liquid (under special conditions, the transformation of liquid into vapor can occur throughout the entire volume of the liquid. This process is well known to you - this is boiling).
It may happen that after some time the vapor molecule returns back to the liquid.
The process of vapor molecules changing into liquid is called condensation. Vapor condensation is the reverse process of liquid evaporation.
Dynamic balance
What happens if a vessel with liquid is hermetically sealed? The vapor density above the liquid surface will begin to increase; vapor particles will increasingly interfere with other liquid molecules flying out, and the evaporation rate will begin to decrease. At the same time, the condensation rate will begin to increase, since as the vapor concentration increases, the number of molecules returning to the liquid will become more and more.
Finally, at some point the rate of condensation will be equal to the rate of evaporation. A dynamic equilibrium will occur between liquid and vapor: per unit time, the same number of molecules will fly out of the liquid as return to it from the vapor. Starting from this moment, the amount of liquid will stop decreasing, and the amount of vapor will stop increasing; the steam will reach “saturation”.
Saturated vapor is vapor that is in a state of dynamic equilibrium with its liquid. Vapor that has not reached a state of dynamic equilibrium with the liquid is called unsaturated.
The pressure and density of saturated steam are denoted by and . Obviously, and are the maximum pressure and density that steam can have at a given temperature. In other words, the pressure and density of saturated steam always exceeds the pressure and density of unsaturated steam.
Properties of saturated steam
It turns out that the state of saturated steam (and even more so of unsaturated steam) can be approximately described by the equation of state of an ideal gas (Mendeleev-Clapeyron equation). In particular, we have an approximate relationship between saturated vapor pressure and its density:
(1)
This is a very surprising fact, confirmed by experiment. Indeed, in its properties, saturated steam differs significantly from an ideal gas. Let us list the most important of these differences.
1. At a constant temperature, the density of saturated steam does not depend on its volume.
If, for example, saturated steam is compressed isothermally, then its density will initially increase, the condensation rate will exceed the evaporation rate, and part of the vapor will condense into liquid - until dynamic equilibrium occurs again, in which the vapor density will return to its previous value .
Similarly, during isothermal expansion of saturated steam, its density will initially decrease (the steam will become unsaturated), the rate of evaporation will exceed the rate of condensation, and the liquid will further evaporate until dynamic equilibrium is established again - i.e. until the steam becomes saturated again at the same density.
2. The pressure of saturated steam does not depend on its volume.
This follows from the fact that the density of saturated vapor does not depend on volume, and pressure is uniquely related to density by equation (1).
As we see, the Boyle-Mariotte law, which is valid for ideal gases, does not hold true for saturated steam. This is not surprising - after all, it was obtained from the Mendeleev-Clapeyron equation under the assumption that the mass of the gas remains constant.
3. At a constant volume, the density of saturated vapor increases with increasing temperature and decreases with decreasing temperature.
Indeed, as the temperature increases, the rate of liquid evaporation increases.
At the first moment, the dynamic equilibrium is disrupted, and additional evaporation of some part of the liquid occurs. The pair will be added until dynamic equilibrium is restored again.
Likewise, as the temperature decreases, the rate of liquid evaporation becomes slower, and some of the vapor condenses until dynamic equilibrium is restored—but with less vapor.
Thus, when saturated steam is heated or cooled isochorically, its mass changes, so Charles’s law does not work in this case. The dependence of saturated vapor pressure on temperature will no longer be a linear function.
4. Saturated vapor pressure increases with temperature faster than according to a linear law.
In fact, with increasing temperature, the density of saturated vapor increases, and according to equation (1) the pressure is proportional to the product of density and temperature.
The dependence of saturated vapor pressure on temperature is exponential (Fig. 2). It is represented by section 1–2 of the graph. This dependence cannot be derived from the ideal gas laws.
Rice. 2. Dependence of steam pressure on temperature
At point 2 all liquid evaporates; with a further increase in temperature, the steam becomes unsaturated, and its pressure increases linearly according to Charles’s law (section 2–3).
Let us recall that the linear increase in pressure of an ideal gas is caused by an increase in the intensity of impacts of molecules on the walls of the vessel. When saturated steam is heated, the molecules begin to beat not only harder, but also more often - because the steam becomes larger. The simultaneous action of these two factors causes an exponential increase in saturated vapor pressure.
Saturated and unsaturated steam
Under natural conditions, steam is considered a gas. It can be saturated or unsaturated, depending on its density, temperature and pressure.
Vapor that is in dynamic equilibrium with its own liquid is saturated .
Dynamic equilibrium between liquid and vapor occurs when the number of molecules leaving the free surface of the liquid is equal to the number of molecules returning to it.
In an open vessel, the dynamic equilibrium is disturbed and the vapor becomes unsaturated, since a certain number of molecules evaporate into the atmosphere and do not return to the liquid.
Saturated steam is formed in a closed vessel above the free surface of the liquid.
Saturated and unsaturated steam have different properties. Let's explore them.
| Rice. 3.2. Isothermal vapor compression |
saturated vapor molecules does not depend on its volume.
Let unsaturated steam at temperature T be in a cylinder with a piston (Fig. 3.2). Let's begin to slowly compress it to ensure an isothermal process (section AB). First, if the vapor is significantly rarefied, the dependence of pressure on volume will correspond to the Boyle-Mariotte law for an ideal gas: pV = const. However, with a decrease in the volume of unsaturated vapor (increase in its density), a deviation from it begins to be observed. Further isothermal compression of the steam leads to the fact that it begins to condense (point B), liquid droplets form in the cylinder and the steam becomes saturated. Its density, and therefore the concentration of molecules, reaches its maximum value for a given temperature. They do not depend on the volume occupied by saturated steam and are determined by its pressure and temperature.
When saturated steam (section BC), its pressure will not change (p = const). This is explained by the fact that as the volume decreases, the saturated vapor condenses, forming a liquid. Its share in the volume of the cylinder increases all the time, and the volume occupied by saturated steam decreases. This happens until all the saturated vapor turns into a liquid state (point C).
A further decrease in volume causes a rapid increase in pressure (DC section), since the liquids are almost not compressed. Material from the site https://worldofschool.ru
So, during isothermal compression of unsaturated vapor, at first (at low density) it exhibits the properties of an ideal gas. When steam becomes saturated , its properties obey other laws. In particular, at low temperatures its state is approximately described by the equation p = nkT, when the concentration of molecules does not depend on the volume occupied by the gas. The graph of pressure p versus volume V, shown in Fig. 3.2 is called the isotherm of real gases .
Isotherms of a real gas characterize its equilibrium state with a liquid. Their compatibility makes it possible to determine the dependence of saturated vapor on temperature.
On this page there is material on the following topics:
Saturated steam and its properties briefly
Saturated steam physics in brief
Isothermal increase in unsaturated steam pressure
Scientific knowledge brief summary
Saturated steam brief summary
Questions about this material:
What kind of steam is called saturated?
What is characteristic from a molecular point of view of saturated steam?
What is the difference between the properties of saturated and unsaturated steam?
Why can steam be considered a gas?
Air humidity
Air containing water vapor is called humid. The more vapor there is in the air, the higher the air humidity.
Absolute humidity is the partial pressure of water vapor in the air (that is, the pressure that water vapor would exert on its own, in the absence of other gases). Sometimes absolute humidity is also called the density of water vapor in the air.
Relative humidity is the ratio of the partial pressure of water vapor in it to the pressure of saturated water vapor at the same temperature. Typically, this ratio is expressed as a percentage:
From the Mendeleev-Clapeyron equation (1) it follows that the ratio of vapor pressures is equal to the ratio of densities. Since equation (1), we recall, describes saturated steam only approximately, we have an approximate relation:
One of the devices that measures air humidity is a psychrometer. It includes two thermometers, the reservoir of one of which is wrapped in a wet cloth. The lower the humidity, the more intense the evaporation of water from the fabric, the more the reservoir of the “wet” thermometer cools, and the greater the difference between its readings and the readings of the dry thermometer. From this difference, air humidity is determined using a special psychrometric table.
Lesson on the topic “Air Humidity” 10th grade lesson plan in physics (10th grade) on the topic
Lesson on the topic “Air Humidity” 10th grade
Lesson objectives:
- Educational: formulate the concept of relative air humidity, introduce students to methods of measuring relative humidity, show the practical dependence of relative humidity in human life.
- Developmental: to develop the ability to analyze, compare, express one’s thoughts, draw conclusions, and establish connections between physical quantities.
- Educational: to create a desire to connect the knowledge and skills acquired in physics lessons, to develop observation skills, skills and culture of conducting a physical experiment; cultivate an attentive attitude to the surrounding world; promote the ability to work in a group.
Equipment: instruments and materials for conducting experiments to determine humidity, humidity indicators, computer.
Forms of work: frontal, individual, group.
- Organizational moment (1 min.).
- Updating knowledge: mutual survey (5 min.).
Presentation “Find the mistake”
- Learning new material (10 min.).
Problematic issues:
1. How does air humidity affect a person?
2. How to take into account air humidity?
If there is one subject that is of interest to everyone, it is probably the weather. The weather is not only the topic of many idle conversations, but it also often determines our behavior. Depending on the weather, we decide whether to go on a picnic, go to the skating rink, ride a sailboat, go swimming or skiing. The climate can be used to judge what kind of clothes people wear, what they eat, and what kind of homes they live in. Depending on the weather, the holidays can be very pleasant or disastrous. Weather affects the health, well-being and well-being of the entire population.
The moisture content of the atmosphere is an important factor in determining weather. The actual amount of moisture contained in 1 m3 of air is called absolute humidity. The amount of moisture depends on the temperature. Air that contains the maximum amount of moisture it can hold is called saturated air. Relative humidity is characterized by the following values:
p o – saturated vapor pressure (Pa)
R. – partial pressure (Pa) – the pressure that water vapor would produce if all other gases were absent.
U – air humidity.
Relative air humidity is the ratio of the partial pressure p of water vapor contained in the air at a given temperature to the pressure p0 of saturated vapor at the same temperature, expressed as a percentage (Slide 4).
What is used to measure air humidity? The main device for measuring air humidity is a hygrometer (from “hygro” and “meter”), hygro means moisture-dependent. There are several types of hygrometers, the operation of which is based on different principles: (Slide 5)
1. Hair hygrometer. 3. Psychrometric hygrometer.
2. Practical work “Determination of air humidity” (4 min)
| Temperature, ° C | Relative humidity, % |
| 18 | 40 |
| 22 | 35 |
Let's look at the table “Optimal microclimate standards”:
Based on the results of the work of all groups, it turned out that the microclimate of our premises is favorable for normal life activity: relative humidity is within the normal range of 40–60%.
3. Report with presentation “The value of air humidity” (5 min)
A person’s well-being depends on many factors, one of which is the required level of humidity and air purity. The humidity and cleanliness of the air in an ordinary apartment are far from ideal, and this happens because we live in large cities, where the air is far from natural purity; in addition, heating and concrete walls absorb the last moisture. In winter, the humidity in the apartment drops to 20–30%, while the natural air humidity is 55–60%. This leads to discomfort, fatigue, and illness.
Dry air prevents oxygen from entering the circulatory system. Symptoms of insufficient oxygen consumption include exhaustion, fatigue, and increased susceptibility to infection. The mucous membrane of the respiratory system dries out, resulting in increased susceptibility to infection and various respiratory diseases. Children especially suffer if the air in the room is too dry, the child's mucous membranes dry out, his nose gets stuffy, and he often wakes up at night. Dry air and children's airways are bad neighbors. Also, due to insufficient air humidity, problems may arise during breastfeeding - the baby’s oral mucosa dries out and, as a result, pain when sucking. At a temperature of 25 degrees and air humidity of 20% (namely, this is the humidity in winter in a room with central heating), the child spends about 30-50 ml of liquid per hour just with breathing to humidify the inhaled air (calculate how much per day). But every gram counts for a baby (Slide 13).
Dry skin. Lack of moisture in the air accelerates the evaporation of water from the skin. It becomes dry, rough and begins to peel. It becomes old and ugly. Skin is 70% water. About 12 percent of them are located in the stratum corneum, which is only a hundredth of a millimeter thick. It protects the underlying layers, which retain the main moisture reserves. Therefore, maintaining indoor humidity at 50–60% and regular care with moisturizing preparations will help you extend its longevity and keep your skin youthful (Slide 15).
Dry air is contraindicated for allergy sufferers and asthmatics. Dry skin needs masks with a moisturizing effect. Any dairy products, fruits, vegetables will be useful. It is necessary to use moisturizer and oxygen-containing masks.
Dry air affects furniture and parquet. They gradually lose their original appearance. They begin to shrink and cracks appear over time. Dry air causes the paint to peel off from the painting. Dry air is especially dangerous for musical instruments, which dry out, crack and fail (Slide 16).
Dry air causes plants to deteriorate. The tips of their leaves begin to dry out, the plants lose their decorative appearance or even gradually die. It is useless to water such plants intensively; it is even harmful. In nature, roots are not the main source of moisture for these plants; the roots are simply unable to saturate the plant with moisture. And the plant will gradually die even with proper and timely watering.-
The body of insects is covered with fluff and various hairs, which are extremely sensitive to changes in humidity in the atmosphere and are capable of condensing moisture on their surface. All weather changes usually begin in the upper layers of the atmosphere, and the accumulation of moisture on the surface of the body of small insects makes it difficult for them to fly and forces them to sink lower and lower, where the air is still dry. Another version: insects’ wings become heavy from moisture. Following them, insectivorous birds descend - swallows, swifts. If swallows and swifts fly high, then there will be dry sunny weather, if low, there will be rain - this is a popular sign. Frogs and fish are also not averse to feasting on insects that appear above the water, frogs jump on the shore and croak during the day, and fish jump out of the water, grabbing gaping midges - let it rain! Bees strictly monitor the air humidity in the hives, maintaining it within 65–88%. In dry summer weather, they place freshly brought liquid nectar (50% water) around the cells with brood, from which the water easily evaporates, and the cells with nectar are filled only 25–30%, which increases the area of evaporation. In extreme heat, bees bring water to the hive.
Air humidity indicators:
a) library 50–60%. b) museum 50–60%. c) pharmacy 50%. d) computer class 45–60%. e) music school 50%. e) lyceum canteen 60%.
Air humidity is of paramount importance for food storage. Too humid air contributes to product spoilage - molding, rotting, too dry - drying out of the product. Relative air humidity during food storage (stand):
- Fresh fruits 75–85% P. Fresh vegetables 85–90% III. Fish 95–98% IV. Meat, meat products 75–95% V. Milk, dairy products 80–87% VI. Eggs, egg products 60–88% VII. Starch, sugar, confectionery 70–75% VIII. Flavored products 70–75%
- Consolidation, skills development (12 min.).
Exercise 14 (2,3,4,5)
5. Homework:
6. Relaxation.
To summarize the lesson, we are convinced that taking into account air humidity is necessary in any area of human activity. Excess and lack of moisture has a detrimental effect on human health, household items, material and cultural values. To regulate optimal humidity, it is necessary to use special devices - psychrometers, hygrometers, humidifiers of various types.
