Thermophysical properties of vapors

When a cup of water is standing for a long time, then in the end all the water will simply evaporate in it. In this article, we will just talk about why this is happening, and discuss the properties of the vapors.

Evaporation and condensation

Water molecules move at different temperatures at different temperatures. Of course, most adhere to a single value of speed, but in some of them the indicators are significantly different.

Under these conditions, one of the fastest molecules hits one's free surface of water.

The free surface of water is the boundary where the liquid is in contact with air. After hitting there, the molecule can overcome the attraction of other, slower molecules and leave the water itself. This process is called evaporation. Molecules that fly out of water are converted into steam. Now let's move on to the terminology.

Evaporation - the transformation of water into steam. This process can only take place at the border with the air.

The properties of water vapor also mean that after a certain period of time the molecule can turn back into water. This is called condensation.

Condensation is a phenomenon opposite to evaporation.

Dynamic balance

The properties of the vapors are varied, and now we will talk about one of these.

We discussed earlier what is done when the molecule leaves the liquid, but the example was brought with an open cup of water. Now we will discuss what will happen if the cup is tightly closed. In this case, the vapor density over the water will increase. Because of this, the particles will prevent each other from leaving the boundary with the air, as a result of which the evaporation process will decrease. At the same time, the condensation rate will increase, because due to the accumulation of steam, the number of molecules that are converted back to water will be greater.

Sooner or later, in the given circumstances, the condensation rate becomes equal to the rate of evaporation. These properties of water and steam are called dynamic equilibrium .

Dynamic equilibrium is when, at the same time, the number of molecules that have turned into steam equals the number of molecules that have passed back into the water. Proceeding from this, it follows that the volume of water will not decrease, as will the amount of steam. This means that the steam has become "saturated".

Saturated steam is when it is in a dynamic equilibrium with the water from which it came out. Similarly, steam that is not in a state of dynamic equilibrium is called unsaturated.

The properties of vapors mean that saturated vapor always has a higher value of pressure and density than unsaturated vapor . This is so, because the saturated vapor has the maximum value of pressure and density. In physics, these quantities are denoted as p and _n, respectively.

Saturated steam properties

It follows from the information above that the state of saturated vapor can be described by the same equation as the state of an ideal gas. At the minimum, the relationship between density and pressure is observed.

The properties of water and water vapor are surprising, at least because of this. And this fact, about the similarity of a saturated vapor with an ideal gas, was tested experimentally. It is striking because the properties of the vapor differ significantly from the properties of ideal gases. It is worth mentioning the main differences between them.

Dependence of density on temperature

It is worth initially making a remark and saying that using the word "pairs", it means "saturated steam". Thus, the thermophysical properties of steam imply that its density at the same temperature does not depend on the volume. Thus, if artificial pressure is created in a sealed vessel, then the vapor density will increase for some time. And also the condensation will accelerate and at times exceed the evaporation process. This will continue until a dynamic equilibrium occurs. With its onset, the density again normalizes.

The same will happen if the pressure is lowered, only the place where the vapor density increases will decrease it. This is due to the acceleration of evaporation. But this process will proceed until all processes are fully normalized.

And also the volume of steam does not influence its pressure in any way. This is so, because the volume does not affect the density. And according to the formula, the density and pressure are reciprocal in this case. From this and this judgment follows.

Effect of temperature on density

The thermophysical properties of water and steam also imply that, with the same volume of water, its density increases with heating and, on the contrary, decreases with decreasing temperature.

When the temperature rises, the process of evaporation increases many times. And as in the previous example, the dynamic equilibrium is violated, due to excessive evaporation, but only for a while. Sooner or later the processes of evaporation and condensation again normalize.

Similarly occurs when the temperature falls. Only in this case will the rate of evaporation decrease, and the condensation will proceed until such time as there is a balance between them. But, of course, this happens with a smaller number of steam.

Proceeding from this, it can be stated that the law of Charles with saturated steam does not work. So this is because when the water is heated and cooled, its mass changes, and this, in turn, means that the function is not linear.

Pressure versus temperature

Continuing this topic, it is worth mentioning another relationship. The fact is that with increasing temperature, the vapor pressure increases several times faster. In fact, this dependence is observed with density, but this conclusion is drawn from the fact that the density and pressure are interrelated values in the presented formula.

The dependence of pressure on temperature can not be distinguished from the law of an ideal gas, since the dependence presented is exponential.

Air humidity

It's time to talk about the humidity. Air is called wet when it contains steam. And it is clear that this dependence is directly proportional. That is, the more steam, the more humid the air.

Also there is the concept of " absolute humidity " - this is a phenomenon when the created pressure in the air is equal to the vapor pressure. This phenomenon also works with the vapor density.

Relative humidity refers to the ratio of absolute humidity in the air to the pressure of saturated steam, provided that the temperature is the same.

Psychrometer is a device for measuring air humidity. It consists of two thermometers, only one of them is wrapped in a damp cloth. The principle of its work is that with a low moisture evaporation from the tissue flows faster, because of which the shrouded thermometer is significantly cooled. In view of this, a difference in the readings between the two instruments appears. Based on this, the moisture content of the air is already calculated.