When a cup of water stands 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 happens and discuss the properties of vapors.
Evaporation and Condensation
Water molecules at the same temperature move at different speeds. Of course, most adhere to a single value of speed, but for some of them the indicators differ significantly.
Under these conditions, it happens that one of the fastest molecules gets on the free surface of the water.
The free surface of the water is the boundary where the liquid comes into contact with air. After getting there, the molecule’s speed 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 to steam. Now let's move on to the terminology.
Evaporation is the conversion of water to steam. This process can proceed only at the border with 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 the opposite of evaporation.
Dynamic balance
The vapor properties are diverse, and now we will talk about one of these.
We previously discussed what happens when a molecule leaves a liquid, but an example was given with an open cup of water. Now let's discuss what happens if the cup is tightly closed. In this case, the vapor density above water will increase. Because of this, the particles will interfere with each other leaving the border with air, as a result of which the evaporation process will be reduced. At the same time, the condensation rate will increase, because due to the accumulation of steam, the number of molecules that are converted again into water will be greater.
Sooner or later, in the circumstances, the rate of condensation will become 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 converted into steam equals the number of molecules transferred back to water. Based on this, it follows that the volume of water will not decrease, as well as the amount of steam. This means that the steam has become “saturated”.
Saturated steam is when it is in dynamic equilibrium with the water from which it exited. Similarly, steam that is not in a state of dynamic equilibrium is called unsaturated.
Vapor properties imply that saturated steam always has a higher pressure and density than unsaturated. This is because saturated steam has a maximum pressure and density. In physics, these quantities are denoted as p n and ρ n, respectively.
Saturated Steam Properties
From the information above it follows that the state of saturated vapor can be described by the same equation as the state of an ideal gas. At least there is a relationship between density and pressure.
The properties of water and water vapor are amazing, at least because of this. And this fact, about the similarity of saturated steam with an ideal gas, was verified experimentally. This is striking because the properties of vapors are significantly different from the properties of ideal gases. It is worth listing their main differences.
Density versus temperature
It is worth initially making a remark and saying that using the word "steam", it means precisely "saturated steam". So, the thermophysical properties of steam mean that its density at the same temperature does not depend on volume. Thus, if you create artificial pressure in a sealed vessel, the vapor density will increase for some time. And also condensation will accelerate and at times exceed the evaporation process. This will continue until a dynamic equilibrium occurs. With its onset, the density returns to normal.
The same thing happens if you lower the pressure, only the place where the vapor density increases will decrease. This is due to the acceleration of evaporation. But this process will proceed until all processes are completely normalized.
And also the volume of steam in no way affects its pressure. This is because volume does not affect density either. And according to the formula, density and pressure are mutual values in this case. From this 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 when heated, and when the temperature drops, it decreases.
When the temperature rises, the evaporation process increases significantly. And as in the previous example, the dynamic equilibrium is violated, due to excess evaporation, but only for a while. Sooner or later, evaporation and condensation will return to normal.
It happens similarly when the temperature drops. Only in this case the evaporation rate will decrease, and condensation will continue until the balance between them occurs. But, of course, this happens with much less steam.
Based on this, it can be stated that Charles’s saturated steam law does not work. This is so 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 dependence. 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 density and pressure are interrelated values in the presented formula.
The dependence of pressure on temperature cannot be distinguished from the law of an ideal gas, since the presented dependence is exponential.
Air humidity
It's time to talk about air humidity. The air is called moist when it contains steam. And it is clear that this dependence is directly proportional. That is, the more steam, the wetter the air.
There is also 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 vapor density.
Relative humidity is the ratio of absolute humidity in air to saturated vapor pressure, provided that the temperature is the same.
A psychrometer is a device for measuring air humidity. It consists of two thermometers, only one of them is shrouded in a damp cloth. The principle of its operation is that, at low humidity, evaporation from the fabric proceeds faster, which is why the shrouded thermometer cools more significantly. In view of this, a difference appears in the readings between the two devices. Based on this, the air humidity itself is already calculated.