In everyday life, few people think about what is the density of the air, and how important this index is generally the existence of all things on the planet.Meanwhile, every day we see the planes flying, soaring birds fly up and falling down objects and do not reflect on the fact that it is the air density parameter determines the presence of these phenomena.
Often, spending holidays in different places, we say in one place was damp air, and the other - dry.At the same time add that in the first case, you easily move, breathe, and in the second you have experienced the severity of motion, discomfort, which did not experience when you stay in a normal climatic environment for you.At this time, we forget what we were told the teacher during his school years at lessons of physics - the humid air has a density less than that of the air is dry, and, consequently, its mass is less than in the dry.
At first glance, it seems paradoxical if we take into account our sensory impressions, which were discussed above.Because really as air, to which further water is added in the form of steam may be lighter than water which does not contain?
But it is really so, and the answer to this, paradoxical at first glance, the question, scientists have known for a long time.
first hypothesis that the density of moist air is lower than that of dry air, expressed great Isaac Newton in his famous book "Optics", which was published in London in 1717.Nevertheless, the hypothesis of the great Englishman was not able to succeed - until the eighteenth century, scientists do not just take it, but generally have little interest in this issue.
to somehow get closer to understanding the problem - why the density of air depends on the humidity - should recall several well-known natural laws.
For example, at the beginning of the last century, Amedeo Avogadro, the famous Italian physicist, found that regardless of the type of gas, if you take it a fixed amount, then at the same temperature and the same pressure, the number of molecules in the gas is constant.This value, and later it was named Avogadro constant, as was called, and he discovered the law of gases.
As this law manifests itself, it depends on the density of air temperature, pressure and humidity can make for quite a simple example.
Typically, dry clean (in the chemical sense) the air contains in its composition approximately 78% nitrogen molecules, and the atomic weight of each of these molecules is equal to 28. As part of the air is still 21% owned by oxygen molecules, the atomic weight of molecules which,equals 32. One percent as part of the air accounts for some other gases that are present in it, but for our calculation of this indicator, we assume negligible.
gas molecules known to possess the property free to go beyond the tank in which the gas.So, Avogadro established the following rule: if our volume of dry gas, containing, as we agreed, nitrogen and oxygen molecules, water molecules are added, then they will make our air is less dense.The explanation is simple - the water molecules have an atomic weight smaller than that of molecular nitrogen and hydrogen, it is equal to 18. Since the number of molecules in a given volume of gas to be constant, the water molecules are simply displaced molecules of nitrogen and oxygen in the air, replacingthey are.So here's a way, and it becomes moist air density is lower than the dry.
In this example, however, there is a contradiction.It consists in the fact that any average person can cry, how can that be, if the density of water is higher than the density of air.The answer is also easy: the water is present in the air in the form of steam, which is easier and nitrogen and oxygen, and because such a "water" covers all patterns derived brilliant Avogadro.
important to note that a greater air density depends on the temperature and pressure than the humidity.Because moist air has a lower density than dry only while maintaining the same temperature and pressure.
