DEFINITION
Atmospheric air is a mixture of many gases. Air has a complex composition. Its main components can be divided into three groups: constant, variable and random. The former include oxygen (the oxygen content in the air is about 21% by volume), nitrogen (about 86%) and the so-called inert gases (about 1%).
The content of the components is practically independent of where in the world the dry air sample is taken. The second group includes carbon dioxide (0.02 - 0.04%) and water vapor (up to 3%). The content of random components depends on local conditions: near metallurgical plants, noticeable amounts of sulfur dioxide are often mixed into the air, in places where organic residues decompose - ammonia, etc. In addition to various gases, the air always contains more or less dust.
Air density is a value equal to the mass of gas in the Earth's atmosphere divided by a unit volume. It depends on pressure, temperature and humidity. There is a standard value for air density - 1.225 kg/m 3, corresponding to the density of dry air at a temperature of 15 o C and a pressure of 101330 Pa.
Knowing from experience the mass of a liter of air at normal conditions(1.293 g), we can calculate the molecular weight that air would have if it were an individual gas. Since a gram molecule of any gas occupies a volume of 22.4 liters under normal conditions, the average molecular weight of air is equal to
22.4 × 1.293 = 29.
This number - 29 - should be remembered: knowing it, it is easy to calculate the density of any gas relative to air.
Density of liquid air
When sufficiently cooled, the air turns into a liquid state. Liquid air can be stored for quite a long time in vessels with double walls, from the space between which the air is pumped out to reduce heat transfer. Similar vessels are used, for example, in thermoses.
Liquid air that evaporates freely under normal conditions has a temperature of about (-190 o C). Its composition is not constant, since nitrogen evaporates more easily than oxygen. As the nitrogen is removed, the color of the liquid air changes from bluish to pale blue (the color of liquid oxygen).
In liquid air, ethyl alcohol, diethyl ether and many gases easily turn into solids. If, for example, carbon dioxide is passed through liquid air, it turns into white flakes similar in appearance. appearance to the snow. Mercury immersed in liquid air becomes hard and malleable.
Many substances cooled by liquid air dramatically change their properties. Thus, chink and tin become so brittle that they easily turn into powder, a lead bell makes a clear ringing sound, and a frozen rubber ball shatters if dropped on the floor.
Examples of problem solving
EXAMPLE 1
EXAMPLE 2
| Exercise | Determine how many times heavier than air is hydrogen sulfide H 2 S. |
| Solution | The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure is called relative density the first gas over the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken to be 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of “relative molecular mass of air” is used conditionally, since air is a mixture of gases. D air (H 2 S) = M r (H 2 S) / M r (air); D air (H 2 S) = 34 / 29 = 1.17. M r (H 2 S) = 2 × A r (H) + A r (S) = 2 × 1 + 32 = 2 + 32 = 34. |
| Answer | Hydrogen sulfide H 2 S is 1.17 times heavier than air. |
This is the mass (weight) of 1 cubic meter of a mixture of air and water vapor at a certain temperature and relative humidity. Specific volume is the volume of air and water vapor per 1 kg of dry air.
Moisture and heat content
The mass in grams per unit mass (1 kg) of dry air in their total volume is called air moisture content. It is obtained by dividing the density of water vapor contained in the air, expressed in grams, by the density of dry air in kilograms.To determine the heat consumption for moisture, you need to know the value heat content of moist air. This value is understood as contained in a mixture of air and water vapor. It is numerically equal to the sum:
Although we cannot feel the air around us, air is not nothing. Air is a mixture of gases: nitrogen, oxygen and others. And gases, like other substances, consist of molecules, and therefore have weight, although small.
Experiments can be used to prove that air has weight. In the middle of a stick about sixty centimeters long, we will attach a rope, and we will tie two identical balloons to both ends. Let's hang the stick by a string and see that it hangs horizontally. If you now pierce one of the inflated balloons with a needle, the air will come out of it, and the end of the stick to which it was tied will rise up. If you pierce the second ball, the stick will again take a horizontal position.
This happens because there is air in the inflated balloon. tighter, and therefore heavier than the one around it.
How much air weighs depends on when and where it is weighed. The weight of air above a horizontal plane is Atmosphere pressure. Like all objects around us, air is also subject to gravity. It is this that gives the air a weight that is equal to 1 kg per square centimeter. The density of air is about 1.2 kg/m 3, that is, a cube with a side of 1 m filled with air weighs 1.2 kg.
A column of air rising vertically above the Earth stretches for several hundred kilometers. This means that a column of air weighing about 250 kg presses on a person standing upright, on his head and shoulders, the area of which is approximately 250 cm 2!
We would not be able to withstand such a weight if it were not resisted by the same pressure inside our body. The following experience will help us understand this. If you stretch a sheet of paper with both hands and someone presses a finger on it on one side, the result will be the same - a hole in the paper. But if you press with two index fingers on the same place, but with different sides, nothing will happen. The pressure on both sides will be the same. The same thing happens with the pressure of the air column and the counter pressure inside our body: they are equal.
Air has weight and presses on our body from all sides.
But it cannot crush us, because the counter pressure of the body is equal to the external one.
The simple experiment depicted above makes this obvious:
if you press your finger on a sheet of paper on one side, it will tear;
but if you press on it from both sides, this will not happen.
By the way...
In everyday life, when we weigh something, we do it in the air, and therefore we neglect its weight, since the weight of air in the air is zero. For example, if we weigh an empty glass flask, we will consider the result obtained to be the weight of the flask, neglecting the fact that it is filled with air. But if the flask is sealed and all the air is pumped out of it, we will get a completely different result...
03.05.2017 14:04
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How much does air weigh?
Even though we cannot see some things that exist in nature, this does not mean that they do not exist. It’s the same with air - it’s invisible, but we breathe it, we feel it, which means it’s there.
Everything that exists has its own weight. Does air have it? And if so, how much does air weigh? Let's find out.
When we weigh something (for example, an apple by holding it by a branch), we do it in the air. Therefore, we do not take into account the air itself, since the weight of air in air is zero.
For example, if we take an empty glass bottle and weigh it, we will consider the result obtained to be the weight of the flask, without thinking about the fact that it is filled with air. However, if we tightly close the bottle and pump out all the air from it, we will get a completely different result. That's it.
Air consists of a combination of several gases: oxygen, nitrogen and others. Gases are very light substances, but they still have weight, although not much.
To make sure that air has weight, ask adults to help you carry out the following simple experiment: Take a stick about 60 cm long and tie a string in the middle of it.
Next, we will attach 2 inflated balloons of the same size to both ends of our stick. Now let’s hang our structure by a rope tied to its middle. As a result, we will see that it hangs horizontally.
If we now take a needle and pierce one of the inflated balloons with it, the air will come out of it, and the end of the stick to which it was tied will rise up. And if we pierce the second ball, then the ends of the stick will be even and it will hang horizontally again.
What does it mean? And the fact is that the air in an inflated balloon is denser (that is, heavier) than the air around it. Therefore, when the ball deflated, it became lighter.
The weight of air depends on various factors. For example, air above a horizontal plane is atmospheric pressure.
The air, like all objects that surround us, is subject to gravity. It is this that gives the air its weight, which is equal to 1 kilogram per square centimeter. In this case, the air density is about 1.2 kg/m3, that is, a cube with a side of 1 m filled with air weighs 1.2 kg.
A column of air rising vertically above the Earth stretches for several hundred kilometers. This means that directly standing man, on his head and shoulders (the area of which is approximately 250 square centimeters), a column of air weighing about 250 kg presses!
If such a huge weight were not opposed by the same pressure inside our body, we simply would not be able to withstand it and it would crush us. There is another one interesting experience, which will help you understand everything we said above:
Take a sheet of paper and stretch it with both hands. Then we ask someone (for example, a younger sister) to press on it with a finger on one side. What happened? Of course, a hole appeared in the paper.
Now let's do the same thing again, only now you will need to press on the same place with two index fingers, but from different sides. Voila! The paper remained intact! Want to know why?
It’s just that the pressure on the sheet of paper on both sides was the same. The same thing happens with the pressure of the air column and the counter pressure inside our body: they are equal.
Thus, we found out that: air has weight and presses on our body from all sides. However, it cannot crush us, since the counter pressure of our body is equal to the external, that is, atmospheric.
Our latest experiment showed this clearly: if you press on one side of a sheet of paper, it will tear. But if you do it on both sides, this won't happen.
Physics at every step Perelman Yakov Isidorovich
How much does the air in the room weigh?
Can you say at least approximately how much weight the air contained in your room represents? A few grams or a few kilograms? Are you able to lift such a load with one finger, or would you barely be able to hold it on your shoulders?
Now, perhaps, there are no longer people who think, as the ancients believed, that air weighs nothing at all. But even now many people will not be able to say how much a certain volume of air weighs.
Remember that a liter mug of air of the same density as it has nearby earth's surface with normal room temperature, weighs about 1.2 g. Since a cubic meter contains 1 thousand liters, a cubic meter of air weighs a thousand times more than 1.2 g, namely 1.2 kg. Now it is not difficult to answer the question posed earlier. To do this, you just need to find out how many cubic meters are in your room, and then the weight of the air contained in it will be determined.
Let the room have an area of 10 m2 and a height of 4 m. In such a room there are 40 cubic meters of air, which weighs forty times 1.2 kg. This will be 48 kg.
So, even in such a small room, the air weighs a little less than you. You would be able to carry such a load on your shoulders with difficulty. And the air of a room twice as spacious, loaded onto your back, could crush you.
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