Many may be surprised by the fact that air has a certain non-zero weight. The exact value of this weight is not so easy to determine, since factors such as chemical composition, humidity, temperature and pressure strongly influence it. Let us consider in more detail the question of how much air weighs.
What is air
Before answering the question of how much air weighs, it is necessary to understand what this substance is. Air is the gas envelope that exists around our planet, and which represents a homogeneous mixture of various gases. The following gases are part of the air:
- nitrogen (78.08%);
- oxygen (20.94%);
- argon (0.93%);
- water vapor (0.40%);
- carbon dioxide (0.035%).
In addition to the gases listed above, there are also minimal amounts of neon (0.0018%), helium (0.0005%), methane (0.00017%), krypton (0.00014%), hydrogen (0.00005%) in air ), ammonia (0,0003%).
It is interesting to note that it is possible to separate these components if the air is condensed, that is, to turn it into a liquid state by increasing pressure and decreasing temperature. Since each air component has its own condensation temperature, in this way it is possible to isolate all components from the air, which is used in practice.
Air weight and factors that influence it
What prevents to answer exactly the question, how much does a cubic meter of air weigh? Of course, a number of factors that can greatly influence this weight.
The first is the chemical composition. The above are the data for the composition of clean air, however, at present this air is heavily polluted in many places on the planet, and accordingly, its composition will be different. So, near large cities in the air contains more carbon dioxide, ammonia, methane than in the air of the countryside.
Secondly, humidity, that is, the amount of water vapor that is contained in the atmosphere. The more humid the air, the less it weighs, all other things being equal.
Thirdly, the temperature. This is one of the important factors, the smaller its value, the higher the density of air, and, accordingly, the greater its weight.
Fourth, atmospheric pressure, which directly reflects the number of air molecules in a certain volume, that is, its weight.
To understand how the combination of these factors affects the weight of air, we give a simple example: the mass of one meter of cubic dry air at a temperature of 25 ° C located near the surface of the earth is 1.205 kg, if we consider the same volume of air near the surface of the sea at a temperature of 0 ° C, then its mass will already be equal to 1.293 kg, that is, it will increase by 7.3%.
Change in air density with height
With increasing height, the air pressure drops, respectively, its density and weight decrease. Atmospheric air at pressures that are observed on the Earth, in a first approximation can be considered an ideal gas. This means that air pressure and density are mathematically related to each other through the equation of state of an ideal gas: P = ρ * R * T / M, where P is pressure, ρ is density, T is temperature in kelvins, M is the molar mass of air, R is the universal gas constant.
From the above formula, we can obtain the formula for the dependence of air density on height, given that the pressure changes according to the law P = P 0 + ρ * g * h, where P 0 is the pressure at the surface of the earth, g is the acceleration of gravity, h is the height . Substituting this formula for pressure into the previous expression, and expressing the density, we obtain: ρ (h) = P 0 * M / (R * T (h) + g (h) * M * h). Using this expression, you can determine the density of air at any height. Accordingly, the air weight (it is more correct to say mass) is determined by the formula m (h) = ρ (h) * V, where V is the given volume.
In the expression of the dependence of density on height, one can notice that temperature and acceleration of gravity also depend on height. The last dependence can be neglected if we are talking about heights of not more than 1-2 km. As for temperature, its dependence on height is well described by the following empirical expression: T (h) = T 0 -0.65 * h, where T 0 is the air temperature near the surface of the earth.
In order not to constantly calculate the density for each altitude, below is a table of the dependence of the main characteristics of air on altitude (up to 10 km).
Which air is the heaviest
Having considered the main factors that determine the answer to the question of how much air weighs, you can understand which air will be the heaviest. In short, cold air always weighs more than warm because the density of the latter is lower, and dry air weighs more than wet. The last statement is easy to understand, since the molar mass of air is 29 g / mol, and the molar mass of the water molecule is 18 g / mol, that is, 1.6 times less.
Determination of air weight under given conditions
Now we will solve a specific problem. We will answer the question of how much air occupies a volume of 150 liters at a temperature of 288 K. We take into account that 1 liter is a thousandth of a cubic meter, that is, 1 l = 0.001 m 3 . As for the temperature of 288 K, it corresponds to 15 ° C, that is, it is typical for many regions of our planet. Next, you need to determine the density of air. There are two ways to do this:
- Calculate using the above formula for a height of 0 meters above sea level. In this case, the result is ρ = 1.227 kg / m 3
- Look in the above table, which is built on the basis of T 0 = 288.15 K. The table contains the value ρ = 1.225 kg / m 3 .
Thus, we got two numbers that are in good agreement with each other. A slight difference is associated with an error of 0.15 K in determining the temperature, as well as with the fact that air is still not ideal, but real gas. Therefore, for further calculations, we take the average of the two values obtained, that is, ρ = 1.226 kg / m 3 .
Now, using the relationship of mass, density and volume, we get: m = ρ * V = 1.226 kg / m 3 * 0.150 m 3 = 0.1839 kg or 183.9 grams.
You can also answer how much a liter of air weighs under given conditions: m = 1.226 kg / m 3 * 0.001 m 3 = 0.001226 kg or approximately 1.2 grams.
Why don't we feel the air pushing us
How much does 1 m3 of air weigh? A little over 1 kilogram. The whole atmospheric table of our planet puts pressure on a person with a weight of 200 kg! This is a fairly large mass of air, which could cause a lot of trouble to a person. Why don't we feel her? This is due to two reasons: firstly, inside the person himself there is also internal pressure that counteracts external atmospheric pressure, and secondly, air, being a gas, exerts pressure in all directions equally, that is, pressures in all directions cancel each other out.