Air refers to the mixed gas in the earth's atmosphere, so air is a mixture, mainly composed of nitrogen, oxygen, rare gases (helium, neon, argon, krypton, xenon, radon), carbon dioxide and other substances (such as water vapor and impurities). Among them, the volume fraction of nitrogen is about 78%, the volume fraction of oxygen is about 2 1%, the volume fraction of rare gases (helium, neon, argon, krypton, xenon and radon) is about 0.934%, the volume fraction of carbon dioxide is about 0.04% (data 20 17), and other substances (such as The composition of air is not fixed. With the change of altitude and air pressure, the composition ratio of air will also change. But for a long time, people always thought that air was a single substance, until French scientist lavoisier first came to the conclusion that air was composed of oxygen and nitrogen through experiments. At the end of 19, scientists found that rare gases such as helium, argon and xenon existed in the air through a large number of experiments.
In the natural state, air is colorless and tasteless.
Oxygen in the air is necessary for all aerobic organisms. All animals need to breathe oxygen, so do green plants. In addition, green plants use carbon dioxide in the air for photosynthesis, and air is almost the only source of carbon dioxide needed by all plants.
Chinese name
air
Foreign name
Air, atmosphere
molecular weight
28.959 (average)
water-solubility
slight soluble
density
1.293kg/m3
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physical features
ingredient
floor
Discover history
quality grade
relative humidity
function
introduce
This layer of air is the bottom of the atmosphere from the ground to 10~ 12 km, which is called the troposphere. Major weather phenomena such as clouds, rain, snow and hail all occur in this layer. The layer about 50 kilometers above the troposphere is called the stratosphere. The air in the stratosphere is much thinner than that in the troposphere, and there is little water vapor and dust in the troposphere, so there are few weather phenomena. From above the stratosphere to 80 kilometers, some people call it the mesosphere, and the temperature of this layer decreases with height. The space between 80 km and 500 km is called the thermosphere, and the temperature in this layer is very high, which varies greatly from day to night.
This layer from about 50 km above the ground to about 1000 km high is called the ionosphere. Beautiful auroras appear in the ionosphere.
More than 500 kilometers above the ground, it is called the outer atmosphere, also called the magnetosphere. It is the outermost layer of the atmosphere and the transition zone from the atmosphere to interstellar space. There is no clear boundary outside. In general, the upper limit is low near the geomagnetic pole, and it is about 9~ 10 radius of the earth high above the near-magnetic equator, in other words, it is about 65,000 kilometers high. The air here is extremely thin.
physical features
Air is a colorless and odorless gas.
At 0℃ and standard atmospheric pressure (1.013x105pa), the air density is 1.29 kg/m3. The state of gas at 0℃ and a standard atmospheric pressure is called standard configuration, and air can be regarded as an ideal gas at standard configuration, with a molar volume of 22.4 L/ mol.
The specific heat capacity of air is related to temperature. When the temperature is 250K, the specific heat capacity of air at constant pressure cp= 1.003kJ/(kg*K). At 300K, the specific heat capacity of air at constant pressure CP =1.005 kj/(kg k).
The impedance of air is about 377 ohms.
The relative molecular mass of air is 29.
Air at room temperature is colorless and odorless gas, while liquid air is light blue liquid, which is easy to flow. Generally, when air is liquefied, carbon dioxide has been removed, so the components of liquid air are 20.95% oxygen, 78. 12% nitrogen and 0.93% argon, and other components are few, which can be omitted.
In the standard state, the speed of sound of air is 331.5m/s.
The molar mass of dry air was 28.9634 g/mol.
In the standard state, the refractive index of air to visible light is about 1.00029. It varies with air pressure, temperature and air composition. In particular, humidity has a great influence on the refractive index, and the corresponding speed of light will also change in the air.
Specific heat capacity:
=1.005kj/(kg k) = 0.279kwh/(tonnek) (isostatic pressing process)
= 0.718kj/(kg k) = 0.199 kwh/(tonnek) (isovolumetric process)
Air without water vapor is called dry air. The gas constant of dry air is 2.8704× 106, the average molecular weight is 28.966 g mol-1,the specific heat at constant pressure is 7r/2 = 0.240 cal g-1k-1,and the specific heat at constant volume of dry air is =
ingredient
Volume content
Nitrogen: 78.09%
Oxygen: 20.95%
Rare gas: 0.932%
Carbon dioxide: 0.034%
Water vapor and impurities: 0.02%
Air is a mixture of many gases. Its invariable components are oxygen, nitrogen and rare gases, and its variable components are carbon dioxide and water vapor. Their content in the air will change slightly in a small range with different geographical locations and temperatures. As for the uncertain components in the air, they vary from region to region. For example, places near metallurgical plants will contain sulfur dioxide, and places near chlor-alkali plants will contain chlorine gas. In addition, there are traces of hydrogen, ozone, nitrous oxide, methane and more or less dust in the air.
Experiments show that the percentage of constant components in the air is almost unchanged in the height range from the ground 100km. According to the volume content, oxygen accounts for about 20.95%, nitrogen accounts for about 78.09%, and rare gas accounts for about 0.934%.
Generally speaking, the composition of air is relatively fixed. This is very important for the survival of human beings and other animals and plants. However, with the development of modern industry, harmful gases and smog discharged into the air have changed the composition of the air and caused air pollution. Polluted air will seriously damage human health, affect crop growth, and cause damage to natural resources and buildings.
Harmful substances discharged into the air can be roughly divided into dust and gas. Globally, sulfur dioxide, carbon monoxide and nitrogen dioxide are the most common gas pollutants discharged into the air. These gases mainly come from the combustion of fossil fuels (coal and oil) and the waste gas from factories.
floor
Air wraps around the earth, reaching a thickness of several Qian Qian meters. This thick layer of air is called the atmosphere. The atmosphere is divided into troposphere, stratosphere (stratosphere), mesosphere, ionosphere (warm layer) and stratosphere. We live in the lowest layer (troposphere). In the stratosphere, the air is much thinner. There is a gas called ozone (oxygen allotrope O3), which can absorb harmful ultraviolet rays from the sun. The mesosphere, also known as the mesosphere, extends from the top of the stratosphere to the atmosphere for 85 kilometers. Above it is the ionosphere, and the air is partially ionized or completely ionized. Ionosphere is a partially ionized atmospheric region, and a completely ionized atmospheric region is called magnetosphere. The ionosphere plays a very important role. It can reflect radio waves all over the world. If water vapor, carbon dioxide and various hydrocarbons are not considered, the average composition of air from the ground to the height of 100km remains unchanged. At an altitude of 25 kilometers, the ozone content increases. At higher altitudes, the composition of air varies with altitude and is obviously related to the time of day and solar activity.
Discover history
More than 200 years ago, French scientist lavoisier (1743 ~ 1794) measured the air composition through quantitative tests.
He put a small amount of mercury in a closed container and heated it for 12 days. He found that some mercury turned into red powder HgO, and at the same time, the air volume decreased by about 1/5. Through the study of residual gas, he found that this part of gas could not be supplied for breathing or combustion, and he mistakenly thought it was all nitrogen.
Lavoisier collected the red powder produced by heating and reheated it in another smaller container to obtain mercury and oxygen, the volume of which is exactly equal to the volume of air reduced in the closed container. He introduced the oxygen he got into the front container, and the gas he got was exactly the same as air.
Lavoisier concluded through experiments that air consists of oxygen and nitrogen, of which oxygen accounts for 1/5. [ 1]
/kloc-Before the 9th century, people thought that there were only nitrogen (N2) and oxygen (O2) in the air. Until 1892, British physicist Rowley found that there was always a slight difference between the density of "nitrogen" obtained by separating oxygen from air (1.2572g/L) and the density obtained by decomposing nitrogen-containing substances (1.2505g/L). Rowley didn't let go of this tiny difference. Later, he cooperated with the British chemist William Ramsay 1852- 19 16) and finally found that there is an inert gas in the air-Argon (ar). In the next few years, Ramsey and others discovered helium (he), neon (Ne), xenon (Xe), krypton (Kr) and radon (Rn) one after another.
People have been able to measure the composition of air accurately.
quality grade
Without air, our earth would be a barren desert and lifeless. Green plants use air, sunlight and carbon dioxide in water to synthesize nutrients. In this process, oxygen is released. Humans and other animals get oxygen by breathing air. Animals also need oxygen to get energy from the food they eat.
The development of industry has discharged harmful substances into the air, polluted the air and increased harmful components in the air. When harmful substances in the air reach a certain concentration, it will seriously damage human health and the growth of crops, destroy some substances, reduce people's visibility and affect traffic safety. Therefore, efforts must be made to prevent air pollution.
Harmful substances discharged into the air can be divided into the following categories: dust (such as carbon particles, etc. ), metal dust (such as iron, aluminum, etc. ), wet fog (such as oil fog, acid fog, etc. ), and harmful gases such as carbon monoxide, hydrogen sulfide and nitrogen oxides. ). From a global perspective, sulfur dioxide and carbon monoxide are harmful gases with large emissions and great harm. Sulfur dioxide is produced when coal and oil are burned. Carbon monoxide is mainly emitted when the car is driving. Globally, it is estimated that carbon monoxide emissions exceed sulfur dioxide.
According to the unified regulations of the State Environmental Protection Bureau, China's air quality is divided into five grades. It processes a series of complex air monitoring data according to a certain method, calculates the specific air pollution index, and then determines its air quality grade.
The specific standards are as follows:1when the air pollution index reaches 0-50; The second level is at 51-100; The third level is at101-200; Level 4 of 20 1-300; Level 5 when it is above 300. Among them, Grade 3 belongs to mild pollution, Grade 4 belongs to moderate pollution and Grade 5 belongs to severe pollution.
20 12 February, the State Council agreed to issue the newly revised Ambient Air Quality Standard, which increased the monitoring index of PM2.5, which refers to particles with a diameter less than or equal to 2.5 microns in the atmosphere, also known as particles that can enter the lungs. The index of PM2.5 indicates the content of this particle per cubic meter of air. The higher this value, the more serious the air pollution.
relative humidity
Usually used to indicate the humidity level, and the unit is %RH. It reflects the ability of air to approach saturation. The greater the humidity, the closer the air is to saturation. On the contrary, the smaller the humidity, the drier the air.
function
Due to the strong gravity of the earth, 80% of the air is concentrated in the average range of 15 km from the ground. This air layer has a great influence on human life and production activities. What people usually say about air pollution refers to the air pollution within this range.
Air is a necessary condition for the survival of animals and plants on the earth, and the respiration of animals and the photosynthesis of plants are inseparable from air; The atmosphere can keep the temperature on the earth relatively stable. If there is no atmosphere, the temperature will be high during the day and low at night. The ozone layer can absorb the ultraviolet rays of the sun and protect life on earth from harm; The atmosphere can prevent high-energy particles from entering the earth too much and prevent meteorites from hitting the earth, because meteorites can slow down and burn when they rub against the atmosphere; The formation of wind, clouds, rain and snow is inseparable from the atmosphere; The spread of sound should use air; Parachutes, parachutes and airplanes also use the power of air.
reference data
[1] Chemistry (Grade 9) People's Education Press (April 3rd edition, 2006)
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