In ancient times, air was once considered as a simple substance. 1669, You Mei inferred that the composition of air was very complicated according to the experiment of candle burning. About 1700, Starr of Germany put forward a general chemical theory, namely "phlogiston theory". He believes that there is an invisible so-called phlogiston, which exists in combustible substances. For example, when a candle burns, phlogiston escapes, and the candle shrinks and collapses into ashes. He believes that burning loses phlogiston, that is, candle-phlogiston = ashes. However, phlogiston theory cannot explain some phenomena in natural changes after all, and there are serious contradictions. The first is that no one has ever seen phlogiston; Incredibly, the mass of the second metal increases after combustion, so "phlogiston" must be negative. 1774, French chemist lavoisier put forward the theory of combustion oxidation, which denied the phlogiston theory. When lavoisier was doing the combustion experiments of metals such as lead and mercury, he found that some metals turned into colored powder, and the volume of air in the bell jar decreased by 65,438+0.5 of the original volume. The remaining air could not support the combustion, and animals would suffocate in it. He called the remaining 4/5 gas nitrogen (meaning it doesn't support life). After he proved that the gas produced by priestley and Scheler from the decomposition of mercuric oxide was oxygen, the composition of air was determined as nitrogen and oxygen.

The composition of air is mainly nitrogen and oxygen, which is caused by various changes in nature for a long time. Before the appearance of primitive green plants, the primitive atmosphere was dominated by carbon monoxide, carbon dioxide, methane and ammonia. After the emergence of green plants, the free oxygen released by plants during photosynthesis oxidizes carbon monoxide in the original atmosphere into carbon dioxide, methane into water vapor and carbon dioxide, and ammonia into water vapor and nitrogen. Later, due to the continuous photosynthesis of plants, most of the carbon dioxide in the air was absorbed in the process of plant photosynthesis, which made more and more oxygen in the air, and finally formed a modern air dominated by nitrogen and oxygen.

Air is a mixture and its composition is very complicated. The constant components in the air are nitrogen, oxygen and rare gases, which are almost constant, mainly because of the mutual compensation of various changes in nature. The variable components of air are carbon dioxide and water vapor. The uncertain composition of air varies from region to region. For example, the air near the factory will contain ammonia, acid vapor and so on due to different production projects. In addition, the air also contains trace amounts of hydrogen, ozone, nitrogen oxides, methane and other gases. Dust is more or less an impurity suspended in the air. Generally speaking, the composition of air is relatively fixed.

Physical characteristics of air:

Air is the gas around us. We can't see it or taste it, but when there is wind, we can feel the flow of air.

At 0℃ and standard atmospheric pressure (1.013x10.5pa), the air density is1.293g/l.

Air state:

Air at room temperature is colorless and odorless gas, while liquid air is light yellow 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.

As a mixed gas, when air condenses at constant pressure, the temperature decreases continuously. For example, under the standard atmospheric pressure (10 1.3KPa), air begins to condense at 8 1.7K (dew point), and when the temperature drops to 78.9K (bubble point), it becomes a saturated liquid. This is because the high-boiling components (oxygen and argon) condense more at the beginning, while the low-boiling components (oxygen) condense more at the end of the process.