There are different opinions about when humans began to know how to use fire. According to archaeology, man knew how to use fire about 300 years ago. The power of fire has left a deep impression on human beings. The use of fire has brought great changes to human life. For example, fire can be used for lighting, baking food, warming the body, driving away wild animals, protecting safety and so on. Chemistry also develops with the use of fire. Human understanding of fire has gone through a long history, from myth to legend, to phlogiston theory, and then to burning theory.

Humans have gone through the following steps with fire:

First, use natural fire. Volcanic eruption, lightning bombardment, meteorite landing, long-term dry welding, spontaneous combustion of coal and trees, etc. , can form a natural fire. This process has been repeated many times, so that people can see the power and function of fire and gradually learn to use it. It may be to lead the fire into a cave and often put firewood in it to form a fire that is not easy to extinguish for people to use.

Second, drill wood for fire. Make a fire by drilling wood friction, and then ignite the combustible material to get the fire, and ignite the fire.

Third, make a fire with flint, sickle and tinder. Legend has it that primitive people threw stones at their prey when hunting, and sparks burst out because of the collision of stones. Over time, they learned to hit each other with stones, and then lit the fluff of plants to make a fire. Later, this method was improved in many aspects, and a systematic fire-making tool of flint, sickle and tinder was formed.

Because people know how to use fire, they gradually learned to burn ceramics, smelt metals and make glass.

Phlogiston theory

People have been exploring the nature of fire for a long time. The phlogiston theory in Europe once occupied a dominant position in18th century and has been circulating for more than 100 years.

Leonardo da vinci, an Italian scientist in the Renaissance, once pointed out that when a substance burns, it can't burn without fresh air, which is very close to the conclusion of "air supports combustion".

1630, French doctor Rey found that tin and lead gained weight after burning with air.

It was German Becher and Dr. Shtal who really put forward phlogiston theory. They all think that combustion is due to the action of phlogiston, because combustible substances contain phlogiston, and when burning, phlogiston is in and out, so the formula should be as follows:

Combustible phlogiston = ash

However, it is found that most substances will become lighter after burning, while metals will not. After combustion, their weight increases instead, so the phlogiston theory has the following formula:

Metal+particles of fire (phlogiston) = ashes

These two formulas are contradictory, which leads to the paradox of phlogiston theory. Later, some theorists put forward various theories to solve this paradox. One of them said that phlogiston has a negative weight, so when the metal burns, the negative weight runs away, so the weight of ash increases.

Although the phlogiston theory is far-fetched, it can explain most of the known chemical phenomena at that time, and it has been popular so far 100 years. The accumulated chemical knowledge during this period liberated the theory of chemical fuel supply from alchemy. The phlogiston theory is based on the image of chemistry, which reverses the real chemical relationship and causes many mistakes.

The phlogiston theory also makes the concepts of "element" and "force" empty. Why does matter burn-because there is phlogiston. As for why creatures live-because of "vitality". Why do substances combine-because of "chemical affinity". And so on, it seems that all the questions have been answered, but in fact nothing has been answered. This is essentially the same as the theology of "everything is arranged by God", so in fact, science is still imprisoned in theology, but God's will is replaced by imaginary things.

The discovery of oxygen

/kloc-after the 0/7th century, people gradually discovered many gases and understood the complex composition of air. For example, CO2 is found in 1755, and the following formula is known:

Limestone-fixed air (CO2)= caustic lime

Soda-fixed air (CO2)= caustic

Cavendish and later Lhemery discovered hydrogen; Scheler discovered nitrogen and so on. But what revolutionized chemistry was the discovery of oxygen.

The discovery of oxygen is due to priestley and Scheler.

1733, Pliester was born in a rural area near Ferhad, England. His income was meager and he lived in poverty. When I was a child, I was put in foster care at Aunt Sarah's house. /kloc-entered school at the age of 0/2. 1755 graduated from the seminary with excellent results and became a priest in Salfi. Written in1767: > In the same year, he was elected as a member of the Royal Society of London.

In 1774, we began to study the use of condensers to heat various chemicals. On August 1 day of the same year, priestley used a condenser to heat silver ash (HgO). He put the silver ash in a glass dish and heated it with a condenser, and found that gas was released. At first, he thought this gas was ordinary air, but later his curiosity drove him to study it seriously.

After research, priestley found that the candle burned in the gas he made, and the flame was very bright. Later, he put the mice in this gas again and found that there was nothing unusual about them at first, but after a while, they were all very active. He tried to inhale this gas by himself, which made him feel particularly comfortable.

Priestley made a mistake in explaining the gas he found. Because he is a believer in phlogiston theory, he thinks that the air he got is the so-called "phlogiston-free air" and he discovered oxygen, but he denied it. The wrong phlogiston theory made him proceed along a wrong theoretical road from a distorted, one-sided and wrong premise, so that when the truth touched his nose, he had not yet got the truth. His candle experiment should easily draw the conclusion that oxygen helps to burn, but he didn't. On the contrary, he tried to interpret the old theory with new experimental results and missed the opportunity to present the truth.

Another scientist who discovered oxygen at the same time was Scheler of Sweden. Scheler was an apprentice in a pharmacy in his early years. Before 1773, he did many experiments in his spare time and systematically studied the combustion phenomenon. His work was completed on1775 < <; Fire and air >>, but it was not published until 1777. There are two main ways for him to make oxygen: one is to heat some oxygen-containing compounds, such as potassium nitrate, mercury chloride and silver carbonate. The second is prepared by the reaction of manganate (MnO2) with sulfuric acid.

Scheler discovered oxygen, but he made the same mistake as priestley in explaining his discovery, because he also believed in phlogiston. He believes that it is a process in which fire and gas components in the air combine with phlogiston in fuel. Fire is a compound in which fire and gas combine with phlogiston. He blindly interprets the wrong phlogiston theory and never wants to draw practical conclusions from his own findings.

French chemist lavoisier really studied the combustion process theoretically, and his theory brought a revolution to chemistry.

Lavoisier's combustion theory

Lavoisier was born on August 26th, 1743, and graduated from Matlin School with honors at the age of 18. 176 1 year entered Ponsona College to study law, 1763 graduated. Although he studies law, he is not interested in it. He takes a nap in the law office every day when he goes to work, but he does his own chemistry experiment when he comes home. At the age of 23, lavoisier wrote a paper to solve the problem of urban lighting, and won an award from the Academy of Sciences.

1766, lavoisier gave up his lawyer work and concentrated on studying chemistry. At first, he studied the properties of gypsum and wrote two papers. 1768 was elected as an assistant professor of the Academy of Sciences.

177 1 year, lavoisier has proved through many experiments that the total weight of substances before and after the reaction remains unchanged, which is a famous law of conservation of mass. His main contribution is to prove that air is not a single simple materials, but a mixture of many gases.

Lavoisier's experiment proves that air is mainly composed of two parts, which can maintain combustion and can't. He called the part that can maintain combustion "living part", which he called "aerobic" and later renamed oxygen.

Lava's research on air composition is different from that of Pliester and Scheler. It is characterized by the use of scales and seals for strict quantitative research. Through research, the total mass of metals such as charcoal, aluminum, mercury, etc. remains unchanged after burning in a sealed glass container, no matter whether the mass of their ashes increases or decreases. Therefore, he believes that combustion is only a chemical reaction phenomenon of substances, and there is no "phlogiston" at all. The so-called "phlogiston" is just something invented because people don't understand the burning phenomenon.

In the process of studying combustion, lavoisier also studied other components in the air, such as carbon dioxide, nitrogen, etc., and pointed out that our experimental equipment can not be separated from the interference of air, and it must be closed to explain the problem strictly and quantitatively.

After studying the oxidation process in detail, lavoisier put forward the theory of oxidative combustion, which overthrew the phlogiston theory that ruled people's thoughts for more than 100 years and brought a revolution to chemistry. However, his theory was not recognized at first, and some people accused him of stealing priestley's research results. 1785, lavoisier's combustion theory was supported by Laplace, a famous scientist, and finally recognized.

Why is the sky blue?

The sky we see is often blue, especially after a heavy rain. The sky is as blue as autumn water, which makes people feel relaxed and eager to fly. Why is the sky blue?

The atmosphere itself is colorless. The blue of the sky is a picture created by atmospheric molecules, ice crystals, water droplets and sunlight.

When sunlight enters the atmosphere, long-wavelength colored light, such as red light, has great transmission power and can penetrate the atmosphere and shoot to the ground; However, violet, blue and cyan light with shorter wavelength will easily scatter when it touches atmospheric molecules, ice crystals and water droplets. Scattered purple, blue and cyan light fills the sky, making it appear blue.

Why is the sky blue, not green or red?

First of all, you have to understand a truth: the things around us are colored only because the sun shines on them. Although sunlight looks white, all colors: red, orange, yellow, green, cyan, blue and purple exist in sunlight.

There are so many colors in the sky, why do I usually only see blue? You might ask.

If you think of light as a wave, you will solve the mystery. Light actually moves like a wave. Let's imagine a drop of rain falling in a puddle. When this drop of rain falls on the water, it will produce small waves, which will fall into a larger circle and spread in all directions. If these waves hit pebbles or other obstacles, they will bounce back and change the direction of the waves.

When the sun falls from the sky, it will continue to encounter some obstacles. Because the air that light must penetrate is not empty, it is made up of many tiny particles. Ninety-nine percent of them are either nitrogen or oxygen, and the rest are other gas particles and tiny floating particles, which come from automobile exhaust, factory smoke, forest fires or volcanic ash. Although oxygen and nitrogen particles are only one millionth of a drop of rain, they can still block the path of sunlight. The light bounced back from these small stumbling blocks and naturally changed direction.

But so many colors of light have changed direction, why only see blue? You probably still don't understand.

We must go back to the puddle we just talked about.

In a puddle, if a small wave meets pebbles, the water surface will be chaotic; But if it is a "huge wave", just like the kind of "huge wave" that you lift by hand at the edge of the puddle, it may simply overflow from the stone and reach the edge opposite the puddle unimpeded. Then, just like there are big waves and small waves, all kinds of light waves have different "waves", that is, wavelengths: but unlike water waves, their size is invisible to the naked eye, because they are incredibly small, only 1% of a hair! You have to use very sensitive measuring instruments to measure accurately.

According to the determination of scientists, the wavelengths of blue light and purple light are relatively short, which is equivalent to "wavelet"; The wavelengths of orange light and red light are relatively long, which is equivalent to "big waves". When encountering obstacles in the air, blue light and purple light are "scattered" everywhere because they can't cross those obstacles, covering the whole sky-the sky, so they are "scattered" into blue.

The scientist who discovered this "scattering" phenomenon is called Rayleigh. He discovered it about 130 years ago, and he is also a Nobel Prize winner.

With the phenomenon of "scattering", the following astronomical phenomena can be explained:

For example, the sky above you is blue, but on the horizon, where heaven meets the earth, the sky looks almost white. Why? This is because sunlight travels far in the air from the horizon to where you are than it falls directly from the air-and it naturally eats more particles along the way. These large particles scatter light many times in this way, so it appears light blue in white. I suggest you do a small experiment to verify it: take a glass of water, put it in a dark background, put a drop of milk, and then illuminate one end of the cup with a flashlight and get close to it. The light of a flashlight will appear light blue in water. If you put more milk into water, the water will be whiter, because the light is repeatedly scattered by many milk particles, and the result is white. It is as white as the horizon.

When the sun goes down in the evening, the sky turns red instead of blue, and the setting sun turns dark red. Due to the countless particles that the sunset glow encountered on the way to your place, the purple and blue parts of the sunlight scattered in all directions, leaving only a little orange-red light visible to the naked eye-because of their long wavelength and "big waves", they crossed the obstacles on the road.

However, if you are careful, you will find that the sky will be dark blue for some time after sunset. This used to be a strange thing that scientists cared about, but several physicists solved the mystery 50 years ago: the blue color that caused the dusk sky was a special substance. This special substance gathers into a thick layer at a height of 20 to 30 kilometers from the earth's surface, which is called the ozone layer. This gas acts as a color filter for the falling sunlight: it intercepts the yellow and orange parts of the sunlight, but lets the blue part pass almost unimpeded. When the last light disappears, all the colors disappear in the night.

Ozone not only creates the blue sky at dusk, but also swallows a special kind of light that you can't see: ultraviolet light, or ultraviolet light. You must have heard how dangerous ultraviolet rays are to all living things, including you. If it shines on your bare skin for too long, you will get sunburned. The ozone layer everywhere is thick enough to intercept as much ultraviolet rays as possible: this is extremely important for all life on our planet.

Unfortunately, today, this life-threatening protective layer has become thinner in many places, and even a big hole has been formed over the South Pole. The killer of ozone destruction is "Freon"-a substance that people use to spray hair mousse or for refrigeration in refrigerators and air conditioners. This is a substance that is particularly harmful to the ozone layer, so many countries no longer use this "ozone killer".

Today we know why the sky in our eyes is blue. In fact, it is the same from the outside of the earth: the sea water covering two-thirds of our earth also emits blue light. Although there are brown land or green forests on the land, the sky is always blue-from the perspective of the universe, the whole earth is wrapped in a soft blue veil. Astronomers who have seen the earth from outside the atmosphere have reported this situation.

So it is absolutely right that the earth is called "blue planet". Its unique blue is the color of life.