It is clear that in the case of the unlucky researcher (he stayed alive, but decided to study chemistry), we are dealing with some new phenomenon - not physical, but chemical. Sodium in this experience ceased to be itself - sodium metal, turning into another substance. There was a typical chemical transformation, while in other experiments (with the heating of sodium or measurement of its electrical conductivity) the substance remained the same as silver sodium metal, although it was undergoing physical transformations: sodium melted, conducted current, etc.
Chemical phenomena include such phenomena in which one substance turns into another. Chemical phenomena are called differently chemical reactions. Physical phenomena are not accompanied by the transformation of some substances into others.
Our researcher, without wishing to do so, has made a typical chemical reaction between sodium and water. Sodium and water have become other substances. Fortunately, only a few chemical transformations are as rapid. When we study chemistry at school, we will be dealing with "calm" and long-studied transformations. But professional chemists often have to be wary: even when the first studied chemical transformations go without explosions, it is necessary to remember that in the experiment may turn out new, previously unknown substances with some unusual properties.
Chemistry is the science of substances and the laws by which they are transformed into other substances.
If we wanted to follow when chemistry appeared, we would have found its traces at the very origins of human civilization - at a time when crafts and art were born. Since then, the history of mankind has been inextricably linked to the production of the substances that mankind needs - be it gunpowder, medicine, paper, steel, paint, fuel and car lubricants, cement, glass, porcelain and much more - that do not exist in nature in their finished form. This led to the constant accumulation of chemical knowledge, which, in turn, pushed the further development of civilization.
Now, when the world population has reached 7 billion, it is difficult to imagine how one could feed, protect from diseases, dress and warm in the cold time such a huge number of people without using synthetic materials, medicines, artificial fertilizers, products of oil processing, coal, gas, wood, various ores. Equally important are many things that simply decorate our lives, making them easy and comfortable. These are modern airliners made of the latest materials, which carry us without landing on the other side of the Earth, lively colors of cinema and television, artificial treadmills of stadiums, bright colors of light and comfortable clothes, beautiful fragrances of perfume and shampoos. And many, many other things that in one way or another are connected with chemistry.
Unfortunately, the uncontrolled growth of consumption, which is often not kept up with the improvement of production technologies, in many countries has caused many problems. Nature is experiencing unbearable "stresses" on the environment, which is created by man. If the consumption of new and new materials will grow at the same rate, and the technology of production of these substances will leave mountains of waste on the Earth, the nature will pay for it by climate change, termination of fruiting of the earth, unpredictable genetic changes of the man himself, disappearance of many species of animals, birds, fish, plants.
To avoid serious trouble in the future, people should constantly look for ways to produce the substances they need so that the environment is not destroyed. And it is still necessary to constantly invent more and more effective materials instead of the old ones that created problems. It is necessary to invent ways of their non-waste production, to use reasonably irreplaceable sources of raw materials, to learn to process wastes of industry and life activity. And it should be done in such a way that mankind does not lose the achieved opportunities to enjoy a full life. And here again, the decisive word for chemistry.
But this is just one thing - the applied side of chemistry. Even more interesting is its role in the knowledge of the world around it. There is still a lot of obscure, even mysterious things here. One should not think that modern science already knows everything. For example, our planet can be compared to a powerful chemical "machine". On the surface of the Earth and in its depths, the most complicated chemical transformations are constantly taking place. How does this "machine" work? How are the conditions for its uninterrupted operation maintained? How does the Earth manage to maintain an approximately constant (and best!) temperature for complex chemical reactions over billions of years? Why has the oxygen content of the Earth's atmosphere remained constant (to the nearest percent!) for the past half a billion years?