Some of the ancient scholars believed that the world consists of one substance in a different form. The general concept of matter exists now, but still the ancients were wrong. They proceeded from this error when they tried to make lead turn into gold. However, alchemists switched from physical processes to chemical ones, creating a new science. Now, every well-performing student knows the signs of the difference between these two processes. What are chemical reactions?
Examples of them can be seen even in everyday situations. Many children watch their mother quench soda with vinegar, and so they get the very first impressions of chemical reactions. But in some situations, you can draw the attention of the child to other manifestations. For example, rusting iron and darkening silver. These are also chemical reactions.
Let us return to the difference in the processes mentioned at the beginning of the article. So, during the physical process, the molecules of the substance do not change. The body can be forced to change shape, it can be crushed into particles, its state of aggregation can be changed. But the composition of the particles will remain unchanged. You can crush an aluminum spoon into pieces or with the help of colossal temperatures and pressure make it evaporate - but the body will remain aluminum.
In a chemical reaction, bonds are rearranged, although the composition of the substance may remain the same. For example, if diamond is obtained from graphite, this should not be considered a physical process, but a chemical one, since the bonds are rearranged in this case. Although, as a rule, the composition of substances varies, the example with diamond is an exception.
Typically, chemical reactions are associated with the release or absorption of energy. The latter manifests itself as heat. There are also thermoneutral reactions in which the thermal effect is zero. But there are very few of them, most either require "injections" of energy, or go on their own and are accompanied by a change in temperature. The combustion process is additionally accompanied by the release of light.
But far from always changing the deep composition of a substance can be called a chemical process. There are also nuclear reactions. They affect matter at the atomic level. In this case, the latter are transformed into atoms of another variety - a different chemical element. And this is a very complex process in which a tremendous amount of energy is released. It is enough to recall nuclear explosions to understand what it is about.
To fix the ongoing processes, people began to use a special tool: the equation of a chemical reaction. On the left are substances that carry the name of the reagents, on the right, the products. Not everyone liked to put coefficients in equations in school. I must say that these coefficients simply reflect real processes, and are not created “out of the head” by a person. After all, atoms do not arise from nowhere. Modern students are lucky, they can learn the formulas of substances, but the ancients had to rely only on the amount of product and reagent, which is not always obvious.
How to distinguish between physical and chemical processes? Most often, the release of heat, gas or precipitation, discoloration. But this approach does not always work, because even a pre-dissolved gas comes out of an open bottle with soda, mercury oxide can change color when chopped, and zinc oxide changes color when heated. The thermal effect can be very weakly expressed, so it is not easy to fix, so that such chemical reactions can be detected using ... other reactions.
In the science of substances there is an interesting section - mechanochemistry. As a result of mechanical influences in special mills, free radicals appear in the substance , and it acquires new properties. For example, this is how aspirin is made in pharmaceuticals. Mechanically activated substances are more chemically active, which is important for drugs.