What is chemical and physical change

Basic concept of chemical reaction

Processes in which new substances are created are referred to as chemical processes or chemical reactions. The basic concept of chemical reaction describes the changes in substances from a macroscopic and a submicroscopic point of view. Macroscopically, a chemical reaction can be characterized with the help of a number of indicators that relate to the material change and the energy expenditure during this change. Chemical reactions - i.a. also for a more precise delimitation to physical changes - define as processes in which bonds between atoms or ions are broken and re-established and these undergo changes in their arrangement. Significant chemical reactions can be recorded with the acceptor-donor model and described from the perspective of redox reactions and acid-base reactions. The depth of the level of interpretation, especially with regard to the course of chemical reactions, is determined by the introduced atomic model.

Using the example of everyday processes, the students first learn that chemical reactions can be distinguished from physical changes. A change in the physical state due to thermal effects is a physical process that is reversible. Evaporates z. If, for example, a liquid is supplied with energy and becomes a gas, which becomes a liquid again through condensation, the substance and its properties have been retained. These physical changes in a substance can also be understood at the sub-microscopic level on the basis of a simple particle model.

The students learn that the effects of temperature can also lead to chemical reactions. Using the example of well-known processes, they learn that chemical reactions create new substances. These new substances are called reaction products. Their properties differ from the starting materials (also called educts) from which they emerged. The speed at which the starting materials are converted into the products can be influenced.

With the help of simple experiments, the students learn that the mass does not change during chemical reactions. With this knowledge, the lesson takes on the quantitative consideration of chemical reactions. A simple atomic model makes it possible to explain the law of the conservation of mass. In the further course, the students learn that the particles involved in chemical reactions always react with one another in defined proportions.

The process of reorganization - or, in general, the reorganization of particle arrangements in chemical reactions - is first described in the form of reaction schemes, later in reaction equations.

A distinction can be made between different types of reactions in the classroom. Initially, redox processes are described as the uptake and release of oxygen, then later applied as electron donor and acceptor reactions, e.g. B. in the processing of technical metal extraction processes and electrochemical processes.

The students get to know acids as substances whose aqueous solutions contain hydrogen ions. Alkaline solutions are solutions that contain hydroxide ions. This idea is later expanded to include the definition of acids as hydrogen ion donors and bases as hydrogen ion acceptors. These ideas are used in the development of a number of phenomena relevant to everyday life as well as in the treatment of technical processes.

On the basis of the introduced atomic model and the knowledge of the periodic table, the students learn in advanced lessons that atoms are retained in a chemical reaction, but that they experience changes in the electron shell during this process. They learn that different types of bonds exist and that there is a different type of bond between atoms than between ions. The depth of the level of interpretation is determined by the introduced atomic model.

On the basis of the knowledge that atoms are retained in chemical reactions, a system of successive reactions can also result, whereby one type of atom is inherent in all subsequent reactions and a material cycle can thus be described (e.g. so-called carbon cycle or so-called nitrogen cycle ). The concept of cycle is closely linked to the aspect of the reversibility of chemical reactions.

The knowledge that chemical reactions are reversible and can run incompletely should be initiated in advanced lessons in order to facilitate the understanding of the dynamic equilibrium that is developed in the upper school level.

These materials are part of the curriculum navigator offer of the Quality and Support Agency - State Institute for Schools (QUA-LiS NRW)