👉 Koppen is a scientific system used to classify and describe physical properties of substances. It was introduced by German chemist Fritz Klein in 1905, and it is based on the idea that certain elements have certain chemical properties or "kinds" (e.g., alkali, alkaline earth, alkaline metal). The classification is divided into two categories: bulk and phase. In bulk, substances are classified by their physical state (solid, liquid, gas) and their temperature