Chemie Grundlagen: Oxidation und Nachweise

Observations of Chemical Reactions in Everyday Life

A common example of a chemical reaction can be observed in daily life when an apple is cut into pieces. After a short period, the exposed surface of the apple begins to turn brown. This discoloration occurs because oxygen ($O_2$) from the air reacts with the substances contained within the apple. As a result of this specific interaction, new substances known as brown pigments or brown dyestuffs are produced on the surface of the fruit.

The Fundamental Concept of Oxidation

Oxidation is defined as a specific type of chemical reaction. During this process, a substance absorbs or takes up oxygen ($O_2$). This reaction leads to the creation of an entirely new substance, which is categorized as an oxide.

Core Laws of Chemical Composition and Gases

The Law of Constant Proportions, established by Lewis Proust in the year 1794, states that a chemical compound always consists of elements in a fixed and characteristic mass ratio. This means the proportions by mass of the constituent elements in a specific chemical compound are always the same, regardless of how the compound was prepared or its source.

Avogadro's Law, also known as the Principle of Avogadro, specifies the behavior of gaseous substances. It dictates that equal volumes of different gases, when measured at the same temperature and the same pressure, contain an equal number of particles. This physical law remains true regardless of the identity or nature of the gas involved in the comparison.

Qualitative Detection Methods for Hydrogen Gas

The oxyhydrogen test, known in German as the "Knallgasprobe," is a procedural method used to detect the presence of hydrogen. To perform this test, the gas in question is collected inside a test tube. A flame is then brought to the opening of the tube. If hydrogen ($H$) is present, a distinct "plopp" sound is heard. This phenomenon occurs because the hydrogen reacts extremely quickly with oxygen ($O_2$) to form water ($H_2O$). During this rapid reaction, a significant amount of energy is released. This energy causes the gases to heat up and expand rapidly, creating a pressure wave that manifests as the audible "plopp" sound.

Experimental Identification of Oxygen and Carbon Dioxide

To detect oxygen, the glowing splinter test (known as "Glimmspanprobe") is utilized. This involves taking a wooden splinter that is glowing or smoldering but not actively burning and placing it into the container holding the gas. If oxygen ($O_2$) is present in the container, the splinter will reignite into a flame and begin to glow with much greater intensity.

The detection of carbon dioxide ($CO_2$) is achieved through the limewater test (known as "Kalkwasserprobe"). When carbon dioxide is introduced to limewater (a solution of calcium hydroxide), the water turns cloudy or takes on a milky appearance. This change happens because the carbon dioxide ($CO_2$) reacts chemically with the calcium hydroxide ($Ca(OH)_2$) dissolved in the water, producing a visible precipitate.