Chemical change is change to the chemical composition of a substance. This can be contrasted with physical change that transforms the physical properties of a substance but not its chemical composition. Chemical change includes synthesis, decomposition, combustion and replacement reactions that change a substance. The following are illustrative examples of a chemical reaction.
Electrolysis of water to produce hydrogen and oxygen
Leaves changing color in Autumn
Melting ice with salt
Mixing an acid & base such as vinegar and baking soda
Nuclear fusion by the Sun
Oxidation of copper to form a distinctive green coating known as patina
Painting a wall (drying paint oxidises)
Ripening of fruit
Synthesis of table salt from sodium and chloride
Synthesis of water with hydrogen and oxygen
Washing hands with soap
Chemical Change vs Physical ChangeChemical change transforms the chemical composition of a substance and a physical change transforms its physical properties such as density, size or state. Traditional lessons regarding this topic state that chemical changes generally aren't reversible but physical changes are typically reversible. This is unhelpful because many physical changes such as chopping wood are not reversible. Likewise, chemical reactions such as the electrolysis of water into hydrogen and oxygen is reversible. Chaotic chemical changes such as fire aren't reversible but neither are chaotic physical changes such as a car crash. The classification of physical change as "mostly reversible" and chemical change as "mostly irreversible" is questionable.Traditionally, chemical change is also described as producing odor, color changes, energy and other things you can sense such as the formation of gases. This is also a strange distinction as physical change can also produce all of these effects. For example, cutting an onion causes an odor as does burning an onion. The differentiation between chemical change and physical change requires an investigation into what is actually happening as these rules of thumb are unhelpful. It should also be noted that many processes include both physical and chemical changes such that the two aren't mutually exclusive. For example, the eruption of a volcano often involves chemical reactions but also includes potentially massive physical processes such as the release of pressure.
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