Final answer:
Chemical reactions in Column A change the properties of materials, resulting in new substances with different characteristics.
Step-by-step explanation:
The reactions listed in Column A are classified as chemical reactions because they change the properties of the materials. A chemical reaction involves forming new substances with different properties from the originals. This means choice C, 'They changed the properties of the materials,' is the correct answer. For instance, during a chemical reaction, substances might display changes such as color transformation, temperature variation or light production, gas generation, and formation of a precipitate. Additionally, chemical reactions are often categorized into types like precipitation, acid-base, and oxidation-reduction based on certain characteristics and patterns, and these transformations involve a significant shift in energy and are not easily reversible.