Final answer:
Chemical reactions triggered by external factors such as UV light, heat, or moisture can involve compounds like epoxies that harden upon stimulation. Initiators start the chain reactions leading to polymers like polyethylene, while oxidizing chemicals can drive combustion. Compounds are created or destroyed exclusively through chemical reactions, which change atomic bonds.
Step-by-step explanation:
A chemical reaction that is initiated by an external trigger like UV light, heat, or moisture involves compounds that can respond to such stimuli. For example, epoxies, which are reactive ethers used in various applications, can harden when exposed to an appropriate catalyst or environmental condition. These are types of chain reactions, which can be started by an initiator that provides the necessary energy or active species to propagate a sequence of bond-forming events, leading to a polymer like polyethylene.
In the case of polyethylene production, a free-radical initiator donates an electron to a monomer like ethylene, which then becomes reactive and can join with other monomers to form a polymer. Oxidising chemicals, an example being potassium dichromate, can also cause chemical reactions by providing oxygen to fuel combustion. Furthermore, compounds can undergo a variety of reactions, such as the reaction of calcium oxide with water to form calcium hydroxide, a type of synthesis reaction.
Overall, a compound is a substance composed of two or more elements combined in a fixed ratio, exhibiting properties different from those of the constituent elements. Compounds can only be created or broken down through chemical reactions, which involve changes in the bonding between atoms. An example in combustion is the reaction of methane with oxygen to produce carbon dioxide and water.