Overview:
Chemical reactions change one or more types of substances into completely different substances with different physical and chemical properties from the components that existed before the reaction. For example, a shiny copper wire exposed to air will eventually rust, because the oxygen in the air will combine with copper to form copper oxide. Types of reactions include, direct composition reactions, decomposition reactions, single replacement reactions, and double replacement reactions.
What Are Direct Combination Reactions?
Direct combination reactions occur when one or more chemicals combine to form one product. For example, in the formation of table salt, sodium and chlorine combine directly to form sodium chloride. The product is very different than either sodium or chlorine. Besides chemical elements, chemical compounds can join together in direct combination reactions. For example, carbon dioxide (CO2) reacts with water (H2O) to form carbonic acid (H2CO3).
What Are Decomposition Reactions?
A decomposition reaction is just the opposite. One product is broken down into several components. For example, water (H2O) can be broken into hydrogen and oxygen. Similarly, calcium carbonate (CaCO3) can be broken down when heated into calcium oxide (CaO) and carbon dioxide (CO2). When mercury (II) oxide (HgO) is heated, it breaks down into mercury (Mg) and oxygen (O2). This was one of the early ways that free oxygen was produced.
What Are Single-Replacement Reactions?
In single-replacement reactions, one element that is not already part of a compound replaces an element that is already a part of a compound. Those reactions differ from the direct combination reactions and the decomposition reactions because the free element is not part of an existing compound. Metals replace other metals and nonmetals replace other nonmetals. For example, if an iron nail (Fe) is placed in a solution of copper II sulfate (CuSO4), the iron will replace the copper in the copper (II) sulfate, as FeSO4, leaving free copper.
What Are Double-Replacement Reactions?
In double-replacement reactions, the elements in two different compounds switch places to form two different compounds. For example, calcium carbonate (CaCO3) in an antacid tablet reacts with the hydrochloric acid (HCl) in the stomach to produce calcium chloride (CaCl2) and carbonic acid (H2CO3), which harmlessly decomposes further into water (H2O) and carbon dioxide (CO2). The calcium and the hydrogen replace each other in compounds.
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