When an electric current is passed through acidified water, the bonds between the atoms break, forming ions of hydrogen and oxygen. The positively charged hydrogen ions migrate to the negative electrode, where they acquire electrons and are reduced to hydrogen gas-new molecules of H2. Negatively charged ions migrate to the positive electrode, where they release electrons and oxygen gas (02) is liberated. (This process, discovered in 1800 by two English scientists, William Nicholson and Anthony Carlisle, was called electrolysis by English chemist Michael Faraday (1791-1867). If the gases produced in this way are mixed, and then a spark is applied, the mixture explodes and the gases recombine to form water. Both changes, the decomposition and the recombination, are examples of chemical reactions, in contrast to changes in physical state. They illustrate two important principles:
■ Substances will change into entirely different substances if conditions are right. Such changes are called reactions. When a chemical reaction occurs, the molecules at the end of the reaction are different from the molecules at the beginning.
■ Chemical reactions can be reversed. As in the case of water, a compound can be changed into different compounds and these can then be changed back into the original compound. Under the right conditions, nearly all chemical reactions are reversible.
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