chemical reactions proceed such that less stable materials are transformed into more stable materials. A ball rolling down a slope is a useful analogy. Chemical reactions proceed in a ‘downhill direction’, where downhill signifies toward more stable products, products that are characterized by what is termed lower ‘free energy’. Since the free energy of water is lower than the free energy of a mixture of hydrogen and oxygen gases, the two gases react to form water, and the energy that was stored in the higher-energy hydrogen and oxygen molecules is released as heat. The reverse reaction in
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