Oxidation is the gain in oxygen, loss of hydrogen, loss of electron or increase in oxidation number, while reduction is the loss of oxygen, gain in hydrogen, gain in electron or decrease in oxidation number.
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- Gain/loss of oxygen, e.g.
The reactant H2 is oxidised as it gains an oxygen atom to form H2O, while O2 is reduced as it loses an oxygen to form H2O.
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- Gain/loss of hydrogen, e.g.
The reactant H2S is oxidised as it loses hydrogen atoms to form S, while Cl2 is reduced as it gains hydrogen atoms to form HCl.
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- Gain/loss of electron, e.g.
The reactant Zn is oxidised as it loses two electrons to form Zn2+, while Cu2+ is reduced as it gains two electrons to form Cu.
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- Increase in oxidation state or oxidation number (see next article for details), e.g.
The reactant Zn is oxidised as its oxidation state/number increases from 0 in Zn to +2 in Zn2+, while Cu2+ is reduced as its oxidation state/number decreases from +2 in Cu2+ to 0 in Cu.
Next article: Oxidation state or oxidation number
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