Some chemical reactions are accompanied by formation of precipitate. A precipitate is a solid substance formed by mixing two solutions. for example,
AgNO₃+NaCl give rise to AgClppt.+NaNO₃
When an acid and a base react with each other salt and water are obtained this reaction is called neutralization reaction.for example,
HCl+NaOH gives rise to NaCl+H₂O
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Complement-fixation reactions are serological tests based on the depletion of a fixed amount of complement in the presence of an antigen- antibody reaction.
In neutralization reactions, the harmful effects of a bacterial exotoxin or virus are eliminated by a specific antibody.

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