Substances that prevent certain chemical reactions are called

Substances that prevent or reduce the rate of a chemical reaction are called as inhibitors.

Explanation:

Inhibitors are those substances that either prevent or reduce the rate of a chemical reaction. Ex: Acetanilide slows down the decomposition of hydrogen peroxide solution.  

Inhibitors even reduce the effectiveness of catalysts without hindering their composition. There are primarily two kinds of inhibitors- reversible and irreversible. Where, reversible inhibitors slows the rate of any reaction, irreversible inhibitors completely prevents the unwanted reactions.

Where catalysts are used to increase the rate of a reaction, inhibitors stand as a wall for the unwanted reactions to occur. For example, in case of silver tarnishing, to prevent silver jewellery being tarnished, a thin layer of rhodium is applied on it which works like an inhibitor for formation reaction of silver sulphides (the tarnish).

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