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).

when heated, the valance electrons increased their speed and transferred the heat from electron to electron.   i just took the test.

80cm/s.

24cm/s.

53.3cm/s.