What is catalyst?

A catalyst is a chemical substance which increases or decreases the rate of chemical reaction out itself remaining chemically unchanged . There are two types of catalyst,they are: i) positive catalyst: A catalyst which increases the rate of chemical reaction is called a positive catalyst. For eg. manganese dioxide (MnO2), Iron powder(Fe) ii) Negative catalyst: A catalyst which decreases the rate of chemical reaction is called negative catalyst. For eg. Glycerine