Why is desorption important for a substance to act as good catalyst ?

Why is ozone layer important ?

Why is melting point of a substances used as a criterion for testing the purity of a substances?

Why does V_2O_5 act as catalyst ?

Read the given passage and answers following questions : A substance which alters the rate. of chemical reaction without itself undergoing any change in mass and chemical composition at the end' of reaction is called catalyst. It may be noted that a catalyst which increase the speed of a reaction are called positive catalyst and catalyst which decrease the speed of reaction are called negative catalyst. The promoters are substances that enhance activity of catalyst and poisons which decrease the activity of catalyst. Amines act as Lewis bases.