A catalyst is a chemical substance that increases or decreases the rate of a chemical reaction without taking part in a chemical reaction.

The +ve catalyst is the chemical substance that increases the rate of a chemical reaction. For example MnO₂. The catalyst is the chemical substance that decreases the rate of a chemical reaction. For example glycerine.

The elements or compounds that take part in a chemical reaction are reactants and the elements or compounds that are formed as a result of a chemical reaction are called products.

for example

H₂SO₄ + Zn ----------> ZnSO₄ + H₂

Reactants                    Products

When we move from top to bottom in a group of non-metal, the atomic size increases gradually. Hence, nuclear attraction for incoming electrons decreases. As a result, the tendency of non-metallic atoms to gain electrons decreases. Therefore, chemical reactivity of non-metals decreases on moving down in a group.

We know, every element of the zero group has achieved stable electronic configuration being duplet stable or octet stable They have the valency zero and do not take part in any chemical reaction. So, elements of the zero group are called inert gas.

Any three pieces of info that can be obtained from a balanced chemical eqn are:

1. The type of chemical reaction.
2. The name and symbol of reactants and products.
3. The total number of atoms of reactants and products.

The amount of a substance that contains 6.023 X 10²³ atoms or ions is called moles.

Avogadro's number is the number of particles in one mole of a substance.

N_A = 6.023 X 10²³