The combination or displacement that occurs in themolecules of matter during a chemical changecalled a chemical reaction.Any two examples are:

1. 2H₂ + 0₂ -------> 2H₂0
   
2. H₂SO₄ + Zn ---------> ZnSO₄ + H₂

A chemical reaction expressed by writing the fullnames of reactants and products is called a wordequation. For example:

Hydrogen + oxygen ----------->Water

A chemical equation is an equation that representsthe chemical reaction with the symbols and molecularformulae of the reactants and products.

For example: 2H₂+ 0₂ --------> 2H₂O

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

for example

H₂SO₄ + Zn ----------> ZnSO₄ + H₂

ReactantsProducts

A chemical eqn. which contains an equal number ofatoms of each element in both reactant and productside is called balanced chemical eqn.

Example: 2H₂ + O₂ --------> 2H₂O

Anythree pieces of info that can be obtained froma 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.

Four factors that bring about a chemical change are:

> Heat > Catalyst
> Light >Solution
> Pressure > Electricity

A catalyst is a chemical substance that increases ordecreases the rate of a chemical reaction without takingpart in a chemical reaction.

The +ve catalyst is the chemical substance thatincreases the rate of a chemical reaction. For example MnO₂.Thecatalyst is the chemical substance thatdecreases the rate of a chemical reaction. For exampleglycerine.

A chemical reaction in which two or more reactantsare combined together to give a single product is calleda combination reaction.Examples are:

C + O₂ --------> CO₂
C + 2H₂ ---------> CH₄

A chemical reaction in which a single reactant isbroken down into two or more products called adecomposition reaction.For example:

CaCO₃ -------> CaO + CO₂
CuCO₃ -------> CuO +CO₂

A chemical reaction in which an atom of a compoundis displaced by anotherelement is called a displacementreaction. Its types are:

1. Single displacement rxn.
2. Double displacement rxn.

A chemical reaction in which aid and base reacttogether to gee salt and water is called an acid-basereaction.

It is called a neutralization reaction because the acidicand basic nature of compounds is neutralized duringthe reaction.

The atomic weight of an element expressed in grams iscalled gram atomic weight.

The molecular weight of a substance expressed ingrams is calledmolecular weight.

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

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

N_A =6.023 X 10²³

The burning of magnesium in the air conducts that reactionwhere magnesium and oxygencombine to formmagnesium oxide. As two elements combine each other, it iscalled a combination reaction.

2Mg + O₂ ------> 2MgO

We know, a chemical reaction in which an acid and basereact together to give salt and water is called an acid-basereaction. Here acidic and basic nature et compounds areneutralized. So, it called a neutralization reaction.

We know, water breaks down into hydrogen andoxygen during its electrolysis. As two elementsdecompose themselves in this chemical reaction, it is called adecomposition reaction.

2H₂0 --------> 2H₂ + O₂

The positive change in the concentration of areactant or a product per unit time is called therate of a chemical reaction.Any four factors that affect the rate of a chemicalreaction are:

1. The concentration of reactants
2. Temperature
3. Physical nature of reactants
4. Chemical nature of reactants

An increase in temperature increases the rate e chemicalreaction whereas a decrease in temperature decreases the rateof a chemicalreaction.

Positive catalyst increases and negative catalystdecreases the rate of a chemical reaction.

a. Reactants and products.

b. Word equation and formulae equation.