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

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.

When theconcentration of Sodium thiosulphatewas increased therateofreactionincreased and the time taken to reach equilibrium decreased, so therefore therateofreactionis directly proportional to theconcentration.So rate of reaction increases.

a. Reactants and products.

b. Word equation and formulae equation.