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Chemical Reactions are associated with chemical change.

Whenever a chemical change occurs chemical reaction is said to take place.

Chemical reactions are usually irreversible and a new product is formed.

Signs of Chemical Reactions

There are four main signs that indicate a chemical reaction has taken place:

change in color Evolution of a gas Change in temperature Change in state

A WORD EQUATION describes chemical change using the names of the reactants and products.

The word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water.

methane + oxygen

Reactant Product

CH4 O2 CO2 H2O+ + 22

carbon dioxide + water

Showing Phases in Showing Phases in Chemical EquationsChemical Equations

(i) STATES: 1:solid NaCl(s)

2:liquid H2O(l)

3:gaseous Cl2(g)

(ii) Medium & Pressure: atm pressure is denoted over the arrow sometimes whereas medium for reaction like heat is written over the arrow

(iii) Blancing.

Cl

Cl

ClH

H

H

ClClCl

Cl HHH

H

H2 + Cl2 HCl H2 + Cl2 2 HCl

reactants products

H

Cl

reactants products

H

Cl

22

2 22 2

11

(unbalanced) (balanced)

Unbalanced and Balanced Equations

• According to the law of conservation of mass, total mass must be equal on the both sides of the equation.

• This type of equation is known as a balanced chemical reaction. E.g.-

2Mg + O2 2MgO

• Here both sides have two atoms of Magnesium and two atoms of Oxygen.

1. Combination reaction

2. Decomposition reaction

3. Single-displacement reaction

4. Double-displacement reaction

A + B AB

AB A + B

A + BC AC + B

AB + CD AD + CB

5. Redox reaction

These type of reactions occur when two reactants combine to form one or more products. They are generally exothermic reactions which involve evolution of heat during reaction.

They are of three types:1)Element + element = compound

2)Element + compound= compound

3)Compound + compound=compound

• The types of reaction in which a single reactant breaks down to give simpler products are called decomposition reaction.

2 H2O 2 H2 + O2

Decomposition ReactionIt is mainly of three types:

(i)Photo Decomposition

(ii)Thermal Decomposition.

(iii)Electrolytic Decomposition.