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Chemical Reaction : – Whenever a chemical change occurs we can say that a chemical reaction has taken place
eg – Food gets digested in our body
– Rusting of iron.

Chemical Equation :– A chemical reaction can be expressed symbolically by using chemical equation 

eg magnesium is burnt into air to form magnesium oxide can be represented
as
Mg + O2 -> MgO
– We can observe or recognise a chemical reaction by observing change in state, colour, by evolution of gas or by change in temperature.

- Physical state of the reactant and products are mentioned to make chemical reaction more informative. eg we use (g) for gas, (l) for liquid, (s) for solid and (aq) for aqueous.

Balancing Equation :– We balance the chemical equation so that no. of atoms of each element involved in the reaction remain same at the reactant and product side.

eg Fe + H2O --> Fe2O3 + H2 can be written as
    3 Fe(s) + 4H2O(g) --> Fe2O3(s) +4H2(g)

Combination Reaction :– The reaction in which two or more substances combine to form a new single substance

eg CaO(s) + H2O(l)  --->  Ca(OH)2 (aq)
    Calcium      Water          Calcium hydroxide
    oxide      (slaked lime)
    Quick lime

- Ca(OH)2 slaked lime is used for white washing walls. It reacts will CO2 to form CaCO3 and gives a shiny finish to the walls.
Ca(OH)2(aq)  +  CO2(g) ---> CaCO3(s)+ H2O (l)
Calcium                             Calcium
hydroxide                         Carbonate

– Burning of Coal
C(s) + O2(g) ---> CO2(g) + heat + light
– Formation of water
2H2(g) + O2(g) --> 2H2O(l)

Exothermic Reactions :– Reaction in which heat is released along with the formation of products.
eg. CH4(g) + 2O2(g)  -->  CO2(g) + 2H2O(g)
– Respiration is also exothermic reaction.
– De composition of vegetable matter into compost.

De compositon Reactions :– The reaction in which a single substance decomposes to give two or more substances. De composition reactions can be of three types
Thermal Decompositon :– When a decompositon reaction is carried out by heating

Silver bromide behaves similarly
2Ag Br + sunlight ---------> 2Ag(s) + Br2(g)
– The above two reactions are used in black and white photography.

Endothermic Reactions – The reactions which require energy in the form of heat, light or electricty are called Endothermic Reactions.
2Ba(OH)2 + NH4Cl ---->  2BaCl2 + NH4OH
Displacement Reaction : The chemical Reaction in which an element displaces another element from its solution
Fe(s) + CuSO4(aq)  ---> FeSO4 + Cu(s)
Copper (aq)
Sulphate                     Iron Sulphate

Double Displacement Reaction : The reaction in which two different atoms or group of atoms are mutually exchanged
eg. Na2 SO4 + BaCl2 ---> BaSO4(s) + 2NaCl
(aq) (aq) (aq)
Sodium          Barium      Barium        Sodium
Sulphate        Chloride     Sulphate     Chloride
A white substance is formed due to above reaction. The insoluble substance
is called precipitate.
Precipitation Reaction – Any reaction that produces a precipitate is called a precipitation reaction.

eg. Pb(NO3)2 + 2KI ---> PbI2 (down arrow) +2KNO3
(aq) (aq) (aq)
Lead Nitrate    Potassium Lead                   Potassium
                      Iodide      Iodide                   Nitrate
– Oxidation : Oxidation is the gain of oxygen or loss of hydrogen
eg. 2Cu + O2 heat--> 2CuO
When Copper is heated a black colour appears. If this CuO is reacted with
hydrogen gas then again Cu becomes brown as reverse reaction takes place
CuO + H2  heat--> Cu + H2O
Reduction : Reduction is the loss of oxygen or gain of hydrogen.
– Redox Reaction : The reaction in which one reactant gets oxidised while other gets reduced
eg. ZnO + C  ---> Zn + CO
MnO2 + 4HCl ---> MnCl2 + 2H2O + Cl2

Corrosion : When a metal is attacked by substances around it such as
moisture, acids etc.
eg. Reddish brown coating on iron.
(ii) Black coating on Silver.
– Rancidity : When fats and oils are oxidised they become rancid and their
smell and taste change.
– Antioxidants are added to foods containing fats and oil.

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