Rancidity, types of reactions. Class 10th science chapter1

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TYPES OF REACTIONS: DETAILED

  • Combination reaction:

Those reaction in which two or more chemical substances (elements & compounds) combine together to form a single chemical substance.
 
Ex.    Mg  + O2 ➡    2MgO

• Decomposition reaction:

Those reactions in which a single chemical substance breaks down to give two or more simpler substances. 

Ex. • Thermal decomposition:
      FeSO4. 7H2O   ➡  FeSO4  + 7H2O
   
•Electrolytic decomposition:
       2H2O   ➡   2H2  +  O2

•photo decomposition:
    2AgCl   ➡   2Ag   +   Cl2


• Displacement reaction:

                     Reactivity series.

Those reactions in which a more active element displaces a less active element from its compound.

Ex.   Fe  + CuSO4   ➡  FeSO4  +    Cu


• Double Displacement:

 Those reactions in which two compound react with each other by exchanging their ions to form two new compounds.

Ex.    BaCl2  + Na2SO4 ➡  BaSO4  +  2NaCl

• Oxidation & Reduction:

OXIDATION:
• It is process which involves gain or addition of oxygen.
• It is also a process which involves removal or loss of hydrogen.


REDUCTION:
•It is process which involves gain or addition of hydrogen.
•It is a process which involves removal or loss of oxygen.

Oxidising & Reducing Agents

An oxidising agent is a substance which 
• supplies oxygen 
• takes up hydrogen.
An reducing agent is a substance which 
• supplies hydrogen
• takes up oxygen.

Effects of Oxidation & Reduction:

• Corrosion of metals 
• Rancidity of food

 CORROSION

Oxidation of metals in the presence of air, moisture or chemicals called corrosion of metals.
Ex.   Rusting 
   4Fe + 3O2 + 2xH2O ➡ Fe2O3 . XH2O

Preventions of Rusting:

Rusting of iron can be prevented by keeping the iron objects away from contact with the damp air.

• painting: By painting the iron articles like iron gates, steel furniture, window grills, bodies of Cars etc.

• Galvanisation: By coating the surface of iron objects with a thin layer of zinc.

• Greasing: By greasing and oiling the iron articles like machine parts, mechanical tools etc.

RANCIDITY

Oxidation of oils/fats in a food by atmospheric oxygen resulting into an unpleasant smell and taste, called Rancidity.

Preventions of Rancidity:

• By addition of antioxidants: In food industry, there are a number of special types of substances which are added before packing in order to slow down the process of oxidation.

• Vaccum packing: In a number of cases, after packing of food, the container is evacuated before sealing so that no air is available to bring about the oxidation of food.

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