Answer Chemistry

 

Answer Key

Section A 

1. Chemical reaction:
   A process in which one or more substances undergo a chemical change to form new substances with different properties.
2. Observable sign:
   Change in colour / evolution of gas / change in temperature / formation of precipitate (any one).
3. Balanced chemical equation:
   An equation in which the number of atoms of each element is the same on both sides.
4. Combination reaction:
   A reaction in which two or more substances combine to form a single product.
   Example:
   CaO + H₂O → Ca(OH)₂
5. Type of reaction:
   Decomposition reaction.
6. Corrosion:
   The gradual destruction of metals due to reaction with environment.
   Example: Rusting of iron.
7. Chemical formula of rust:
   Fe₂O₃·xH₂O (hydrated iron(III) oxide)
8. Rancidity:
   Oxidation of fats and oils leading to unpleasant smell and taste.

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Section B (2 × 6 = 12 marks)

9. Balanced equation:
   3Fe + 4H₂O → Fe₃O₄ + 4H₂

10. Steps to balance a chemical equation:

• Write the skeletal equation
• List atoms on both sides
• Balance one element at a time
• Balance hydrogen and oxygen last
• Check if atoms are equal
• Write physical states (if needed)

11. Type of reaction:
    Displacement reaction
    Justification: Zinc displaces copper from copper sulphate solution.

12. Reactions:

       a) Magnesium burns with a bright white flame forming magnesium oxide:
        2Mg + O₂ → 2MgO

       b) Iron displaces copper from copper sulphate solution:
       Fe + CuSO₄ → FeSO₄ + Cu

13. Difference:

Exothermic Reaction	Endothermic Reaction
Releases heat	Absorbs heat
Example: Combustion	Example: Decomposition of CaCO₃

14. Reason:
    Magnesium ribbon is cleaned to remove the oxide layer (MgO) so that it burns properly.

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Section C 

       a) 2Na + 2H₂O → 2NaOH + H₂
       b) Combination reaction
       c) Exothermic reaction

                     OR

       a) Oxidation: Addition of oxygen or loss of hydrogen
       b) Reduction: Removal of oxygen or gain of hydrogen
      c)

•    Oxidising agent: CuO
•    Reducing agent: H₂

     a) Decomposition reaction: A reaction in which a compound breaks down into simpler substances.
     Example: CaCO₃ → CaO + CO₂

     b) Electrolysis of water:
     2H₂O → 2H₂ + O₂

        OR

     a) Displacement reaction: A more reactive element displaces a less reactive element.
     Example: Zn + CuSO₄ → ZnSO₄ + Cu

     b) It is called displacement reaction because one element replaces another in a compound.

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Section D (Case-Based) (2 marks)

a) Pb(NO₃)₂ + 2KI → PbI₂ (yellow ppt) + 2KNO₃

b) Double displacement (precipitation) reaction