Questions - Answers

2 Marks Questions 

1. Lithium salts are mostly hydrated. Why?
A: Due to small ionic size of Li⁺, can polarise more water molecules easily. So Li salts are mostly hydrated.  
e.g.: LiCl. 3 H₂O
 

2. Which of the alkali metal shows abnormal density? Give the order of the variation of density among IA - elements.
A: K shows abnormal density due to unusual increase in its atomic size due to vacant d orbitals. Order of density: Li < K < Na < Rb < Cs.
 

3. What happens when 'Mg' metal is burnt in air?
A: 'Mg' gives MgO and Mg₃N₂ with air.
          2 Mg + O2   2 MgO          3 Mg + N₂    Mg₃N₂.

4. What happens if gypsum is heated upto 500°C?
A: CaSO₄.2H₂O -1   H₂O  

 CaSO₄.   H₂O   CaSO₄  CaO + SO₃
           Gypsum                                           Plaster of paris                 Anhydrite
 

5. How does plaster of paris set to a hard mass?
A: It becomes hard in two stages.
         CaSO₄ .   H₂O + 1   H₂O       CaSO₄ . 2 H₂O
                                                                                         Orthorhombic dihydrate

           CaSO₄ . 2 H₂O         CaSO₄ . 2 H₂O
      Orthorhombic dihydrate                Monoclinic dihydrate
 

6. Why is gypsum is added to cement?
A: 3% gypsum is added to cement in order to slow down the setting process.
 

7. Give the important uses of plaster of paris.
A: * In building industry.
         * In plastering the fractured bones.
         * For making statues.
         * In dentistry.

4 Marks Questions:

1. Write a short note on cement.
A: Cement is an important building material, consists CaO (50 - 60%), SiO₂ (20 - 25%), Al₂O₃ (5 - 10%), MgO (2 - 3%), Fe₂O₃ (1 - 2%), SO₃ (1 - 2%). For a good quality of cement, the ratio of SiO₂ to Al₂O₃ must be between 2.5 - 4.0. Cement is basically a mixture of clinker (formed by strong heating of clay & lime) and 2 - 3% gypsum (to regulate the setting time). Cement is used in concrete and plastering. It is used in the construction of dams, bridges, roads, tunnels etc.