Give reasons for the following :
(i) `Be(OH)_(2)` dissolves in NaOH but `Mg(OH)_(2)` does not . (ii) Magnesium oxide is used as a refractors material , (iii) During electrolysis of molten sodium chlorides , calcium chloride and potassium fluoride are added .

Explanation

(i) `Be(OH)_(2)` is amphoteric and hence it dissolves in NaOH forming sodium beryllate
`Be(OH)_(2) + 2NaOH to Na_(2)BeO_(2) + 2H_(2)O`
`Mg(OH)_(2)` on the other hand , basic does not dissolve in NaOH.
(ii) MgO has high lattice energy due to greater charge and smaller ionic size of `Mg^(2+)` and `O^(2-)` ions . As a result , it has a very high melting point and hence is used as a refractory material .
(iii) Pure sodium chloride metals at 1084 K . The cost of electricity to attain and maintain this temperature is very high . Furthermore , `Cl_(2)` gas evolved during electrolysis has corrosive action on the cell at high temperatures . Therefore , to lower the fusion temperature to 850-875 K , some `CaCl_(2)` and KF are added to molten NaCl.