Contrast the action of heat on the following with reason:
a. `Na_(2)CO_(3) and CaCO_(3)`
b. `MgCl_(2).6H_(2)O and CaCl_(2).6H_(2)O`
c. `Ca(NO_(3))_(2)` and `NaO_(3)`

Explanation

(a). On heating, the decahydrate loses water of crystallisation to form monohydrate. Above `373 K`, they completely become anhydrous and become a powder.
`Na_(2)CO_(3).10H_(2)Ooverset(373 K)(rarr)Na_(2)CO_(3).H_(2)O+9H_(2)O`
`NaCO_(3).H_(2)O overset(above)underset(373 K)(rarr)underset((soda ash))(Na_(2)CO_(3))+H_(2)O`
The `Na_(2)CO_(3)` or soda ash is thermally very stable and does not decompose upon heating.
When `CaCO_(3)` is heated above `1200 K`, it decompose to evolve carbon dioxide.
`CaCO_(3)overset(1200 K)(rarr)CaO+CO_(2)`
(b). When `MgCl_(2).6H_(2)O` is heated, it decomposes to give magnesium oxide `(MgO)`.
`MgCl_(2).6H_(2)OrarrMgO+5H_(2)O+2HCl`
The halide of calcium is hyproscopic. The `CaCl_(2)6 H_(2)O` get dehydrated upon heating.
`CaCl_(2).6 H_(2)O overset(heat)(rarr)CaCl_(2)+6 H_(2)O`
On this basis it is used as a dehydrating agent.
(c ). When calcium nitrate is heated, it decompose and forms the oxide.
`2Ca(NO_(3))_(2)overset(Heat)(rarr)2CaO+4NO_(2)+O_(2)`
When sodium nitrate is heated strongly it gives nitrates and evolve oxygen.
`2NaNO_(3)overset(Heat)(rarr)2NaNO_(2)+O_(2)uarr.`