203.

Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals. (a) Nitrates (b) Carbonates (c) Sulphates. 

Carbonates of metal: Thermal stability

The carbonates of alkali metals except lithium carbonate are stable to heat. The carbonates of group-2 metals and that of lithium decompose on heating, forming an oxide and carbon dioxide . For example,
Li2CO3 +heat -> Li_­2O +CO2
MgCO3 +Heat -> MgO +CO2
Na2CO3 +heat -> no effect.
The stabilities of carbonates of alkaline earth metals increase on moving down the group.For example, BeCO3 decompose at 373K, MgCO3 at 813K, CaCO3 at 1173K, SrCO₃ at 1563K .

solubilities in water:

carbonates of alkali metals, except Li2CO3, are soluble in water. Their solubilities increase on moving down the group. Carbonates of group-2 metals are almost insoluble in water and their solubilities further decrease on moving down the group. In fact, these metals can precipitate from their salt solutions as carbonates.

Nitrates: Thermal stability
Nitrates of alkali metals,except LiNO3, decompose on strong heating forming nitrites and oxygen. for example,
2KNO3 -> 2KNO₂ +O₂
Nitrates of alkaline-earth metals and LiNO3 decompose on heating to form oxides, nitrogen to form oxides, nitrogen dioxide and oxygen.
2LiNO3 +Heat -> Li₂O +2NO₂ +O₂
2Ca(NO₃)₂ +Heat -> 2CaO +4NO₂ +O₂
Thermal stabilities of nitrates of group-1 and group-2 metals increase on moving down the group from top to bottom.

solubility :
Nitrates of group -1 and group-2 metals are all soluble in water. The solubilities of these salts further increase on descending the group.


Sulphates: Thermal stability
The sulphates of group-1 and group-2 metals are all thermally stable.

solubility: 
sulphates of alkali metals are soluble in water. The sulphate of alkaline earth metals is less soluble. Their solubilities decrease on moving down the group from Be to Ba.