Q.Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals.

1. Nitrates All alkali metal and alkaline earth metal nitrates are water-soluble. On heating, alkali metal nitrates (except LiNO$_3$) decompose only to the nitrite plus oxygen:

   $2KNO_3 \xrightarrow{\Delta} 2KNO_2 + O_2$

   whereas LiNO$_3$ and all alkaline earth metal nitrates decompose further, to the oxide, NO$_2$, and O$_2$:

   $2Mg(NO_3)_2 \xrightarrow{\Delta} 2MgO + 4NO_2 + O_2$

   showing alkali nitrates (other than Li) are thermally more stable.
2. Carbonates Alkali metal carbonates (except Li$_2$CO$_3$) are soluble in water and thermally very stable, not decomposing on ordinary heating; Li$_2$CO$_3$ behaves like the alkaline earth carbonates and decomposes on heating (Q14). Alkaline earth metal carbonates are, in general, insoluble (or only sparingly soluble) in water and decompose readily on heating to the oxide and CO$_2$, with thermal stability increasing down the group (BeCO$_3$ least stable, BaCO$_3$ most stable) as the cation's polarising power falls.
3. Sulphates …

For each anion class, first state the general solubility (alkali salts almost always soluble; alkaline earth carbonates/sulphates progressively less soluble down/along the series) and then the thermal-decomposition products, applying the small-cation- …

• Forgetting the two named exceptions — LiNO$_3$ and Li$_2$CO$_3$ — which behave like alkaline earth salts rather than typical alkali salts.
• Stating that all alkaline earth carbonates/sulphates are equally insoluble, missing the graded trend down the group (Be/Mg more soluble, Ba least soluble for hydroxides is opposite — here for carbonate/sulphate, solubility decreases down th …