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

Question:

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.

Solution:

(i) Nitrates

Thermal stability

Nitrates of alkali metals, except $\mathrm{LiNO}_{3}$, decompose on strong heating to form nitrites.

$2 \mathrm{KNO}_{3(s)} \longrightarrow 2 \mathrm{KNO}_{2(s)}+\mathrm{O}_{2(g)}$

$\mathrm{LiNO}_{3}$, on decomposition, gives oxide.

Similar to lithium nitrate, alkaline earth metal nitrates also decompose to give oxides.

$2 \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{(s)} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{CaO}_{(s)}+4 \mathrm{NO}_{2(g)}+\mathrm{O}_{2(g)}$

As we move down group 1 and group 2, the thermal stability of nitrate increases.

Solubility

Nitrates of both group 1 and group 2 metals are soluble in water.

(ii) Carbonates

Thermal stability

The carbonates of alkali metals are stable towards heat. However, carbonate of lithium, when heated, decomposes to form lithium oxide. The carbonates of alkaline earth metals also decompose on heating to form oxide and carbon dioxide.

$\mathrm{Na}_{2} \mathrm{CO}_{3} \stackrel{\Delta}{\longrightarrow} \mathrm{Noeffect}$

$\mathrm{Li}_{2} \mathrm{CO}_{3} \stackrel{\Delta}{\longrightarrow} \mathrm{Li}_{2} \mathrm{O}+\mathrm{CO}_{2}$

$\mathrm{MgCO}_{3} \stackrel{\Delta}{\longrightarrow} \mathrm{MgO}+\mathrm{CO}_{2}$

Solubility

Carbonates of alkali metals are soluble in water with the exception of $\mathrm{Li}_{2} \mathrm{CO}_{3}$. Also, the solubility increases as we move down the group.

Carbonates of alkaline earth metals are insoluble in water.

(iii) Sulphates

Thermal stability

Sulphates of both group 1 and group 2 metals are stable towards heat.

Solubility

Sulphates of alkali metals are soluble in water. However, sulphates of alkaline earth metals show varied trends.

$\mathrm{BeSO}_{4}$ Fairly soluble

$\mathrm{MgSO}_{4}$ Soluble

$\mathrm{CaSO}_{4}$ Sparingly soluble

$\mathrm{SrSO}_{4}$ Insoluble

$\mathrm{BaSO}_{4}$ Insoluble

In other words, while moving down the alkaline earth metals, the solubility of their sulphates decreases.