Comparison: Solubility and Thermal Stability of Compounds of Alkali and Alkaline Earth Metals
Alkali metals (Group 1) and alkaline earth metals (Group 2) show distinct trends in solubility and thermal stability of their nitrates, carbonates, and sulphates. These trends arise due to differences in cation size, charge density, hydration enthalpy, lattice enthalpy, and polarizing power.
(a) Nitrates (NO₃⁻ salts)
| Property | Alkali Metals (Group 1) | Alkaline Earth Metals (Group 2) | Comparison / Key Trend |
|---|---|---|---|
| Solubility in water | Highly soluble (all nitrates are very soluble). Solubility generally increases slightly down the group (LiNO₃ < NaNO₃ < KNO₃ etc.). | Soluble in water. Solubility decreases down the group (Be(NO₃)₂ > Mg(NO₃)₂ > Ca(NO₃)₂ > Sr(NO₃)₂ > Ba(NO₃)₂). | Alkali metal nitrates are generally more soluble than alkaline earth metal nitrates (especially lower down the groups). |
| Thermal stability / Decomposition on heating | Most decompose to nitrite + O₂ (except LiNO₃): 2MNO₃ → 2MNO₂ + O₂ (M = Na, K, Rb, Cs). LiNO₃ decomposes to oxide: 4LiNO₃ → 2Li₂O + 4NO₂ + O₂. Stability increases down the group. |
Decompose to oxide + NO₂ + O₂: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂ (M = Be, Mg, Ca, Sr, Ba). Stability increases down the group (Mg(NO₃)₂ least stable, Ba(NO₃)₂ most stable). |
Alkali metal nitrates (except Li) are more thermally stable than alkaline earth metal nitrates. Group 1 nitrates require higher temperature for decomposition and often form nitrites (not oxides). |
Note: Lithium shows anomalous behaviour (diagonal relationship with Mg) – its nitrate decomposes like Group 2 nitrates.
(b) Carbonates (CO₃²⁻ salts)
| Property | Alkali Metals (Group 1) | Alkaline Earth Metals (Group 2) | Comparison / Key Trend |
|---|---|---|---|
| Solubility in water | Soluble (except Li₂CO₃ sparingly soluble). Solubility increases down the group (Li₂CO₃ < Na₂CO₃ < K₂CO₃ etc.). | Generally insoluble / sparingly soluble. Solubility decreases down the group (BeCO₃ > MgCO₃ > CaCO₃ > SrCO₃ > BaCO₃). | Alkali metal carbonates are much more soluble than alkaline earth metal carbonates. |
| Thermal stability / Decomposition on heating | Stable to heat (except Li₂CO₃): Li₂CO₃ → Li₂O + CO₂. Others (Na₂CO₃, K₂CO₃ etc.) do not decompose easily. |
Decompose to oxide + CO₂: MCO₃ → MO + CO₂. Stability increases down the group (BeCO₃ least stable, BaCO₃ most stable). |
Alkali metal carbonates (except Li) are more thermally stable than alkaline earth metal carbonates. Group 2 carbonates decompose more readily due to higher polarizing power of M²⁺ ions. |
(c) Sulphates (SO₄²⁻ salts)
| Property | Alkali Metals (Group 1) | Alkaline Earth Metals (Group 2) | Comparison / Key Trend |
|---|---|---|---|
| Solubility in water | Highly soluble. Solubility increases slightly down the group. | Soluble to insoluble. Solubility decreases sharply down the group (BeSO₄ and MgSO₄ soluble, CaSO₄ sparingly soluble, SrSO₄ and BaSO₄ insoluble). | Alkali metal sulphates are much more soluble than alkaline earth metal sulphates (especially lower members like BaSO₄). |
| Thermal stability / Decomposition on heating | Highly stable and do not decompose easily (except Li₂SO₄ decomposes like MgSO₄). | Stable to heat, but lower members (BeSO₄, MgSO₄) decompose to oxide + SO₃ on strong heating. Stability increases down the group (BaSO₄ most stable). |
Alkali metal sulphates are generally more thermally stable than alkaline earth metal sulphates (especially upper members like Be and Mg). |
Summary of Differences
- Solubility: Alkali metal salts (nitrates, carbonates, sulphates) are generally more soluble in water than the corresponding alkaline earth metal salts.
- Thermal stability: Alkali metal compounds (especially carbonates and sulphates) are more thermally stable than alkaline earth metal compounds due to lower charge density and weaker polarization of the anion by M⁺ ions compared to M²⁺ ions.
- Trends within groups: Solubility and thermal stability often increase down the group for alkali metals, while solubility decreases and thermal stability increases down the group for alkaline earth metals.