↪ Solubility, the extent of dissolution is defined as the amount of solute required to saturate the given amount of solvent at a given temperature.
It is commonly expressed as g/100g.
↪ Dissolution of a solute or ionic crystal in water is due to the absorption energy evolved in course of hydration of ionic species present in the crystal.
↪ Hydration energy is the heat evolved when one mole of ionic species is hydrated completely by water molecules.
↪ The energy required to dissolve a solute is used up to break the crystal lattice held together by lattice energy and lattice energy is the energy required to break one mole of the crystal into free gaseous ionic species.
↪ The energy required to break the lattice is endothermic process and for hydration of ions the process is exothermic.
↪ Increase of lattice energy decreases the solubility whereas increase of hydration energy increases the solubility.
↪ The extent of dissolution of a solute in water is therefore decided by solute or ionic substance in water.
Based upon this information the trend of variation of solubilities of alkaline earth metal hydroxides, carbonates and sulphates is discussed as follows:
⁕ Solubility of alkaline earth metal hydroxides
It has been observed the following solubility chart of the hydroxides at 20°C.
|Hydroxides||Solubility (g/100g) in water|
|Mg(OH)2||3.08 × 10-3|
↪ It shows that alkaline earth metal hydroxides have low solubility in water.
↪ This is because they possess high lattice energy relative to hydration energy.
↪ Further it shows that solubility increases down the group i.e., solubility of hydroxides in water increases in general from magnesium to barium.
Ba(OH)2 > Sr(OH)2 > Ca(OH)2 > Mg(OH)2 > Be(OH)2
⁕ Solubility of alkaline earth metal carbonates and sulphates
Solubility chart of alkaline earth metal carbonates and sulphates in water at 20°C is given as:
|MCO3||3.85 × 10-2||6.9 × 10-3||1.54 × 10-4||1.3 × 10-5|
|MSO4||35.5||0.204||1.04 × 10-2||2.4 × 10-4|
↪ It shows that the solubility of alkaline earth metal carbonates and sulphates decreases from magnesium to barium.
↪ Further it shows that solubility of alkaline earth metals is decreased from top to bottom in a group.
BeCO3 > MgCO3 > CaCO3 > SrCO3 > BaCO3
BeSO4 > MgSO4 > CaSO4 > SrSO4 > BaSO4