Group 2 Compounds and Uses

Learning Objectives (OCR Spec 3.1.2)

By the end of this lesson you should be able to:

• State and explain the trend in solubility of Group 2 hydroxides (increasing down the group)
• State and explain the trend in solubility of Group 2 sulfates (decreasing down the group)
• Recall the reactions of Group 2 oxides with water
• Know key uses of Group 2 compounds: Mg(OH)2 in antacids, Ca(OH)2 in liming acidic soils, CaCO3 for flue-gas desulfurisation, BaSO4 in barium meals
• Link solubility and basicity to specific applications

Group 2 Oxides with Water

Group 2 oxides (MO) are ionic and react with water to form metal hydroxides:

MO(s) + H2O(l) → M(OH)2(aq or s)

Oxide	Reaction with water	Solubility of product
MgO	Slow, partial reaction	Mg(OH)2 is only sparingly soluble
CaO	Exothermic - "slaking of lime"	Ca(OH)2 slightly soluble ("limewater")
SrO	Vigorous	Sr(OH)2 more soluble
BaO	Very vigorous	Ba(OH)2 readily soluble

The resulting solutions are alkaline because hydroxide ions (OH-) are released. As you go down the group, the oxides react more vigorously and the resulting hydroxides are more soluble → higher OH- concentration → higher pH.

Hydroxide	Approximate pH of saturated solution
Mg(OH)2	~10
Ca(OH)2	~12
Sr(OH)2	~13
Ba(OH)2	~13-14

Solubility of Group 2 Hydroxides: Increasing Down the Group

graph TD
  A[Group 2 Hydroxides] --> B[Mg OH 2: insoluble 'milk of magnesia']
  A --> C[Ca OH 2: slightly soluble 'limewater']
  A --> D[Sr OH 2: more soluble]
  A --> E[Ba OH 2: readily soluble]
  F[Going down: solubility INCREASES] --> A

Hydroxide	Solubility (mol/100g water at 25 degC)
Mg(OH)2	~0.00002 (insoluble)
Ca(OH)2	~0.002 (sparingly soluble)
Sr(OH)2	~0.008
Ba(OH)2	~0.05 (soluble)

OCR does not require you to explain the thermodynamic reason for this trend in detail (that belongs to A-Level energetics/lattice enthalpy in the enthalpy chapter); you need to recall the direction and its chemical consequences.

Solubility of Group 2 Sulfates: Decreasing Down the Group

The trend for sulfates is opposite to that of hydroxides:

graph TD
  A[Group 2 Sulfates] --> B[MgSO4: very soluble 'Epsom salts']
  A --> C[CaSO4: slightly soluble 'plaster of Paris']
  A --> D[SrSO4: almost insoluble]
  A --> E[BaSO4: insoluble 'barite']
  F[Going down: solubility DECREASES] --> A

Sulfate	Solubility (mol/100g water)
MgSO4	~0.3 (very soluble)
CaSO4	~0.0002 (sparingly soluble)
SrSO4	~6 x 10-6
BaSO4	~1 x 10-8 (insoluble)

BaSO4's extreme insolubility has two famous applications:

1. Qualitative test for sulfate ions: add acidified BaCl2 solution to the unknown. A white precipitate of BaSO4 confirms SO4^2- is present.
2. Barium meals in medical imaging (see Uses below).

Key Uses of Group 2 Compounds

1. Mg(OH)2 - "Milk of Magnesia" Antacid

Mg(OH)2 is a weak, insoluble base. Because it is insoluble, it does not raise the pH of the stomach too far - it only reacts with the excess acid present. It neutralises hydrochloric acid in the stomach:

Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)

Used to treat indigestion and heartburn. Its insolubility is an advantage: it avoids over-alkalinising the stomach.

2. Ca(OH)2 - "Slaked Lime" for Liming Acidic Soils

Agricultural soils become acidic over time (due to acid rain, nitrogen fertiliser, organic matter decay). Ca(OH)2 is spread on fields to neutralise acidity and raise the pH to a level favourable for crops:

Ca(OH)2(s) + 2H+(aq) → Ca2+(aq) + 2H2O(l)