2.2.2 Solubility of Group 2 Compounds: Hydroxides & Sulfates

Group 2 elements, known as the alkaline earth metals, exhibit trends in the solubility of their hydroxides and sulfates as you move down the group from magnesium to barium. These trends are important in both theoretical and practical contexts, such as laboratory tests and industrial applications.

Solubility of Group 2 Hydroxides

General trend: The solubility of hydroxides increases down the group from magnesium hydroxide to barium hydroxide.

Magnesium Hydroxide, $Mg(OH)₂$

• Sparingly soluble in water.
• This means it forms a suspension rather than dissolving fully, which is why it is often used as an antacid or a mild laxative (known as "milk of magnesia").

Calcium Hydroxide, $Ca(OH)₂$

• More soluble than magnesium hydroxide.
• In solution, it is known as "limewater" and is used in agriculture to neutralise acidic soils.

Barium hydroxide, $Ba(OH)₂$

• Highly soluble in water, forming a strong alkaline solution.
• This property makes it useful for laboratory testing of sulfates.

Solubility of Group 2 Sulfates

General trend: The solubility of sulfates decreases down the group from magnesium sulfate to barium sulfate.

Magnesium sulfate, $MgSO₄$

• Highly soluble in water and is often used in medicine as Epsom salts.

Calcium sulfate, $CaSO₄$

• Sparingly soluble, forming a weak solution.
• It is used in the production of plaster and in other construction materials.

Barium sulfate, $BaSO₄$

• Insoluble in water.
• Due to its insolubility, it is used in medical imaging as a contrast agent for X-rays of the digestive system, despite barium ions being toxic in other compounds.

Testing the Solubility of Hydroxides and Sulfates

Students can investigate the solubility trends of Group 2 hydroxides and sulfates using simple laboratory experiments:

Hydroxide test

Mix solutions of soluble Group 2 salts with sodium hydroxide. As the hydroxides precipitate, observe the formation of a solid and the clarity of the solution.

For example, adding sodium hydroxide to magnesium chloride will produce a suspension of sparingly soluble $Mg(OH)₂$, while adding sodium hydroxide to barium chloride will result in a clearer solution due to $Ba(OH)₂$**'**s higher solubility.

Sulfate test

Mix solutions of soluble Group 2 salts with sulfuric acid. The formation of a precipitate indicates the solubility of the sulfate.

Barium sulfate will precipitate out as it is insoluble in water. This reaction is used as a test for sulfate ions in a solution ($Ba²⁺ + SO₄²⁻ → BaSO₄(s)$).