Magnesium Compounds (Mg) - Revision Guide

Introduction to Major Compounds of Magnesium

Magnesium (Mg), an alkaline earth metal, forms several important compounds widely studied in high school chemistry. These compounds exhibit diverse chemical and physical properties and find applications in various fields, from medicine to construction. Key compounds include its oxide, hydroxide, and sulfate.

Magnesium Oxide (MgO)

Magnesium oxide is a highly stable inorganic compound of magnesium.

• Chemical Formula: MgO

• Common Name: Magnesia, Periclase (mineral form)

• Nature: Basic oxide

• Laboratory Preparation: Magnesium oxide is typically prepared in the laboratory by the combustion of magnesium metal ribbon in air or oxygen.

  Balanced Chemical Equation:

  2Mg(s) + O2(g) → 2MgO(s)

  It can also be obtained by the thermal decomposition of magnesium carbonate or magnesium hydroxide at high temperatures.

  Balanced Chemical Equations:

  MgCO3(s) \xrightarrow{\Delta} MgO(s) + CO2(g)

  Mg(OH)2(s) \xrightarrow{\Delta} MgO(s) + H2O(g)

Magnesium Hydroxide (Mg(OH)₂)

Magnesium hydroxide is a white, amorphous, slightly soluble compound.

• Chemical Formula: Mg(OH)₂

• Common Name: Milk of Magnesia (aqueous suspension), Brucite (mineral form)

• Preparation Process: Magnesium hydroxide can be prepared by the precipitation reaction of a soluble magnesium salt with a strong base, or by the hydration of magnesium oxide.

  1. From Magnesium Chloride: When an aqueous solution of magnesium chloride (or any soluble Mg salt) is treated with an aqueous solution of sodium hydroxide:

     MgCl2(aq) + 2NaOH(aq) → Mg(OH)2(s) + 2NaCl(aq)

  2. From Magnesium Oxide: Magnesium oxide reacts slowly with water to form magnesium hydroxide. This reaction is exothermic.

     MgO(s) + H2O(l) → Mg(OH)2(aq)

• Properties:

  • White amorphous solid.
  • Very sparingly soluble in water, forming a suspension known as “Milk of Magnesia.”
  • Weak base: The aqueous suspension is slightly alkaline.
  • Antacid properties: Neutralizes excess stomach acid.

• Exam-Relevant Reactions:

  1. Reaction with Acids (Neutralization): Magnesium hydroxide reacts with acids to form the corresponding magnesium salt and water. This is why it acts as an antacid.

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

     Mg(OH)2(s) + H2SO4(aq) → MgSO4(aq) + 2H2O(l)

  2. Thermal Decomposition: Upon heating, magnesium hydroxide decomposes to form magnesium oxide and water vapor.

     Mg(OH)2(s) \xrightarrow{\Delta} MgO(s) + H2O(g)

Magnesium Sulphate (MgSO₄)

Magnesium sulphate is a widely used magnesium salt, often found as a heptahydrate.

• Chemical Formula: MgSO₄ (anhydrous), MgSO₄·7H₂O (Epsom salt)

• Common Name: Epsom Salt (heptahydrate form)

• Brief Guide: Magnesium sulphate can be prepared by the reaction of magnesium metal, magnesium oxide, or magnesium carbonate with dilute sulphuric acid.

  Balanced Chemical Equations:

  1. From Magnesium Metal:

     Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)

  2. From Magnesium Oxide:

     MgO(s) + H2SO4(aq) → MgSO4(aq) + H2O(l)

  3. From Magnesium Carbonate:

     MgCO3(s) + H2SO4(aq) → MgSO4(aq) + H2O(l) + CO2(g)

  • Properties: White crystalline solid, highly soluble in water. The heptahydrate form loses water of crystallization upon heating.
  • Uses: Used in medicine as a laxative, anticonvulsant, and in bath salts. Also used in agriculture to correct magnesium deficiencies in soil.

Comparative Properties of Important Magnesium Compounds

Property	Magnesium Oxide (MgO)	Magnesium Hydroxide (Mg(OH)₂)	Magnesium Sulphate (MgSO₄)
Physical State	White solid	White amorphous solid	White crystalline solid
Solubility in H₂O	Sparingly soluble	Very sparingly soluble	Highly soluble
Nature	Basic oxide	Weak base	Neutral salt
Thermal Stability	Very stable, high melting point	Decomposes upon heating	Decomposes at high temperatures
Reaction with Acid	Reacts to form salt and water	Reacts to form salt and water	No reaction (already a salt)
Key Use	Refractories, antacid, insulation	Antacid, laxative	Laxative, bath salts, agriculture