Group 2 Oxides & Water
- Group 2 oxides react with water, releasing hydroxide ions, to form alkaline solutions:
CaO (s) + H2O (l) → Ca2+ (aq) + 2OH- (aq)
- These solutions get more alkaline going down the group
- This is due to increasing solubility meaning the resulting solutions formed contain more OH- ions
- When the solution becomes saturated any further metal and hydroxide ions will form a precipitate:
Ca2+ (aq) + 2OH- (aq) → Ca(OH)2 (s)
- Solid magnesium hydroxide, Mg(OH)2 (s), is only slightly soluble in water
- The solution has a low OH-(aq) concentration and a pH ≅ 10
- Barium hydroxide, Ba(OH)2 (s), is much more soluble in water
- The solution has a greater OH-(aq) concentration and a pH ≅ 13
Uses of Group 2 Compounds
- Due to their basic properties and ability to neutralise acids, Group 2 oxides, hydroxides and carbonates have many uses
Group 2 compounds in agriculture
- Calcium hydroxide, Ca(OH)2, is reasonably soluble in water and is used in agriculture to neutralise acidic soils
Ca(OH)2 (s) + 2H+ (aq) → Ca2+ (aq) + 2H2O (l)
- It is important to regulate how much is added as If too much is added then this will result in soil becoming too alkaline to sustain crop growth
Group 2 compounds in medicine
- Magnesium hydroxide, Mg(OH)2, is partially soluble in water and is used in suspension (known as 'milk of magnesia') to neutralise excess acid in the stomach and treat constipation
Mg(OH)2 (s) + 2HCl (aq) → MgCl2 (aq) + 2H2O (l)
- It is safe to use as the magnesium hydroxide is only partially soluble making the solution only slightly alkaline (pH ≅ 10) due to the low OH- concentration
