The strong bases of the alkaline earth metals are primarily the hydroxides of calcium, strontium, and barium. These compounds, though varying in solubility, completely dissociate in water, releasing hydroxide ions (OH⁻) and making them highly effective at increasing the pH of a solution.
Understanding Strong Bases from Alkaline Earth Metals
A strong base is defined as a chemical species that completely dissociates in an aqueous solution, releasing all of its hydroxide (OH⁻) ions (or reacting completely to form OH⁻ ions). This complete dissociation leads to a significant increase in the solution's pH. The hydroxides of Group 1A (alkali metals) and Group 2A (alkaline earth metals) are generally recognized as strong bases. Examples of strong bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).
While all alkaline earth metals form hydroxides, not all are considered "strong bases" in the practical sense due to differences in their solubility and other chemical properties down the group. The strength of these bases generally increases as you move down Group 2A due to increasing atomic radius and decreasing charge density of the metal ion, which weakens the bond to the hydroxide and allows for easier dissociation.
Key Strong Bases Among Alkaline Earth Metals
The principal strong bases derived from alkaline earth metals are:
- Calcium Hydroxide (Ca(OH)₂)
- Strontium Hydroxide (Sr(OH)₂)
- Barium Hydroxide (Ba(OH)₂)
These compounds share the characteristic of complete dissociation of the dissolved portion into their respective metal cations and hydroxide ions.
Detailed Overview of Strong Alkaline Earth Hydroxides
The following table summarizes the strong bases of the alkaline earth metals:
| Alkaline Earth Metal Hydroxide | Chemical Formula | Solubility in Water (at 20°C) | Classification | Key Characteristics |
|---|---|---|---|---|
| Calcium Hydroxide | Ca(OH)₂ | Sparingly soluble | Strong Base | "Slaked lime," common in industry, agriculture, and water treatment. |
| Strontium Hydroxide | Sr(OH)₂ | Moderately soluble | Strong Base | Less common but a strong base. Used in refining beet sugar. |
| Barium Hydroxide | Ba(OH)₂ | Highly soluble | Strong Base | Most soluble and strongest of the common alkaline earth hydroxides. Toxic. |
Let's explore each in more detail:
Calcium Hydroxide (Ca(OH)₂)
Commonly known as "slaked lime" or "calcium dihydroxide," calcium hydroxide is a white powder that is sparingly soluble in water. Despite its limited solubility, the portion that does dissolve completely dissociates, making it a strong base. Its saturated solution is known as "limewater" and is mildly alkaline.
- Applications:
- Used in wastewater treatment to neutralize acidic waste.
- A component in mortars and plasters in construction.
- Soil amendment in agriculture to reduce soil acidity.
- A food additive (e.g., in nixtamalization of corn).
For more information, refer to the Calcium Hydroxide Wikipedia page.
Strontium Hydroxide (Sr(OH)₂)
Strontium hydroxide is a stronger base than calcium hydroxide and is moderately soluble in water. It is less commonly encountered than calcium hydroxide but is a significant strong base.
- Applications:
- Used in the refining of beet sugar.
- As a source of strontium salts.
For further reading, visit the Strontium Hydroxide Wikipedia page.
Barium Hydroxide (Ba(OH)₂)
Barium hydroxide is the most soluble and, consequently, the most commonly encountered strong base among the alkaline earth metal hydroxides. It dissolves readily in water to form a highly alkaline solution. It is important to note that barium compounds are toxic.
- Applications:
- Used in analytical chemistry for titrations of weak acids.
- As a component in some industrial lubricants.
- In the production of glass and ceramics.
Learn more about its properties on the Barium Hydroxide LibreTexts page.
Other Alkaline Earth Metal Hydroxides
Not all alkaline earth metal hydroxides are classified as strong bases due to their unique properties:
Beryllium Hydroxide (Be(OH)₂)
Beryllium hydroxide is unique among Group 2A hydroxides because it is amphoteric, meaning it can act as both an acid and a base. It is also quite insoluble. Due to its amphoteric nature, it is not considered a strong base.
Magnesium Hydroxide (Mg(OH)₂)
Magnesium hydroxide (Mg(OH)₂) is sparingly soluble in water. While the small amount that does dissolve dissociates completely, its very low solubility significantly limits the concentration of hydroxide ions it can produce in solution. Consequently, it is often practically considered a weak base or a mild base in many contexts (e.g., "Milk of Magnesia" as an antacid). However, it is technically a strong electrolyte because the dissolved ions are fully dissociated.
Radium Hydroxide (Ra(OH)₂)
Radium hydroxide (Ra(OH)₂) is expected to be a strong base, similar to barium hydroxide, given its position below barium in Group 2A. However, due to radium's high radioactivity and extreme scarcity, it is not practically used or discussed in the context of common strong bases.
Practical Insights
When determining the pH of a strong base solution, such as those made from calcium, strontium, or barium hydroxide, we assume complete dissociation. For instance, a 0.01 M solution of Ba(OH)₂ would produce 0.02 M OH⁻ ions (since each Ba(OH)₂ produces two OH⁻ ions), allowing for direct calculation of pOH and subsequently pH. This complete dissociation is what defines their "strength."
