Hydroxides: The Basic Building Blocks
What is a Hydroxide Ion?
At the heart of every hydroxide compound is the hydroxide ion. Imagine a water molecule, H$_2$O. If it loses one of its hydrogen nuclei (a proton), what remains is the hydroxide ion, written as OH$^-$. This ion has a negative charge because it now has one more electron than it has protons. This negative charge makes the hydroxide ion highly attractive to positive ions, called cations, leading to the formation of ionic compounds known as hydroxides.
Meet the Group 2 Hydroxides
The periodic table is organized into groups (vertical columns). Group 2 is also known as the alkaline earth metals. This group includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Each of these metals can form a hydroxide with the general formula M(OH)$_2$, where 'M' represents the metal atom.
For example:
- Magnesium hydroxide: Mg(OH)$_2$
- Calcium hydroxide: Ca(OH)$_2$
- Barium hydroxide: Ba(OH)$_2$
These compounds are all crystalline solids at room temperature and are classic examples of bases.
Trends in Solubility and Basicity
One of the most fascinating aspects of Group 2 hydroxides is how their properties change predictably as you move down the group. This is known as a trend.
| Hydroxide | Chemical Formula | Solubility in Water | Strength as a Base |
|---|---|---|---|
| Beryllium Hydroxide | Be(OH)$_2$ | Insoluble | Amphoteric[1] |
| Magnesium Hydroxide | Mg(OH)$_2$ | Very Slightly Soluble | Weak Base |
| Calcium Hydroxide | Ca(OH)$_2$ | Sparingly Soluble | Medium-Strength Base |
| Strontium Hydroxide | Sr(OH)$_2$ | Soluble | Strong Base |
| Barium Hydroxide | Ba(OH)$_2$ | Soluble | Strong Base |
The trend is clear: Solubility increases down the group. Beryllium hydroxide is practically insoluble, while barium hydroxide dissolves quite readily. This directly affects their strength as bases. A base must produce OH$^-$ ions in water to act as one. The more soluble the hydroxide, the more OH$^-$ ions it can release, making it a stronger base. Therefore, basicity also increases down the group.
How Are Group 2 Hydroxides Made?
There are two primary methods for producing these important compounds:
1. Reaction of the Metal with Water: Metals like calcium, strontium, and barium react with water to form their respective hydroxides and hydrogen gas. The reaction with calcium is a classic example:
Ca (s) + 2H$_2$O (l) → Ca(OH)$_2$ (aq) + H$_2$ (g)
This is a relatively vigorous reaction. Magnesium reacts very slowly with cold water but reacts with steam. Beryllium does not react with water at all.
2. Reaction of the Metal Oxide with Water: This is a more common industrial method. The metal is first burned in air to form its oxide, which is then reacted with water (a process called slaking).
CaO (s) + H$_2$O (l) → Ca(OH)$_2$ (s)
This reaction is highly exothermic (releases heat) and is used to produce calcium hydroxide, or slaked lime, on a large scale.
Hydroxides at Work: From Medicine to Construction
Group 2 hydroxides are not just laboratory curiosities; they are workhorse chemicals in many industries.
Other key applications include:
- Calcium Hydroxide (Slaked Lime):
- Agriculture: Used to neutralize acidic soils, creating a better environment for crops to grow.
- Water Treatment: Adjusts the pH of water to prevent pipe corrosion and to help remove impurities.
- Construction: A key ingredient in mortar and plaster. It is also used to make whitewash.
- Barium Hydroxide: Used in laboratories to test for the presence of sulfate ions. It also helps in the purification of sugar.
- Magnesium Hydroxide: Besides being an antacid, it is used as a flame retardant in plastics.
Important Questions
A: No, it is not. Sodium (Na) is a Group 1 metal, also known as an alkali metal. Group 1 hydroxides, like NaOH and KOH, are generally more soluble and stronger bases than Group 2 hydroxides.
A: Beryllium is a very small atom with a high charge density. This gives its compounds, including Be(OH)$_2$, unique properties. Unlike the other Group 2 hydroxides which are purely basic, beryllium hydroxide is amphoteric. This means it can react with both acids and bases. It dissolves in strong bases to form complexes like [Be(OH)$_4$]$^{2-}$.
A: A simple and safe way is to use an indicator. A universal indicator or litmus paper will change color. Bases turn red litmus paper blue and universal indicator green, blue, or purple, depending on the strength. For example, a solution of calcium hydroxide (limewater) will turn red litmus paper blue, confirming it is a base.
Footnote
[1] Amphoteric: A substance that can act as both an acid and a base, depending on the conditions. Beryllium hydroxide (Be(OH)$_2$) can react with strong acids as a base and with strong bases as an acid.